Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
BULK HEATING A SUBSURFACE FORMATION
Cross-Reference To Related Application
[0001] This application claims the benefit of U.S. Provisional Patent
Application
62/087,655 filed December 4, 2014 entitled BULK HEATING A SUBSURFACE
FORMATION, the entirety of which is incorporated by reference herein.
Field
[0002] The present disclosure relates to systems and methods for bulk
heating a
subsurface formation. More specifically, the present disclosure relates to
systems and
methods for mitigating subsurface shunts during bulk heating of a subsurface
formation.
Background
[0003] Certain subsurface formations may include organic matter, such as
shale oil,
bitumen, and/or kerogen, which has material and chemical properties that may
complicate
production of fluid hydrocarbons from the subsurface formation. For example,
the organic
matter may not flow at a rate sufficient for production. Moreover, the organic
matter may not
include sufficient quantities of desired chemical compositions (typically
smaller
hydrocarbons). Hence, recovery of useful hydrocarbons from such subsurface
formations
may be uneconomical or impractical.
[0004] Heating of organic matter-containing subsurface formations may be
particularly
useful to generate producible hydrocarbons from immature organic-rich source
rocks in situ.
For example, heating organic matter-containing subsurface formations may
pyrolyze kerogen
into mobile liquids and gases, and may reduce the viscosity of heavy oil to
enhance
hydrocarbon mobility.
[0005] One method to heat a subsurface formation is to conduct electricity
through the
formation and, thus, resistively heat the subsurface formation. This method of
heating a
subsurface formation may be referred to as "bulk heating" or "volumetric
heating" of the
subsurface formation. Bulk heating of the subsurface formation may be
accomplished by
conducting electricity between electrode assemblies in the subsurface
formation and through
a subsurface region (volume) of naturally electrically-resistive rock between
the electrode
assemblies. The electrode assemblies may be contained in wellbores and/or
manmade
fractures, and the electrode assemblies may include electrical conductors,
such as metal rods
and/or granular electrically conductive materials. Bulk heating may include
applying a
1
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
voltage gradient across the subsurface region to initiate a relatively uniform
electrical current
flow through the subsurface region. Heat may be generated within the volume of
the
subsurface region due to electrical resistive loss resulting from the current
flow through the
volume of the subsurface region (Joule heating). Bulk heating performance may
not be
dependent on applied thermal gradients or rock thermal conductivity ¨ physical
constraints
that can impede feasibility of subsurface formation heating schemes based on
thermal
conduction.
[0006] As
heating occurs in subsurface regions between the pairs of electrode
assemblies,
the electrical conductivity (or alternatively, resistivity) of the subsurface
regions may change.
This change in the electrical conductivity (or resistivity) of the subsurface
regions may be due
to physical and/or chemical changes within the subsurface regions, for
example, due to
temperature sensitivity of the electrical resistance of the native rock, due
to native brine
boiling off, due to disassociation and boil off of chemically bound water,
and/or due to
pyrolysis (and/or coking) of native hydrocarbons.
[0007]
Heating a subsurface region via electrical conduction through the subsurface
region may not occur uniformly and may suffer from instabilities, in
particular if conductivity
within the subsurface region increases strongly with increasing temperature.
The
conductivity increase within the subsurface region may result from pyrolysis
occurring and
may lead to the formation of electrically conductive coke or other graphitic
materials. When
electrical conductivity increases strongly with increasing temperature, hotter
regions will
become even hotter, since electricity may channel through the hotter (and more
conductive)
regions. Ultimately, this positive correlation between temperature and
electrical conductivity
may lead to the formation of a narrow, highly conductive shunt (also called a
channel)
between the electrode assemblies that will short-circuit the electrical flow
between the
electrode assemblies. Although the electrode assemblies may be large in extent
or area, the
bulk of the electrical flow may occur through a very small zone, and heating
of the
subsurface region between the electrode assemblies may be quite uneven. This
phenomenon
is analogous to viscous fingering that may occur when a low viscosity fluid is
driven through
a higher viscosity fluid. In bulk heating, the tendency for shunting
instabilities to occur and
the rate of shunt growth may be dependent on the heating rate and the extent
to which
electrical and physical property heterogeneities exist within the subsurface
regions.
[0008]
Conventional methods to minimize the effects of subsurface shunts during bulk
heating include disconnecting at least one of the affected electrode
assemblies (electrode
2
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
assemblies that conduct current into a shunted region). Disconnecting the
affected electrode
assembly stops the generation of heat in the shunted region, and any other
(unaffected)
subsurface regions, served by the affected electrode.
[0009] In view of the aforementioned disadvantages, there is a need for
alternative
methods and systems for bulk heating a subsurface formation. More
specifically, there is a
need for alternative methods and systems for mitigating the effects of
subsurface shunts
during bulk heating of a subsurface formation.
Summary
[0010] It is an object of the present disclosure to provide systems and
methods for bulk
heating of a subsurface formation. More specifically, it is an object of the
present disclosure
to provide systems and methods for mitigating effects of subsurface shunts
during bulk
heating of a subsurface formation.
[0011] A method for bulk heating a subsurface formation with at least a
pair of electrode
assemblies in the subsurface formation may include electrically powering the
pair of
electrode assemblies to resistively heat a subsurface region between the pair
of electrode
assemblies with electrical current flowing through the subsurface region
between the pair of
electrode assemblies; flowing a shunt mitigator into at least one of the pair
of electrode
assemblies; and, responsive to a shunt indicator, mitigating a subsurface
shunt between the
pair of electrode assemblies with the shunt mitigator, wherein the shunt
indicator indicates a
presence of the subsurface shunt.
[0012] A method for bulk heating a subsurface formation with at least a
pair of electrode
assemblies in the subsurface formation may include electrically powering the
pair of
electrode assemblies to resistively heat an in situ resistive heater, wherein
the in situ resistive
heater is a subsurface region of the subsurface formation between the pair of
electrode
assemblies; upon determining a presence of a subsurface shunt between the pair
of electrode
assemblies, forming a modified in situ resistive heater by mitigating the
subsurface shunt; and
electrically powering the pair of electrode assemblies to resistively heat the
modified in situ
resistive heater.
[0013] A subsurface formation may include at least a pair of electrode
assemblies,
wherein each electrode assembly of the pair of electrode assemblies may
include an
electrically conductive material, and wherein at least one electrode assembly
of the pair of
3
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
electrode assemblies may include a shunt mitigator that is selected to undergo
a state change
in response to a shunt indicator.
[0014] The foregoing has broadly outlined the features of the present
disclosure so that
the detailed description that follows may be better understood. Additional
features will also
be described herein.
Description of the Drawings
[0015] These and other features, aspects and advantages of the present
disclosure will
become apparent from the following description and the accompanying drawings,
which are
briefly discussed below.
[0016] Fig. 1 is a schematic representation of electrode assemblies in a
subsurface
formation.
[0017] Fig. 2 is a schematic representation of bulk heating methods to
mitigate
subsurface shunt formation.
[0018] Fig. 3 is a schematic representation of the system of Fig. 1 during
the application
of a shunt mitigator.
[0019] Fig. 4 is a schematic representation of the system of Fig. 3 after
the subsurface
shunt is mitigated.
[0020] Fig. 5 is a schematic representation of bulk heating methods that
are responsive to
subsurface shunt formation.
[0021] It should be noted that the figures are merely examples and no
limitations on the
scope of the present disclosure are intended thereby. Further, the figures are
generally not
drawn to scale, but are drafted for purposes of convenience and clarity in
illustrating various
aspects of the disclosure.
Detailed Description
[0022] For the purpose of promoting an understanding of the principles of
the disclosure,
reference will now be made to the features illustrated in the drawings and
specific language
will be used to describe the same. It will nevertheless be understood that no
limitation of the
scope of the disclosure is thereby intended. Any alterations and further
modifications, and
any further applications of the principles of the disclosure as described
herein, are
contemplated as would normally occur to one skilled in the art to which the
disclosure relates.
4
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
It will be apparent to those skilled in the relevant art that some features
that are not relevant
to the present disclosure may not be shown in the drawings for the sake of
clarity.
