Process for enhanced production of heavy oil using microwaves

Abstract

A process for utilizing microwaves to heat solvent within a subterranean region wherein the heated solvent, vapor, contacts heavy oil in the subterranean region to lower the viscosity of the heavy oil and improve production of the heavy oil.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

FIELD OF THE INVENTION

The present invention relates generally to a process for recovering heavy oil from a reservoir.

BACKGROUND OF THE INVENTION

Heavy oil is naturally formed oil with very high viscosity but often contains impurities such as sulfur. While conventional light oil has viscosities ranging from about 0.5 centipoise (cP) to about 100 cP, heavy oil has a viscosity that ranges from 100 cP to over 1,000,000 cP. Heavy oil reserves are estimated to equal about fifteen percent of the total remaining oil resources in the world. In the United States alone, heavy oil resources are estimated at about 30.5 billion barrels and heavy oil production accounts for a substantial portion of domestic oil production. For example, in California alone, heavy oil production accounts for over sixty percent of the states total oil production. With reserves of conventional light oil becoming more difficult to find, improved methods of heavy oil extractions have become more important. Unfortunately, heavy oil is typically expensive to extract and recovery is much slower and less complete than for lighter oil reserves. Therefore, there is a compelling need to develop a more efficient and effective means for extracting heavy oil.

Viscous oil that is too deep to be mined from the surface may be heated with hot fluids or steam to reduce the viscosity sufficiently for recovery by production wells. One thermal method, known as steam assisted gravity drainage (SAGD), provides for steam injection and oil production to be carried out through separate wellbores. The optimal configuration is an injector well which is substantially parallel to and situated above a producer well, which lies horizontally near the bottom of the formation. Thermal communication between the two wells is established and, as oil is mobilized and produced, a steam chamber or chest develops. Oil at the surface of the enlarging chest is constantly mobilized by contact with steam and drains under the influence of gravity.

In order to initiate a SAGD production, thermal communication must be established between an injection and a production SAGD well pair. Initially, the steam injected into the injection well of the SAGD well pair will not have any effect on the production well until at least some thermal communication is established because the hydrocarbon deposits are so viscous and have little mobility. Accordingly, a start-up phase is required for the SAGD operation. Typically, the start-up phase takes about three months before thermal communication is established between the SAGD well pair, depending on the formation lithology and the actual inter-well spacing.

The traditional approach to starting-up the SAGD process is to simultaneously operate the injection and production wells independently of one another to circulate steam. The injection and production wells are each completed with a screened (porous) casing (or liner) and an internal tubing string extending to the end of the liner, forming an annulus between the tubing string and casing. High pressure steam is simultaneously injected through the tubing string of both the injection and production wells. Fluid is simultaneously produced from each of the injection and production wells through the annulus between the tubing string and the casing. In effect, heated fluid is independently circulated in each of the injection and production wells during the start-up phase, heating the hydrocarbon formation around each well by thermal conduction. Independent circulation of the wells is continued until efficient thermal communication between the wells is established. In this way, an increase in the fluid transmissibility through the inter-well span between the injection and production wells is established by conductive heating. The pre-heating stage typically takes about three to four months. Once sufficient thermal communication is established between the injection wells, the upper, injection well is dedicated to steam injection and the lower, production well is dedicated to fluid production.

A variant of SAGD is expanded solvent steam-assisted gravity drainage (ES-SAGD). In ES-SAGD a solvent is used in conjunction with steam from water. The solvent then evaporates and condenses at the same condition as the water phase. By selecting the solvent in this matter, the solvent will condense with the condensed steam, at the boundary of the steam chamber. Condensed solvent around the interface of the steam chamber dilutes the oil and in conjunction with heat, reduces its viscosity.

There are several patents on the improvements to SAGD operation. U.S. Pat. No. 6,814,141 describes applying vibrational energy in a well fracture to improve SAGD operation. U.S. Pat. No. 5,899,274 teaches addition of solvents to improve oil recovery. U.S. Pat. No. 6,544,411 describes decreasing the viscosity of crude oil using ultrasonic source. U.S. Pat. No. 7,091,460 claims in situ, dielectric heating using variable radio frequency waves.

