Note: Descriptions are shown in the official language in which they were submitted.
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APPLICATION FOR PATENT
SLANTED WELL ENHANCED EXTRACTION
PROCESS FOR THE RECOVERY OF HEAVY
OIL AND BITUMEN USING HEAT AND SOLVENT
Field of the Invention
This invention relates to a process for the recovery of high viscosity
hydrocarbon
resources from subterranean hydrocarbon bearing reservoirs with the
application of heat
and solvent using a special well configuration.
Background of the Invention
Highly viscous hydrocarbons, known as heavy oil and bitumen, exist inside the
fine pores
of the subterranean porous formation called reservoir. To extract the
hydrocarbons from
these reservoirs, the hydrocarbons have to be mobilized inside the porous
formation and
allowed to flow to holes drilled inside the formation called wells. The
mobilized
hydrocarbon is then brought to the surface and processed for its end use. The
efficient
extraction of these hydrocarbons out of the porous formation is the objective
of this
disclosure.
Although there is an enormous amount of resources available in the form of
heavy oil and
bitumen, their high viscosity prevents its flow in the formation. Depending on
the
reservoir temperatures and the type of resources the mobility of the fluid
varies. In large
areas of the underground Alberta oil sand the temperature is in the range of l
OoC and the
hydrocarbons in the formation would be a few million times more viscous than
water at
ambient conditions. Under this reservoir condition the hydrocarbon has a
thick, semi-
solid appearance and is substantially immobile even outside the formation. In
the heavy
oil reservoirs the mobility is usually higher than that at the Athabasca oil
sand reservoirs,
however, the conventional recovery techniques have been unsuccessful in
recovering
these resources. The high viscosity of these resources demands special
recovery
techniques.
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Certain prior art publications of interest in this field are noted as follows:
References
Butler, R.M., "Method for Continuously Producing Viscous Hydrocarbons by
Gravity
Drainage while Injecting Heated Fluid", Canadian Patent 1,130,201, August 24,
1982,
US patent 4,344,485, Aug 17, 1982.
Das, S.K., "In Situ Recovery of Heavy Oil and Bitumen Using Vaporized
Hydrocarbon
Solvents", Ph.D. Thesis, University of Calgary, 1995.
Nzekwu, B.I., Sametz, P.D. and Pelensky, P.J., "Single Horizontal Wellbore
Gravity
Drainage Assisted Steam Flooding Process", US patent 5,626,193, May 6, 1977.
Anderson, D.J., "Method of Recovering Viscous Petroleum From an Underground
Formation", US patent 4,037,658, July 26, 1977.
Dewell, R.R., "In Situ Extraction of Asphaltic Sands by Counter-current
Hydrocarbon
Vapor", US patent 4,067,391, Jan 10, 1978.
Sanchez, J.M., "Process/Apparatus for the In Situ Extraction of Viscous Oil by
Gravity
Action Using Steam Plus Solvent Vapor", US patent 5,148,869, Sep 22, 1992.
Jensen, E.M., Uhrich, K.D. and Hassan, D.J., "Single Well Vapor Extraction
Process",
US patent 5,771,973, June 30, 1998.
Mobility of these hydrocarbons increases with increase in temperature. Based
on this
principle various thermal recovery techniques have been applied for the
recovery of these
hydrocarbons. One of the more successful processes, the steam assisted gravity
drainage
(SAGD), Canadian Patent 1,130,201, 1982 noted above, a steam based process
using a
pair of horizontal wells drilled into the reservoir and placed one vertically
above the
other, has been used successfully for the recovery of these high viscosity
resources. In
this process steam is injected in the upper well of the well pair. The
injected steam
condenses inside the reservoir and heats the formation and hydrocarbons. The
hot
mobilized oil and the condensed water drains to the lower horizontal well by
gravity.
This hot fluid is produced to the surface by natural or artificial lift. Heat
transfer over a
huge surface area along the edge of the steam chamber formed during the
process and the
gravity head are the key factors in achieving a high extraction rate in this
process.
Various other well configurations have also been attempted.
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Although the SAGD process has met with commendable success, it suffers from
the
inherent disadvantages of higher energy requirement, environmental pollution
(emission
from steam generation), high capital costs for the surface facilities for
water treatment,
etc.
