Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR ENHANCED OIL RECOVERY
FROM CARBONATE RESERVOIRS
Muhammad Haroun, Ph.D.
J. Kenneth Wittle, Ph.D.
George V. Chilingar, Ph.D.
BACKGROUND OF THE INVENTION
This invention relates to the use of direct current (DC) electrokinetics to
enhance oil
production from carbonate reservoirs.
Carbonate formations occur naturally as sediments of carbonate materials,
especially
calcite (CaCO3) and dolomite (CaMg(CO3)2). They are anionic complexes of
(CO3)2- and
divalent metallic cations such as calcium, magnesium, iron, zinc, barium,
strontium and copper,
along with a few other less common elements. Carbonates form within the basin
of deposition
by biological, chemical and detrital processes and are largely made up of
skeletal remains and
other biological constituents that include fecal pellets, lime mud (skeletal)
and microbially
mediated cements and lime mud. A main difference between carbonates and
silicious soils is
that in carbonates chemical constituents, including coated grains such as
ooids and pisoids,
cement and lime mud are common, whereas they are not present in most
siliciclastic sediments.
Carbonate reservoirs owe their porosity and permeability to processes of
deposition, diagenesis
or fracturing.
Petroleum reservoirs in carbonate formations are porous, permeable rock bodies
that
contain significant amounts of hydrocarbons. It has been estimated that as
much as 60% of the
world's oil reserves are present in carbonate reservoirs. However, a
substantial portion of these
reserves is considered unrecoverable. Among many factors that have contributed
to the low
recovery rates experienced in these reservoirs, the oil-wettable nature of
carbonate rock is
particularly problematic. Wettability is generally referred to as the tendency
of one fluid to
spread on or adhere to a solid surface in the presence of other immiscible
fluids. A published
report of an evaluation of carbonate reservoir rock cores obtained from all
over the world
showed that a vast majority of carbonates are oil-wet. Chilingar and Yen,
Energy Sources, 7(1):
21-27 (1992).
Knowledge of the wettability of reservoir rock is important, e.g., for making
an informed
decision about the use of gas injection or water flooding as an appropriate
secondary oil recovery
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means. A water flooding application to stimulate oil-wet rock would be
considerably less
efficient than if applied to water-wet rock.
Various attempts have been made to alter the wettability and thereby provide
enhanced
oil recovery from carbonate reservoirs. One such approach involves chemically-
enhanced oil
recovery from in which a surfactant is used to modify wettability of the
matrix rock to be more
water-wet, as described in U.S. Patent 7,581,594. Another technique entails
the use of imbibing
fluids which have the effect of modifying the concentration of potential
determining ions that
influence the surface charge of carbonate rock, so as to improve its water-
wetting nature. Zhang
and Austad, Colloids and Surfactants A: Physicochemical and Engineering
Aspects, 279(1-3):
179-87 (2006). See also U.S. Patent 4,491,512.
A number of methodologies have been considered for enhanced recovery of high
viscosity or "heavy" oil. Low-frequency alternating current (AC) heating has
been evaluated in
Canadian heavy oil fields. Electro-magnetic (EM) and radiofrequency (RF)
induction have been
proposed for near well bore heating to reduce oil viscosity. Down-hole
resistive heaters have
also been suggested for heating the near well bore reservoir rocks. The
research and
development affiliates of several major oil companies have investigated
various AC, RF and
down-hole heaters for enhanced oil recovery. None of these approaches have
produced
consistent results.
Enhanced oil recovery has been achieved by DC electrical stimulation. See,
e.g., U.S.
Pats Nos. 6,877,556, 7,322,409 and 7,325,604, which are commonly owned with
the present
application. To date, this technique has been shown to be effective in
formations composed
primarily of either sandstone or unconsolidated sand.
Insofar as is known, the use of DC electrokinetics for hydrocarbon recovery
enhancement
in a carbonate rock reservoir has not previously been proposed.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides an efficient and effective
method of
enhancing oil recovery from a carbonate reservoir.
This method comprises selecting an underground formation comprising an oil-
bearing
carbonate reservoir, positioning two or more electrically conductive elements
at spaced apart
locations in proximity to the formation, at least one of the conductive
elements being disposed in
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or adjacent to a bore hole affording fluid communication between the bore hole
interior and the
formation, passing a controlled amount of electric current along an
electrically conductive path
through the formation and withdrawing oil from at least one of the bore holes.