In a recent patent publication (U.S. Patent Publication 20070289736/US-A1, filed May 25, 2007), it is disclosed to extract hydrocarbons from a target formation, such as a petroleum reservoir, heavy oil, and tar sands by utilizing microwave energy to fracture the containment rock and for liquification or vitalization of the hydrocarbons.

In another recent patent publication (US Patent Publication 20070131591/US-A1, filed Dec. 14, 2006), it is disclosed that lighter hydrocarbons can be produced from heavier carbon-base materials by subjecting the heavier materials to microwave radiations in the range of about 4 GHz to about 18 GHz. This publication also discloses extracting hydrocarbons from a reservoir where a probe capable of generating microwaves is inserted into the oil wells and the microwaves are used to crack the hydrocarbons with the cracked hydrocarbon thus produced being recovered at the surface.

Despite these disclosures, it is unlikely that direct microwave cracking or heating of hydrocarbons would be practical or efficient. It is known that microwave energy is absorbed by a polar molecule with a dipole moment and bypasses the molecules that lack dipole moment. The absorption of the microwave energy by the polar molecule causes excitation of the polar molecule thereby transforming the microwave energy into heat energy (known as the coupling effect). Accordingly, when a molecule with a dipole moment is exposed to microwave energy it gets selectively heated in the presence of non-polar molecules. Generally, heavy oils comprise non-polar hydrocarbon molecules; accordingly, hydrocarbons would not get excited in the presence of microwaves.

Additionally, while the patent publication above claims to break the hydrocarbon molecules, the energy of microwave photons is very low relative to the energy required to cleave a hydrocarbon molecule. Thus, when hydrocarbons are exposed to microwave energy, it will not affect the structure of a hydrocarbon molecule. (See, for example, “Microwave Synthesis”, CEM Publication, 2002 by Brittany Hayes).

BRIEF SUMMARY OF THE DISCLOSURE

A process of injecting a solvent into a subterranean region through a first wellbore of a solvent assisted gravity drainage operation. Microwaves are introduced into the region at a frequency sufficient to excite the solvent molecules and increase the temperature of at least a portion of the solvent within the region to produce a vapor. At least a portion of the heavy oil in the subterranean region is heated by contact with the vapor to produce heated heavy oil. The heated heavy oil is then produced through a second wellbore of the solvent assisted gravity drainage operation. Heavy oil is then recovered with the solvent assisted gravity drainage operation from the subterranean region. In this embodiment a portion of the solvent is injected as vapor and the vapor contacts with at least a portion of the heavy oil in the region so as to heat the portion of the heavy oil and reduce its viscosity so that it flows generally towards the second wellbore.

In an alternate embodiment a process is taught of injecting a solvent into a region through a first wellbore of a solvent assisted gravity drainage operation. Microwaves are introduced into a subterranean region at a frequency sufficient to excite the liquid solvent molecules and increase the temperature of at least a portion of the liquid solvent within the region to produce a vapor. At least a portion of the heavy oil is heated by contact with the vapor to produce a heated heavy oil. The heated heavy oil is produced through a second wellbore of the solvent assisted gravity drainage operation, thereby recovering heavy oil with the solvent assisted gravity drainage operation from a subterranean region. In this embodiment a portion of the solvent is injected as vapor and the vapor contacts with at least a portion of the heavy oil in the region so as to heat the portion of the heavy oil and reduce its viscosity so that it flows generally towards the second wellbore.

In yet another embodiment a process is taught of injecting a solvent into a subterranean region though an injection wellbore of a solvent assisted gravity drainage operation. Microwaves are introduced into the region at a frequency sufficient to excite the solvent molecules and increase the temperature of at least a portion of the solvent within the region to produce a vapor. At least a portion of the bitumen is heated to below 3000 cp in the region by contacting with the vaport to produce a heated heavy oil and an imposed pressure differential between the injection wellbore and a production wellbore. Heated heavy oil is produced through the production wellbore of the solvent assisted gravity drainage operation, thereby recovering heavy oil with the solvent assisted gravity drainage operation form the subterranean region. In this embodiment a portion of the solvent is injected as vapor and the vapor contacts with at least a portion of the heavy oil in the region so as to heat the portion of the heavy oil and reduce its viscosity so that it flows generally towards the second wellbore. Additionally the injection wellbore and the production wellbore are from 3 meters to 7 meters apart and the injection wellbore is located higher than the production wellbore.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram illustrating a heavy oil heating process, wherein wave guides are used to introduce the microwaves to the reservoir.

