WO2000066882A1 - Process for the producing of viscous oil with vapex using a vertical well - Google Patents

Abstract

In one aspect, the method of the invention involves providing one or more vertical or substantially vertical wells which partially or fully penetrate a viscous oil reservoir. A non-thermal recovery process may be initiated whereby a hydrocarbon vapor is introduced into the reservoir so as to bring it into contact with the oil in the reservoir. The solvent is selected so that it is in the vapor phase at ambient temperature and pressure in the reservoir in the absence of heating. The hydrocarbon vapor may increase the mobility of the oil by means of diffusion or other mixing mechanisms, whereupon the oil may be permitted to drain under the influence of gravity to the lower reaches of the wellbore. From there, the oil may be lifted to the surface.

Description

PROCESS FOR THE PRODUCING OF VISCOUS OIL WITH VAPEX USING A VERTICAL WELL

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. In this context, viscous oil or bitumen reservoirs may be those which are not amenable to economic conventional primary production by virtue of the very high viscosity and correspondingly substantial immobility of the oil at original reservoir conditions.

BACKGROUND OF THE INVENTION

Thermal and non-thermal viscous oil recovery processes are known. A thermal recovery process, as defined herein, is one in which the principal means of mobilizing the viscous oil in the reservoir is by heating. Such heating is generally accomplished by means a heated fluid injected into the reservoir. The most common example of a thermal recovery process involves the injection of steam into the reservoir. Correspondingly, in this context, a non- thermal recovery process is one in which the principal mechanism for mobilizing the viscous oil in the reservoir is not heating.

The prior art includes descriptions of non-thermal recovery processes 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 well under the influence of gravity. The art teaches that such processes should be carried out using horizontal wells so that a large area of the reservoir may be exposed to the solvent vapor in order to achieve effective recovery of oil despite the relatively slow pace of gravity drainage.

Widely accepted calculation techniques which compare horizontal well flow rates to those in vertical wells yield results which point to a large flow rate advantage in favor of horizontal wells because they possess a much greater surface area exposed to the reservoir and available for flow. In a horizontal well in which the principal drive mechanism is thermally facilitated gravity drainage, the existing body of theory indicates that it is possible to compensate for the limited potential energy available under gravity drainage by drilling a sufficiently long wellbore and thereby exposing a larger cross-sectional area to flow. However, in the case of a vertical or substantially vertical well, the top and bottom of the productive formation limit the cross-sectional area that can be opened to flow. Therefore, in the case of a vertical well undergoing a thermally facilitated gravity drainage process, theory indicates that the limited potential energy and the restricted flow area combine to diminish productivity. Furthermore, when non-thermal gravity drainage processes are considered, overall productivity levels are expected to be still lower than their thermal counterparts, so that the productivity levels of vertical wells will be further reduced.

For example, a seminal publication by Butler and Mokrys in 1991 (Reference 1) described a process, referred to thereafter in the industry literature as "NAPEX". The process involves utilizing a solvent, such as propane, in the vapor form, as a means of dissolving the viscous oil in the reservoir. Butler and Mokrys proposed the use of a solvent which is introduced in the vapor form, and which is close to its dew point. In the NAPEX process, the solvent is introduced into the reservoir at a horizontal injection well, and the mobilized oil drains into an underlying horizontal producing well parallel to the injector.

Liquid solvents have been proposed for mobilizing viscous oil, however, the lower diffusivity of liquid solvent, and the reduced density difference between the liquid solvent and the oil, slow the rate at which the recovery process occurs. As a consequence, the rate at which oil is mobilized by the solvent, and the rate at which the mobilized oil subsequently drains down to the producer, may be very slow with a liquid solvent, a problem which is avoided in the VAPEX process through the use of a vapor solvent.

