Note: Descriptions are shown in the official language in which they were submitted.
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COMPLETION PROCEDURE FOR PADS SUBJECTED
TO THERMAL STFAM STIMULATI ON
Field of the Invention
The invention relates generally to the production of
hydrocarbons from subterranean reservoirs. This invention
relates more particularly to the production of bitumens from
underground tar sand beds.
Back~round of the Invention
Throughout the world there exist numerous subterranean
tar sand formations containing high-density, high-viscosity
bitumens which resist recovery by conventional means. The vast
Athabasca tar sand field in Alberta Province, Canada represents
one of the most notable examples of such formations. The Cold
Lake deposits in the same province represent a similar
formation.
A variety of methods have been proposed for improving
the production of hydrocarbons from these formations by
increasing their mobility, including both solvent injection and
thermal steam stimulation processes. One currently utilized
method is the so-called "huff-and-puff" process. In this
method, a well is sunk into the bitumen-bearing formation and
completed at a given depth, usually by perforation, so as to
establish fluid communication between the well and the
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formation. Steam is injected through the producing well into
the formation to mobili~e the bitumens. Steam injection is then
terminated and, often following some shut-in period, hydrocarbon
containing fluids are produced from the same well. In a
representative process, steam is injected at high pressures
(approximately 10 MPa) into a well for approximately six weeks,
until about 10,000 m3 water equivalent steam has been
injected. The steam injection pressure applied exceeds the
fracture pressure of the formation in order to achieve the
required injectivity. The injection of steam is then terminated
and the well ls produced for a period of three months or more.
Typical practice is to complete "huff-and-puff" wells
at depths determined more or less individually on the basis of
geological considerations for that portion of the tar sand bed
which they penetrate. ~ence, if the geological considerations
indicate that an individual "huff-and-puff" well can most
effectively drain a reservoir if completed at a certain true
vertical depth (typically somewhere near the lower oil horizon),
then each such "huff-and-puff" well penetrating that area of the
bed is completed at that same true vertical depth.
Recent study of reservoir behavior during huff-and-puff
operations by Applicants indicates that several material
disadvantages inhere in the typical completion procedure when
applied to tar sand beds of the Athabasca or Cold Lake type.
Applicants studied a cluster or "pad" of "huff-and-puff" wells
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which, consistent with the practice described above, had all
been completed at about the same true vertical depth in order to
drain a portion of an Athabasca- or Cold Lake- type tar sand
bed. Through the use of a number of observation wells
associated with this particular pad, Applicants obtained data
strongly indicative of well-to-well communication over
considerable distances via horizontal fractures at the nominal
completion depth of the huff-and-puff wells. As summarized in
FIGURE 5, significant temperature increases were noted
throughout the pad in each of the observation wells at the
nominal completion depth of 455 meters. Furthermore,
measurements indicated that the minimum horizontal in situ
stress exceeded the overburden stress at each of the production
wells and positive wellhead pressures were observed on each of
the unsteamed wells.
Such well-to-well communication via horizontal
fracturing is disadvantageous in that it results in poor
vertical conformance in the tar sand bed. That is, the thermal
and other mobilizing effects of the steam stimulation process
are not evenly distributed through the formation, resulting in
reduced thermal efficiency and a lower recovery factor per well,
which further results in the need for additional wells to
effectively drain a given area of tar sand bed.
Accordingly, prior to the present invention, there was
believed to be no huff-and-puff method for production of bitumen
from a subterranean tar sand bed which achieved an improved
vertical sweep efficiency by preventing well-to-well
communication via horiæontal fracturing.
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Summarv of the Invention
The present invention concerns a method for recovering
bitumens from a subterranean reservoir penetrated by more than
one production well, comprising the steps of establishing fluid
communication between a first well and said reservoir at at
least one level in said reservoir, establishing fluid
communication between a second we].l and said reserYoir at at
least one other level in said reservoir, said at least one other
level being arranged so as to be a vertically spaced distance
from every level at which fluid communication is established in
said first well, whereby the intersection of horizontal
fractures between said wells at said at least one other level is
prevented, injecting fluid via each of said wells into said
reservoir at a pressure exceeding the fracture pressure of said
reservoir, terminating the injection of fluid into at least onè
of said wells, and recovering bitumen-containing fluids from
said reservoir via said at least one of said wells.
Another feature of the present invention is that method
in which the steps of injectlng fluid into and recovering
bitumen from the tar sand bed via each of said wells is repeated
cyclically.