FIG. 2 is a schematic diagram illustrating a heavy oil heating process wherein the microwaves are introduced into the reservoir using a microwave generator located within the reservoir.

DETAILED DESCRIPTION

Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.

The selection of solvent to be used in the gravity drainage operation includes those with a dipole moment so that the solvent can be heated by the microwave frequencies. Types of solvents that can be used include water, butane, pentane, hexane, diesel and mixtures thereof. In another embodiment the selection of the solvent does not include water to appease environmental and costs concerns. In another embodiments the solvent contains 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20% even 10% water.

Turning now to FIG. 1, wellbores 14, 15 and 16 are illustrated. Wellbore 14 extends from the surface 10 into a lower portion of subterranean region 12. Wellbore 16 extends from the surface 10 into subterranean region 12 and generally will be higher than wellbore 14. Wellbore 16 will be used to inject solvent and it is preferred that it is located higher than wellbore 14 so that when the injected solvent heats the heavy oil, the heavy oil will flow generally towards wellbore 14, which is used to extract the heavy oil from the reservoir. In one embodiment a portion of the H₂O is injected as steam and the steam contacts with at least a portion of the heavy oil in the region so as to heat the portion of the heavy oil and reduce its viscosity so that it flows generally towards the second wellbore. Wellbore 15 is used to introduce microwaves to the reservoir and it is preferred that wellbore 15 be located intermittent to wellbores 14 and 15; although, other arrangements are possible.

In operation, vapor generated in boiler 11 is provided into the reservoir 12 through upper wellbore leg 16. The vapor heats the heavy oil within zone 17 of the oil-bearing portion 13 of reservoir 12 causing it to become less viscous and, hence, increase its mobility. The heated heavy oil flows downward by gravity and is produced through wellbore leg 14. While FIG. 1 illustrates a single wellbore for injection and a single wellbore for extraction, other configurations are within the scope of the invention, for example, there can be two or more separate wellbores to provide steam injection and two or more separate wellbores for production. Similarly, multiple wellbores can be used for microwave introduction to the reservoir, as further discussed below.

Generally, the wellbore for steam injection, wellbore 16, will be substantially parallel to and situated above the wellbore for production, wellbore 14, which is located horizontally near the bottom of the formation. Pairs of vapor injection wellbores and production wellbores will generally be close together and located at a suitable distance to create an effective steam chamber and yet minimizing the preheating time. Typically, the pairs of injection and production wellbores will be from about 3 meters to 7 meters apart and preferably there will be about 5 meters of vertical separation between the injector and producer wellbores. In other embodiments it is possible for the injection and production wellbores be anywhere from 1, 3, 5, 7, 12, 15, 20 even 25 meters of horizontal separation apart. Additionally, in other embodiments it is possible for the injection and production wellbores be anywhere from 1, 3, 5, 7, 12, 15, 20 even 25 meters of vertical separation apart. In this type of SAGD operation, the zone 17 is preheated by steam circulation until the reservoir temperature between the injector and producer wellbore is at a temperature sufficient to drop the viscosity of the heavy oil so that it has sufficient mobility to flow to and be extracted through wellbore 14. Generally, the heavy oil will need to be heated sufficiently to reduce its viscosity to below 3000 cP; however, lower viscosities are better for oil extraction and, thus, it is preferable that the viscosity be below 1500 cP and more preferably below 1000 cP. Preheating zone 17 involves circulating vapor inside a liner using a tubing string to the toe of the wellbore. Both the injector and producer would be so equipped. Vapor circulation through wellbores 14 and 16 will occur over a period of time, typically about 3 months. During the steam circulation, heat is conducted through the liner wall into the reservoir near the liner. At some point before the circulation period ends, the temperature midway between the injector and producer will reach a temperature wherein the bitumen will become movable typically around 3000 cP or less or from about 80 to 100° C. Once this occurs, the steam circulation rate for wellbore 14 will be gradually reduced while the steam rate for the injector wellbore 16 will be maintained or increased. This imposes a pressure gradient from high, for the area around wellbore 16, to low, for the area around wellbore 14. With the oil viscosity low enough to move and the imposed pressure differential between the injection and production wellbores, vapor (usually condensed to hot solvent) starts to flow from the injector into the producer. As the vapor rate is continued to be adjusted downward in wellbore 14 and upward in wellbore 16, the system arrives at solvent assisted gravity drainage operation with no vapor injection through wellbore 14 and all the vapor injection through wellbore 16. Once hydraulic communication is established between the pair of injector and producer wellbores, steam injection in the upper well and liquid production from the lower well can proceed. Due to gravity effects, the vapor tends to rise and develop a solvent chamber at the top section 19 of zone 17. The process is operated so that the liquid/vapor interface is maintained between the injector and producer wellbores to form a vapor trap which prevents live vapor from being produced through the lower wellbore.