[0023] At
the outset, for ease of reference, certain terms used in this application and
their
meanings as used in this context are set forth below. To the extent a term
used herein is not
defined below, it should be given the broadest definition persons in the
pertinent art have
given that term as reflected in at least one printed publication or issued
patent. Further, the
present processes are not limited by the usage of the terms shown below, as
all equivalents,
synonyms, new developments and terms or processes that serve the same or a
similar purpose
are considered to be within the scope of the present disclosure.
[0024] As
used herein, the term "hydrocarbon" refers to an organic compound that
includes primarily, if not exclusively, the elements hydrogen and carbon.
Hydrocarbons may
also include other elements, such as, but not limited to, halogens, metallic
elements, nitrogen,
oxygen, and/or sulfur. Hydrocarbons generally fall into two classes:
aliphatic, or straight
chain hydrocarbons, and cyclic, or closed ring hydrocarbons, including cyclic
terpenes.
Examples of hydrocarbon-containing materials include any form of natural gas,
oil, coal,
heavy oil and kerogen that can be used as a fuel or upgraded into a fuel.
[0025]
"Heavy oil" includes oils which are classified by the American Petroleum
Institute ("API"), as heavy oils, extra heavy oils, or bitumens. The term
"heavy oil" includes
bitumen. Heavy oil may have a viscosity of about 1,000 centipoise (cP) or
more, 10,000 cP
or more, 100,000 cP or more, or 1,000,000 cP or more. In general, a heavy oil
has an API
gravity between 22.3 API (density of 920 kilograms per meter cubed (kg/m3) or
0.920 grams
per centimeter cubed (g/cm3)) and 10.0 API (density of 1,000 kg/m3 or 1
g/cm3). An extra
heavy oil, in general, has an API gravity of less than 10.0 API (density
greater than 1,000
kg/m3 or 1 g/cm3). For example, a source of heavy oil includes oil sand or
bituminous sand,
which is a combination of clay, sand, water and bitumen. The recovery of heavy
oils is based
on the viscosity decrease of fluids with increasing temperature or solvent
concentration.
Once the viscosity is reduced, the mobilization of fluid by steam, hot water
flooding, or
gravity is possible. The reduced viscosity makes the drainage or dissolution
quicker and
therefore directly contributes to the recovery rate.
[0026] As
used herein, the term "fluid" refers to gases, liquids, and combinations of
gases
and liquids, as well as to combinations of gases and solids, and combinations
of liquids and
solids.
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
[0027] As used herein, the term "formation hydrocarbons" refers to both
light and/or
heavy hydrocarbons and solid hydrocarbons that are contained in an organic-
rich rock
formation. Formation hydrocarbons may be, but are not limited to, natural gas,
oil, kerogen,
oil shale, coal, tar, natural mineral waxes, and asphaltenes.
[0028] As used herein, the term "gas" refers to a fluid that is in its
vapor phase at 1
atmosphere (atm) and 15 degrees Celsius ( C).
[0029] As used herein, the term "kerogen" refers to a solid, insoluble
hydrocarbon that
may principally contain carbon, hydrogen, nitrogen, oxygen, and/or sulfur.
[0030] As used herein, the term "oil" refers to a hydrocarbon fluid
containing primarily a
mixture of condensable hydrocarbons.
[0031] As used herein, the term "oil shale" refers to any fine-grained,
compact,
sedimentary rock containing organic matter made up mostly of kerogen, a high-
molecular
weight solid or semi-solid substance that is insoluble in petroleum solvents
and is essentially
immobile in its rock matrix.
[0032] As used herein, the term "organic-rich rock" refers to any rock
matrix holding
solid hydrocarbons and/or heavy hydrocarbons. Rock matrices may include, but
are not
limited to, sedimentary rocks, shales, siltstones, sands, silicilytes,
carbonates, and diatomites.
Organic-rich rock may contain kerogen.
[0033] As used herein, the term "organic-rich rock formation" refers to any
formation
containing organic-rich rock. Organic-rich rock formations include, for
example, oil shale
formations, coal formations, oil sands formations or other formation
hydrocarbons.
[0034] As used herein, "overburden" refers to the material overlying a
subsurface
(subterranean) reservoir. The overburden may include rock, soil, sandstone,
shale, mudstone,
carbonate and/or ecosystem above the subsurface reservoir. During surface
mining, the
overburden is removed prior to the start of mining operations. The overburden
may refer to
formations above or below free water level. The overburden may include zones
that are
water saturated, such as fresh or saline aquifers. The overburden may include
zones that are
hydro carbon bearing.
[0035] As used herein, the term "pyrolysis" refers to the breaking of
chemical bonds
through the application of heat. For example, pyrolysis may include
transforming a
compound into one or more other substances by heat alone or by heat in
combination with an
oxidant. Pyrolysis may include modifying the nature of the compound by
addition of
6
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
hydrogen atoms which may be obtained from molecular hydrogen, water, carbon
dioxide, or
carbon monoxide. Heat may be transferred to a section of the formation to
cause pyrolysis.
[0036] As used herein, "reservoir," "subsurface reservoir," or
"subterranean reservoir" is
a subsurface rock or sand formation from which a production fluid or resource
can be
harvested. The rock formation may include sand, granite, silica, carbonates,
clays, and
organic matter, such as oil shale, light or heavy oil, gas, or coal, among
others. Reservoirs
can vary in thickness from less than one foot (0.3048 meter (m)) to hundreds
of feet
(hundreds of meters).
[0037] As used herein, the term "solid hydrocarbons" refers to any
hydrocarbon material
that is found naturally in substantially solid form at formation conditions.
Non-limiting
examples include kerogen, coal, shungites, asphaltites, and natural mineral
waxes.
[0038] As used herein "subsurface formation" refers to the material
existing below the
Earth's surface. The subsurface formation may interchangeably be referred to
as a formation
or a subterranean formation. The subsurface formation may comprise a range of
components,
e.g. minerals such as quartz, siliceous materials such as sand and clays, as
well as the oil
and/or gas that is extracted.
[0039] As used herein, "underburden" refers to the material underlaying a
subterranean
reservoir. The underburden may include rock, soil, sandstone, shale, mudstone,
wet/tight
carbonate and/or ecosystem below the subterranean reservoir.
[0040] As used herein, "wellbore" is a hole in the subsurface formation
made by drilling
or inserting a conduit into the subsurface. A wellbore may have a
substantially circular cross
section or any other cross-section shape, such as an oval, a square, a
rectangle, a triangle, or
other regular or irregular shapes. The term "well," when referring to an
opening in the
formation, may be used interchangeably with the term "wellbore." Further,
multiple pipes
may be inserted into a single wellbore, for example, as a liner configured to
allow flow from
an outer chamber to an inner chamber.
[0041] The terms "approximately," "about," "substantially," and similar
terms are
intended to have a broad meaning in harmony with the common and accepted usage
by those
of ordinary skill in the art to which the subject matter of this disclosure
pertains. It should be
understood by those of skill in the art who review this disclosure that these
terms are intended
to allow a description of certain features described and claimed without
restricting the scope
of these features to the precise numeral ranges provided. Accordingly, these
terms should be
7
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
interpreted as indicating that insubstantial or inconsequential modifications
or alterations of
the subject matter described and are considered to be within the scope of the
disclosure.
[0042] The articles "the", "a" and "an" are not necessarily limited to mean
only one, but
rather are inclusive and open ended so as to include, optionally, multiple
such elements.
[0043] "At least one," in reference to a list of one or more entities
should be understood
to mean at least one entity selected from any one or more of the entity in the
list of entities,
but not necessarily including at least one of each and every entity
specifically listed within
the list of entities and not excluding any combinations of entities in the
list of entities. This
definition also allows that entities may optionally be present other than the
entities
specifically identified within the list of entities to which the phrase "at
least one" refers,
whether related or unrelated to those entities specifically identified. Thus,
as a non-limiting
example, "at least one of A and B" (or, equivalently, "at least one of A or
B," or, equivalently
"at least one of A and/or B") may refer, to at least one, optionally including
more than one,
A, with no B present (and optionally including entities other than B); to at
least one,
optionally including more than one, B, with no A present (and optionally
including entities
other than A); to at least one, optionally including more than one, A, and at
least one,
optionally including more than one, B (and optionally including other
entities). In other
words, the phrases "at least one," "one or more," and "and/or" are open-ended
expressions
that are both conjunctive and disjunctive in operation. For example, each of
the expressions
"at least one of A, B and C," "at least one of A, B, or C," "one or more of A,
B, and C," "one
or more of A, B, or C" and "A, B, and/or C" may mean A alone, B alone, C
alone, A and B
together, A and C together, B and C together, A, B and C together, and
optionally any of the
above in combination with at least one other entity.
