Recovering oil from a subterranean carbonaceous formation

Abstract

In a process for recovering oil from oil shale which comprises injecting a fluid A, such as steam, into contact with finely-divided, unconsolidated, solid particles in a rubbled oil shale chimney; withdrawing fluids from the chimney from a point above the injection point of fluid A, and adjusting the rate of fluid injection and withdrawal so that a circulation current is established within the chimney sufficient to suspend at least a portion of the particles in the fluids in the chimney, the improvement which comprises radially extending the region of current upflow throughout a wide region of the chimney. Preferably the current upflow is extended radially by increasing the permeability at the base of the chimney over a wide area.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an in situ process for obtaining hydrocarbons from a subterranean carbonaceous formation.

2. Prior Art:

It is known, from U.S. Pat. No. 3,759,574 to Beard and U.S. Pat. No. 3,759,328 to Ueber et al, that an oil shale formation containing nahcolite can be treated with a nahcolite solvent (preferably water or condensed steam at high temperatures) to leach out the nahcolite and at the same time form a rubbled oil shale chimney, which then can be further treated with a pyrolyzing fluid, e.g., steam, to pyrolyze the oil shale to form hydrocarbon products. The resulting fluids may then be pumped to the surface and hydrocarbon recovered therefrom. Generally the known processes involve creating a rubbled oil shale volume in the nahcolite solvent, having a production tubing inserted into the rubbled area near the bottom of the rubbled zone, and injecting the pyrolyzing fluid at the top of the rubbled zone to force the pyrolyzed products through the zone and out the production tubing.

Although the Ueber and Beard processes described above are real advances in the in-situ recovery of hydrocarbons from oil shale, the processes have been plagued by problems wherein the lower part of the process zone or the production tubing has been plugged by an accumulation of fine particles. As a result of this plugging, injection was reduced to levels which lead to inefficient heat distribution within the rubbled chimney.

It was proposed, in Pat. application U.S. Pat. Ser. No. 476,973 filed June 6, 1974 that the plugging and heat distribution problems could be overcome by employing the process of that invention, which process comprises forming at least some finely-divided, fluid-surrounded particles and some bitumen in a rubbled oil shale cavity, injecting a fluid into the rubbled cavity at a level under at least a portion of the finely-divided particles, withdrawing fluids containing at least a portion of the bitumen from the rubbled cavity from a level higher than where a fluid A is injected and adjusting the temperature and rate of fluids injected and withdrawn so that a circulation current is established within the formation sufficient to suspend at least a minor portion of the particles then recovering hydrocarbons from the fluids which are withdrawn.

Although the process of Ser. No. 476,973 filed June 6, 1974 overcame some of the problems of the prior process, it is clear that not all the problems have been solved. For example, it is difficult to obtain uniform heat distribution throughout the cavity and growth of the rubbled oil shale chimney in the underground formation is not easily controlled. By the improvement of the process of this invention which involves radially extending the region of fluid up-flow farther from the ejection point of the fluidizing fluid A, the heat distribution problem is substantially overcome and the rubbled oil shale chimney can be expanded at a generally more uniform rate.

SUMMARY OF THE INVENTION

In a process for recovering hydrocarbons from a permeable subterranean carbonaceous formation which comprises

injecting a disaggregating fluid into contact with said permeable carbonaceous formation under conditions sufficient to form at least a minor amount of finely-divided, unconsolidated, fluid surrounded, solid particles, and at least some free bitumen;

injecting fluid A into contact with at least some of said particles;

withdrawing fluids containing at least a minor portion of the bitumen from the permeable formation from a level higher than that where fluid A is injected;

adjusting the rate of fluids injected and withdrawn so that a circulation current is established within said formation sufficient to suspend at least a minor portion of the particles within the fluid; and