During operation, vapor will come into contact with the heavy oil in zone 17 and, thus, heat the heavy oil and increase its mobility by lessening its viscosity. Heated heavy oil will tend to flow downward by gravity and collect around wellbore 14. Heated heavy oil is produced through wellbore 14 as it collects. Vapor contacting the heavy oil will lose heat and tend to condense into solvent. The solvent will also tend to flow downward toward wellbore 14. In past SAGD operations, water would also be produced through wellbore 14. Such produced water would need to be treated to reduce impurities before being reheated in the boiler for subsequent injection. As the process continues operation, zone 17 will expand with heavy oil production occurring from a larger portion of oil-bearing portion 13 of subterranean formation 12.

Turning again to FIG. 1, the current invention provides for microwave generator 18 to generate microwaves which are directed underground and into zone 17 of the reservoir through a series of wave guides 20. The diameter of the wave guides will preferably be more than 3 inches in order to ensure good transmission of the microwaves. Within the reservoir, the microwaves will be at a frequency substantially equivalent to the resonant frequency of the water within the reservoir so that the microwaves excite the water molecules causing them to heat up. Optimally, the microwaves will be introduced at or near the liquid vapor interface so that condensed vapor is reheated from its solvent state back into vapor further supplying the steam chamber. In some embodiments the microwave frequency will be not greater than 3000 megahertz and/or at a resonant frequency of water. Based on the resonant frequency of water, the optimum frequency will be 2450 megahertz; however, power requirements and other factors may dictate that another frequency is more economical. Additionally, salt and other impurities may enhance the coupling effect (production of heat by resonance of a polar or conductive molecule with microwave energy); thus, the presence of salt is desirable.

Turning now to FIG. 2, a further embodiment of the invention is illustrated wherein, instead of using wave guides, power is supplied through electrical wire 22 to microwave generating probe 24. The electrical power can be supplied to wire 22 by any standard means such as generator 26.

In still another embodiment of the invention, also illustrated in FIG. 2, no vapor boiler is used. Instead solvent is introduced directly into wellbore 16 through pipe 28 and valve 30. Wellbore 16 then introduces solvent into the reservoir instead of vapor and the entire vapor production would be accomplished through use of the microwave generators. This embodiment of the invention has the added advantage of avoiding costly water treatment that is necessary when using a boiler to generate steam because, as discussed above, salt and other impurities can aid in heat generation. In a preferred embodiment, the solvent introduced into the reservoir would have a salt content greater than the natural salt content of the reservoir, which is typically about 5,000 to 7,000 ppm. Accordingly, it is preferred that the introduced solvent has a salt content greater than 10,000 ppm. For enhanced heat generation 30,000 to 50,000 ppm is more preferred.