[0044] Where two or more ranges are used, such as but not limited to 1 to 5
or 2 to 4, any
number between or inclusive of these ranges is implied.
[0045] As used herein, the phrase, "for example," the phrase, "as an
example," and/or
simply the term "example," when used with reference to one or more components,
features,
details, structures, and/or methods according to the present disclosure, are
intended to convey
that the described component, feature, detail, structure, and/or method is an
illustrative, non-
exclusive example of components, features, details, structures, and/or methods
according to
the present disclosure. Thus, the described component, feature, detail,
structure, and/or
method is not intended to be limiting, required, or exclusive/exhaustive; and
other
components, features, details, structures, and/or methods, including
structurally and/or
8
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
functionally similar and/or equivalent components, features, details,
structures, and/or
methods, are also within the scope of the present disclosure.
[0046] As used herein, the terms "adapted" and "configured" mean that the
element,
component, or other subject matter is designed and/or intended to perform a
given function.
Thus, the use of the terms "adapted" and "configured" should not be construed
to mean that a
given element, component, or other subject matter is simply "capable of"
performing a given
function but that the element, component, and/or other subject matter is
specifically selected,
created, implemented, utilized, programmed, and/or designed for the purpose of
performing
the function. It is also within the scope of the present disclosure that
elements, components,
and/or other recited subject matter that is recited as being adapted to
perform a particular
function may additionally or alternatively be described as being configured to
perform that
function, and vice versa. Similarly, subject matter that is recited as being
configured to
perform a particular function may additionally or alternatively be described
as being
operative to perform that function.
[0047] Figs. 1-5 provide examples of systems and methods for bulk heating
of a
subsurface formation. More specifically, Figures 1-5 provide examples of
systems and
methods for mitigating subsurface shunts during bulk heating of a subsurface
formation.
Elements that serve a similar, or at least substantially similar, purpose are
labeled with
numbers consistent among the figures. The corresponding elements with like
numbers in
each of the figures may not be discussed in detail herein with reference to
each of the figures.
Similarly, all elements may not be labeled in each of the figures, but
associated reference
numerals may be utilized for consistency. Elements, components, and/or
features that are
discussed with reference to one or more of the figures may be included in
and/or utilized with
any of the figures without departing from the scope of the present disclosure.
[0048] In general, elements that are likely to be included are illustrated
in solid lines,
while elements that are optional are illustrated in dashed lines. However,
elements that are
shown in solid lines may not be essential. Thus, an element shown in solid
lines may be
omitted without departing from the scope of the present disclosure.
[0049] Fig. 1 is a schematic representation of a bulk heating system 10.
Bulk heating
systems 10 may include at least two electrode assemblies 50 that extend into a
subsurface
formation 20. The at least two electrode assemblies 50 may form, or define, at
least a pair of
electrode assemblies 50. More specifically, electrode assemblies 50 are in
electrical
communication with a subsurface formation 20, and the electrode assemblies are
configured
9
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
in adjacent pairs to form electrical circuits with a subsurface region 32
between each pair of
electrode assemblies 50. Individual electrode assemblies 50 may be a member of
more than
one pair of electrode assemblies 50 and may be in electrical communication
with more than
one subsurface region 32. For clarity, Fig. 1 illustrates in solid lines two
spaced-apart
electrode assemblies 50. As schematically illustrated with dashed lines, bulk
heating systems
may include more than two electrode assemblies 50, for example, 3, 4, 5, 6, or
more than 6
electrode assemblies 50.
[0050] Subsurface formation 20 is a finite subsurface (subterranean)
region. Subsurface
formation 20 may be of any geologic form and may contain one or more organic
matter-
containing regions (e.g., layers, intervals, etc.), one or more regions with
little to no organic
matter, an overburden, and/or an underburden. Subsurface formation 20 may be
below an
overburden and/or above an underburden. In Fig. 1, subsurface formation 20 is
schematically
indicated to include organic matter 30 (e.g., a solid, liquid, and/or gaseous
hydrocarbon
mineral such as hydrocarbon compounds, shale oil, bitumen, bituminous coal,
and/or
kerogen). Subsurface formation 20 may be at least 100 m, at least 200 m, at
least 500 m, at
least 1,000 m, at least 2,000 m, at least 5,000 m, at most 20,000 m, at most
10,000 m, at most
5,000 m, at most 2,000 m, at most 1,000 m, and/or at most 500 m below the
Earth's surface
22. Suitable depth ranges may include combinations of any upper and lower
depth listed
above or any number within or bounded by the preceding depth ranges.
[0051] Subsurface regions 32 may be portions of the subsurface formation 20
that are in
electrical contact with at least two electrode assemblies 50, i.e., subsurface
regions 32 adjoin
at least two adjacent electrode assemblies 50. Subsurface regions 32 generally
may extend
between at least a pair of electrode assemblies 50.
[0052] Subsurface regions 32 may be the regions of subsurface formation 20
that are
heated by the bulk heating system 10 via electrical resistive heating (Joule
heating).
Subsurface regions 32 may be electrically powered (also called energized) to
cause resistive
heating, i.e., electrical power dissipated within a given subsurface region 32
may heat the
given subsurface region 32. Electrically powering (also referred to as
transmitting electricity)
may be the result of connecting different voltages to different electrode
assemblies 50 and
applying the voltages to cause current to flow through the subsurface region
32 between the
electrode assemblies 50. When electrically powered and resistively heating,
subsurface
regions 32 may be referred to as in situ resistive heaters 34. The heating
and/or the power
dissipated within the subsurface regions 32 may be expressed as power
deposited and/or
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
dissipated per volume (or length cubed). For illustration purposes, in situ
resistive heaters 34
are depicted schematically via example electrical flow lines between adjacent
electrode
assemblies 50. It should be understood that electricity flow is occurring over
the entire
exposed surface of an electrode assembly 50 and not just where flow lines are
shown.
[0053] Electrode assemblies 50 may include at least one wellbore 40 and/or
fracture 44.
Electrode assemblies 50 may include electrically conductive material
sufficient to conduct
electricity from the surface 22 to the adjoining subsurface region(s) 32
without undue power
loss (due to electrical resistive heating). An electrode assembly 50 may be
electrically
connected to one or more subsurface regions 32 of the subsurface formation 20
that adjoin
the electrode assembly 50. An electrode assembly 50 may include a wellbore 40
that
includes an electrically conductive wire, cable, casing, tubular, rod, etc.,
and that is
electrically connected to at least one subsurface region 32 adjoining the
wellbore 40. An
electrode assembly 50 may include a fracture 44 that includes conductive
media, such as
electrically conductive particulate and/or electrically conductive fluid.
[0054] Wellbores 40 may be substantially vertical, substantially
horizontal, any angle
between vertical and horizontal, deviated or non-deviated, and combinations
thereof, for
example, a vertical well with a non-vertical segment. As used herein,
"substantially vertical"
means within 15 of true vertical and "substantially horizontal" means within
15 of true
horizontal. Wellbores 40 may include and/or may be supported, lined, sealed,
and/or filled
with materials such as casings, linings, sheaths, conduits, electrically
conductive materials
(e.g., metal rods, metal cables, metal wires, metal tubulars, electrically
conductive particulate,
electrically conductive granular materials, and/or electrically conductive
liquid). Wellbores
40 may be configured to be in electrical and/or fluidic communication with the
subsurface
formation 20 and/or one or more subsurface regions 32.
[0055] Fractures 44 may be natural and/or manmade cracks, or surfaces of
breakages,
within rock in the subsurface formation 20. Fractures 44 may be induced
mechanically in
subsurface regions, for example, by hydraulic fracturing (in which case, the
fracture 44 may
be referred to as a hydraulic fracture). Another example of a method of
forming of fractures
44 is steam fracturing (in which case, the fracture 44 may be referred to as a
steam fracture).
Fractures 44 may be referred to as hydraulic fractures and steam fractures,
respectively.
