Method For Producing Retorting Channels In Shale Deposits

Abstract

A method for producing in situ retorting channels in subterranean shale deposits. Electro-pneumatic treatment is used for lean shale. Additional electro-chemical treatment is required for rich shale.

Description

BACKGROUND OF THE INVENTION

While oil shale deposits include a substantial percentage of our nations mineral energy resources, compared to oil sands the recovery of shale-borne oil is difficult and expensive. Open pit recovery techniques are at present the most highly developed for practical oil shale utilization, but these techniques are undesirable since they are inefficient and permanently mar the landscape. In situ recovery techniques, because they are more efficient and less destructive, are a more acceptable alternative.

One method for in situ recovery of oil from subterranean shale deposits is described in U.S. Pat. No. 3,106,244, issued to H. W. Parker. Into a pre-formed fracture system, Parker injects air to advance a direct drive combustion zone which is generated by simultaneously applying high voltage electricity. Hydrocarbons are produced from the shale adjacent to the fractures.

Another method useful for in situ recovery is described in U.S. Pat. No. 3,103,975, issued to A. W. Hanson. By electrical and chemical treatment of an oil shale bed, Hanson enlarges fractures between spaced wellbores. Once a fracture has been produced between the wellbores, Hanson floods the fracture with an electrolyte through which an electric current, preferably a.c., is passed. Chemical and thermal reaction of the electrolyte with the walls of the fracture causes crumbling and sloughing, enlarging the communicating passageway between the wellbores.

The methods of both parker and Hanson require fracturing of the oil shale bed with high fluid fracturing pressures. To create fractures, the entire body of shale must be moved by the pressure applied. The power applied must be proportional to the weight of the overbearing formation. For deep fracturing, power requirements are excessive. To obviate the need for fracturing in in situ recovery of oil from shale deposits, this invention was made.

SUMMARY OF THE INVENTION

My invention is a method for in situ production of retorting channels in oil shale. Electro-pneumatic treatment is used in lean shale. Additional electro-chemical treatment is applied to rich shale. In many respects, both forms of treatment employ similar apparatus.

As a primary distinction from previous methods for producing retorting channels, using my invention no attempt is made to fracture the shale bed. Retorting channels are produced by actually removing material from the bed between spaced wellbores without significantly altering the strength of the surrounding formation. Unlike fractured channels which close up as the shale settles, channels produced by my invention have little inclination to close up. Because of their relatively large size they are less subject to plugging than fractured channels. Since no attempt is made to fracture, no overburden need be moved, and power requirements are independent of the depth of the channels below the surface.

For electro-pneumatically producing retorting channels in lean shale, two spaced wellbores are sunk into a subterranean shale bed. An electrode is inserted into each wellbore in contact with the shale bed, preferably vertically aligned with a single bedding plane. By applying a high, preferably a.c., electrical voltage to the electrodes, an electrical current is caused to flow through the shale between the wellbores. Sufficient current heats the shale intensely, forming a viscous, molten fluid core. This fluid core is forced to flow out of the surrounding shale by injecting high pressure gas into one of the wellbores. An open retorting channel results.

For rich shales, additional treatment is required between the electric heating and gas injecting steps. Conducting cores formed in rich shale are not sufficiently molten for immediate removal by high pressure gas injection. Before removal, the rich shale core must be reduced to lean shale by electro-chemical reaction. After electrically forming a conducting core by the process used for lean shale, one electrode is removed from contact with the shale bed. An electrolytic solution is poured into the vacant wellbore and an acid-resistant electrode suspended within it. A high d.c. voltage is applied between the acid-resistant electrode and the other electrode, causing electrolysis and forming free oxygen where the conducting carbon core intersects the solution. Using sufficient voltage to cause a high electric current with intense heating and arcing, combustion of organic materials results in the presence of the oxygen. Vigorous percolation of the electrolyte in the combustion zone renews the spent electrolyte while removing the combustion products. Application of the electrical voltage continues until the combustion zone has completely penetrated the conducting path between the wellbores.

Sometimes this electro-chemical treatment alone is sufficient to create a usable retorting channel. Often, however, the resulting lean shale core requires additional electro-pneumatic treatment to form a usable channel. This additional treatment follows the steps described above for lean shale.

Therefore, one object of this invention is a method for producing retorting channels in subterranean shale.

Another object of this invention is an electro-pneumatic method for producing retorting channels in lean shale.

Another object of the invention is an electro-chemical method for producing retorting channels in rich shale.

These and other objects of this invention are evident in the following specification and drawing.

DESCRIPTION OF THE DRAWING

FIG. 1 shows an arrangement of apparatus for electro-pneumatically producing retorting channels in subterranean shale.