Microwave generators useful in the invention would be ones suitable for generating microwaves in the desired frequency ranges recited above. Microwave generators and wave guide systems adaptable to the invention are sold by Cober Muegge LLC, Richardson Electronics and CPI International Inc.

Solvent to oil ratio is an important factor in SAGD operations and typically the amount of solvent required will be 2 to 3 times the oil production. Higher solvent to oil production ratios require higher solvent and natural gas costs. The present invention reduces solvent and natural gas requirements and reduces some of the solvent handling involving recycling, cooling, and cleaning up the water.

In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as additional embodiments of the present invention.

Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.

Claims

1. A process comprising: (a) injecting a solvent into a subterranean region through a first wellbore of a solvent assisted gravity drainage operation;(b) introducing microwaves into the region at a frequency sufficient to excite the solvent molecules and increase the temperature of at least a portion of the solvent within the region to produce a vapor(c) heating at least a portion of the heavy oil in the subterranean region by contact with the vapor to produce heated heavy oil; and(d) producing the heated heavy oil through a second wellbore of the solvent assisted gravity drainage operation;thereby recovering heavy oil with the solvent assisted gravity drainage operation from the subterranean region;wherein a portion of the solvent is injected as vapor and the vapor contacts with at least a portion of the heavy oil in the region so as to heat the portion of the heavy oil and reduce its viscosity so that it flows generally towards the second wellbore.

2. The process of claim 1 wherein at least a portion of the vapor condenses to a liquid state to form solvent as a result of its contact with the heavy oil and wherein the microwaves excite the molecules of at least a portion of the solvent so that the solvent is heated and becomes vapor.

3. The process of claim 1 wherein the microwaves are generated at the surface and introduced into the region through at least one waveguide.

4. The process of claim 1, wherein the solvent does not include water.

5. The process of claim 1 wherein the microwaves are generated within the subterranean region.

6. The process of claim 1, wherein the solvent comprises 10% water.

7. The process of claim 1 further comprising injecting at least a portion of the solvent as water and wherein the microwaves excite the molecules of at least a portion of the thus injected water so that the water is heated and becomes steam.

8. The process of claim 1, wherein the solvent comprises 20% water.

9. The process of claim 1 wherein the vapor contacts at least a portion of the heavy oil in the region so as to heat the heavy oil and reduce its viscosity so that it flows generally towards the second wellbore.

10. A process comprising: (a) injecting a solvent into a region through a first wellbore of a solvent assisted gravity drainage operation;(b) introducing microwaves into a subterranean region at a frequency sufficient to excite the liquid solvent molecules and increase the temperature of at least a portion of the liquid solvent within the region to produce a vapor(c) heating at least a portion of the heavy oil in the region by contact with the vapor to produce a heated heavy oil; and(d) producing the heated heavy oil through a second wellbore of the solvent assisted gravity drainage operation;thereby recovering heavy oil with the solvent assisted gravity drainage operation from the subterranean region;wherein a portion of the liquid solvent is injected as vapor and the vapor contacts with at least a portion of the heavy oil in the region so as to heat the portion of the heavy oil and reduce its viscosity so that it flows generally towards the second wellbore.

11. A process comprising: (a) injecting a solvent into a subterranean region through an injection wellbore of a solvent assisted gravity drainage operation;(b) introducing microwaves into the region at a frequency sufficient to excite condensed solvent molecules and increase the temperature of at least a portion of the solvent within the region to produce a vapor;(c) heating at least a portion of the bitumen to below 3000 cp in the region by contact with the vapor to produce a heated heavy oil and a imposed pressure differential between the injection wellbore and a production wellbore; and(d) producing the heated heavy oil through the production wellbore of the solvent assisted gravity drainage operation;thereby recovering heavy oil with the solvent assisted gravity drainage operation from the subterranean regionwherein the injection wellbore and the production wellbore are from 3 meters to 7 meters apart and the injection wellbore is located higher than the production wellbore;wherein the solvent is injected as vapor and the vapor contacts with at least a portion of the heavy oil in the region so as to heat the portion of the heavy oil and reduce its viscosity so that it flows generally towards the second wellbore.