Note: Descriptions are shown in the official language in which they were submitted.
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TS 9140
IN-SITU PRODUCTION OF BITUMEN
Background of the invention
The invention relates to a method and system for in-
situ production of bitumen wherein steam is injected into
the bitumen bearing formation to transfer heat to the in-
situ bitumen so that it will melt and move to one or more
producer wells and ultimately to surface.
Many steam stimulation, steam drive and gravity
drainage schemes (and combinations thereof) have been
suggested to produce in-situ bitumen. Designers of these
schemes have typically sought to promote the mass
transfer of the steam into the areas of the reservoir in
which bitumen is thought to be present. Steam stimulation
typically involves drilling wells into an underground
bitumen deposit and using the wells sequentially for
steam injection and then for production, which is called
a steam soak cycle. Mass transfer of steam into the
deposit may be promoted by fracturing the reservoir.
Steam drive and gravity drainage schemes make use of
steam injection wells and production wells
simultaneously. Mass transfer of the steam can take the
form of a communication path or steam breakthrough from
the injection well(s) to the production well(s). The flow
of steam warms the adjacent bitumen and encourages it to
flow toward the production well(s). Alternatively the
steam is encouraged into the deposit to create a steam
chamber that grows from the injection well(s) to the
production well(s). In any event, leakage of steam to a
basal water transition zone or elsewhere in the reservoir
where the bitumen saturation level is low (hereinafter
called "thief zones") has been avoided as being a waste
of heat energy and as being unproductive.
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.
Canadian patent specification 2,015,459 discloses a
process for confining steam injected into a heavy oil
reservoir having a thief zone, wherein a pressurized non-
condensable gas is injected into the thief zone to
inhibit the escape of injected steam through that zone.
US patent specification 4,344,485 and Canadian patent
specification 1,304,287 disclose steam assisted gravity
drainage processes wherein steam is injected via an upper
horizontal well section to transfer heat to the normally
immobile heavy oil so that it will melt and will drain by
gravity to a lower horizontal well section where the oil
is recovered.
US patent specification 4,390,067 discloses the use
of a rectangular grid of horizontal steam injection wells
to create heated corridors in a viscous oil or bitumen
bearing formation from which viscous oil or bitumen is
then produced via vertical production wells.
US patent specification 4,702,314 discloses an oil
production system comprising a rectangular four spot
production well pattern and a vertical steam injection
well at the centre of the pattern, wherein the production
wells comprise horizontal inflow sections that point
towards the steam injection well.
US patent specification 4,283,088 discloses a thermal
oil mining method wherein a series of steam injection and
oil production wells is drilled in an upward direction
and in a star-shaped configuration into the oil bearing
formation from a ring-shaped working tunnel which is
located near the bottom of said formation.
It is an object of the present invention to provide a
method and system for in-situ production of bitumen which
promotes the mobility of bitumen in larger volumes of the
bitumen bearing formation than has been previously
possible.
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Summary of the invention
The system according to the invention thereto
comprises a steam injection well system which comprises a
number of lateral sections that traverse the bitumen
bearing formation partly above a thief zone such that a
tip of each lateral section protrudes from the bitumen
bearing formation into the thief zone.
The method according to the invention comprises
injecting steam via the steam injection well system into
the bitumen bearing formation and the thief zone so as to
build up a steam chamber which grows from the tip towards
a heel of each lateral section of the steam injection
well system.
The injection of steam into the thief zone is opposed
to the previous teachings and has been adapted to
promote heat distribution in the vicinity of the end of
the lateral sections of the steam injection well system
and to reduce the amount of water in the produced fluids.
It is thought that this technique is more efficient
because it enables steam chamber growth at an
unprecedented rate from the tip towards the heel of each
lateral section of the steam injection system and
therefore promotes the mobility of bitumen in larger
volumes of the reservoir than has been previously
possible.
Preferably, the steam injection well system comprises
a plurality of substantially radial lateral sections
which are linked to a wellhead via a number of
substantially vertical upper sections such that, when
seen from above, said lateral sections traverse the
bitumen bearing formation in a star-shaped pattern away
from the wellhead.