Viscosity of heavy oil and bitumen may also be reduced by injection of solvent
into the
reservoir. This forms the theoretical basis for any solvent recovery process.
One of the
prior art processes (named VAPEX) injects vaporized hydrocarbon solvent into
the
reservoir through an injector and the diluted oil, due to its lower viscosity,
drains to a
production well (Das, 1995). This process has so far been investigated in
laboratory
experiments. In the solvent-based extraction processes, the solvent recovery
at the surface
is crucial for the economic success of the process and requires significant
capital
investment.
A combination of heat and solvent has been visualized as a solution to this
problem in
our pending Canadian application Serial No. 2,281,276 filed August 31, 1999. A
near
well bore heating mechanism in the production well is used to revaporize the
solvent and
the solvent vapor condenses at the solvent-bitumen interface inside the
reservoir. This
solvent action dilutes the oil that drains to the horizontal producer. Thus
the same solvent
is effectively recycled and utilized inside the reservoir. A small amount of
make up
solvent is injected through a horizontal producer. Production through these
horizontal
wells, especially in case of two or three phase flow situation, is quite
complicated and it
is difficult to maintain a gravity stable drainage process along the entire
length (-1000
m) of the production well. Much of the time this may end up in solvent vapor
bypassing
(as it happens in the steam process, SAGD).
The above techniques use a pair or more of horizontal wells for the process.
There are '
prior techniques in which a single well is used for both injection of the
heated fluid or
solvent and production of the mobilized oil. Nzekwu et al. (US patent 5626193)
presented a single wellbore SAGD process, which was applied in the field
operation.
Results indicated a limited growth of the steam chamber and primarily the near
well bore
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heating phenomenon in this process.
Anderson presented a proposal (US patent 4037658) for creating a horizontal
passage in
the formation by delivering heat through a tubular carrying hot fluid. A drive
fluid
(steam) is injected into the formation through one end of the heated passage
to promote
movement of the viscous hydrocarbon along the passage, external to heat
delivery
tubular. The displaced fluid is produced through a recovery shaft at the other
end of the
passage.
Dewell (US patent 4,067,391) proposed a special design of a horizontal conduit
for
delivering heated hydrocarbon vapor to the formation and the recovery of
viscous
hydrocarbon. A Plurality of multilaterals is drilled from a vertical shaft to
distribute the
solvent vapor and recover the hydrocarbon.
In another proposal (Sanchez, US patent 5148869)a similar conduit was proposed
to be
used for simultaneous injection of steam and hydrocarbon solvent vapor into
the
formation. Thus steam is allowed to heat the reservoir by conductance while
hydrocarbon
vapor enters the hydrocarbonaceous reservoir fluids. Heated hydrocarbonaceous
fluids
with dissolved solvent, having a reduced viscosity flow from the reservoir
around the
horizontal wellbore.
Jensen et al. (US patent 5771973) presented a proposal in which a tubing
string placed
inside a horizontal well with raised end (toe) is used for injection of
unheated
hydrocarbon solvent as saturated vapor. Viscous reservoir hydrocarbon,
mobilised due
to dilution by the solvent, drains to the horizontal section of the same well
and is
produced through a second tubing string.
Summary of the Invention
Accordingly, the invention in one aspect provides a method for the in-situ
recovery of
viscous petroleum hydrocarbons from an underground formation comprising:
(a) providing a slanted well within the formation, (b) providing solvents in
the formation
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above the slanted well which are capable of dissolving in and diluting the
hydrocarbons
to reduce the viscosity and promote drainage of said hydrocarbons toward the
slanted
well, with a vapour chamber being created above the slanted well in the
hydrocarbon-
depleted formations resulting from said drainage, (c) maintaining a hot zone
adjacent to
or at the slanted well to vaporize solvents contained in the downwardly
draining
hydrocarbons prior to or upon their entry into the slanted well such that the
vaporized
solvents move upwardly through the vapour chamber in a continuing manner to
again
contact said hydrocarbons in the formations at the boundaries of the vapour
chamber to
promote the continued dilution and drainage of the hydrocarbons toward said
slanted well
whereby a substantial portion of the solvents are recycled within the
formation, and
wherein (d) said slanted well is sloped downwardly from a first point to a
second point
sufficiently that the downwardly draining hydrocarbons entering said well pass
along said
well by gravity from the first point to the second point; and (e) producing to
the surface
the hydrocarbons which have reached the second point of the slanted well.