recovering hydrocarbon materials from the fluids withdrawn, the improvement which comprises radially extending the region of current up-flow throughout a wider region of the permeable carbonaceous formation. The region of current up-flow is extended radially throughout a wider region of the permeable formation by increasing the permeability of the formation in the lower region radially extending horizontally from the injection point of fluid A. Generally, the carbonaceous formation is oil shale, and fluid A is steam which is injected at a level under at least a portion of the unconsolidated particles. Preferably, the permeability of the formation is radially extended horizontally from the point of injection of fluid A by positioning large inert objects in an evacuated region radially extending horizontally from the point of injection of fluid A. This is most effectively done by first forming a cavity extending radially under a substantial portion of an oil shale formation, then filling said cavity with inert objects to make a radially-extending region substantially permeable to fluids, and then carrying out the rest of the process steps.

DETAILED DESCRIPTION OF THE INVENTION

Although the process of this invention is particularly useful in the recovery of hydrocarbons from a subterranean oil shale formation, it is applicable to any carbonaceous formation, that is, a geological formation which has hydrocarbon or hydrocarbon-forming substances closely associated with the inorganic matrix of the formation. Thus the process is applicable in oil reservoirs, tar sand reservoirs, coal deposits, and of course, oil shale. For brevity, "carbonaceous formation" will be referred to in the following discussion in terms of oil shale formation, but it is understood that the process is not to be limited to such a formation, but has broad application in any of the carbonaceous formations aforementioned.

The process of which this invention is an improvement, is set forth in co-pending U.S. Pat. application Ser. No. 476,973 filed June 6, 1974 and that application is incorporated into this application by reference. In that process, it appeared that heat was being distributed to the formation nonuniformly and thus inefficiently. This appeared to be affecting the rate of cavity growth and formation of bitumen and other free hydrocarbons adversely. We realized that inefficient distribution and poorly controlled cavity growth was probably caused by channeling within the cavity. That is, it was thought that the particles suspended in the circulation current established by the process described in U.S. Pat. application Ser. No. 476,973 filed June 6, 1974 tend to circulate in a rather narrow pattern and, after this pattern continues, some particles tend to swell and pack around the fringes of the circulation current. This would lead to decreased or stopped cavity growth and inefficient heat distribution.

This invention is premised, at least in part, on the realization that such channeling could be substantially decreased if the current upflow of fluid and solid particles could be radially spread out over a wider region in the cavity. One method of assuring such radial spreading of the current upflow is to increase the permeability of the cavity's lower region which extends horizontally out from the point where the injection fluid A enters so that fluid A more readily penetrates horizontally farther into the cavity before starting its upward flow.

The invention of U.S. Pat. application Ser. No. 476,973 filed June 6, 1974 comprises at least in part:

injecting a disaggregating fluid into contact with a subterranean oil shale formation under conditions sufficient to form at least a minor amount of finely-divided, unconsolidated, solid particles, and at least some free hydrocarbon material;

injecting fluid A into contact with at least some of said particles;

withdrawing fluid containing at least a minor portion of said hydrocarbon material from the permeable subterranean oil shale formation from a higher level than that where fluid A is injected;

adjusting the rate of fluids injected and withdrawn so that a circulation current is established within said formation sufficient to suspend at least a minor portion of the solid particles in the fluid; and

recovering hydrocarbon materials from the fluids withdrawn. The improvement which is the process of this invention involves changing the circulation current within the formation by radially extending the current up-flow through a wider region of the subterranean formation. This is best accomplished by increasing the permeability of the formation in the lower region radially extending horizontally from the point of injection of fluid A so that the upflow of the fluids and particles in the cavity is properly extended over the wider region.

Fluid A is a heat carrying fluid which may be a condensable or non-condensable gas. Preferably, fluid A will be steam, hot water or a mixture of the two. Other fluids may be mixed with the steam, such as air or oxygen in small quantities, methane, CO.sub.2, and others as disclosed in Ser. No. 476,973 filed June 6, 1974.

This invention can be seen by contrasting the pictorial representation of the process of U.S. Pat. application Ser. No. 476,973 filed June 6, 1974 in FIG. 1 and the improved process of this invention depicted in FIG. 2.