Furthermore it is preferred that a group of four
lateral steam injection sections is linked to the
wellhead and adjacent lateral steam injection sections
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traverse the bitumen bearing formation in substantially
orthogonal directions and that the system comprises a
pair of substantially parallel and horizontal production
wells and a steam injection well system which comprises,
when seen from above, a plurality of wellheads which are
located at substantially equal distances from the
production wells and which are each linked to four
lateral steam injection sections which cross said
production wells at an angle between 30~ and 60~.
In that case it is also preferred that one or more
substantially horizontal production wells are drilled
after several steam soak cycles have been carried out via
the steam injection well system and that these production
wells cross the lateral steam injection sections at
selected distances.
These and further features, objects and advantages of
the method and system according to the invention will
become apparent from the following claims, abstract and
detailed description with reference to the drawings.
Description of the drawings
Fig. 1 shows a plan view, seen from above, of a well
system according to the invention which comprises two
parallel bitumen production wells and three steam
injection well systems;
Fig. 2 shows, at an enlarged scale, a vertical
sectional view of the system of Fig. 2, taken along
phantom line II-II; and
Fig. 3 shows a plan view, seen from above, of two
adjacent clusters of four well systems according to the
invention.
Referring now to Fig. 1 there is shown a gravity
assisted drainage system for in-situ bitumen production
according to the invention.
The system comprises two substantially parallel
horizontal production wells 1 and 2 and three
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multilateral steam injection well systems 3, 4 and 5.
Each system 3, 4 and 5 comprises a wellhead 6, 7 and 8,
respectively, which is connected to four lateral steam
injection sections 9, 10 and 11, respectively.
These lateral steam injection sections 9, 10 and 11
extend in a radial direction away from the wellheads 6, 7
and 8 in orthogonal directions such that each of the
lateral sections 9, 10 and 11 crosses one of the
production wells 1, 2 at an angle of between 30~ and 60~,
which angle is in the example shown 45~.
The distance between the parallel production wells 1
and 2 is about 100 m and the wellheads 6, 7 and 8 of the
steam injection systems 3, 4 and 5 are located halfway
between the production wells 1 and 2 and at mutual
distances of about 200 m so that the lateral sections 9,
10 and 11 cross the production wells at regular intervals
of about 100 m.
Fig. 2 shows in more detail the steam injection
system 4 at the centre of Fig. 1. As shown in Fig. 2 the
lateral steam injection sections 10 are connected to the
wellhead 7 by substantially vertical upper sections 12.
Furthermore, a substantially vertical pilot hole 13 is
connected to the wellhead 7, which hole serves to
accurately locate the depths of the bitumen bearing
formation 14 and basal water zone 15.
The lateral steam injection sections 10 trend
downwards over their length from their heel 16 towards
their tip 17. Each lateral steam injection section 10 and
vertical section 10 is completed with an un-cemented
liner (not shown) which is tied back to the pilot
hole 13. The liner is slotted over the length of the
lateral steam injection section 10 to permit injection of
steam into the bitumen bearing formation 14 and basal
water zone 15.
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The tip 17 of each lateral steam injection section 10
dips towards the basal water zone 15, which is a zone of
increasing water saturation and steam injectivity.
The thermal recovery process is initiated before the
horizontal wells 1 and 2 are drilled by injecting steam
at high rate via the wellheads 6, 7 and 8 into the
lateral steam injection sections 9, 10 and 11. Initially
the majority of steam will flow via the tips 17 of the
lateral steam injection sections 9, 10 and 11 into the
basal water zone 15, whereas lack of bitumen mobility
limits heat transfer to conduction along the length of
the other parts of these sections 9, 10 and 11.
The heat transferred by conduction into the bitumen
bearing formation 14 will warm up and gradually mobilize
bitumen in the vicinity of the lateral steam injection
sections 9, 10 and 11.
Injection of steam via the wellheads 6, 7 and 8 is
stopped after some time whereupon fluids are produced
back via the steam injection well systems 3, 4 and 5, so
that a steam-soak cycle is performed.