U.S Patent No. 5,899,274 issued May 4, 1999 (hereinafter referred to as the AOSTRA patent, Reference 2) describes a solvent-assisted method for mobilizing viscous oil. in which a mixture of at least two solvents may be employed. The AOSTRA patent discusses the use of vertical wells to recover conventional oil, i.e. mobile oil at original reservoir conditions which is capable of cold primary production at economic rates. As used herein, the term "viscous oil" distinguishes substantially immobile heavy oils or oil sands from conventional oils. Viscous oil, as the term is used herein, is not sufficiently mobile oil at original reservoir conditions to be capable of cold (ambient reservoir temperature) primary production at economic or practical rates. Vertical wells have also been suggested for use in methods of thermal recovery of viscous oils where solvent is pre-heated by steam, and the solvent is further heated by subsequent injection of the steam into the reservoir (Duerkson and Eloyan, 1995, Reference 3), so that the solvent is vaporized by heating.

SUMMARY OF THE INVENTION

It has unexpectedly been discovered that the flow efficiencies of horizontal wells used in thermal methods of viscous oil recovery may be substantially less than those expected on the basis of existing theory. The current body of knowledge related to gravity drainage recovery processes favors horizontal over vertical wells by a considerable margin. However, the present inventors have discovered that in pilot experiments at the AEC Foster Creek pilot located in Alberta, Canada, calculations based on observed temperature profiles along horizontal well P3 indicate that flow is occurring over less than one half of the wellbore length. In contrast, temperature surveys at a nearby vertical observation well which monitors vertical distribution of the same heat source show that substantially all of the formation thickness has been heated. This represents an unexpected result, and suggests that, for a thermal gravity drainage process, while horizontal wells might still be expected to exhibit a higher productivity than their vertical counteφarts, the contrast between vertical and horizontal well productivity levels may not be as pronounced as the prior art would indicate.

In one aspect, the method of the invention involves providing one or more vertical or substantially vertical wells which partially or fully penetrate a viscous oil reservoir. A non- thermal recovery process may be initiated whereby a hydrocarbon vapor is introduced into the reservoir so as to bring it into contact with the oil in the reservoir. The solvent is selected so that it is in the vapor phase at ambient temperature and pressure in the reservoir in the absence of heating. The hydrocarbon vapor may increase the mobility of the oil by means of diffusion or other mixing mechanisms, whereupon the oil may be permitted to drain under the influence of gravity to the lower reaches of the wellbore. From there, the oil may be lifted to the surface. In one aspect, the present invention involves using a solvent in the vapor phase that does not rely on heating to achieve or maintain this vapor state. In the description of this invention, reference to a solvent includes a single-component solvent, such as propane for example, or a succession of single component solvents, as distinct from a solvent consisting of two or more components that have been mixed prior to introduction into the reservoir. In the subject invention, the solvent vapor contacts the viscous oil and mobilizes it. The oil, thus mobilized, drains downward under the influence of gravity. The process of the invention may applied at one or more vertical, or substantially vertical wells. That is, the solvent may be injected into a vertical well, and the mobilized oil may be produced either through that same vertically oriented wellbore or through another nearby vertically oriented wellbore.

As noted above, the well may partially or fully penetrate a given reservoir or zone. In addition, the well may be completed over a multiplicity of reservoirs or zones, and the process applied to each jointly or severally.

In various aspects, the invention may be characterized by one or more of the following elements:

1. The utilization of a single-component hydrocarbon, or succession of single- component hydrocarbons, to mobilize the oil, without the injection of an associated heating fluid, such as steam.

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. In various embodiments, the invention accordingly provides novel processes for recovering viscous oils from reservoir lodgements. In one aspect, the present invention facilitates non- thermal production of a viscous oil, that is oil which is substantially immobile at original reservoir conditions so that there is no economic primary production capability.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration showing a single vertical well gravity assisted vapor extraction process of the invention. Figure 2 is a schematic illustration showing a multiple zone single well operation of a gravity assisted vapor extraction process of the invention.

Figure 3 is a schematic illustration showing a wellbore completion in accordance with one aspect of the invention.

Figure 4 is a schematic illustration showing a single vertical well gravity assisted vapor extraction process of the invention with refluxing.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments Pertaining to Enhancement of the Recovery Mechanism In some embodiments, there are circumstances that can improve the attractiveness of the vertically oriented oil recovery process of the invention, in addition to the generally lower cost of vertical wells compared to horizontal wells. Three such embodiments are for example 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 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 zones as conduits. Thus, by imposing a convection mechanism, and not simply relying on molecular diffusion, 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. 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, so that vertical wells may be advantageously used in accordance with this aspect of the invention to provide an unexpectedly high rate of oil recovery.