The method thus provides for the in-situ recovery of viscous petroleum
hydrocarbons
from an underground formation using a combination of heat and vaporized
solvents
comprising lighter hydrocarbons. Liquid solvent is injected through the
substantially
slanted well, placed near the base of the reservoir, and is vaporized in situ
by applying
heat through an indirect heating system placed inside the slanted well. Hot
solvent vapor
rises inside the reservoir, contacts the cold viscous hydrocarbons, condenses,
dissolves
and diffuses into the hydrocarbon in the reservoir. As a result, the viscous
hydrocarbon
is diluted to a lower viscosity fluid, which drains to the slanted well.
Although the
primary effect of viscosity reduction is achieved due to the solvent dilution,
the increased
temperature inside the reservoir also aids the process by causing additional
reduction in
viscosity. Depending on the solvent composition used there may be some in situ
deasphalting and upgrading of the viscous reservoir hydrocarbon. This may also
cause
some reduction in viscosity.
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The hot/diluted oil in one embodiment is drained into the slanted well, flows
to the toe,
and is collected in a vertical drain hole drilled at the toe of the slanted
well. The vertical
well is equipped with artificial lift to produce the oil to the surface
facility for treatment.
The process is named SWEEP (Slanted Well Enhanced Extraction Process). More
than
one slanted well may drain to a single vertical well. This may reduce the
number of
vertical wells required and reduce the capital and operating costs.
Alternatively several
slanted wells may drain to a single horizontal well drilled at the base of the
reservoir.
The hot bitumen from this horizontal well is produced to the surface using an
artificial
lift.
The slanted well, drilled down-dip from the heel is typically completed with a
closed
loop circulation heating arrangement. Steam or any other hot fluid is
circulated through
a pair of concentric tubings of the closed loop circulation heating system to
transmit heat
to the reservoir fluid without contacting the reservoir with steam (termed as
indirect
heating). Due to the indirect nature of the heating system, the return
condensed water is
not contaminated by reservoir fluids and does not require elaborate surface
treatment for
the purpose of recycling or disposal, thereby significantly reducing the
surface facilities.
This also eliminates the operating pressure and temperature constraints, which
are
dictated by the fracture pressure of the formation. A high pressure and
temperature
heating fluid system may be used inside the closed loop system without
subjecting the
reservoir to the same high pressure.
The purpose of this energy supply into the reservoir is to boil off the
solvent dissolved
in the diluted oil. This vaporized, previously injected solvent along with the
makeup
amount of solvent moves to the unextracted formation, contacts the viscous
hydrocarbon
and continues the extraction process. This process is repeated in a continuous
fashion and
the solvent component is recycled again and again inside the reservoir. This
in situ "
solvent recycle reduces the size of the surface treatment facility and the
solvent recovery
unit. The capital and operating costs are also reduced significantly. Several
of the slanted
wells may drain to a single vertical/horizontal well reducing the overall
number of well
and pads requirement. Also, in a solvent-based gravity drainage extraction
process the
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solvent has to be present in the vapor phase in the extraction chamber. The in
situ supply
of heat provides a superior way of maintaining the vapor phase in the
reservoir compared
to injection of vaporized solvent at the surface, which may condense on its
way to the
reservoir.
It should be noted that when a mixed solvent such as naphtha is used, only the
lighter
hydrocarbon components are vaporized and refluxed into the reservoir. The
heavier
fractions of the solvent mixture remain in the liquid phase and act as the
diluent aiding
in the lifting process. This also reduces the requirement of diluent blending
at the surface,
necessary for bringing down the viscosity of the crude oil to the pipeline
specification.
In a variation of the process a single well may be drilled from a surface
location that goes
down to the hydrocarbon reservoir, travels the desired length inside the
reservoir with a
desired dip and comes out of the ground at another surface location. The
segment of the
well inside the reservoir is completed with a slotted liner or wire-wrapped
screen. The
closed loop heating system is placed inside the slanted section of the well
through one
of the vertical shafts. The oil is produced using an artificial lift placed in
the other shaft
of the well. This reduces the uncertainty of intersecting the vertical drain
hole or the
producer with a slanted well drilled from another pad location.
