In situ production of bitumen from oil shale

Abstract

Hydrocarbons are recovered from oil shale deposits by introducing hot fluids into the deposits through wells and then shutting in the wells to allow kerogen in the deposits to be converted to bitumen which is then recovered through the wells after an extended period of soaking.

Description

BACKGROUND OF THE INVENTION

This invention relates to the recovery of bitumen from oil shale and more particularly to an in situ process for conversion of kerogen to bitumen and recovery of the resulting bitumen.

Oil shale deposits are found in many locations throughout the world and are a potential source of extremely large quantities of hydrocarbon products. Oil shale is generally a laminated, nonporous, impermeable, fine-grained dolomitic marlstone containing variable but relatively large amounts of organic matter known as kerogen. Kerogen is a high molecular weight substance largely insoluble in benzene and which is dispersed throughout an inorganic matric composed principally of carbonates along with other minor constituents. The kerogen in oil shale is relatively rich in hydrogen and will yield a benzene soluble material (bitumen) on heating.

Many proposals have been made for recovering usuable hydrocarbons from oil shales, most of which involve the use of heat in one form or another to soften or liquefy the kerogen for conversion to bitumen or for further conversion to produce both liquid and gaseous products. The heat may be applied in situ or the shale may be mined by conventional mining methods with subsequent heating or retorting of the mined shale. In conventional in situ retorting, a heating agent is injected into one or more wells extending into the shale deposit and product is produced through the same or separate wells. It is also known to inject air into the formation to ignite the kerogen and form a combustion front which is then moved through the formation in a conventional manner to liquefy and partially gasify the kerogen and carry the liquid and gaseous product through the formation to wells from which it may be recovered. In situ processes frequently involve fracturing the shale deposit to facilitate contact between heating agents and kerogen.

In all of the previously known in situ processes for recovery of bitumen from shale deposits, thermal efficiency has been extremely low because, once formed from the kerogen, bitumen has been recovered at relatively high temperatures. Also a significant amount of the bitumen that has been formed migrates through the formation and is not recovered. It is therefore an object of the present invention to recover bitumen from shale deposits by means of a novel in situ recovery process which involves recovery of substantial quantities of bitumen with a high degree of thermal efficiency and to reduce the migration of the bitumen out of the formation.

SUMMARY OF THE INVENTION

Hydrocarbon product is recovered from a subterranean deposit of oil shale by introducing heat energy into the deposit through one or more wells extending into the deposit. Heat energy is introduced in quantities sufficient to heat the deposit in the vicinity of the wells to more than 50F.degree. (Fahrenheit Degrees) above its transition temperature (the temperature at which exfoliation of the shale structure commences). The wells are then shut in until the temperature in the vicinity of the wells drops to less than 50.degree. above transition temperature, at which time the wells are again opened and bitumen is produced therefrom.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a vertical cross-sectional view illustrating use of the present invention in recovery of hydrocarbons from an oil shale deposit.

FIG. 2 is a horizontal cross-sectional view further illustrating use of the invention in recovering hydrocarbons from oil shale deposits.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, the prior art in situ processes for recovery of hydrocarbons from shale deposits have suffered from extreme inefficiency. By use of the process of the present invention it is possible to convert lkerogen to bitumen and recover the bitumen from the shale deposit at significantly lower temperatures than those at which bitumen is normally recovered from such deposits while achieving at the same time recovery of substantial quantities of the total possible recoverable hydrocarbons. As mentioned above, this is accomplished by introducing heat energy into the deposit through one or more wells extending into the deposit with the heat energy being introduced in quantities sufficient to heat the deposit in the vicinity of the wells to more than 50F.degree. above its transition temperature. The transition temperature is considered to be that temperature at which exfoliation (swelling) of the shale structure begins to take place. The wells are then shut in until the temperature of the deposit in the vicinity of the wells drops to less than 50F.degree. above the transition temperature of the deposit at which point the wells may be opened and bitumen produced therefrom or additional heat energy may be introduced for conversion of additional kerogen to bitumen. If the bitumen is not removed, it will act as a solvent and tend to solubilize more of the kerogen.