FIG. 1 shows a subterranean oil shale formation 12 in which a well 17 has been completed through the overburden 16 into communication with the oil shale 23, and equipped with conduits 11 and 18 and packer 19. Fluid is circulated into and out of contact with the oil shale by an in-flow through conduit 11 and an out-flow through conduit 18. The outflow conduit may contain a flow-resistance means to provide a backpressure if desired. The conduits may extend into an impermeable zone or a permeable zone which has been made so by natural or man-made means. If the underground zone is impermeable, disaggregation will be very slow and thus the process will require a long time period to operate.

In order for the process to be particularly valuable, the oil shale formation is preferably permeable so that the fluid initially injected into the formation may contact a greater surface area of oil shale so that the oil shale will disaggregate rapidly. The permeability generally will be man made. Permeability can be obtained by any means known in the art, such as exploding high energy devices within the formation -- such as dynamite (e.g. as disclosed in U.S. Pat. No. 3,661,423 to Garret or U.S. Pat. No. 3,434,757 to Prats) or nuclear explosives; using mechanical means to break down the formation, such as hydraulic jets, grinders or cutters, electric or electropneumatic means such as the method disclosed in U.S. Pat. No. 3,696,866; decomposing heat-sensitive minerals such as nahcolite, trona, or dawsonite, to spall off portions of the oil shale such as described in U.S. Pat. No. 3,759,574 to Beard; or any combination of the above-mentioned means can be used. Generally, these means will result in a cavity or cavern within the formation that can be visualized as substantially filled with rubbled oil shale of various sizes. On the other hand these methods may result in what is a cavity which is a zone of interconnected fractures, or the like, through which fluids may flow with minimum impedance.

It is particularly suitable to establish communication with a nahcolite-containing oil shale formation so that a nahcolite solvent (hot water) may be injected into contact with the nahcolite through conduit 11 to leach out the nahcolite and form a rubbled oil shale chimney indicated generally as within the circle 10. (See co-pending U.S. Pat. application Ser. No. 476,973 filed June 6, 1974.

Steam is injected through the injection tubing 11, preferably at the bottom of the chimney, and condenses to dissolve nahcolite and form a basic solution which then acts upon the oil shale to disaggregate and continually rubble the oil shale to form the finely-divided particles indicated by small dots 13. The disaggregated particles generally will settle out in the bottom of the chimney as an unconsolidated agglomeration of particles 14, while fluids are withdrawn from the production conduit 18. The rate of fluid injection and fluid withdrawal is then adjusted to set up a circulation current indicated generally by the arrows 15, so that at least a minor portion of the particles are suspended and circulated, and a certain amount of the particles will be suspended, i.e., the upward force of the circulating fluid is sufficient to overcome the forces of gravity until the particles reach an apex at which point they fall back towards the bottom of the cavity to be re-circulated again. After the fluids, which contain at least some hydrocarbon material, are withdrawn through conduit 18, they are transferred to a separating means not shown in FIG. 1, at which point the hydrocarbons are removed. The particles 13 are thought to be circulating up the middle of the chimney generally around the conduit 11, and coming down around the outside of the chimney. The particles tend to return to the lower region toward the outside of the chimney, and may swell and pack in that area, thus forcing the current closer in towards injection tubing as shown by dotted arrows 20, and causing a certain amount of channeling. This causes inefficient heat distribution in the cavity and curtails cavity growth and formation of hydrocarbons.

According to the improved process of this invention, as shown in FIG. 2, the region which radially extends horizontally from the bottom of the injection tubing 11 is increased in permeability, thus causing the current upflow to spread over a wider region. The same numbers are used in FIG. 2 as in FIG. 1 to indicate the same subject matter. Thus, when the lower region 21 of the cavity 10 is increased in permeability the injected fluid spreads out through the region of increased permeability and the upflow indicated by the arrows 22, will be spread out over a greater region, thus allowing more particles to be suspended and the heat to be distributed over a wider range thus causing a faster decomposition of the kerogen to form bitumen of shale oil, and further speeding up the process of recovering hydrocarbons from the oil shale formation. Cavity growth will also be improved.