The closed loop indirect heating system may be replaced by any other heat
energy supply
system such as an electrical or electromagnetic induction heating (EMI)
system. In EMI
heating system electric current is passed through a set of magnetic coils,
attached to the
outside of the tubing inside the slanted well. This induces heating of the
liner of the
slanted well. The liquid solvent, injected in the annular region, is vaporized
by the heat
energy delivered by this process. This heat energy also boils off the
dissolved solvent
from the oil drained to the slanted well.
A variation of the process utilizing electrical/EMI heating system may use an
electrical
submersible pump (ESP) attached to the toe of the tubing inside the slanted
well, to lift
the oil to the surface through the tubing. The exterior of the same tubing is
used to place
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the electromagnetic element along the slanted segment of the well for
induction heating.
With this configuration it is possible to eliminate the requirement of the
vertical well.
In another variation of the process the slanted section is drilled up-dip from
the heel. The
well is completed in a very similar manner as in a down-dip well. In this
configuration
the diluted and hot oil flows along the length of the slanted well to the heel
section of the
well which is then produced to the surface by using an artificial lift. Along
the length of
the slanted well the dissolved solvent vaporizes and refluxes back into the
reservoir. The
make up solvent is either injected near the toe of the well or at the middle
of the slant
section through a tubing, concentric to the closed loop heating system.
The overall purpose of the process is to sufficiently saturate the bitumen
with solvent in
the colder section of the reservoir, such that its viscosity is reduced enough
to allow the
oil to flow by gravity drainage to the production well. The energy supplied in
the slanted
well further reduces the viscosity of the oil through heating, and re-vaporize
the dissolved
solvent in the oil. This causes a "reboil" effect in the reservoir, as the re-
vaporized
solvent rises and combines with the injected solvent vapor to continue the
extraction of
the viscous hydrocarbon. The heated oil is produced to surface free of the
lighter fractions
of the dissolved solvent, and potentially free of water, which will remain in
the reservoir.
As such, this becomes a continuous process for the recovery of heavy oil and
bitumen
from oil sands using smaller number of wells or replacing a pair of horizontal
wells used
in the prior arts with one slanted well and one or no vertical well.
A considerable volume of the bitumen resources are present at a depth (-100
m), which
deeper than the mining capability and shallower than the capability of the
currently
available in situ recovery technologies. Some of the conventional in situ
recovery
technologies using a pair ofhorizontal wells may not be applicable in these
reservoirs due '
to the challenges in drilling horizontal wells in the shallow reservoirs. Use
of slanted
wells in the present invention may be useful for recovering these reserves.
Since the heat
transfer fluids do not contact the reservoir directly the pressure of the
SWEEP operation
can be controlled to a lower range suitable for this shallow reservoir.
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Brief Description of the Drawings
Figure 1 is a diagram illustrating the operation of the SWEEP process;
Figure 2 is a schematic cross-section through a formation showing details of
one
embodiment of the well configuration;
Figure 2A shows a finned section of external tubing;
Figures 3 and 3A are schematic elevation and plan views of the process using
multiple
slanted well for each vertical well;
Figure 4 is a schematic of the SWEEP process using electrical / EMI heating;
Figure 5 is a schematic of the process using electrical/EMI heating and ESP;
Figure 6 is a schematic of the process using a single up-dip slanted well;
Figure 7 is a schematic of the process using a U-shaped well.
Detailed Description of Preferred Embodiments
This invention is a method of in-situ recovery of heavy oil and bitumen, using
a
combination of heat and vaporized solvents as explained in Figure 1. Heavy oil
and
bitumen are present in underground reservoir 1, depicted in Figure 1, in a
highly viscous
and immobile form. The solvent is injected into the reservoir through a
slanted well 2.
The solvent consists of a combination of hydrocarbon vapors containing one or
more of
methane, ethane, propane, butane, pentane, hexane, heptane, octane, xylene,
toluene,
distillate, natural gas condensate, naphtha and all of their isomers. The
solvent may
include one or more of non- hydrocarbon compounds such as carbon dioxide,
sulfur
dioxide, and hydrogen sulfide, including small amounts of nitrogen and oxygen.
The
solvent may be injected either as a vapor, liquid or a vapor-liquid mixture
and may be
vaporized inside the wellbore and the reservoir by the application of heat.
