Note: Descriptions are shown in the official language in which they were submitted.
CA 02270703 1999-04-29
A PROCESS FOR NON-THERMAL VAPOR EXTRACTION OF VISCOUS OIL
FROM A HYDROCARBON RESERVOIR USING A VERTICAL WELL
CONFIGURATION
FIELD OF THE INVENTION
This invention relates to the processes and apparatus for the recovery of
hydrocarbons
from hydrocarbon deposits, and specifically of viscous oil from viscous oil or
bitumen
reservoirs, by means of wells drilled into or through the reservoir.
to
BACKGROUND OF THE INVENTION
This invention is concerned with the application of hydrocarbon vapor in a
recovery
process whose principal flow mechanism is gravity drainage, carried out using
vertical
i 5 wells.
Other patents and publications describe a non-thermal recovery process in oil
sands and
heavy oil reservoirs wherein a hydrocarbon vapor is used to mobilize the
bitumen or
viscous oil, and the mobilized oil is allowed to drain to a producing point
under the
2o influence of gravity. The prior art teaches that such processes should be
carned out using
horizontal wells because the relatively slow pace of gravity drainage dictates
a
requirement for a large area of exposure of the hydrocarbon solvent to the
reservoir, and
this can be best achieved with horizontal wells.
25 For example, a seminal publication by Butler and Mokrys in 1991 (Reference
1)
described a process, referred to thereafter in the industry literature as
"VAPEX". The
process involves utilizing a hydrocarbon solvent, such as propane, in the
vapor form, as a
means of dissolving the viscous oil in the reservoir. The solvent is
introduced into the
reservoir at a horizontal injection well, and the mobilized oil drains into an
underlying
30 horizontal producing well parallel to the injector. It is important to note
that liquid
solvents had been proposed prior to the disclosure of the VAPEX process.
However, the
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CA 02270703 1999-04-29
lower diffusivity of liquid solvents, and the smaller density difference
between the liquid
solvent and the oil, produce a slower rate of oil recovery using a liquid
solvent. In other
words, the rate at which oil is mobilized by the solvent, and the rate at
which the
mobilized oil subsequently drains down to the producer well, are typically
very slow with
a liquid solvent. Butler and Mokrys proposed the use of a solvent which is
introduced in
the vapor form, and which is close to its dew point. The introduction of a
solvent in the
vapor form was proposed to mitigate the abovementioned problems associated
with
liquid solvents.
1o One publication disclosing a solvent-based gravity drainage process
utilizes vertical
wells. However, it includes, as an integral part of the process, pre-heating
of the solvent.
Specifically, a publication by Duerkson and Eloyan in 1995 (Reference 2)
describes a
solvent-based process involving vertical wells in which recovery occurs by
gravity
drainage. However, in their process, the solvent is vaporized by heating.
In summary, the prior art discloses non-thermal solvent-based gravity drainage
processes
for horizontal wells, and a thermal solvent-based gravity drainage process for
vertical
wells.
SUMMARY OF THE INVENTION
The present invention provides methods of viscous oil recovery using a solvent
in the
vapor phase, wherein heating is not required to achieve or maintain the vapor
state of the
solvent. The solvent vapor contacts the viscous oil and mobilizes it. The oil,
thus
mobilized, drains downward under the influence of gravity. The invention is
applied at
one or more vertical, or substantially vertical wells. That is, the solvent is
injected into a
vertical well, and the mobilized oil is produced either through that same
vertically
oriented wellbore or through another nearby vertically oriented wellbore.
More specifically, the method involves providing one or more vertical, or
substantially
3o vertical, wells which partially or fully penetrate the reservoir. At one or
more of the
vertical wells, a non-thermal process is initiated whereby a hydrocarbon vapor
solvent is
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CA 02270703 1999-04-29
injected so as to bring it into contact with the oil in the reservoir. The
hydrocarbon vapor
increases the mobility of the oil by means of diffusion or other mixing
mechanisms,
whereupon the oil is permitted to drain under the influence of gravity to the
lower reaches
of the wellbore. From there, the oil is lifted to the surface through one or
more of the
vertical wells.
The process comprises three elements as follows:
1. The utilization of an unheated hydrocarbon solvent in the vapor phase to
mobilize
the oil.
2. Gravity drainage of the mobilized oil to a collection point.
3. Application of the process described in points 1. and 2. above by means of
one or
more vertical wells.
The combination of these elements in appropriate ways, as described herein,
provides a
process for recovering viscous oils from their reservoir lodgements.
EMBODIMENTS OF THE INVENTION
Embodiments Pertaining to Enhancement of the Recovery Mechanism
In many viscous oil reservoirs, the area capable of being exposed to a
vertical wellbore is
much less than the area which can be exposed to a horizontal wellbore. Thus
diffusion
2o and oil mobilization will be slower in the vertical wellbore, as will the
rate at which
mobilized oil enters the wellbore. Consequently, one might expect the overall
recovery
process to be slower in a vertical well.
There are, however, some important circumstances that can improve the
attractiveness of
the vertically oriented process, in addition to the generally lower cost of
vertical wells
compared to horizontal wells. Three such embodiments are described as follows:
1. If there is a mobile phase, such as gas and/or water, in the form of either
distinct
zones in contact with the viscous oil reservoir, or in the form of saturations
within
3o the pore structure of the oil-bearing zone, or both, the solvent vapor may
be injected
so as to penetrate further into the reservoir matrix using these mobile fluid
phases or
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CA 02270703 1999-04-29
zones as conduits. Thus, by imposing a convection mechanism, and not simply
relying on molecular diffusion to disperse a solvent, an effectively large
area of
contact is generated between the solvent vapor and the viscous oil, even in a
vertical
well. This accelerates the rate at which viscous oil is mobilized by the
solvent.