Fractures 44 may be substantially planar. Fractures 44 may be substantially
vertical,
substantially horizontal, any angle between vertical and horizontal, branched,
networked, and
combinations thereof, for example, a planar vertical fracture with a non-
vertical branch. The
11
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
length of a fracture 44 may be a distance from the source of the fracture
(e.g., a wellbore 40
used to establish the fracture) to a fracture tip (the furthest point of the
fracture from the
source) or the distance along the fracture between the two farthest spaced
fracture tips.
Fractures 44 may be configured to be in electrical and/or fluidic
communication with the
subsurface formation 20 and/or one or more subsurface regions 32. For
illustration purposes,
the widths of the fractures 44 are exaggerated compared to the length of the
fractures. For
example, fracture widths may be on order of a few millimeters or centimeters,
whereas
fracture lengths may be on order of tens or hundreds of meters.
[0056] Fractures 44 may be held open with granular material called
proppant.
Fractures 44 may include and/or may be supported, lined, sealed, and/or filled
with other
materials, such as electrically conductive materials, particulate, granular
materials, liquids,
and/or gases. Proppant may be electrically conductive. Electrically conductive
materials
may include at least one of granular material, granules, particles, filaments,
metal, granular
metal, metal coated particles, coke, graphite, electrically conductive gel,
and electrically
conductive liquid. For example, the proppant may include, and/or may be,
graphite particles.
As other examples, the proppant may include, and/or may be, an electrically
conductive
material, such as metal particles, metal coated particles, and/or coke
particles.
[0057] Electrode assemblies 50 may be arranged in pairs of adjacent
electrode assemblies
50 within the subsurface formation. The pair of electrode assemblies 50 in
each pair of
adjacent electrode assemblies 50 may be nearer to each other than to other,
non-adjacent
electrode assemblies 50. Relative to a given electrode assembly 50, an
adjacent electrode
assembly 50 may be the closest electrode assembly 50 or one of the closest
electrode
assemblies 50. Pairs of adjacent electrode assemblies 50 are not necessarily
within a small
distance of each other and may be separated by distances of hundreds of
meters. The
distance between electrode assemblies 50 is the shortest distance between the
electrode
assemblies 50 through the subsurface region 32 that separates the electrode
assemblies 50.
Electrode assemblies 50 may be deemed adjacent when no other electrode
assembly 50
intersects a line spanning the shortest distance between the electrode
assemblies 50.
[0058] Electrode assemblies 50 may be arranged in pairs, groups, rows,
columns, and/or
arrays. The electrode assemblies 50 may be spaced apart and may have a
substantially
uniform spacing (at least in one direction). For example, electrode assemblies
may be spaced
apart with a spacing of at least 5 m, at least 10 m, at least 20 m, at least
50 m, at least 100 m,
at least 200 m, at most 500 m, at most 200 m, at most 100 m, at most 50 m,
and/or at most
12
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
20 m. Groups, rows, columns, and arrays of electrode assemblies 50 may include
inside
electrode assemblies 52 and outer electrode assemblies 54. Outer electrode
assemblies 54
may be adjacent and/or connected to fewer electrode assemblies 50 than inside
electrode
assemblies 52. For example, rows and columns of electrode assemblies 50 may
include a
first outer electrode assembly 54 at one end of the row or column and a second
outer
electrode assembly 54 at the other end of the row or column. The first outer
electrode
assembly 54 may be adjacent to only one electrode assembly 50; the second
outer electrode
assembly 54 may be adjacent to only one electrode assembly 50; and the inside
electrode
assemblies 52 may each be adjacent to two electrode assemblies 50 of the
electrode
assemblies in the row or column. The inside electrode assemblies 52 may be
referred to as
middle electrode assemblies 52, central electrode assemblies 52, intermediate
electrode
assemblies 52, inner electrode assemblies 52, and/or interior electrode
assemblies 52. The
outer electrode assemblies 54 may be referred to as edge electrode assemblies
54 and/or end
electrode assemblies 54.
[0059] Electrode assemblies 50 may be oriented with respect to each other.
For example,
two or more electrode assemblies 50 (or portions thereof) may be at least
substantially
parallel to each other and substantially facing each other. In particular, two
electrode
assemblies 50 may each include a generally planar fracture 44, and the
fractures 44 of the
electrode assemblies 50 may be substantially parallel to each other, with each
electrode
assembly 50 including a face, or generally planar fracture surface, 46 that
faces a
corresponding face 46 of the other electrode assembly 50. In the example of
Fig. 1, two
substantially parallel fractures 44 (shown in solid lines) each form a portion
of two separate
electrode assemblies 50. The two solid-line electrode assemblies 50
illustrated in Fig. 1 may
be deemed parallel electrode assemblies 50.
[0060] Adjacent electrode assemblies 50 may be configured to transmit
electricity and/or
to electrically power the subsurface region(s) 32 between the adjacent
electrode assemblies
50. The electrode assemblies 50 may be configured to apply a voltage across
and/or to
supply an electrical current through the corresponding subsurface region(s)
32. Electrical
power supplied to the subsurface region(s) 32 may be DC (direct current) power
and/or AC
(alternating current) power. The electrical power may be supplied by an
electrical power
source 70. As indicated in Fig. 1, electrical power source 70 may be
electrically connected to
the electrode assemblies 50 from a surface (above-ground) location 22.
13
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
[0061] DC power may be supplied by applying a voltage difference (gradient)
across the
subsurface region 32. In a DC powered configuration, one of the electrode
assemblies 50
contacting the subsurface region 32 may have a higher voltage (called the high
voltage and/or
the high polarity), and another electrode assembly 50 contacting the
subsurface region 32
may have a lower voltage (called the low voltage and/or the low polarity). If
the high
polarity is a positive voltage and the low polarity is a negative voltage, the
high polarity and
the low polarity may be referred to as the positive polarity and the negative
polarity,
respectively. Where DC power is supplied, the voltages of the electrode
assemblies 50 may
be occasionally (e.g., periodically) switched, for example, to avoid
electrochemical effects
and electrode degradation at the electrode assemblies 50.
[0062] AC power may be supplied by applying different voltage waveforms
(also called
alternating voltages) to different electrode assemblies 50 in contact with the
same subsurface
region 32. Generally, the applied alternating voltages are periodic, have the
same frequency,
and have differing phase angles. Suitable AC frequencies include at least 10
Hz (hertz), at
least 30 Hz, about 50 Hz, about 60 Hz, about 100 Hz, about 120 Hz, at least
100 Hz, at
least 200 Hz, at least 1,000 Hz, at least 10,000 Hz, at most 100,000 Hz, at
most 300,000 Hz,
at most 1,000,000 Hz, at most 5,000,000 Hz, and/or at most 15,000,000 Hz.
Suitable ranges
may include combinations of any upper and lower AC frequency listed above or
any number
within or bounded by the AC frequencies listed above. The AC frequency may be
selected to
be below a frequency at which radio-frequency (dielectric) heating dominates
over resistive
(Joule) heating of the subsurface formation 20.
[0063] AC power may be supplied as one or more alternating voltages, and
each
electrode assembly 50 may have an alternating voltage or a DC voltage applied.
For
example, AC power may be supplied in a single-phase configuration where an
alternating
voltage is applied to one electrode assembly 50 and a DC voltage (also
referred to as a neutral
voltage) is applied to another electrode assembly 50. As other examples, AC
power may be
supplied in a two-phase configuration, a three-phase configuration, and/or in
a multi-phase
configuration. The 'electrical phases' available in a multi-phase
configuration are alternating
voltages having the same frequency and different phase angles (i.e., nonequal
phase angles).
Generally, the phase angles are relatively evenly distributed within the
period of the AC
power (the period is the inverse of the shared frequency of the alternating
voltages). For
example, common phase angles for a two-phase configuration are 0 and 180 (a
phase angle
difference of 180 , i.e., of 180 in absolute value), and 0 and 120 (for
example, two of the
14
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
three poles from a 3-phase generator). Common phase angles for a three-phase
configuration
are 00, 120 , and 240 (phase angle differences of 120 , i.e., of 120 in
absolute value).
Though less common, other multi-phase configurations (e.g., 4, 5, 6, or more
'electrical
phases') and/or other phase angles, and other phase angle differences, may be
utilized to
supply AC power.