The exfoliation temperature for a particular shale deposit varies depending on the amount of kerogen contained in the shale between about 600.degree. and about 700.degree.F. with the lower transition temperatures occuring in connection with relatively richer shale deposits. When an oil shale deposit is heated to above its transition temperature in the absence of oxygen, exfoliation is accompanied by a marked increase in permeability. If the shale is allowed to remain above the transition temperature for a few hours, the permeability decreases again to the original value, usually essentially zero. If, however, the shale deposit is maintained above its transition temperature for a substantial length of time, such as weeks or months, significant portions of the kerogen are converted to bitumen which has substantially lower viscosity than the kerogen and can flow freely through the inorganic matrix of the shale deposit. In accordance with this invention, the shale deposit is heated to more than 50F.degree. and preferably at least about 100F.degree. above its transition temperature and then allowed to cool to less than 50F.degree., preferably to about 25F.degree. or less above its transition temperature before recovery of any bitumen therefrom. This allows ample time for substantial quantities of kerogen to convert to bitumen and allows heat to be transferred to further portions of the formation to avoid loss of thermal efficiency in recovery of bitumen from the deposit. It also allows the bitumen to act as a solubilizing agent on the undissolved or unconverted kerogen.

Introduction of heat energy to a shale deposit in accordance with the invention can be by any suitable means with use of hot fluids at temperatures between about 700.degree. and about 2,000.degree.F. being preferred. Preferred fluids include steam and hot water although other fluids not containing free oxygen, such as liquid or vaporous hydrocarbons, flue gas, etc. may be used.

Because of the extremely low permeability of oil shale deposits, it is usually not possible to inject hot fluids at normal injection rates without increasing pressure in the injection well to an undesirable degree before the desired rise in temperature has taken place in significant portions of the surrounding shale deposit. It is therefore preferred that the initial step of heating the deposit in the vicinity of the wellbores to more than 50F.degree. above its transition temperature be done in stages. In this preferred embodiment of the invention, hot fluid is injected through the wells until the pressure is raised to between about 200 and about 1,000 psi above normal formation pressure of the deposit. The wells are then shut in for a period of time necessary to allow the pressure to drop to less than about 50 psi above the formation pressure of the deposit. This frequently takes between about 2 weeks and about six months. Additional hot fluid is then injected until the pressure again rises more than 200 psi above formation pressure. Similar cycles of injection and shut in are continued until the temperature in the vicinity of the injection wells reaches the desired range of more than 50F.degree. above the transition temperature of the deposit. The injection wells are then shut in until the temperature in the vicinity of the wells drops to less than 50F.degree. above the transition temperature of the deposit at which time bitumen may be produced from the wells or, preferably, injection of hot fluids as described above is again resumed. By so resuming injection of hot fluids, the affected area of the shale deposit may be extended beyond that possible by merely raising the temperature in the immediate vicinity of the wells. This is possible because of the increased permeability of the formation in the vicinity of the wells due to conversion of kerogen to lower viscosity bitumen during the injection and shut in cycles mentioned above and also because the bitumen tends to solubilize additional kerogen.

Once the deposit in the immediate vicinity of the injection wells has been heated to more than 50F.degree. above its transition temperature, it is preferred that the wells be shut in for between about 6 months and about 1 year to allow the temperature to drop to less than 50F.degree. and more preferably to less than 25F.degree. above transition temperature. Bitumen may then be produced from the wells or more preferably injection of hot fluids may be resumed to extend the affected area of the deposit before production of any bitumen therefrom. Such expansion of the effected area of the deposit preferably is carried out in the same manner as the original heating of the deposit described above, i.e., hot fluids are injected into the deposit until the well pressures rise to between about 200 and about 1,000 psi above normal formation pressures, the wells are shut in for between about 2 weeks and about 6 months to allow pressure to return to less than 50 psi above formation pressure and injection of hot fluids is then resumed on the same basis until temperatures in the previously unaffected portions of the shale deposit surrounding the wells have been raised to more than 50.degree.F. above their transition temperatures. The increase in temperature of previously unaffected shale deposit may in part or in whole be achieved by indirect transfer of the heat from the injected hot fluids through previously formed bitumen.