FIG. 2 shows an arrangement of apparatus for electro-chemically producing retorting channels in subterranean shale.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A stratified geological formation 10 with an oil shale bed 12 is shown in cross-section in FIG. 1. For producing a horizontal retorting channel 14 through the subterranean bed, two spaced wellbores 16 and 18 extend downward from the surface 20 to deep within the formation. The wellbores penetrate the shale bed to a depth beyond the optimum level for producing and operating the retorting channel. Open ended tubular casings 22-24 line the walls of each wellbore, preventing collapse of softer overburden into the underlying portion of the bore.

In electrical contact with a wall of each wellbore 16-18, expandable, conductive electrodes 26-28 depend from insulated electrical conductors 30-32. Through an intermediate on-off switch 34, conductor 30 connects electrode 26 to one output terminal of an electrical voltage source 36. Conductor 32 connects electrode 28 to the other output terminal of the source. When switch 34 closes, an electrical voltage appears between the two electrodes situated deep within the wellbores. Depending upon the electrical resistance of the particular shale bed 12, a sufficiently high voltage between the electrodes causes a powerful electrical current to flow through the bed. The electrical current heats the path between the electrodes, forming a charred conducting core 38 within the surrounding shale. By removing this charred core, an open retorting channel is formed. Depending upon whether the shale is rich or lean in oil content, the removal process includes a somewhat different series of steps.

If the shale bed 12 is lean in oil content, the charred core 38 is removed by pressurized gas. For lean shale, continued application of electrical current melts the core, forming a very viscous stream of molten liquid slag within the surrounding bed of impervious shale. Injecting pressurized gas into one wellbore while maintaining the other wellbore at atmospheric pressure forces the molten stream to flow out of the retorting channel. This operation is performed in the following manner.

On the surface end of casing 22 a cap 40, as shown in FIG. 1, seals wellbore 16 to form a closed subterranean chamber. Through a sealed opening in the cap, a supply conduit 42 injects high pressure gas into the sealed wellbore. Depending upon the physical resistance of the particular lean shale bed 12, a sufficiently high pressure gradient between wellbores 16 and 18 forces the viscous stream to flow along the heated path and into wellbore 18 in a manner analogous to fluid flow within a pipe. When the pressure gradient lowers rapidly, evidencing retorting channel breakthrough, electrical switch 34 is opened and gas injection stopped. The retorting channel is then available for in situ oil recovery by established procedures.

Both a.c. and d.c. electrical voltage sources are suitable for producing conducting cores and melting oil shale in this manner. A.c. is preferred since shale offers less resistance to breakdown from an a.c. than from a d.c. voltage. Since shale is often a good insulator, initial electrical breakdown usually requires extreme voltages. In actual tests, sources producing as high as 22,400 volts have been used, although a.c. voltages in the range of 3,000 volts have produced successful results. Higher voltage requirements are expected for many in situ operations. As current flow begins, resistance decreases sharply so voltage requirements diminish. Subsequent current loads are governed by the power required to char the core 38 and maintain it in a molten state. Because the amount of heating is directly proportional to current flow, the core diameter produced increases directly with current intensity.

Numerous designs are suitable for electrodes 26 and 28. Important design criteria for the electrodes include firm contact with the shale bed and a small contact area. Firm contact is required for electrical continuity. A small contact area prevents dissipation of electrical power and assures ample space for discharging molten material past the low pressure electrode. In addition, the electrodes should be resistant to both heat and electrical arcing resulting from high potential operation.

Air is a suitable gas for injection into well 16. Variable pressures are satisfactory, but rapid injection is required, so gas pressure must remain sufficiently high to support molten flow once it has begun. With high gas pressure the molten slag is prevented from solidifying at the effluent end of the channel 38. The molten materials discharge as finely divided particles, hardening into tiny pellets with minimum tendency to plug the wellbore 18.

Placing electrodes 26 and 28 in contact with the same bedding plane in shale bed 12 minimizes electrical resistance along the conducting path and insures optimum efficiency. For best results, wellbores 16 and 18 are analyzed by coring during drilling and the electrodes placed in a bedding plane of lowest oil content. Laboratory tests indicate that although charred slag cores are formed in most shales, melting is limited to shales containing less than about twenty gallons of oil per ton. In richer shales the electrical current causes insufficient heating for treatment in this manner. For rich shale beds, an alternate operation is required.