[0064] When electrical power is supplied to subsurface regions 32, the
subsurface regions
32 may resistively heat and become more electrically conductive. As the
subsurface regions
32 are heated, the electrical conductivity may increase (and the electrical
resistivity may
decrease) due to physical and/or chemical changes within the subsurface
regions 32, for
example, due to temperature sensitivity of the electrical resistance of the
native rock, due to
native brine boiling off, and/or due to pyrolysis (and/or coking) of native
organic matter
and/or native hydrocarbons. Before heating, the subsurface regions 32 may be
relatively
poorly electrically conductive, for example, having an average electrical
conductivity of less
than 1 S/m (Siemens/meter), less than 0.1 S/m, less than 0.01 S/m, less than
0.001 S/m, less
than 10-4 S/m, less than 10-5 S/m, less than 10-6 S/m, less than 10-7 S/m,
and/or within a range
that includes or is bounded by any of the preceding examples of average
electrical
conductivity. Upon heating, the subsurface regions 32 may become more
electrically
conductive, achieving an average electrical conductivity of at least 10-5 S/m,
at least 10-4 S/m,
at least 10-3 S/m, at least 0.01 S/m, at least 0.1 S/m, at least 1 S/m, at
least 10 S/m, at least
100 S/m, at least 1,000 S/m, and/or within a range that includes or is bounded
by any of the
preceding examples of average electrical conductivity.
[0065] Where electrical conduction and/or resistive heating is not uniform
within
subsurface region 32, a subsurface shunt may form between electrode assemblies
50 that
serve the subsurface region 32. An example of such a subsurface shunt is
schematically
illustrated in Fig. 1 at 60. The subsurface shunt 60 may form because
electrical conductivity
increases with increasing temperature and/or may form due to inhomogeneities
(such as
electrically-conductive and/or fluidically-conductive regions) within the
subsurface region
32. The subsurface shunt 60 may be a region, a pathway, and/or a channel that
extends
between two electrode assemblies 50 within the subsurface region 32, and which
has a higher
electrical conductivity than the rest of the subsurface region 32. Subsurface
shunts 60 may be
electrical shorts between electrode assemblies 50. Subsurface shunts 60 may
divert electrical
current supplied by the electrode assemblies 50 away from the bulk of the
subsurface regions
32 and into the subsurface shunts 60. Subsurface shunts 60 may be, and/or may
include, a
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
fluid path between electrode assemblies 50. Subsurface shunts 60 may transmit
fluid injected
into one electrode assembly 50 to another, connected, electrode assembly 50.
[0066]
When subsurface shunts 60 become sufficiently electrically conductive, the
majority of electrical current passing between the electrode assemblies 50 may
travel through
the subsurface shunts 60. The positive correlation between temperature and
electrical
conductivity may reinforce and/or concentrate the subsurface shunts 60 as
electrical current
flows through the subsurface shunts 60. Subsurface shunts 60 may be very small
as
compared to the corresponding subsurface regions 32. The electrical current
and the
consequent heating may be very highly concentrated within the subsurface
shunts 60.
[0067] The
average electrical conductivity of subsurface shunts 60 may be at least
10-5 S/m, at least 10-4 S/m, at least 10-3 S/m, at least 0.01 S/m, at least
0.1 S/m, at least 1 S/m,
at least 10 S/m, at least 100 S/m, at least 300 S/m, at least 1,000 S/m, at
least 3,000 S/m, at
least 10,000 S/m, and/or within a range that includes or is bounded by any of
the preceding
examples of average electrical conductivity. The electrical conductivity of
the subsurface
shunt 60 may be so great, relative to the remainder of the subsurface region
32, that the
average electrical conductivity of the subsurface region 32 may be dominated
by the average
electrical conductivity of the included subsurface shunt 60.
[0068] The
presence of a subsurface shunt 60 within the subsurface region 32 may
increase the electrical power flowing through the subsurface region 32 and/or
through the
localized region corresponding to the subsurface shunt 60. The increased
electrical power
flowing through the subsurface shunt 60 may increase resistive heating within
the subsurface
shunt 60 and/or may decrease electrical power flowing and/or resistive heating
outside of the
subsurface shunt 60. The presence of a subsurface shunt 60 within the
subsurface region 32
may be indicated by one or more thermal, mechanical, and/or electrical
parameters (referred
to as shunt indicators) relating to the bulk heating system 10, the subsurface
region 32, one or
more of the electrode assemblies 50, and/or the subsurface shunt 60 (at least
the region of the
subsurface region 32 corresponding to the subsurface shunt). Shunt indicators
may be the
value of, and/or changes in, one or more thermal parameters, mechanical
parameters,
electrical parameters, and/or related quantities. Thermal parameters may
include the average
temperature, a point (localized) temperature, a temperature difference, and/or
a temperature
gradient (temperature difference per length). Mechanical parameters may
include fluid
permeability and/or porosity. Electrical parameters may be electrical
conductivity-related
parameters, which may include, and/or may be, at least one of conductivity (a
material's
16
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
intrinsic ability to conduct electrical current), conductance (the ease with
which electrical
current may flow through an object or defined region), resistivity (a
material's intrinsic
ability to oppose electrical current flow), resistance (the opposition to the
flow of electrical
current through an object or defined region), current (electrical current
flow), voltage
(electrical potential), and/or a density and/or gradient of any of the
preceding examples of
electrical conductivity-related parameters.
[0069] Electrical conductivity may be referred to as specific electrical
conductance and/or
volume conductivity. Electrical resistivity may be referred to as specific
electrical resistance
and/or volume resistivity. Electrical conductivity, conductance, resistivity,
and resistance
each may be an AC and/or a DC quantity, i.e., each may be described as a
complex quantity,
a magnitude, a phase angle, and/or a frequency-dependent quantity. When
specifically
referring to AC quantities, electrical conductivity may be called electrical
admittivity and/or a
real part of the complex electrical admittivity, electrical conductance may be
called electrical
admittance and/or a real part of the complex electrical admittance, electrical
resistivity may
be called electrical impeditivity and/or a real part of the complex electrical
impeditivity, and
electrical resistance may be called electrical impedance and/or a real part of
the complex
electrical impedance.
[0070] Bulk heating systems 10 may include a shunt mitigator 64 in and/or
near the
electrode assemblies 50, the subsurface region 32, and/or the subsurface shunt
60. The shunt
mitigator 64 may be a material configured to selectively attenuate and/or
eliminate electrical
current flow through the subsurface shunt 60 in response to and/or in the
presence of the
subsurface shunt 60. The shunt mitigator 64 may be, optionally selectively,
located and/or
placed in the electrode assemblies 50, the subsurface region 32, and/or the
subsurface shunt
60 to attenuate and/or eliminate electrical current flow through the
subsurface shunt 60. The
shunt mitigator 64 may be, optionally selectively, located and/or placed
before, during, and/or
after the electrode assemblies 50 are formed. The shunt mitigator 64 may be,
optionally
selectively, located and/or placed before, during, and/or after the subsurface
shunt 60 is
formed.
[0071] The shunt mitigator 64 may be a solid (e.g., particles, granules,
etc.), a liquid, a
gas, and/or a combination of solid, liquid, and/or gas. The shunt mitigator 64
may be placed
in (e.g., into porous regions within) the electrode assemblies 50, the
subsurface region 32,
and/or the subsurface shunt 60 by flowing and/or injection under pressure.
Where the shunt
mitigator includes solids, the solids may be suspended and/or dispersed in a
carrier fluid.
17
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
[0072] Shunt mitigator 64 may be within the electrode assemblies 50. For
example, a
solid and/or liquid shunt mitigator may be electrically conductive and may be
at least a
portion of the electrical conductive material that forms an electrically
conductive path from
the surface 22 to the subsurface region 32. Whether electrically conductive or
not, the solid
and/or liquid shunt mitigator 64 may be flowed into the wellbore 40 and/or the
fracture 44 of
the electrode assembly 50 with (other) electrically conductive materials
(e.g., during
formation of the electrode assembly 50). When electrically conductive, the
solid and/or
liquid shunt mitigator 64 may be flowed into the wellbore 40 and/or the
fracture 44 as the
electrically conductive material of the electrode assembly 50. The solid
and/or liquid shunt
mitigator 64 may be flowed into electrode assembly 50 after the electrode
assembly 50
already includes electrically conductive material.