2. It may be desirable to inject a precursor fluid prior to injecting the solvent. A typical precursor fluid might be natural gas. One purpose of the precursor fluid would be to create a higher mobility path ahead of the solvent. A second puφose of the precursor fluid would be to provide conditions for the solvent to achieve desired phase behavior characteristics. A third puφose of the precursor fluid would be to alter the stress field within the pore matrix so as to dilate the pore structure and alter mobility without exceeding fracturing pressures. 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, so that vertical wells may be advantageously used in accordance with this aspect of the invention to provide an unexpectedly high rate of oil recovery.

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 (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 mixing of the solvent with 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 mixing of the solvent with 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, so that vertical wells may be advantageously used in accordance with this aspect of the invention to provide an unexpectedly high rate of oil recovery.

Embodiments Pertaining to Process Operating Sequence and State The steps of the process, involving a) introduction of the solvent or succession of solvents 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 a given solvent in batches rather than continuously. In that instance, production could either proceed continuously, or could be scheduled to occur in discrete production windows.

Regardless of whether the process proceeds continuously or in discrete steps, one may elect to vary the process state by means of a technique such as pressure cycling without deviating from the essential elements of the invention.

Embodiments Pertaining to Choice of Solvent or Succession of Solvents

As noted above, the invention contemplates the introduction of an individual solvent or succession of solvents. The choice of solvent or succession of solvents can be dictated by numerous factors, including but not limited to the pressure and temperature of the reservoir and the pressures and temperatures at which the process is to be carried out. In the specific instance in which the gravity drainage process is to be aided and abetted by a supplementary or precursor process, such as cyclic stimulation (often referred to as huff-and-puff), wherein large pressure and temperature variations may occur, these large variations in state, with their consequent implications for phase behavior, may influence the choice of solvent or succession of solvents. For example, a solvent such as ethane may be utilized initially because of the proximity of its dew point to the downhole conditions, after which a succeeding solvent such as propane, in either a hot or cold state, may be introduced.

Embodiments Pertaining to Vertical Well Equipment Configuration 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. 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 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.

Figure 3 attached illustrates one of many possible configurations. It shows schematically the present completion at well AEC Dl Fisher 1-21-70-4W4 in Alberta Canada. Figure 3 thus represents 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 3, solvent vapor, or any other precedent injection fluid as described previously, is injected down the 3 V-i inch Injection Tubing. The solvent enters the reservoir through the upper portion of slots in the Slotted Pipe, mixing with 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. Embodiments Pertaining to Solvent Reflux The process of this invention can involve introduction of solvent in such a manner that it mixes with the viscous oil and is subsequently lifted to the surface as part of the resulting liquid blend. In this situation, the option exists to separate the solvent from the oil at the surface and re-use the solvent. In these circumstances, no downhole solvent reflux action would generally be designed into the process.

On the other hand, it may be advisable to introduce a solvent refluxing mechanism into the process so that the solvent is separated from the oil at or near the bottom of the well. The separated solvent, together with any required make-up solvent, then re-enters the reservoir above the point of separation, whereupon it can again blend with and mobilize the oil. A specific embodiment that employs a refluxing mechanism involves placement of a heating source in the wellbore such that there is an exchange of heat between this heating source and the oil-solvent blend, but no associated mixing of fluids.

In various embodiments, the heat source may or may not involve a fluid. For example, electricity may be used as a heat source, in which case no heating fluid is involved, and therefore no mixing can occur. However, in embodiments where the heat source involves a fluid, that fluid may remain part of a closed system, so that heat is exchanged without associated fluid mixing. For example, one may inject a heated fluid such as steam at the wellhead into a closed loop system. The steam would exchange its heat with fluids in the wellbore, and would thereby vaporize solvent for subsequent re-circulation into the reservoir.