Another feature of this invention is that the method
may include the subsequent steps of: terminating the injection
of fluid into the formation via at least one well which is in
fluid communication with the reservoir at a level below the
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median level of communication for those wells in communication
with the local area of said reservoir, and producing bitumen
from the tar sand bed via those wells through which fluid
injection has been terminated.
Yet another feature of this invention is that the
method may include producing bitumen from the tar sand bed via
those wells through which fluid injection has been terminated
while continuously injecting fluid into the tar sand bed via
those wells through which fluid injection has not been
terminated.
`~ The method of the present invention for producing
bitumens from a subterranean tar sand bed has the advantages of
increasing the vertical sweep efficiency by preventing adverse
well-to-well communication via horizontal fractures.
Brief Description of the Drawin~s
FIGS. 1-4 are partial, cross-sectional views of a
formation having a reservoir containing high-density, high-
viscosity bitumens wherein the sequential method steps in
accordance with the present invention are illustrated.
FIG. 5 illustrates vertical temperature distributions
observed in observation wells associated with a pad of
huff-and-puff wells. The horizontal distances between the
observation wells and the nearest steamed huff-and-puff wells
are listed in the upper right corner.
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These drawings are not intended as a definition of the
invention, but are provided solely for the purpose of
illustrating certain preferred embodiments of the invention, as
described below.
Detailed Descri~tion of the Invention
FIGS. 1-4 illustrate the sequential steps in practicing
the method of the present invention for the recovery of high-
density, high-viscosity bitumens. As used in this application,
the term bitumens is used to mean any heavy, viscous
hydrocarbon.
With reference to FIG. l, terrain, or field, 10 is
shown to include overburden 12 lying over reservolr 14
containing high-density, high-viscosity bitumen. Reservoir 14
is underlain by stratum 16. Field 10 is provided with
injection-production wells 18, 20, and 22 in a conventional
manner, which extend from the surface of terrain 10 downwardly
into reservoir 14. Three injection-production wells are shown,
however, it should be understood that the method of the present
lnvention is applicable to use with as few as two
injection-production wells, as well as with any other number of
injection-production wells disposed in any suitable pattern or
configuration.
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Still with reference to FIG. l, it is seen that wells
18, 20J and 22 are provided with a conventional casingB 24~ 26
and 28 which are cemented in a conventional marmer which
prevents the flow of steam along the axis of the wells between
the well casings 24~ 26~ and 28 and the well bores (not shown).
With further reference to FIG. 1, the next steps of the
present invention will be described. After wells 18J 20, and 22
have been provided, each well casing 24~ 26~ and 28 is
perforated in a conventional maTmer to provide it with a
plurality of holes or perforations 30J 32~ aT1d 34J which
establish fluid communication between each well 18, 20J and 22
and reservoir 14. It should be noted that although fluid
communication is here shown as being established by conventional
perforation, such communication could be established by any
suitable method of forming an opening through the well casing
and surrounding cement. Each depth at which each well is
perforated is vertically offset from every depth at which each
other well is perforated. As used in this application, depth,
level, and horizontal refer to relative positions or elevations
in the reservoir itself, which may or may not correspond to the
absolute elevation of the positions. Although wells 18, 20, and
22 are shown with only one perforated interval each, it should
be noted that each well could be provided with any number of
perforation levels, so long as each such level is vertically
offset from each level of perforation in the other wells.
Perforations 30, 32~ and 34 are offset by a sufficient vertical
distance to prevent the intersection of any horizontal fractures
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emanating therefrom. A further consideration in determining the
vertical offset distance is the optimum vertical spacing of the
"huff-and-puff" recovery zones for the subject bitumens.
Techniques for determining such spacing, including reservoir
modeling, are kno~m to those skilled in the art. Data derived
from the workover of existing "huff-and-puff" wells indicates
that the spacing for Cold Lake-type tar sands may be 10 to 20
meters.
After wells 18, 20, and 22 have been provided with
perforations 30, 32, and 34 as described, high-pressure fluid
(that is, fluid at or above fracturing pressure for reservoir
14) is injected into each injection-production well 18, 20, and
22. The high-pressure fluid travels in the directions showm by
arrows 36, passes through perforations 30, 32, and 34 formed in
casings 24, 26, and 28 and forms fractures 38, 40, and 42 as it
enters the formation. Fractures 38, 40 and 42 are each
substantially horzontal and are each vertically offset from the
others so that they do not intersect and so that there is no
communication via the fractures between wells 18, 20, and 22.
It should be understood that, while it is an object of
the present invention to isolate injection fractures from one
another, in some embodiments it may still be desirable to have
one or more fractured intervals common to several wells: for
example, a gravity drainage fracture at the bottom of the
reservoir with which each well communicates.