Although these mobile phases might also be available to a horizontal well, the
state
of the art of horizontal well completions precludes the degree of control of
fluid
injection and production that is routinely achieved with vertical wells.
2. It may be desirable to inject a precursor fluid prior to injecting the
solvent. A typical
1o precursor fluid might be natural gas. The purpose of the precursor fluid
would be to
either create a higher mobility path ahead of the solvent, or to provide
conditions for
the solvent to achieve the desired phase behavior characteristics, or both.
The state
of the art of vertical well completions is such that better control of the
injection
profile in the vicinity of the well, would be afforded by a vertically
oriented well
than by a horizontally oriented well.
3. The mechanism described in item 1 above, and enlarged upon in item 2 above,
can be
further enhanced if the solvent vapor is injected into the vertical well at
pressures that
exceed the fracturing or parting pressure of the formation. This will expose
an even
larger area of reservoir matrix to the wellbore, and will permit a greater
rate of
diffusion of the solvent into the viscous oil. The resulting induced fracture
may be
intentionally propped open using a suitable propping medium, or may simply be
allowed to behave in accordance with the fluid-rock mechanics of a fracture
into
which no proppant has been intentionally introduced. The presence of a
fracture, in
addition to facilitating diffusion of the solvent in the viscous oil, will
also expedite
the rate at which the mobilized oil drains downward to its collection point.
Again, the
state of the art of horizontal well completions with respect to inducing a
fracture and
propping it open has not been developed to any degree, and certainly not to
the degree
to which the technology has been developed for vertical wells.
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Embodiments Pertaining to Process Operating Sequence
The steps of the process, involving a) introduction of the solvent so that it
contacts the
viscous oil, b) mobilization of the oil, c) gravity drainage of the oil to a
collection point,
and d) production of the mobilized oil, may proceed in such a manner that
solvent
injection and fluid production are ongoing concurrently. Alternatively, one
may wish to
inject the solvent in batches rather than continuously. In that instance,
production could
either proceed continuously, or could be scheduled to occur in discrete
production
windows.
to
Embodiments Pertaining to the Vaporized Solvent
The solvent vapor can consist of either a pure substance or a mixture of
substances. For
example, in come circumstances, a pure substance such as ethane may be
suitable.
Alternatively, one might wish to employ a mixture of, say, methane and butane
in
appropriate proportions.
2o Embodiments Pertaining to Vertical Well Equipment Confi urg ation
There are many ways in which a vertical well, or an aggregate of vertical
wells, can be
configured so as to utilize the recovery process described above. The
particular choices
will depend upon the characteristics of the reservoir, including but not
restricted to
lithology, fluid properties, fluid distribution, depth, pressure and
temperature. Those
choices will also depend upon the solvent properties, and upon equipment
constraints and
cost considerations. Configuration will also depend upon recovery process
strategies,
such as the use of induced fractures or precursor fluids.
3o The well can be completed so that only the oil zone is exposed at the
wellbore. Or, if
there are associated gas and/or water zones, it can be completed so that one
or more of
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CA 02270703 1999-04-29
these zones are exposed as well if this is advantageous. The exposure of the
reservoir to
the wellbore can involve one continuous interval. Alternatively, the exposure
may be
designed to occur over one or more segments of selected length which do not
span the
entirety of the target zone(s).
The well can be completed so that there is no isolating mechanism, such as a
packer,
within the wellbore. Or, the well can be completed so that portions of the
open interval
are isolated, one from another, by an isolating mechanism.
1o Figure 1 attached illustrates one of many possible configurations. It shows
schematically
one specific embodiment related to vertical well equipment configuration. It
should be
understood, however, in relation to all of the embodiments herein described,
that the
present disclosure is to be considered an exemplification of certain
principles of the
invention and is not intended to limit the invention to any specific
embodiment so
described.
In the specific example presented in Figure l, solvent vapor, or any other
precedent
injection fluid as described previously, is injected down the 3 %z inch
Injection Tubing.
The solvent enters the reservoir through the upper portion of slots in the
Slotted Pipe,
2o diffusing into and mobilizing the viscous oil. Mobilized oil drains
downward and enters
the wellbore through the lower portion of slots in the Slotted Pipe. Once in
the wellbore,
the oil, together with any other reservoir liquids that have entered the
wellbore, migrates
to the lower reaches of the wellbore, and specifically to the 178 mm
Production Casing,
which is blanked off. The Production Tubing string is bottomed in the lower
reaches of
the wellbore, so that any action taken to raise fluids up the Production
Tubing, as may be
done for example by pumping or gas lifting, will collect the oil and
associated liquids and
bring them to the surface. Note that in this specific example, a Packer is
used to isolate
the active process region of the wellbore from the region above.
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Embodiments Pertaining to Well Groupings
The embodiments described above may be applied on either a single well or a
multiple
well basis. In the case of multiple wells, those wells may be functioning as
an aggregate
of isolated wells, or two or more of the wells may be in communication with
each other
through the reservoir. Where such communication exists, the locations at which
solvent is
injected and fluids are collected may be configured as desired, both laterally
among wells
and vertically within each wellbore.
REFERENCES CITED:
1. BUTLER, R.M., MOKRYS, LJ., A new process (VAPEX) for recovering heavy oils
using hot water and hydrocarbon vapour; Journal of Canadian Petroleum
Technology, Vol. 30, No. l, pp.97-106, January-February 1991.
2. DUERKSEN, J.H., ELOYAN, A., Evaluation of Solvent-Based In Situ Processes
for
Upgrading and Recovery of Heavy Oil and Bitumen, UNITAR Conference, Houston,
Texas, February 12, 1995.
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