With respect to the process in which electricity is employed to achieve solvent reflux, two specific instances are contemplated. In one instance, as described above, electrical heating may be employed within the wellbore as a solvent refluxing mechanism. That is, a heating element within the wellbore would vaporize the solvent for subsequent refluxing. In a second instance, electrical heating may be employed within the proximate reservoir to achieve this same end. In the second instance, which involves heating of the proximate reservoir, an electrical circuit can be configured to include selected portions of the wellbore equipment and the reservoir in the immediate vicinity of the wellbore. An electrical voltage is imposed so that electrical current flows through the primarily resistive circuit involving wellbore equipment and near-wellbore reservoir. The circuit resistance is such that the imposition of a voltage generates heat in the higher resistance reservoir material constituting the near-wellbore vicinity. The heat thus generated is sufficient to effect or assist the vaporization of solvent for subsequent reflux.

Electrical heating may also, for example, be employed to achieve some combination of the above two methods.

In embodiments utilizing downhole solvent reflux, the make-up solvent may be pre-heated prior to its introduction into the reservoir. Also, however, even where no downhole solvent reflux mechanism is employed, the introduction of heated solvent such that heating is not the primary mechanism of oil mobilization may be used in the present methods of non- thermal viscous oil recovery. A possible solvent heating configuration involving downhole electrical heating is for example illustrated schematically in Figure 4. As illustrated, the heating of the solvent, whether in association with solvent reflux or independent of it, whether occurring in the wellbore or in the reservoir or in some combination thereof, may be achieved without mixing of a heated fluid, such as steam, with the produced fluids or solvent. Rather, the heating may be achieved either by non-fluid means (e.g., electricity), or by fluid means in which the heated fluid is isolated from the produced fluids or solvent.

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:

1. BUTLER, R.M., MOKRYS, I.J., A new process (VAPEX) for recovering heavy oils using hot water and hydrocarbon vapour; Journal of Canadian Petroleum Technology, Vol. 30, No. 1, pp.97-106, January-February 1991.

2. U.S Patent No. 5,899,274 Date of Patent: May 4, 1999.

Inventors: J.W. Frauenfeld and Douglas A. Lillico.

Assignee: Alberta Oil Sands Technology and Research Authority, Alberta, Canada.

3. 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.

Claims

What is claimed is:

1. A process for extraction of viscous oil from a reservoir, comprising: a) providing one or more generally vertical wells in a viscous oil reservoir; b) injecting a hydrocarbon solvent into the reservoir through one or more of the wells, wherein the solvent is in the vapor phase at ambient temperature and pressure in the reservoir in the absence of heating; c) allowing the solvent to mobilize the viscous oil, to produce mobilized oil that migrates towards the bottom of one or more of the wells under the influence of gravity; d) extracting the mobilized oil through one or more of the wells.

2. The process of claim 1, wherein there is a mobile phase in the viscous oil reservoir, at least one of the wells through which solvent is injected into the reservoir is located in fluid communication with the mobile phase, and the solvent is injected so as to penetrate into the reservoir through the mobile phase.

3. The process of claim 1 wherein a precursor fluid is injected into the viscous oil reservoir prior to injecting the solvent, so as to create a higher mobility path ahead of the solvent.

4. The process of claim 2 wherein a precursor fluid is injected into the viscous oil reservoir prior to injecting the solvent, so as to create a higher mobility path ahead of the solvent.

5. The process of claim 1, wherein the solvent is injected into the well at pressures that exceed the fracturing pressure of the reservoir to produce fractures in the reservoir.

6. The process of claim 2, wherein the solvent is injected into the well at pressures that exceed the fracturing pressure of the reservoir to produce fractures in the reservoir.

7. The process of claim 3, wherein the solvent is injected into the well at pressures that exceed the fracturing pressure of the reservoir to produce fractures in the reservoir.

8. The process of claim 4, wherein the solvent is injected into the well at pressures that exceed the fracturing pressure of the reservoir to produce fractures in the reservoir.

9. The process of claim 5, wherein the fractures are propped open with a propping medium.

10. The process of claim 6. wherein the fractures are propped open with a propping medium.

1 1. The process of claim 7, wherein the fractures are propped open with a propping medium.

12. The process of claim 8, wherein the fractures are propped open with a propping medium.