During the production phase of the steam soak cycle
the rate of gravity drainage of bitumen into the lateral
steam injection sections 9, 10 and 11 is sufficient to
block condensing steam in the basal water zone 15 from
entering the well systems 3, 4 and 5.
The steam soak cycle is then repeated one or more
times. The bitumen mobility will gradually increase as a
result of the subsequent steam soak cycles. Consequently
steam chambers will build up along the lengths of the
lateral sections 9, 10 and 11 which accelerates reservoir
heating and well production rates.
Within these steam chambers rising steam contacts
cold bitumen, condenses and an emulsion is created.
During the production phase residual heat in the rock
surrounding the lateral sections 9, 10 and 11 will
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vapourize water, thus returning steam to the chambers and
resulting in low water cut production.
Throughout the steam injection phases of the initial
steam soak cycles the majority of fluid loss continues to
be in the basal water zone 15, however, the fraction of
water in the produced fluid increase with steam chamber
development.
Following two or more steam soak cycles, the
horizontal wells 1 and 2 are drilled below the lateral
steam injection sections 9, 10 and 11 such that the
horizontal wells 1 and 2 cross these sections at a
distance of several, preferably at least 3 metres and
intersect the developed steam chambers.
The spacing of the horizontal production wells 1 and
2 at about 100 m and of the wellheads 6, 7 and 8 at about
200 m intervals, with the orthogonal radial steam
injection sections 9, 10 and 11 crossing the horizontal
wells 1 and 2, when seen from above at about 45~ leads to
the further development of steam chambers at 100 m
intervals over the length of the horizontal production
wells 1 and 2 thus reducing the likelihood of less than
full utilization of the horizontal production wells 1 and
2.
The large spacing of 100 m between the parallel
horizontal production wells reduces the drilling capital
and will improve ultimate recovery of the bitumen
resource.
The performance of a set of three steam injection
well systems 3, 4 and 5 drilled into the Peace River
bitumen deposit in Alberta, Canada has been encouraging.
During two steam soak cycles the well systems 3, 4 and 5
have produced more than 50000 m3 of bitumen at a bitumen-
steam ratio more than 0.4.
Subsequently a third steam soak cycle has been
completed. The overall result of the three steam soak
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cycles is that more than 90,000 m3 of bitumen has been
produced at a bitumen-steam ratio of more than 0.42.
The Peace River in-situ oil sand deposit is in a
formation that contains water in varying concentrations.
In the upper regions in the formation, bitumen saturation
levels are high and they decline toward the lower regions
in the formation. Conversely, the water saturation levels
are low in the upper levels in the formation and increase
toward the lower levels of the formation. At a certain
depth, the water saturation levels are sufficiently high
that the water becomes mobile. The lower part of the
formation, containing this mobile water, acts as a thief
for injected steam, and progressed into a basal water
transition zone (15).
Referring now to Fig. 3 there is shown an alternative
well configuration where two sets of four well systems 30
according to the invention traverse in substantially
horizontal directions through a bitumen bearing
formation.
Each well system comprises three radial sections 31,
32, 33 that have been drilled away from a substantially
vertical central riser section 34 that leads to a central
wellhead which in the plan view of the drawing coincides
with the riser section 34.
From at least two of the radial sections 31, 32, 33 a
set of one or two tangential sections 35, 36 has been
drilled in a left hand or other predetermined orientation
such that the tip 40 of each tangential section 35, 36
protrudes downwardly from the bitumen bearing formation
into a thief zone at the bottom of the bitumen bearing
formation or through the thief zone into a basal water
transition zone which is located below the thief zone.
The well pattern shown in Fig. 3 generates clusters
of substantially regularly spaced and distributed radial
and tangential lateral well sections 31-36, via which
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steam is injected in an evenly distributed manner into
the bitumen bearing formation and via the tips 40 of
these wells into the underlying thief and/or basal water
transition zone.
Preferably the same well systems 30 are used for
production of bitumen after steam has been injected
through the well systems 30, so that a steam soak cycle
is carried out. The seam soak cycle may be repeated
several times until a major part of the bitumen has been
mobilized and recovered.