Heat is injected into the reservoir through the closed loop circulation
heating system as
explained below in detail. The primary purpose of the heat injection is to
boil the solvent
out of the diluted oil accumulated at the slanted well; the hot oil with
minimal solvent is
produced to the surface through the production well. The hot solvent vapor
moves
generally upwardly through the vapor chamber 3 which has developed within the
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formation and condenses at the oil-solvent vapor interface 4, dissolves in the
oil and
reduces the viscosity; the heat transfer at the interface also aids in
viscosity reduction due
to increase in temperature. The hot and diluted mobile oil 5 drains by gravity
downwardly through the formation to the slanted well near the base of the
reservoir,
enters the well via suitable openings in the well liner and flows along the
dip of the well
to a vertical drain hole 6. Along its journey to the production end the
dissolved solvent is
vaporized from the mobilized oil; it goes back into the reservoir via the
vapor chamber 3
and again dissolves and leaches the reservoir oil in the colder section at the
interface 4 of
the reservoir. Since, the same solvent is recycled and reused again and again
inside the
reservoir, only a small amount of makeup solvent is injected to fill up the
void space
created inside the reservoir due to oil production. This significantly reduces
the surface
operations, offering an efficient method for the recovery of highly viscous
hydrocarbons
with greatly reduced emissions. The heated mobilized oil is withdrawn
continuously
through the vertical well 6 and is produced to the surface, either by natural
lift or using
any artificial lift 21.
The solvent is selected on the basis of the reservoir temperature and
operating pressure.
The proposed solvent should stay in the vapor phase under the conditions
inside the
vapor chamber. However, it should condense at the solvent vapor-bitumen
interface 4.
Around the injector and the producer due to higher temperature the solvent
will remain
in the vapor phase. However, the reservoir beyond the solvent oil interface 4
will be
close to the initial reservoir temperature and the solvent should have enough
solubility at
the interface. Ideally the solvent composition should be such that at these
temperatures
and pressures it is in the vapor-liquid region and can split into the liquid
and vapor
phases in the required proportions.
The first step in solvent selection involves setting an operating pressure.
This should be
close to the reservoir pressure and obviously should be lower than the
fracture pressure
of the reservoir. Lower operating pressure ensures confinement of the chamber
and
maintains the vapor phase. On the other hand a higher operating pressure
eliminates the
need for artificial lift and increases the solubility of the lighter
components of the solvent
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vapor in the bitumen.
In the second step the range of temperature achievable inside the reservoir
due to the
indirect heating is estimated. The third step involves pre screening of the
possible solvent
composition through PVT calculations with the help of an equation of state.
Any PVT
software package may be used for this purpose. Solvent compositions in vapor-
liquid
region at the operating pressure and the lower end of the temperature range
are short-
listed.
In the fourth step either PVT measurements should be carried out or the same
PVT
software package may be used (when the interaction parameters are known /
approximated) to estimate the solubility and the corresponding viscosity
reduction of
these selections at the solvent oil interface conditions. The composition that
yields the
highest viscosity reduction would be selected. An overall optimization of the
operating
pressure, temperature range and the solvent composition should be carried out
for the best
performance of the process. In a reverse approach, the properties of the
available solvent
may be used to determine the operating conditions.
For example in a reservoir at original temperature of 8 C and 1000 kPa, if it
is established
that the near interface temperature would be close to 25 C, pure propane or
propane with
a small amount of other hydrocarbons may be used as the solvent for the SWEEP
process. The near well bore temperature may be in the range of 150-200 C which
will be
sufficient to boil off all of the dissolved solvent and still the viscosity of
the hot oil will
be low enough to be lifted using a conventional artificial lift. On the other
hand if it is
desirable to operate the process at a lower pressure, naphtha or a condensate
may be used
to match the operating pressure and temperature. Use of heavier solvents will
require a
higher temperature near the well bore to vaporize most of the dissolved
solvent. Even
then some of the heavy components of the solvent mixture may still remain in
the liquid
phase and act as diluent and help in lifting and transportation of the oil.