As described above treating oil shale, whether lean or rich, with sufficient electric current causes a charred carbon core. Two general core types result. Lean shales form slag cores which ultimately melt from sufficient electrically generated heat. Slag cores formed in rich shales, however, remain relatively solid regardless of the heat applied. Since the organic oil content of a rich shale core often exceeds one-third the raw shale volume, the core permeability can be increased by removing the oil from the residual carbon and slag by an electro-chemical process. Cores with high slag content become essentially lean shale once the oil is removed. Often the resulting core is sufficiently permeable for use as a retorting channel without additional treatment. In many cases, however, the slag core resulting from electro-chemical treatment is an impervious as indigenous lean shale. These cases require additional treatment, using the above described method for producing retorting channels in lean shale.

An arrangement for electro-chemically removing the oil from electro-thermally generated carbon cores in rich shale beds is shown in FIG. 2. Elements common to both FIGS. 1 and 2 have identical two digit reference numerals. Unique elements in FIG. 2 have three digit reference numerals. In this Figure a geological formation with a carbon core 138 penetrating a stratum of rich oil shale 112 is shown in cross-section. The carbon core is first produced in the rich shale bed in the manner shown and described with reference to FIG. 1. After the core is produced, as evidenced by a decrease in electrical resistance between the electrodes, electrode 28 and insulated conductor 32 are removed from wellbore 18, and replaced by a long, acid-resistant electrode 128, suspended by an insulated electrical conductor 132. Conductor 132 connects electrode 128 to the positive output terminal 144 of a d.c. electrical voltage source 136, while conductor 30, through switch 34, connects electrode 26 to the negative terminal 146. When switch 34 closes, a d.c. electrical voltage appears between electrodes 26 and 128.

Into wellbore 18 an electrolytic solution 148, such as water or a dilute acid-water mixture, is pumped until electrode 128 is at least partially submerged. At this level the solution completely submerges the proximate end of charred core 138. For improving electrical continuity between electrode 26 and the charred core, plain water can be pumped into wellbore 16 if necessary. When the electrical potential from d.c. source 136 is applied between electrodes 26 and 128, electrical continuity is completed through charred conducting core 138 and electrolytic solution 148. Electrolysis of the solution results, decomposing the water to form free hydrogen at the positive electrode 128, and free oxygen at the electrical extension of the negative electrode 28 -- the end of conducting carbon core 138 in contact with the electrolyte 148.

To avoid confusion regarding the labeling of electrolysis electrodes 26 and 128 as negative and positive, respectively, please note that the electrical, rather than chemical convention for current flow is applied. Following this convention, hydrogen forms at the positive electrode, and oxygen at the negative electrode.

With a negative electrical potential applied to conducting carbon core 138, pure oxygen is formed at the intersection area 150 of the core and electrolytic solution 148. This free oxygen is available for combustion of the carbonaceous material within the core. By raising the voltage of d.c. source 136, sufficient electrical current is generated through core 138 to create heat and electrical arcing in intersection area 150. Since submerged combustion is possible in the presence of pure oxygen, the electrolytic solution does not hinder oxidation, and a fire front moves forward until the entire core is burned. When combustion is completed along the entire length, electrolytic solution 148 fills the core, markedly reducing the resistance to electrical current flow. When this reduction in resistance signals completion of the process, switch 34 is opened to disconnect d.c. source 136 from the electrical circuit. If sufficient slag remains to obstruct retorting channel 114, it is removed by the electro-pneumatic process described above with reference to FIG. 1.

During the electro-chemical process of treating carbon core 138, adequate current flow is important for several reasons. As explained above, adequate current is necessary for heating and arcing at the intersection area 150. Adequate current is also necessary to cause vigorous percolation of the electrolytic solution. Vigorous percolation, caused by rapid oxygen generation and combustion, violently exhausts spent acid and combustion products from the intersection area 150, allowing fresh acid to enter. In the resulting spent electrolyte stream, particles of imbedded clay and other inert debris are carried from the core, ultimately settling to the bottom of wellbore 18. In this way the intersection area 150 advances through the core in a continuously regenerative cycle.

Because of the diverse environments to which this electro-chemical method is applicable, specific electrical and physical operating parameters are difficult to predict precisely. General parameters, however, are sufficient for establishing necessary design criteria. In this regard, electrode 128 is preferably constructed from an acid-resistant material. Since hydrogen is produced adjacent to this electrode, oxidation is not a problem, and a carbon electrode is suitable. For dimensional purposes, in laboratory experiments a current density of 1 ampere per square inch of submerged electrode area yielded satisfactory results. Dilute hydrocloric acid is a suitable electrolytic solution since it is readily available and generally produces solvable chloride salts. Other dilute common acids are equally satisfactory. The amount of dilution is not critical.