The electric
current applied in carrying out this method is produced by a DC source
including a cathode
connected to one of the conductive elements and an anode connected to another
of the
conductive elements, and the electrically conductive path comprises at least
one of connate
formation water and an aqueous electrolyte introduced into the formation.
In another aspect, the present invention provides a method of fracturing an
oil-bearing
carbonate rock formation by subjecting the formation to long term electrical
stress.
The invention described herein is believed to be the first technically
feasible method
using electrokinetic phenomena to enhance oil recovery from a carbonate
reservoir.
BRIEF DESCRIPTION OF THE DRAWINGS
The following summary as well as the following description will be better
understood
when read in conjunction with the accompanying figures in which:
FIG. 1 is a schematic diagram of a DC electrokinetic method for enhancing oil
recovery
from DC oil-bearing carbonate reservoir in accordance with this invention;
FIG. 2 is a schematic diagram of one embodiment of a DC electrokinetic method
for
enhancing oil recovery from an oil-bearing carbonate reservoir; and
FIG. 3 is a schematic diagram of another embodiment of a DC electrokinetic
method for
enhancing oil recovery from an oil-bearing carbonate reservoir.
DETAILED DESCRIPTION OF THE INVENTION
Referring to the figures in general, and to FIG. 1 specifically, there is
shown an
underground formation 11 composed primarily of oil-bearing carbonate rock, in
which the
method of the invention is practiced. A suitable carbonate reservoir may be
selected or
determined using well established geologic sampling techniques. The oil-wet or
water-wet
condition of the carbonate rock present in the formation can be assessed
according to the
previously reported method of Chilingar and Yen, supra. Typically, such oil-
bearing formations
are found beneath the upper strata of earth, commonly referred to as
overburden, at a depth on
the order of 1,000 feet or more below the surface. Communication from the
surface 12 to the
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formation 11 is established through one or more bore holes. In FIG. 1,
communication from the
surface 12 to the formation 11 is established through spaced-apart boreholes
13 and 14.
Borehole 13 functions as an oil-producing well, whereas the adjacent hole 14
is a special access
passage provided for the transmission of electricity to the formation 11..
The present invention can be practiced using a multiplicity of electrically
conductive
elements or electrodes in one or more boreholes. The boreholes may be drilled
in a variety of
vertical, horizontal or angular orientations and configurations. In FIG. 1,
two electrodes are
disposed into vertically drilled boreholes. A first electrode 15 is lowered
through access hole 14
to a location in proximity to formation 11. Preferably, first electrode 15 is
lowered through
access hole 14 to a medial elevation in formation 11, as shown in FIG. 1. By
means of an
insulated cable in access hole 14, the relatively positive terminal or anode
of a high-voltage DC
electric power source 2 is connected to the first electrode 15. The relatively
negative terminal or
cathode of the power source is connected to a second electrode 16 associated
with producing
well 13. Between the electrodes, the electrical resistance of connate water 4
present in the
underground formation 11 is sufficiently low so that current can flow through
the formation
between the first and second electrodes 15, 16. Although the resistivity of
the oil is substantially
higher than that of the overburden, the current preferentially passes directly
through the
formation 11 because this electrically conductive path is much shorter than
any alternative path
traveling through the overburden to "ground."
To create the electric field, a periodic voltage is produced between the
electrodes 15, 16.
In one embodiment, the periodic voltage is established using pulsed DC power.
In another
embodiment, the voltage may be a DC-biased signal with a ripple component
produced under
modulated AC power. See U.S. Patent 6,877,556. The voltage may be produced
using any
technology known in the electrical art. For example, voltage from an AC power
supply may be
converted to DC using a diode rectifier. The ripple component may be produced
using an RC
circuit or through transistor controlled power supplies. Once the voltage is
established, the
electric current is normally carried by connate formation water. Electrical
current is conducted
through the formation by naturally occurring electrolytes in the groundwater.
If necessary or
desirable, aqueous electrolyte may be introduced into the formation to modify
the conductivity of
the connate water. A detailed system and procedure for injecting electrolyte
solution into an
underground formation is described in U.S. Patent 3,782,465. See also, U.S.