The process of this invention can be carried out in several ways. As mentioned previously, if the process of co-pending Pat. application Ser. No. 476,973 filed June 6, 1974 is carried out there is a great chance of channeling and thus inefficient heat distribution in the permeable formation due to settling and swelling of the particles. Thus, before small particles have a chance to settle and pack in the bottom area of the permeable formation, these small particles may be pumped out as they form by increasing the flow in injection fluid A so that the particles are carried up through the formation and out the outflow tube 18. This process may be continued until there is a substantial void in the bottom of the formation. This void region can then be filled with inert objects (24 in FIG. 2) in order to increase the permeability of this region as set forth in U.S. application No. 269,729, filed Oct. 11, 1972 (now Pat. No. 3,804,172), incorporated herein by reference. Disaggregation in the region above the zone of increased permeability is then continued. With the increased permeability to fluid in the lower region, the injection fluid coming through the injection conduit will tend to spread out radially to a greater extend as shown in FIG. 2, and thus will cause an up-flow over a broader range in the zone to be treated. Thus, as the disaggregation of the formation proceeds and there are more and more small consolidated particles formed, these particles will tend to be suspended throughout the region to a greater extend, and thus the heat that is carried in by the injection fluid will be distributed more evenly throughout the chimney, and will act on the oil shale to more readily cause disaggregation and separation of hydrocarbons from the oil shale.

Another means of extending the current upflow in this process over a wider region in the subterranean oil shale formations is to locate an underground region in a nahcolite-containing oil shale formation wherein there is only a minor amount of nahcolite mixed in with a substantial amount of oil shale, that is, a region of moderate nahcolite richness. If this region is leached near what is to be the base of the oil shale chimney, then relatively large blocks of oil shale rubble will be formed at the base, thus causing an area of permeability in the lower region of the rubbled chimney to extend over a longer period of time. The region above the permeable zone is then treated to form a rubbled zone which may then be treated with an oil disaggregating fluid to form at least a minor amount of finely-divided, unconsolidated, fluid-surrounded, solid particles and some free bitumen. With larger rubble or particle size at the base, injected fluid permeates a wider region, causing the up-flow throughout a wider region of the chimney, thus causing a more even heat distribution throughout the chimney and better separation of hydrocarbon material from inorganics.

A preferred method of performing the process of this invention is to first locate an oil shale formatin which is associated with a water soluble mineral or a "heat-sensitive carbonate mineral", i.e., a carbonate mineral that decomposes relatively rapidly at a relatively low temperature, such as between about 250.degree.F and 700.degree.F, to yield fluid products such as carbon dioxide and water. Examples of water-soluble mineral or sensitive carbonate minerals include halite, nahcolite, dawsonite, trona, and the like, which usually contain saline carbonate and/or bicarbonate compounds or groups. Preferably the mineral associated with the oil shale is nahcolite. It can then be determined where a layer which is substantially entirely nahcolite is to be found in the oil shale, and that particular layer can be leached out substantially entirely to form a void region which is substantially free from any other material, after the leaching has taken place. The void region would areally extend as far as the projected chimney growth on subsequent rubblization of the formation. This void region can then be filled with large, insoluble, inert pieces of rock or other matter; the pieces being of such size as to be injected through the well as disclosed in U.S. Pat. application Ser. No. 296,729 filed Oct. 11, 1972, now U.S. Pat. No. 3,804,172, to form a permeable layer which generally fills the cavity and extends under the region that eventually will be treated to recover oil shale. Preferably the void region is filled with packing granules, i.e., relatively large balls or pieces of rock or other substantially inert, solid material, that are small enough to be transported through well conduits into a subterranean location but are large enough and near-enough spherical shaped to form a mass of packed granules within the void region that is substantially rigid and highly permeable, but presents a large surface area and many and diverse passageways, through the interstices between the packed granules.