[0073] Shunt mitigators 64 that are solid may be granular and may be at
least a portion of
the proppant that holds open a fracture 44 of the electrode assembly. For
example, shunt
mitigator 64 may be flowed with, and/or as, proppant into a fracture 44 during
formation
and/or propping of the fracture 44.
[0074] As shunt mitigators 64 that are fluid (e.g., liquid and/or gaseous)
may tend to not
be retained within a selected location within the electrode assemblies 50, the
subsurface
region 32, and/or the subsurface shunt 60, fluid shunt mitigators 64 may be
flowed into at
least one electrode assembly 50 in anticipation of, during, and/or after
formation of a
subsurface shunt 60.
[0075] The shunt mitigator 64 may be configured to change one or more
properties of the
shunt mitigator in response to the presence of a subsurface shunt 60 (e.g., in
response to a
shunt indicator). The shunt mitigator 64 may be configured such that the
change in its
properties results in a decrease in the electrical conductance (i.e., an
increase in the electrical
resistance) of the subsurface shunt 60 and/or a decrease in the electrical
current flowing
through the subsurface shunt 60. The shunt mitigator 64 may be configured such
that the
change in its properties results in a decrease in the electrical conductivity
(i.e., an increase in
the electrical resistivity) of the subsurface shunt 60 and/or at least a
portion of at least one of
the electrode assemblies 50 near the subsurface shunt 60. The properties may
include at least
one of electrical conductivity, electrical admittivity, electrical
resistivity, electrical
impeditivity, electric susceptibility, electric permittivity, magnetic
susceptibility, magnetic
permeability, density, viscosity, volume, and chemical activity.
18
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
[0076] The
shunt mitigator 64 may be configured to decrease its electrical conductivity
(i.e., to increase electrical resistivity) in response to the shunt indicator.
For example, the
shunt mitigator 64 may decrease its electrical conductivity in response to
temperatures above
a predetermined threshold. If electrically powered by a voltage-limited power
source, an
increase in temperature, which may be due to a subsurface shunt 60, may result
in a decrease
in electrical power dissipated in the shunt mitigator 64 and consequently less
heating due to
electricity flowing through the shunt mitigator 64. In response to the shunt
indicator, the
shunt mitigator 64 may be configured to decrease its electrical conductivity
at one or more
frequencies, and/or above or below a cutoff frequency.
[0077] The
shunt mitigator 64 may be configured to chemically react, in response to the
shunt indicator, with at least one of the subsurface region 32, the subsurface
shunt 60, and
one or more of the electrode assemblies 50. As an example, the shunt mitigator
64 may
include, may include a source of, and/or may be molecular oxygen, carbon
dioxide, an
oxidizing gas, and/or a gasification gas. These examples of shunt mitigators
64 may
selectively react with (selectively oxidize) electrically-conductive carbon
(e.g., residual char
or a source of elemental carbon) in the subsurface shunt 60, for example,
because the shunt
mitigator 64 is selectively placed in the subsurface shunt, and/or because
electrically-
conductive carbon is relatively more prevalent in the subsurface shunt 60 than
in the
electrode assemblies 50. A gasification gas is a gas that, when added to
electrically-
conductive carbon under appropriate conditions, reacts to form a gaseous
carbon compound
(such as carbon monoxide). A gasification gas may be carbon dioxide or a gas
that may be
decomposed into a carbon dioxide product. When electrically-conductive carbon
is oxidized,
the amount of electrically-conductive carbon may be reduced and/or the
electrically-
conductive carbon may be transformed into other carbon-containing compounds
that are less
electrically-conductive (e.g., carbon monoxide).
Hence, oxidization of electrically-
conductive carbon within and/or near the subsurface shunt 60 may reduce the
electrical
conductance, i.e., increase the electrical resistance, of the subsurface shunt
60.
[0078] The
shunt mitigator 64 may be configured to decompose in response to the shunt
indicator, to polymerize in response to the shunt indicator, and/or to melt in
response to the
shunt indicator. For example, the shunt mitigator 64 may include, and/or may
be, a carbonate
mineral such as calcite and/or dolomite. Carbonate minerals may decompose at
elevated
temperatures that may be generated within the subsurface region 32 and/or the
subsurface
shunt 60. For example, dolomite may decompose at about 550 C, and calcite may
19
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
decompose at about 700 C. Decomposition of carbonate minerals may result in
the
production of carbon dioxide gas, which may oxidize electrically conductive
carbon in the
subsurface shunt 60 and/or in a region near the subsurface shunt 60. As
discussed,
oxidization of electrically conductive carbon within and/or near the
subsurface shunt 60 may
reduce the electrical conductance of the subsurface shunt 60. The shunt
mitigator 64 may be
electrically conductive and form at least a portion of the electrically
conductive path of an
electrode assembly. When the shunt mitigator 64 decomposes, the shunt
mitigator may
become less electrically conductive and/or may transform into a mobile
material (e.g., a
liquid and/or a gas) that migrates away from the site of decomposition. Such
decomposition
may leave a void and/or a region of higher electrical resistance in the
electrical path to the
subsurface shunt 60 and thereby reduce the electrical conductance through the
subsurface
shunt 60.
[0079] The shunt mitigator 64 may be configured to change volume and/or
density in
response to the shunt indicator. For example, the shunt mitigator 64 may be
electrically
insulating and intermixed within the electrically conductive material that
forms the electrical
path through an electrode assembly to the subsurface region 32. When the
subsurface shunt
60 forms, the shunt mitigator 64 near the subsurface shunt 60 may expand and
displace
electrically conductive material near the subsurface shunt 60 and thereby
reduce the electrical
conductance through the subsurface shunt 60.
[0080] The shunt mitigator 64 may be configured to undergo a state change
in response to
the presence of the subsurface shunt 60 (e.g., in response to a shunt
indicator). The state
change is a change in property of the shunt mitigator 64. The shunt mitigator
64 may be
configured such that the state change results in a decrease in the electrical
conductance (i.e.,
an increase in the electrical resistance) of the subsurface shunt 60 and/or a
decrease in the
electrical current flowing through the subsurface shunt 60. The shunt
mitigator 64 may be
configured such that the state change results in a decrease in the electrical
conductivity (i.e.,
an increase in the electrical resistivity) of the subsurface shunt 60 and/or
at least a portion of
at least one of the electrode assemblies 50 near the subsurface shunt 60.
[0081] The state change may be an electromagnetic state change, an
electromagnetic
phase transition, a paramagnetic transition, and/or a paraelectric transition.
The state change
may be a thermodynamic state change, a thermodynamic phase transition, and/or
a solid-
liquid transition. The state change may be a chemical state change, a chemical
decomposition, and/or a polymerization. For example, the shunt mitigator 64
may be
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
configured to transition, in response to a shunt indicator, to a paramagnetic
state, a
paraelectric state, a liquid state, a decomposed state, and/or a polymerized
state.
[0082] The state change may be associated with a transition temperature of
the shunt
mitigator 64. The transition temperature may be a temperature between the
desired and/or
expected temperature of the subsurface region 32 (upon heating) and the
temperature
associated with an active subsurface shunt 60. For example, the transition
temperature may
be greater than 200 C, greater than 300 C, greater than 400 C, greater than
500 C, greater
than 700 C, less than 1,200 C, less than 1,000 C, less than 900 C, less than
700 C, less than
500 C, less than 400 C, less than 300 C, and/or within a range that includes
or is bounded by
any of the preceding examples of transition temperatures.
[0083] The transition temperature may be a Curie temperature, a
paraelectric transition
temperature, a melting point, and/or a solidus temperature. The Curie
temperature is the
temperature above which a magnetic material becomes paramagnetic (loses its
intrinsic
magnetization). The paraelectric transition temperature is the temperature
above which a
dielectric material becomes paraelectric (loses its intrinsic polarization).