Patent 5,074,986.
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The electric potential required for carrying out electrochemical reactions
varies for
different chemical components in the oil. As a result, the desired intensity
or magnitude of the
ripple component depends on the composition of the oil and the type of
reactions that are
desired. The magnitude of the ripple component must reach a potential capable
of oxidizing and
reducing bonds in the oil compounds. In addition, the ripple component must
have a frequency
range above about 2 hertz and below the frequency at which polarization is no
longer induced in
the formation. The waveshape of the ripple may be sinusoidal or trapezoidal
and either
symmetrical or clipped. Frequency of the AC component is preferably between 50
and 2,000
hertz. However, it is understood in the art that pulsing the voltage and
tailoring the wave shape
may allow the use of frequencies higher than about 2,000 hertz.
Referring still to FIG. 1, the steps for practicing the method for enhancing
oil recovery
from a carbonate reservoir will now be described. An electric potential is
applied to first
electrode 15 so as to raise its voltage with respect to the second electrode
16 and the region of
the formation 11 immediately surrounding it. The voltage between the
electrodes 15, 16 is
preferably no less than 0.4 V per meter of electrode distance. Current flows
between the first and
second electrodes 15, 16 through the formation 11. Connate water 4 and/or
added aqueous
electrolyte, as the case may be, in the interstices of the oil-bearing
formation provides a
conductive path for current flow. Water that collects above the electrodes in
the boreholes does
not cause a short circuit between the electrodes and surrounding casings. Such
short circuiting is
prevented because the water columns in the boreholes have relatively small
cross sectional areas
and, consequently, greater resistances than the formation itself.
As current is conducted across formation 11, electrolysis in the formation
water occurs.
Electrolysis generates agents that promote oxidation and reduction reactions
in the oil. That is,
negatively charged interfaces of oil compounds undergo cathodic reduction, and
positively
charged interfaces of the oil compounds undergo anodic oxidation. These redox
reactions tend to
cause decomposition of split long-chain hydrocarbons and multi-cyclic ring
compounds into
lighter-weight compounds, contributing to lower oil viscosity. Redox reactions
may be induced
in both aliphatic and aromatic oils. As viscosity of the oil is reduced
through redox reactions, the
mobility or flow of the oil through the surrounding formation is increased so
that the oil may be
drawn to the recovery well. Continued application of electric current can
ultimately produce
carbon dioxide through mineralization of the oil. Dissolution of this carbon
dioxide in the oil
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further reduces viscosity and enhances oil recovery.
In addition to enhancing oil flow characteristics, the present invention
promotes
electrochemical reactions that upgrade the quality of the oil being recovered.
Some of the
electrical energy supplied to the oil formation liberates hydrogen and other
gases from the
formation. Hydrogen gas that contacts warm oil under hydrostatic pressure can
partially
hydrogenate the oil, improving the grade and value of the recovered oil.
Oxidation reactions in
the oil can also enhance the quality of the oil through oxygenation.
Electrochemical reactions are sufficient to decrease oil viscosities and
promote oil
recovery in most applications. In some instances, however, additional
techniques may be
required to adequately reduce retentive forces and promote oil recovery from
underground
formations. As a result, the foregoing method for enhanced oil recovery may be
used in
conjunction with other processes, such as electrothermal recovery or
electroosmotic treatment.
For instance, electroosmotic pressure can be applied to the oil deposit by
switching to straight
DC voltage and increasing the voltage gradient between the electrodes 15, 16.
Supplementing
electrochemical stimulation with electroosmosis may be conveniently executed,
as the two
processes use much of the same equipment. See U.S. Patent 3,782,465.
Many aspects of the foregoing invention are described in greater detail in
related patents,
including U.S. Pat. No. 3,724,543, U.S. Pat. No. 3,782,465, U.S. Pat. No.
3,915,819, U.S. Pat.
No. 4,382,469, U.S. Pat. No. 4,473,114, U.S. Pat. No. 4,495,990, U.S. Pat. No.
5,595,644 and
U.S. Pat. No. 5,738,778. Carbonate reservoirs in which the methods described
herein can be
applied include, without limitation, those containing heavy oil, kerogen,
asphaltinic oil,
napthalenic oil and other types of naturally occurring hydrocarbons. In
addition, the methods
described herein can be applied to both homogeneous and non-homogeneous
formations.