Voltage requirements vary greatly, depending upon the particular environment operated upon. Sufficient voltage is necessary to stimulate heating and percolation, as described above. Field tests have successfully employed as low as 143 volts at 5.7 amperes to cause the necessary percolation. By using adequate current flow in the initial formation of the conducting carbon path 138 a low resistance conductor results, reducing electro-chemical processing voltage requirements to a minimum. During electrolysis, the amount of current required varies with the structure of the core. A good rule is to always exceed the maximum current used to form the core. Laboratory tests have successfully employed currents of 8 to 10 ampers for cores from 1 to 2 inches in diameter.

By combining the electro-pneumatic process of FIG. 1 and the electro-chemical process of FIG. 2 it is possible to produce retorting channels in all grades of rich and lean shales. Once the channels are produced, retorting proceeds according to established procedures. Because, within the bounds of these established procedures, modifications of the electro-pneumatic and electro-chemical process steps will be obvious to persons of ordinary skill in the art, the scope of this invention should not be limited by the above description, but only by the following claims:

Claims

I claim:

1. A method for producing an in situ retorting channel in a subterranean shale deposit comprising the steps of:

drilling two spaced wellbores into a shale bed,

electrically forming a charred conducting path through an impermeable portion of the shale between the wellbores,

removing a substantial percentage of the material along the charred conducting path to form a permeable retorting channel.

2. A method for producing an in situ retorting channel in a subterranean shale deposit as claimed in claim 1, in which the step of removing includes:

electrically forming a molten, fluid core along the conducting path, and

forcing the fluid core to flow along the path and out of the surrounding shale, leaving an open retorting channel between the wellbores.

3. A method for producing an in situ retorting channel in a subterranean shale deposit as claimed in claim 1, in which the step of removing includes:

electrolyzing an oxygen bearing electrolyte in one of the spaced wellbores so that free oxygen forms at the intersection of the electrolyte and the conducting path, and

applying a sufficiently high voltage across the conducting path to cause a combustion front to advance between the wellbores, forming a burned out, permeable retorting channel.

4. A method for producing an in situ retorting channel in a subterranean shale deposit as claimed in claim 3, including the additional steps of:

electrically forming a molten, fluid core along the burned out, permeable channel, and

forcing the fluid core to flow along the channel and out of the surrounding shale, leaving an open retorting channel between the wellbores.

5. A method for producing an in situ retorting channel in a subterranean shale deposit as claimed in claim 2 in which the step of electrically forming a charred conducting path further includes:

positioning first and second conducting electrodes, respectively, in each wellbore in electrical contact with the shale deposit, and

applying a high electrical voltage across the first and second electrodes to cause electrical current flow through the shale deposit on a conducting path between the first and second electrodes, the voltage and resulting current flow having sufficient intensity to ultimately melt the shale along the conducting path and form a viscous, molten, fluid core,

and the step of forcing further includes:

Establishing a pressure differential between the spaced wellbores to cause the molten fluid core to flow out of the surrounding shale and into one of the wellbores.

6. A method for producing an in situ retorting channel in a subterranean shale deposit as claimed in claim 1 in which the step of electrically forming a charred conducting path further includes:

positioning first and second conducting electrodes, respectively, in each wellbore in electrical contact with the shale deposit,

applying a high electrical voltage across the first and second electrodes to cause electrical current flow through the shale deposit on a conducting path between the first and second electrodes, and

removing the applied voltage after increased current flow shows that a conducting path has been established between the first and second electrodes,

and the step of removing further includes:

removing one electrode from one of the wellbores,

filling the one wellbore to a level above the conducting path with an oxygen bearing electrolyte,

immersing a third electrode in the electrolyte,

applying a high d.c. electrical voltage between the third electrode and the other electrode to cause electrolysis of the oxygen bearing electrolyte, with the polarity of the electrodes such that free oxygen forms at the intersection area of the conducting path and the electrolyte, whereby continued application of the high d.c. voltage causes a combustion front to advance along the conducting path toward the other electrode, and

interrupting the high d.c. voltage after decreased electrical resistance of the core indicates that combustion has progressed a sufficient distance.

7. A method for producing an in situ retorting channel in a subterranean shale deposit as claimed in claim 6 including, following the step of interrupting, the additional steps of:

removing the third electrode and electrolyte from the one wellbore,

re-positioning the one electrode in its approximate original position within the one wellbore,

applying a high electrical voltage across the first and second electrodes to cause electrical current flow through the shale deposit on the conducting path between the first and second electrodes, the voltage and resulting current flow having sufficient intensity to ultimately melt the shale along the conducting path and form a viscous, molten fluid core.

electrically forming a molten, fluid core along the conducting path, and

establishing a pressure differential between the spaced wellbores to cause the molten fluid core to flow out of the surrounding shale and into one of the wellbores.