It is preferred that the introduction of heat energy into the shale deposit as described above be continued until the deposit has been heated to more than 50F.degree. above its transition temperature throughout a sphere having a radius of at least about 50 feet from the injection point of each well through which hot fluid has been injected. Each portion of the deposit so heated should then be allowed to "soak" with the wells shut in until the temperature again drops to less than 50F.degree. above the transition temperature (usually for a period of at least about 6 months) to allow time for conversion of kerogen to bitumen. For maximum efficiency of recovery, it is preferred that no bitumen be produced from the deposit until all of the above heating and soaking cycles have been completed at least once for each portion of the deposit contained within the spheres mentioned above.

In practicing the invention, it is important to avoid fracturing the shale deposit since any fractures formed beyond the area of the deposit in which kerogen is transformed to bitumen will result in excessive loss of bitumen into other portions of the deposit or surrounding formations. For the same reason, it is not desirable to allow the portions of the deposit in which kerogen is converted to bitumen to extend to the boundaries of the shale deposit if the surrounding or underlying formations or overburden are permeable. For this reason, it is preferred that the invention be practiced in shale deposits having a thickness of at least about 200 feet and that the periphery of each of the spheres of affected area in which kerogen is converted to bitumen remain a minimum of at least about 50 feet from the boundary of the deposit. Overlapping of affected spheres is, of course, permissible and frequently desirable to ensure maximum recovery of hydrocarbons but it is preferred that overlapping be kept to the minimum necessary to obtain desired recovery of hydrocarbons. Otherwise, excessive temperatures may build up in portions of the deposit thereby resulting in thermal inefficiency of undesirably long periods of time being required for heat to be transferred to other portions of the deposit. For this reason, it is preferred that the average temperature of affected portions of the shale deposits not rise above about900.degree.F. and that, to the extent practical, temperatures above about 1200.degree.F. be avoided completely.

In practicing the invention, the temperature of the shale deposit may be determined by temperature sensing means introduced into the deposit such as through the wells used to inject hot fluids or by means such as infrared aerial photography which allows reasonably accurate determination of temperatures throughout the deposit. Most accurate temperature information is usually obtained by a combination of these or other temperature measuring means.

If the invention is carried out using the preferred embodiments described above, it is normally feasible to convert at least about 70 percent and frequently at least about 90 percent of the kerogen in the affected areas of the deposit into bitumen and to recover at least about 65 percent of such bitumen from the deposit. Recovery initially is by merely opening the injection wells as described above but it should be understood that in addition, other conventional primary, secondary and even tertiary recovery processes may be used as desired to recover bitumen.

Referring to the drawings, FIG. 1 shows a well 12 extending from the surface of the earth 14 through overburden formation 16 into a shale oil deposit 18. An underlying formation 20 is also indicated. The well 12 may be suitably lined and equipped with tubing, etc. in a conventional manner. A conduit 22 communicates at one end thereof to the top of the well 12. The other end of conduit 22 may be connected to a source of hot injection fluids (not shown) or may be connected to means for recovering bitumen produced from the shale deposit 18. Means (not shown) are also provided for closing off the conduit 22 completely to shut in the well 12.