The above-described method may be used in combination with one or more pre-
treatments to improve the permeability of the formation. For example, the
present method may
be used in conjunction with an acidizing pre-treatment. A suitable acid is
introduced into one or
more borehole and an electric field is applied, as described above, to drive
the acidizing agent
into the formation. Migration of the acid is promoted by electroosmosis, but
may be assisted by
other means, such as well pumping. The electric field is effective to drive
the acid into regions of
the formation that cannot readily be reached using other available procedures.
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The present invention can be practiced using a multiplicity of cathodes and
anodes placed
in vertical, horizontal or angular orientations and configurations, as stated
earlier. Referring now
to FIG. 2, an alternate system is shown with electrodes installed in well
casing 113, 114. The
well casings 113, 114 extend in a generally horizontal orientation through an
oil-bearing
formation 111. The relatively positive terminal (anode) of high-voltage DC
electric power source
102 is connected to the first well casing 113. The relatively negative
terminal (cathode) on the
power source is connected to the second well casing 114. In this arrangement,
well casing 113
acts as a cathode at the producing well, and well casing 114 acts as an anode.
Insulating
components or breaks 115 are placed in each of the well casings 113, 114 so
that electricity
flows between the horizontal sections of the casings within the oil-bearing
formation 111.
Between the well casings 113, 114, the electrical resistance of the connate
water in the
formation, or any added aqueous electrolyte, as the case may be, is
sufficiently low so that
current can flow through the formation between the casings. Although the
resistivity of the oil is
substantially higher than that of the overburden, the current preferentially
passes directly through
the formation 111 because this path is much shorter than any path through the
overburden to
"ground."
The present method may include one or more electrodes placed at ground level.
See, e.g.,
U.S. Patent 4,495,990. Referring now to FIG. 3, an alternate oil recovery
system is shown with a
first electrode 215 placed below the earth's surface (marked "E") and a second
electrode 216 is
located at ground level in proximity to an underground oil-bearing formation
211. The first
electrode 215 is disposed in a borehole 214 that penetrates the formation 211.
The first electrode
215 is located within the formation, but may be located outside the formation,
depending on the
desired deployment and range of the electric field. The second electrode 216
is constructed on
the earth's surface. By means of an insulated cable in access hole 214, a
terminal on high-voltage
DC electric power source 202 is connected to the first electrode 215. The
opposite terminal on
the power source 202 is connected to the second electrode 216. A voltage
difference is
established between the first and second electrodes 215, 216 to create an
electric field across the
formation 211. It should be noted that the second electrode 216 may be
contained at a shallow
depth just beneath the earth's surface to produce an electric field. For
example, the second
electrode may be installed within fifty feet of the earth's surface to
establish an electric field
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across the formation. Placing the second electrode 216 at a shallow depth
below the earth's
surface may be desirable where space above ground is limited.
Although not wishing to be bound by a specific theory, it is believed that
when oil-
bearing carbonate rock is exposed in situ to DC electrokinetic treatment, as
described herein, the
wettability of the carbonate rock surface is altered. Specifically, the
carbonate rock surface is
rendered more hydrophilic than before electrokinetic treatment, thereby
causing oil to be more
easily displaced from the rock surface, e.g., by water flooding.
The technology described herein may also be beneficially applied to induce
fracturing of
an oil-bearing carbonate rock formation by subjecting the formation to long
term electrical stress.
The fracturing method may be carried out by the steps of positioning two or
more
electrically conductive elements at spaced apart locations in proximity to the
formation; and
passing a controlled amount of electric current along an electrically
conductive path through the
formation, with the electric current being produced by a DC source including a
cathode
connected to one of the conductive elements and an anode connected to another
of the
conductive elements, and the electrically conductive path comprising at least
one of connate
formation water and an aqueous electrolyte introduced into said formation.
Fracturing can be achieved by applying electrical stress in the manner
described above
for a time period of from 1 day to about 12 months, more preferably from about
1 week to about
6 months, and most preferably for at least 2 weeks. The electrical stress may
be applied at 2
volts/cm for the duration of the fracturing treatment, or it may be initiated
and maintained at 2
volts for a predetermined time and thereafter reduced to a lower value, e.g.,
1 volt/cm.