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Figure 2 presents the details of the slanted well 8 with the closed loop
heating system
and the vertical well 9. The slanted well 8 is drilled from the ground surface
10 through
the overburden 11 into the reservoir 12 and completed with a casing 13 from
surface to
the slanted segment and cemented. In the slanted segment the well is completed
with
either of slotted liners 14, wire mesh wrapped screens, prepacked liners,
perforated
casing, open hole 15 or any combination of these. The closed loop circulation
heating
system consists of a pair of concentric tubes placed inside the well and it
extends from
the surface to the end (toe) of the well completion string or open hole inside
the
formation containing the viscous hydrocarbon. The external tube 16 of the pair
of
concentric tubing, may or may not consists of sections of finned tubes 17 of
any design
along the entire or part of the slanted length, and is closed with a plug at
the toe. The
inner tube 18 is of a smaller diameter than the external tube, creating an
annular space
between the two tubings, 16 and 18. Hot fluids including steam is injected
through the
inner tubing and condensed liquid with or without any vapor returns to the
surface
through the closed annulus. Alternatively the hot fluid is injected through
the annulus of
the concentric tubing and the condensed liquid and associated vapor, if any,
is produced
through the inner tubing. The heat is transmitted f:rom the hot fluid inside
the external
tubing to the fluid outside the external tubing through the wall of the
external tubing
without the fluids physically contacting each other and hence the heat
transfer process is
termed as indirect heating. Solvent is injected into the annular region
between the casing
13 and the outer tubing 16 of the closed loop heating system. A perforated
tubing, placed
inside the entire length of the well, may also be used for solvent injection
to ensure the
uniform distribution of the solvent along the entire slanted length of the
well.
The heat transfer process is limited by the amount of heat transfer area (the
wall 16 of the
external tubing). Preliminary calculation shows that only 50 m3/d (condensed
water
equivalent) of steam could be used in a closed loop circulation heating system
of 1000 m
length of the slanted segment, and 1600 m total length with a external and
internal tubing
diameter 5'/2" and 3'/2" respectively. Depending on the extraction rate in
SWEEP and
the solvent concentration in the diluted oil in the mobilized oil, the amount
of heat
transfer may not be sufficient to vaporize or "reboil" all or most of the
solvent dissolved
CA 02304938 2005-08-22
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in the oil. The purpose of the fins on the external tubing is to increase the
heat transfer
area and facilitate the heat transfer process. Addition of the external fins
may result in
utilization of more than 150 m3/d (condensed water equivalent) of steam inside
the same
closed loop system increasing the heat input into the reservoir.
An optional insulating tubing string 19 (Fig. 2) may be placed between the
inner and
outer tubing of the closed loop heating system to prevent heat transfer
between the
injected and return fluid in the vertical and deviated sections of the well
prior to the liner
14 installation. The annular region between the insulating tubing 19 and the
inner steam
injection tubing 18 may be filled with a gas e.g., methane, nitrogen etc.
Alternatively
conventional insulated tubing may be used in the vertical and deviated
sections of the
steam injection tubing 18.
The vertical well 9 is drilled from the ground surface to the base of the
reservoir to create
a sump 20 at the bottom. The vertical well is cased and cemented up to the
base of the
formation to prevent any communication with the solvent vapor chamber. The
slanted
well 8 is drilled afterwards to hit this vertical drain hole as closely as
possible. This
ensures drainage of the mobilized fluid into the vertical wellbore. An
artificial lift (e.g.
pump) 21 is installed in this vertical well 9 to lift the recovered oil to the
surface.
In a typical SWEEP operation the slanted well length may be in the range of
200 to 1000
m with a dip angle between 1/2 to 10 . Depending on the well length, reservoir
and
solvent characteristics about 2-50 m3/day of solvent will be injected into the
slanted well.
Steam will be circulated into the closed loop heating system at a rate of 10-
100 m3/day.
Operating pressure inside the reservoir will be slightly higher than the
saturation pressure
of the lighter fractions of the solvent at the original reservoir temperature.
The estimated
extraction rate is in the range of 15-100 m3/day of bitumen or heavy oil
produced.
Figures 3 and 3A present the schematic of multiple slanted wells 22, 23, 24,
25 and a
single vertical well 26 in a bitumen reservoir. Oil draining into the slanted
wells flows to
the vertical well 26 and is produced by an artificial lift. Similarly a
multitude of slanted
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wells may be drilled to intercept a single horizontal drain hole. The
horizontal well
gathers all oil draining through the slanted wells and is produced to the
surface using
artificial lift. The dip angle 27 of the slanted wells in the elevation view
of Fig. 3 is
highly exaggerated. The dip angle 27 is selected on the basis of the required
gravity
potential to flow the expected liquid volume along the length of the slanted
segment. The
perforated tubing 14 may be used to distribute the solvent uniformly along the
slanted
length of the well.