The magnetic and/or
dielectric properties of a material may affect the electrical conductivity of
the material when
alternating current is applied. Shunt mitigators 64 that undergo a magnetic
state transition
and/or a dielectric state transition (e.g., the transition temperature is a
Curie temperature
and/or a paraelectric transition temperature), may have reduced conductivity,
may interrupt
the electrically conductive path to the subsurface shunt 60, and may reduce
the electrical
conductance through the subsurface shunt 60. Shunt mitigators 64 configured to
undergo a
magnetic state transition and/or a dielectric state transition may include,
and/or may be, a
metal, a metal alloy, and/or a ceramic. For example, the shunt mitigator 64
may include,
and/or may be, a bismuth-manganese alloy and/or a strontium titanate compound.
[0084] The shunt mitigator 64 may be, and/or may include, a composite shunt
mitigator
66. The composite shunt mitigator 66 may include at least two materials with
different
functional relationships between properties of the material and the shunt
indicator (e.g., a
thermal, mechanical, and/or electrical property). The materials of the
composite shunt
mitigator 66 may include one or more of the materials described with respect
to other types of
shunt mitigators 64, and may include other materials. The composite shunt
mitigator 66 may
include a first material with a first functional relationship and a second
material with a second
functional relationship. The property of the first functional relationship may
be an electrical
property such as electrical conductivity, electrical admittivity, electrical
resistivity, electrical
21
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
impeditivity, electric susceptibility, electric permittivity, magnetic
susceptibility, and/or
magnetic permeability. The property of the second functional relationship may
be an
electrical property, a physical property, and/or a chemical property. For
example, the
property of the second functional relationship may be electrical conductivity,
electrical
admittivity, electrical resistivity, electrical impeditivity, electric
susceptibility, electric
permittivity, magnetic susceptibility, magnetic permeability, density,
viscosity, volume,
rigidity, and/or chemical activity. The combination of the functional
relationships of the
materials in a composite shunt mitigator 66 may be configured to produce a
composite
functional relationship between one or more properties of the composite shunt
mitigator 66
and the shunt indicator. The composite functional relationship may be a non-
monotonic
functional relationship, e.g., defining a mathematical extremum (maximum,
minimum,
inflection point, etc.) within the expected operating range of bulk heating
system 10 and/or
near the shunt indicator (e.g., at a predetermined value of a thermal,
mechanical, and/or
electrical property of the bulk heating system 10, the subsurface region 32,
one or more of the
electrode assemblies 50, and/or the subsurface shunt 60).
[0085] The shunt mitigator 64 may be configured to maintain a property of
the shunt
mitigator 64 in the presence of a subsurface shunt 60. The shunt mitigator 64
may be
configured such that the placement and/or location of the shunt mitigator 64
within and/or
near the subsurface shunt 60 results in a decrease in the electrical
conductance (i.e., an
increase in the electrical resistance) of the subsurface shunt 60 and/or a
decrease in the
electrical current flowing through the subsurface shunt 60. The placement
and/or location of
the shunt mitigator 64 may result in a decrease in the electrical conductivity
(i.e., an increase
in the electrical resistivity) of the subsurface shunt 60 and/or at least a
portion of at least one
of the electrode assemblies 50 near the subsurface shunt 60. The property of
the shunt
mitigator 64 may include at least one of electrical conductivity, electrical
admittivity,
electrical resistivity, electrical impeditivity, electric susceptibility,
electric permittivity,
magnetic susceptibility, magnetic permeability, density, viscosity, volume,
and chemical
activity.
[0086] For example, the shunt mitigator 64 may include, and/or may be, an
electrically
insulating liquid, such as mineral oil, transformer oil, and/or a polymer. The
electrically
insulating liquid may be configured to maintain its electrically insulating
property in the
presence of a subsurface shunt 60, e.g., at the temperature and/or electrical
current that may
be associated with the subsurface shunt 60. The electrically insulating liquid
may not be
22
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
present in the electrode assemblies 50 and/or the subsurface region 32 before
the formation of
a subsurface shunt 60. Once the subsurface shunt 60 is formed and the presence
of the
subsurface shunt 60 is detected, the electrically insulating liquid may be
injected into at least
one of the electrode assemblies 50 and flowed to and/or into the subsurface
shunt 60, thereby
applying an electrically insulating mask to the subsurface shunt 60 and
decreasing the
electrical conductance through the subsurface shunt 60.
[0087] Subsurface shunts 60 may be mitigated during bulk heating of
subsurface
formations 20 by performing bulk heating methods 100. In the example of Fig.
2, bulk
heating methods 100 may include electrically powering 110 at least a pair of
electrode
assemblies (such as electrode assemblies 50) that are within a subsurface
formation (such as
subsurface formation 20), to resistively heat at least a subsurface region
(such as subsurface
region 32 and/or in situ resistive heater 34) between the pair of electrode
assemblies with
electrical current flowing through the subsurface region between the pair of
electrode
assemblies. The bulk heating methods 100 may include flowing 112 shunt
mitigator (such as
shunt mitigator 64) into at least one of the electrode assemblies. Responsive
to a shunt
indicator that indicates the presence and/or formation of a subsurface shunt
(such as
subsurface shunt 60) between the pair of electrode assemblies, the bulk
heating methods 100
may include mitigating 114 the subsurface shunt with the shunt mitigator.
[0088] Electrically powering 110 may include applying a voltage across
and/or supplying
an electrical current through the pair of electrode assemblies. Electrically
powering 110 may
include supplying an AC current (i.e., an alternating current) to the pair of
electrode
assemblies. Electrically powering 110 may include electrically powering the
pair of
electrode assemblies while at least one of the electrode assemblies includes
the shunt
mitigator. For example, electrically powering 110 may include electrically
powering the
electrode assembly configuration of Fig. 1, where shunt mitigator 64 may be
present in one or
both of the electrode assemblies 50 and/or in the subsurface region 32 between
the electrode
assemblies 50.
[0089] Electrically powering 110 may include heating the subsurface region
to pyrolyze
organic matter in the subsurface formation, to pyrolyze organic matter to
create hydrocarbon
fluids, and/or to mobilize hydrocarbon fluids within the subsurface formation.
Electrically
powering 110 may include heating the subsurface region to an average
temperature and/or a
point (localized) temperature of at least 150 C, at least 250 C, at least 350
C, at least 450 C,
at least 550 C, at least 700 C, at least 800 C, at least 900 C, at most 1000
C, at most 900 C,
23
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
at most 800 C, at most 700 C, at most 550 C, at most 450 C, at most 350 C, at
most 270 C,
and/or within a range that includes or is bounded by any of the preceding
examples of
temperature.
[0090] Electrically powering 110 to resistively heat the subsurface region
may include
forming an electrical circuit between the electrode assemblies and the
subsurface region.
Electrically powering 110 may include electrically powering the subsurface
region to form an
in situ resistive heater (such as in situ resistive heater 34) between the
electrode assemblies.
[0091] Electrically powering 110 may begin without a subsurface shunt being
present
between the electrode assemblies. Electrically powering 110 may result in a
subsurface shunt
forming between the electrode assemblies within the subsurface region (and/or
within the in
situ resistive heater).
[0092] Bulk heating methods 100 of Fig. 2 may include flowing 112 the shunt
mitigator
into at least one of the electrode assemblies. Flowing 112 may be performed
before, during,
and/or after electrically powering 110 and/or before, during, and/or after the
formation of the
subsurface shunt.
[0093] Flowing 112 may include injecting a slurry and/or a fluid that
includes, and/or is,
the shunt mitigator into at least one of the electrode assemblies. Flowing 112
may include
flowing shunt mitigator into the subsurface region, the in situ resistive
heater, and/or the
subsurface shunt. Flowing 112 may include applying a pressure differential
between the pair
of electrode assemblies (e.g., injecting into one electrode assembly while
drawing a
hydrostatic pressure on the other electrode assembly). As shown in Fig. 1,
flowing 112 may
result in a bulk heating system 10 with shunt mitigator 64 within the
electrode assemblies 50,
the subsurface region 32, the in situ resistive heater 34, and/or the
subsurface shunt 60 (if
present). Flowing 112 may result in shunt mitigator 64 selectively located
near and/or within
the subsurface shunt 60.
[0094] Flowing 112 may be performed before, during, and/or after
determining 116 the
presence of the subsurface shunt between the electrode assemblies. Determining
116 may
include measuring an electrical conductivity-related parameter between the
pair of electrode
assemblies. The electrical conductivity-related parameter may include, and/or
may be,
conductivity, conductance, resistivity, resistance, admittivity, admittance,
impeditivity,
impedance, current, voltage, a point temperature and/or an average
temperature. Determining
24
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
116 may include measuring a fluid permeability-related parameter between the
pair of
electrode assemblies.