The following examples describe the invention in further detail. These
examples are
provided for illustrative purposes only, and should in no way be considered as
limiting the
invention.
In order to show the viability of the use of electrokinetics for the
production of oil from
carbonate rock two tests were run in the laboratory. One of the tests was
conducted on a core
taken from a cap rock and the second on a core taken from a producing
petroleum oil reservoir.
Both cores were first saturated with formation brine and then water flooded
with 39 API Light
Crude oil. Normal laboratory practice for the preparation of these cores was
used in both these
tests.
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Test 1 was performed on the Cap rock which had a diameter of 3.6 cm and a
length of 5.5
cm. The core was placed in a sample holder which allowed for a voltage
gradient to be
established across the core and a Direct Current power supply having a
variable current control
was used during the test. The measured permeability of this rock was 5.53 mD
for water and
94.3 mD to oil. The viscosity of the oil used in this experiment was 19.5. cp.
The pressure to
saturate the core with water was 40.8 psig, while the pressure necessary to
saturate the core with
oil was 54.4 psig. A voltage gradient of 2 volts/cm was imposed across the
sample. Current was
applied from 0 to 244 mA. The flow of oil and water was observed at various
currents and the
test established that the lowest current at which flow could be maintained was
82 mA or at a
current density of 8.16 mA/cm. In addition to the flow established in this low
permeability core
when the core was removed from the sample holder the rock had fractured as a
result of the
current passage through the core which ultimately would have increased the
permeability of the
rock.
Test 2 was performed on the carbonate reservoir formation rock core, which had
a
diameter of 1.8 cm and a length of 5.15 cm. The measured permeability of this
core to water was
measured at 5156 mD and 4204 mD to oil. The pressure needed to saturate this
core was much
lower, 0.2 psig for water and 4.6 psig to oil. The same voltage gradient of 2
volts/ cm was used
in this test with a resultant flow of water and oil being observed with a
current of 18 tO 21 mA.
The test results described above are summarized in the following table.
Cum
c.s.Area L/A PV
visc. Press. K Vol
cm cm2 cm2 cm-1 cc mumin cp Psi
md ml
Por Vol
5.50 3.60 10.18 0.540 11.46 2 1.04
49.80 5.53 25 2.0(
5.15 1.80 2.54 2.024 12.68 2 1.04 0.20
5156.70 50 4.0(
5.50 3.60 10.18 0.540 12.68 2
19.504 54.40 94.93 75 5.0(
5.15 1.80 2.54 2.024 12.68
2 19.504 4.60 4204.77 100 1 6.0(
The results of these tests demonstrate that the use of electrokinetics can be
effective to
move oil and water under a voltage stress and current flow that will depend on
the initial
permeability of the formation, the salinity of the formation and the applied
current.
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A number of patent and non-patent publications are cited in the foregoing
specification in
order to describe the state of the art to which this invention pertains. The
entire disclosure of
each of these publications is incorporated by reference herein.
While certain embodiments of the present invention have been described and/or
exemplified above, various other embodiments will be apparent to those skilled
in the art from
the foregoing disclosure. The present invention is, therefore, not limited to
the particular
embodiments described and/or exemplified, but is capable of considerable
variation and
modification without departure from the scope of the appended claims.
Furthermore, the transitional terms "comprising", "consisting essentially of'
and
"consisting of', when used in the appended claims, in original and amended
form, define the
claim scope with respect to what unrecited additional claim elements or steps,
if any, are
excluded from the scope of the claim(s). The term "comprising" is intended to
be inclusive or
open-ended and does not exclude any additional, unrecited element, method,
step or material.
The term "consisting of' excludes any element, step or material other than
those specified in the
claim and, in the latter instance, impurities ordinary associated with the
specified material(s).
The term "consisting essentially of' limits the scope of a claim to the
specified elements, steps or
material(s) and those that do not materially affect the basic and novel
characteristic(s) of the
claimed invention. All methods of enhancing oil recovery from carbonate
reservoirs that
embody the present invention can, in alternate embodiments, be more
specifically defined by any
of the transitional terms "comprising", "consisting essentially of' and
"consisting of'.