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g
The injected fluid is usually steam. Consequently, the
terms "fluid" and "steam" will hereinafter be used
interchangeably. As the steam passes into fractures 38, 40, and
42, heat and fluids pass into reservoir 14, mobilizing the
high-density, high-viscosity bitwnen, in the directions shown by
small arrows 44.
After a selected quantit~y of high-pressure steam has
been injected into each o the injection-production wells 18,
20, and 22, the injection of steam is terminated. Thereafter,
as shown in FIG. 2, bitwmen-containing fluids are produced from
each of the wells. The bitwmens in reservoir 14 are mobilized
in the directions shown by small arrows 46. Bitwmen-containing
fluids, as shown by arrows 48, pass along ~he fractures 38, 40,
and 42 and through perforations 30, 32, and 34 in well casings
24, 26, and 28 into wells 18, 20, and 22 for production at the
surface of field 10 in a conventional manner. After the
termination of the injection of steam into injection-production
wells 18, 20, and 22 and prior to the production of bitwmen
thereform, the injection-production wells may be shut-in,
whereby for a time no fluids and/or bitwmens flow outwardly from
the injection-production wells.
The injection and production steps illustrated in
FIG. 1 and FIG. 2, respectively, may be repeated cyclically, as
in the "huff-and-puff" technique described above, in order to
optimize the recovery of bitumens as mobilization of the
reservoir increases. After a time, the reservoir may be heated
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to the point where the bitumen viscosity drops enough to permit
the general downward migration of bitumens to lower levels in
the reservoir due to gravity drainage. Consequently, at some
point it may become advantageous to cease the injection of steam
into those wells which are in communication with the lower
levels of the reservoir. Such wells can then be operated more
or less continuously as production wells while cyclical
injection and production continues in those wells in
communication with the reservoir at higher levels. With
reference to FIG. 3, this next step of the invention will be
described. Injection of steam into injection-production well
22, which is in communication with reservoir 14 at the lowest
level of the three wells illustrated, is terminated. Cyclical
injection of steam and production of bltumen continues in
injection-production wells 18 and 20 as described in the
preceding steps, with steam and bitumen-containing fluids moving
alternately into and out of wells 18 and 20, through
perforations 30 and 32, and along fractures 38 and 40 in the
directions shown by arrows 50. Bitumens in reservoir 14 are
mobilized alternately in the directions shown by small arrows
52. A certain amount of mobilized bitumen-containing fluids
migrate downward through reservoir 14 in the direction shown by
arrows 54 and collect in sump 56, through which mobility has
been increased by heating and attendant viscosity reduction,
along with partial depletion of bitumens from the reservoir by
preceding injection and production from well 22.
Bitumen-containing fluids then move along fracture 42 in the
direction shown by arrows 58, through perforations 34 and into
well 22, from which they are produced in a conventional manner.
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Further, after additional depletion of bitumens from
the reservoir through repeated application of the steps
described above, it may prove advantageous to operate the system
so that the wells completed at deeper levels in the reservoir
are permanent producers and the remaining wells are continuous
injectors, using gravity drainage and/or steam drive to mobilize
remaining bitumens to the produc:lng wells. With reference to
FIG. 4, this next step of the present invention will be
described. Following the final productlon cycle using
injection-production wells 18 and 20, production of bitumens
from wells 18 and 20 is terminated and steam is substantially
continuously injected into injection-production wells 18 and
20. The steam travels out through perforations 30 and 32 and
along fractures 38 and 40 in the direction shown by arrows 60,
and out into partially depleted reservoir 14 in the direction
shown by arrows 62. A steam condensate~bitumen interface is
shown by boundary 64, which progresses downward in the direction
shown by small arrows 66 as mobilized bitumens, shown by arrows
68, move towards lower areas of reservoir 14, due to gravity
drainage or steam drive. Bitumen-containing fluids, as shown by
arrows 70, move along fracture 42, through perforations 34 and
into well 22, from which they are produced by a conventional
manner. Techniques for determining optimum steam injection
rates to maximize bitumen recovery and to minimize fingering of
the condensate/bitumen boundary in steam drive recovery systems
are known to those skilled in the art.
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It is to be understood that the invention is not
limited to the exact details of construction or operation, exact
materials, or exact embodiment shown and described, as obvious
modiFications and equivalents will be apparent to one skilled in
the art. ~or example, another hi~h pressure fluid, other than
steam, could be utilized. Accordingly, the invention is
therefore to be limited only by the scope of the appended claims.