Once the permeable region is formed, a disaggregating fluid can be injected into contact with the subterranean oil shale formation above the permeable region to cause at least a minor amount of finely divided, unconsolidated, solid particles and at least some free hydrocarbon material to form. Then the injection fluid A is injected into the permeable region so that the fluid will contact at least some of the unconsolidated particles throughout substantially the entire cavity area. Then fluids are withdrawn from a point above the injection point, and the rates of fluids injected and withdrawn are adjusted so that a circulation current is established wherein the current upflow extends over a substantial region above the inert object-filled permeable region to suspend a maximum portion of the unconsolidated particles within the fluid. As bitumen and other hydrocarbon materials are formed they flow towards the upper regions of the formation being treated and are withdrawn.

Claims

We claim as our invention;

1. In a process for recovering hydrocarbons from a permeable subterranean carbonaceous formation which comprises

injecting a disaggregating fluid into contact with a subterranean carbonaceous formation under conditions sufficient to form at least a minor amount of finely-divided, unconsolidated, fluid-surrounded, solid particles, and at least some bitumen;

injecting fluid A into contact with at least some of said particles;

withdrawing fluids containing at least a minor portion of said bitumen from said cavity at a level higher than that where fluid A is injected;

adjusting the rate of fluids injected and withdrawn so that a circulation current is established within said formation sufficient to suspend at least a minor portion of said solid particles in said fluid; and

recovering hydrocarbon material from the fluids withdrawn;

the improvement which comprises radially extending the region of the current upflow throughout a wider region of the subterranean formation.

2. The process of claim 1 wherein the region of current up-flow is radially extended throughout a wider region of the subterranean formation by increasing the permeability of the formation in the region radially extending horizontally from the point of injection of fluid A.

3. The process of claim 2 wherein the carbonaceous formation is oil shale and fluid A is steam which is injected at a level under at least a portion of said particles.

4. The process of claim 3 which comprises increasing the permeability of the lower portion of the formation which radially extends horizontally from the point of injection of fluid A by positioning large inert objects in the region radially extending horizontally from the point of injection of fluid A.

5. A process for recovering hydrocarbons from a carbonaceous formation which comprises

forming a void region extending radially under a substantial portion of said formation;

filling said void region with inert objects to form a radially extending region substantially permeable to fluid;

injecting a disaggregating fluid into contact with said subterranean carbonaceous formation above said inert object-filled cavity, under conditions sufficient to form at least a minor amount of finely-divided, unconsolidated, solid particles, at least some free bitumen and a fluid-filled cavity;

injecting fluid A into contact with at least some of said particles;

withdrawing fluids containing at least a minor portion of said bitumen from said fluid-filled cavity from a level higher than that where fluid A is injected;

adjusting the rate of fluid injected and withdrawn so that a circulation current is established within said formation sufficient to suspend at least a minor portion of said particles in said fluid; and

recovering hydrocarbon material from the fluid withdrawn.

6. The process of claim 5 wherein the carbonaceous formation is nahcolite-containing oil shale, and said void region is formed by leaching out a region which is substantially entirely nahcolite.

7. A process for recovering oil from oil shale which comprises

locating an underground region in a nahcolite-containing oil shale formation wherein said region is of only moderate nahcolite richness;

leaching said region with a nahcolite solvent to form a first permeable zone of relatively large blocks of oil shale rubble which zone is located under an oil shale formation;

forming a rubbled zone above said first permeable zone;

injecting a disaggregating fluid into contact with said rubbled zone under conditions sufficient to form at least a minor amount of finely-divided, unconsolidated, fluid-surrounded, solid particles, and at least some bitumen;

injecting fluid A into said first permeable zone particles;

withdrawing fluids containing at least a minor portion of said bitumen from said rubbled zone at a level higher than that where fluid A is injected;

adjusting the rate of fluids injected and withdrawn so that a circulation current is established within said formation sufficient to suspend at least a minor portion of said solid particles in said fluid; and

recovering hydrocarbon material from the fluids withdrawn.