[0095] For example, determining 116 may include determining that the
average electrical
conductivity of the subsurface shunt is at least 10-5 S/m, at least 10-4 S/m,
at least 10-3 S/m, at
least 0.01 S/m, at least 0.1 S/m, at least 1 S/m, at least 10 S/m, at least
100 S/m, at least
300 S/m, at least 1,000 S/m, at least 3,000 S/m, at least 10,000 S/m, and/or
within a range
that includes or is bounded by any of the preceding examples of average
electrical
conductivity. Determining 116 may include determining that the average
electrical
conductivity of the subsurface region is at least 10-5 S/m, at least 10-4 S/m,
at least 10-3 S/m,
at least 0.01 S/m, at least 0.1 S/m, at least 1 S/m, at least 10 S/m, at least
100 S/m, and/or
within a range that includes or is bounded by any of the preceding examples of
average
electrical conductivity.
[0096] Bulk heating methods 100 of Fig. 2 may include mitigating 114,
responsive to the
shunt indicator, the subsurface shunt between the pair of electrode assemblies
with the shunt
mitigator. Prior to mitigating 114, and as shown in Fig. 3, the subsurface
shunt 60 has
formed (or has begun to form) and shunt mitigator 64 may be present near
and/or within the
subsurface shunt 60. As discussed, the shunt mitigator 64 may be located near
and/or within
the subsurface shunt 60 by flowing 112 the shunt mitigator into at least one
of the electrode
assemblies 50. The shunt mitigator 64 may be present prior to the formation of
the
subsurface shunt 60 and/or may be located near and/or within the subsurface
shunt 60 after
the formation of the subsurface shunt 60.
[0097] Once the shunt mitigator 64 is present near and/or within the
subsurface shunt 60
and the subsurface shunt 60 exhibits a shunt indicator (e.g., a thermal,
mechanical, and/or
electrical parameter value and/or change in value), the shunt mitigator 64 may
selectively
attenuate and/or eliminate electrical current (and/or the possibility of
electrical current
transmission) through the subsurface shunt 60. The shunt mitigator 64 may
selectively
attenuate and/or eliminate electrical current (and/or the possibility of
electrical current
transmission) through the subsurface shunt 60 by being selectively located
near and/or within
the subsurface shunt 60 and/or by changing a property in response to the shunt
indicator.
[0098] Returning to Fig. 2, mitigating 114 may be performed before, during,
and/or after
determining 116 the presence of the subsurface shunt between the electrode
assemblies.
Upon determining 116 the presence of the subsurface shunt, mitigating 114 may
prompt
flowing 112 the shunt mitigator to mitigate the subsurface shunt.
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
[0099] Mitigating 114 may include decreasing the electrical conductance
(i.e., increasing
the electrical resistance) of the subsurface shunt. Mitigating 114 may include
electrically
isolating the subsurface shunt from one or more of the electrode assemblies.
[00100] Mitigating 114 may include forming a modified subsurface region, as
illustrated in
Fig. 4. Mitigating 114 may include forming a mitigated subsurface shunt 62
from the
subsurface shunt 60 and thereby forming a modified in situ resistive heater
36, which
includes the mitigated subsurface shunt 62, from the in situ resistive heater
34.
[00101] After mitigating 114, the electrode assemblies may have reduced
electrical
conductivity. Bulk heating methods 100 may include, after the mitigating 114,
introducing
electrically conductive material into at least one of the electrode
assemblies. Electrically
conductive material may include granular material, granules, particles,
filaments, metal,
granular metal, metal coated particles, coke, graphite, electrically
conductive gel, and/or
electrically conductive liquid.
[00102] Bulk heating methods 100 may include electrically powering the pair of
electrode
assemblies 50 to resistively heat the modified in situ resistive heater 36
with electrical current
flowing through the modified in situ resistive heater between the pair of
electrode assemblies
50.
[00103] Bulk heating methods 100 may include monitoring the bulk heating
system 10 for
shunt indicators. For example, bulk heating methods 100 may include measuring
one or
more electrical conductivity-related parameters and/or fluid permeability-
related parameters
between the pair of electrode assemblies.
[00104] Fig. 5 schematically represents an example of bulk heating methods 100
which
may or may not utilize a shunt mitigator. Bulk heating methods 100 of Fig. 5
may include
electrically powering 110 the pair of electrode assemblies to resistively heat
an in situ
resistive heater between the pair of electrodes. The bulk heating methods 100
may include
determining 116 the presence of the subsurface shunt between the pair of
electrode
assemblies. Determining 116 may be similar and/or identical to the determining
described
above with respect to Figure 2. Upon determining 116, the bulk heating methods
100 may
include mitigating 114 the subsurface shunt to form a modified in situ
resistive heater. The
bulk heating methods 100 may include electrically powering 118 the pair of
electrode
assemblies to resistively heat the modified in situ resistive heater.
Electrically powering 118
26
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
may be similar and/or identical to the electrically powering described above
with respect to
Figure 2.
[00105] Though mitigating 114 of the example of Fig. 5 may include aspects or
features
described with respect to the example of Fig. 2, mitigating 114 may include
methods of
mitigation that do not utilize a shunt mitigator. Mitigating 114 may include
thermal-electrical
ablation of at least a portion of the subsurface shunt. Thermal-electric
ablation may include
applying a relatively large impulse of electrical power to the subsurface
shunt, by applying
such impulse to the pair of electrode assemblies. The impulse of electrical
power may be
configured to selectively heat the subsurface shunt and/or at least a portion
of the electrode
assemblies near the subsurface shunt due to the electrical conductivity of the
subsurface
shunt. The heating due to the impulse of electrical power may thermally-
electrically ablate at
least a portion of the subsurface shunt, or at least a portion of one of the
electrode assemblies
near the subsurface shunt, much like blowing a fuse. After the thermal-
electric ablation, the
subsurface shunt may be electrically isolated from at least one of the
electrode assemblies
and/or may include an electrical discontinuity. The impulse of electrical
power may be at
least 1,000 V, at least 10,000 V, and/or at least 100,000 V. The impulse of
electrical power
may be applied for less than 10 seconds, less than 1 second, less than 0.1
seconds, and/or less
than 0.01 seconds. In the example of Fig. 2, mitigating 114 may include
thermally-
electrically ablating as described.
[00106] Bulk heating methods 100 may include producing hydrocarbon fluids from
the
subsurface formation. The hydrocarbon fluids may be produced to the surface
via a
production well in the subsurface formation. The production well may be
proximate to one
or more of the electrode assemblies. The production well may be in fluid
communication
with one or more subsurface regions.
[00107] The various disclosed elements of systems and steps of methods
disclosed herein
are not required of all systems and methods according to the present
disclosure, and the
present disclosure includes all novel and non-obvious combinations and
subcombinations of
the various elements and steps disclosed herein. Moreover, one or more of the
various
elements and steps disclosed herein may define independent inventive subject
matter that is
separate and apart from the whole of a disclosed apparatus or method.
Accordingly, such
inventive subject matter is not required to be associated with the specific
systems and
methods that are expressly disclosed herein, and such inventive subject matter
may find
utility in systems and/or methods that are not expressly disclosed herein.
27
CA 02967300 2017-05-10
WO 2016/089498 PCT/US2015/056807
[00108] In the present disclosure, several examples have been discussed and/or
presented
in the context of flow diagrams, or flow charts, in which the methods are
shown and
described as a series of blocks, or steps. Unless specifically set forth in
the accompanying
description, the order of the blocks may vary from the illustrated order in
the flow diagram,
including with two or more of the blocks (or steps) occurring in a different
order and/or
concurrently.
Industrial Applicability
[00109] The systems and methods of the present disclosure are applicable to
the oil and
gas industry.
[00110] It is believed that the following claims particularly point out
certain combinations
and subcombinations that are novel and non-obvious.
Other combinations and
subcombinations of features, functions, elements and/or properties may be
claimed through
amendment of the present claims or presentation of new claims in this or a
related
application. Such amended or new claims, whether different, broader, narrower,
or equal in
scope to the original claims, are also regarded as included within the subject
matter of the
present disclosure.
28