As an example of recovery of hydrocarbons from oil shale in accordance with a preferred embodiment of the invention, superheated steam at a temperature of about 1,000.degree.F. may be introduced through the conduit 22 and well 12 into the shale deposit 18. The shale deposit 18 for this example begins about 2,000 feet below the surface and has a thickness of 200 feet from top to bottom and a normal formation pressure of about 1,000 psi. Injection of steam at the rate of 4,000 pounds per hour through the well 12 for up to 8 hours will increase the well pressure to 2000 psi at which time the well is shut in for 4 weeks to allow the pressure to return to less than 50 psi above the formation pressure. Three subsequent similar cycles of injection and shutting in are required to raise the temperature of the shale deposit in the vicinity of the well (within a sphere 24 as represented in FIG. 1) to a temperature 100.degree.F. above the formation transition temperature of 562.degree.F. The well 12 is then shut in for 6 months during which time the temperature within the sphere 24 diminishes to 587.degree.F. (25.degree. above formation transitions temperature). At this time at least about 20 percent of the kerogen contained within the sphere 24 has been converted to bitumen. A series of injections and shut ins similar to that described immediately above is then used to extend the affected area of the shale deposit to encompass all the material within a sphere 26 (FIG. 1). Also during this period of time more of the kerogen is converted to bitumen by the action of not only the heat energy but also the solubilizing effect of the bitumen itself until about 90% of the kerogen is converted. Another complete series of injections and shut ins as described above is used to extend the effected area of the shale deposit in which kerogen is converted to bitumen to encompass material within the sphere 28 as shown in FIG. 1. At this time (a total of 10 years after the start of the injections) bitumen is produced from the shale deposit through the well 12. A total of 65 percent of the bitumen contained within the deposit is produced by primary recovery and an additional 25 percent is available for production by conventional secondary and tertiary methods.

To recover the maximum amount of hydrocarbons from a shale formation it is generally desirable to use more than one well as is depicted in FIG. 2 which shows an oil shale deposit 32 with a number of wells such as 34 and 40 completed within the shale deposit. By injection of hot fluids and periodic shutting in of wells as described above, kerogen within a wide area of the deposit may be converted to bitumen. The spheres of affected shale deposit in which this occurs are indicated generally in FIG. 2 by dashed circles such as 36 and 40. It will be noted that these spheres overlap so as to convert kerogen to bitumen throughout the maximum possible volume of the deposit and that none of the spheres reaches the boundary of the deposit. By keeping the spheres from reaching the boundary of the shale deposit and avoiding fracturing of the shale, it is possible to take advantage of the extremely low permeability of the natural shale deposit to prevent loss of bitumen before it can be produced from the deposit.

While the invention has been described above with respect to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit or scope of the invention.

Claims

What is claimed is:

1. A process for recovering hydrocarbon product from asubterranean deposit of oil shale which comprises the steps of:

a. introducing heat energy in the form of hot fluids not containing oxygen into said deposit through one or more wells extending into the deposit, said heat energy being introduced in quantities sufficient to heat the deposit in the vicinity of the wells to more than about 50.degree.F above its transition temperature and until the pressure at the bottom of the wells is from at least 200 to about 1,000 psi about the formation pressure of the deposit;

b. then shutting in said wells until the temperature in the vicinity of the wells drops to less than about 50.degree.F above the transition temperature of the deposit until the pressure at the bottom thereof drops to less than about 50 psi above formation pressure, with the shutting in period lasting from about two weeks to about 6 months;

c. repeating steps (a) and (b) for a period of from about 1 to about 10 years; and

d. producing bitumen through said wells.

2. The process of claim 1 in which steps (a) and (b) are repeated until the deposit has been heated to more than 50F.degree. above its transition temperature and then allowed to drop to less than 50F.degree. above its transition temperature throughout a sphere having a radius of at least about 50 feet from the bottom of each of said wells.

3. The process of claim 2 in which at least about 75 percent of the kerogen in the spheres is converted to bitumen before bitumen is produced through the wells.

4. The process of claim 2 in which the deposit of oil shale is at least about 200 feet thick, the radius of each of the spheres is between about 50 and about 500 feet and the periphery of each sphere is at least 50 feet from the boundary of the deposit.

5. The process of claim 1 in which heat energy is injected in the form of hot fluids at a temperature of between about 500.degree.and about 2,000.degree.F.

6. The process of claim 1 in which step (b) takes at least about 6 months.