Figure 4 presents the schematic of a variation of the process in which the
closed loop
heating system is substituted by an alternative source of energy supply to the
slanted
well. Electromagnetic induction heating coils or electrical heaters 29 are
installed inside
the slanted segment 8 of the well. These are supplied with electrical power
from the
surface through insulated or jacketed electrical cables. Depending on the
reservoir
characteristics and well length about 0.3 to 2 MW of electrical energy will be
delivered
to the slanted well by this method. The make up amount of solvent is injected
into the
annulus and vaporized inside the wellbore along with the dissolved solvent.
The
produced oil drains to the vertical well 9 and the hot oil is produced to the
surface using
artificial lift 2. A modification of this variation presented in Figure 5 uses
an electrical
submersible pump (ESP) 30 attached to the tubing string and placed at the toe
of the
slanted we1131 to pump the oil to the surface. In this modification the
vertical well is not
required.
Figure 6 presents a single slanted well SWEEP operation in which the slanted
segment is
drilled up-dip from the heel of the well 35. The well is completed with a
closed loop
heating system with steam injection 32 and return 33 string. Solvent is
injected through
another concentric tubing 34. The solvent diluted hot oil drained from the
reservoir flows
through the well bore from the toe of the well 31 to the heel 35 section. A
production
tubing 36 is landed in the annular space between the closed loop heating
system and the
casing 37. An artificial lift system 38 e.g. ESP, rod pump, progressive cavity
pump or gas
lift is attached to this tubing 36 and is used to withdraw the fluid to the
surface. In this
option also the vertical drain hole is eliminated. The closed loop heating
system in this
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variation may also be eliminated by using an alternative mode of energy supply
e.g.
electrical or electromagnetic induction heating etc.
In another variation of SWEEP, presented in Figure 7, a dual entry U well is
drilled from
one surface location 39, becomes slanted 40 inside the formation and then
drilled
upwards to come out to the surface at another surface location 41. The well is
cased in
the vertical and deviated segment and completed with slotted liner, wire mesh
wrapped
screens, prepacked liners, perforated casing, open hole or any combination of
these 42.
The closed loop heating system comprising steam injector tube 43, steam return
tube 44
and optional insulating string45 are installed inside the well through one
surface location
and extends up to the end of the slanted segment. A production tubing 46 is
landed
through the second surface location up to the deviated section into the fluid
collected in
the slanted well bore. An artificial lift 47 attached to this tubing string 46
lifts the oil to
the surface.
Unlike other proposed methods of using solvent(s) for the extraction of
viscous
hydrocarbon this process employs both heat and solvent vapors. The specific
well
configuration and the presence of the closed loop circulation heating system
in the
present invention eliminates the initial start up phase required in the prior
techniques
using a pair of horizontal wells. The oil production in the present invention
may start
from the very beginning of the project. Inside the slanted wellbore the fluid
flows along
the pressure gradient aided by the gravity head in the slanted segment. This
may be
compared to the prior art using a pair of horizontal wells, where wellbore
pressure losses
in the injector imposes a negative pressure gradient on the fluids flowing in
the producer.
This makes the drainage process gravity unstable and results in the bypassing
of steam
or solvent. Similarly in a single well based process bypassing of steam and
solvent is a
serious problem, which is alleviated using the slanted well design in the
present
invention.
The uniqueness of the present invention lies in (a) the requirement of less
number of
wells or cheaper wells and pad locations, (b) in situ recycling of the solvent
reducing
CA 02304938 2000-04-10
-17-
surface operation, and (c) providing an efficient and economic process for the
recovery
of heavy oil and bitumen.
Preferred embodiments of the invention have been described and illustrated by
way of
example. Those skilled in the art will realize that various modifications and
changes may
be made while still remaining within the spirit and scope of the invention.
Hence the
invention is not to be limited to the embodiments as described but, rather,
the invention
encompasses the full range of equivalencies as defined by the appended claims.
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