US3533469A - Method of insulating the roof of a subterranean cavity during retorting - Google Patents

Description

3.1.11.1 6,1 6- -11 u nyl I rif /2 1,; 3 65336969 2 United States Patent I 1 1 3,533,469

7 2] Inventor Harry Parker [56] References Cited [7]] A I NO 535 23 UNlTED STATES PATENTS 5 12, 12, 1968 3,113,620 12/1963 I-Icmminger..... 166/247)( Patented 0 13 3,193,006 7/1965 Lewis 166/266 [73] Assignee Phillips Petroleum Com 3,223,158 12/1965 Baker 166/259 Bamesvme, Okm 740,4 3,273,640 9/1966 Huntmgton 166/272x corpormion Delaware 3,291,215 12/1966 N1C1'101S 166/259 3,342,257 9/1967 Jacobs et a1, 166/247 3,346,044 10/1967 Slusser 1 1 1 166/259X 3,382,922 5/1968 Needham. 166/272 3,460,620 8/1969 Parker 166/247UX (54] METHOD OF INSULATING THE ROOF OF A Primary ExaminerStephen .1. Novosad SUBTERRANEAN CAVITY DURING RETORTING A norney Pendleton, Neuman, WiIliams and Anderson 12 Claims, 1 Drawing Fig.

[52] U.S.Cl 166/272, ABSTRACT; Premature roof collapse during retorting of 011 /2 1 1 shale rubble in situ within a subterranean cavity, particularly a 1 Int. nuclear chimney may be prevented injecting into the cavi E21b43/26 ty adjacent the top thereof a gas whose molecular weight is [50] Field ofsearch 166/247, significantly lower than that of a gas being used to retort the 272, 274,273, 271. 268, 261, 259, 265, 266, 302 cavity.

Patented Oct. 13, 1970 BLANKET GAS INVENTOR HARRY W. PARKER METHOD OF INSULATING THE ROOF OF A 1 SUBTERRANEAN CAVITY DURING RETORTING BACKGROUND OF THE INVENTION This invention relates to a method of insulating the roof ofa subterranean cavity during retorting in order to prevent premature roof collapse of thecavity. The invention has particular application to the process for retorting oil shale rubble in situ within a subterranean cavity such as a cavity formed by a contained subterranean nuclear explosion.

There has been increasing interest in this country in processes for the recovery of shale oil. One promising shale oil recovery process involves the use of nuclear explosives detonated underground to create a spherical cavity, the size of which will depend upon the nuclear explosive yield, the average density of the overburden and the depth of burial of the bomb. If a sufficiently large spherical cavity has been produced, the shale above the cavity will be unable to support itself, and the roof of the cavity will begin caving in soon after the creation of the cavity by the explosion. Natural factors and factors caused by the shock of the nuclear explosion will aid sloughing of the rock into the cavity until in place of the initial spherical cavity there will be an approximately cylindrical cavity filled with broken rock or rubble. This filled cavity or column of oil shale rubble is known as a nuclear chimney." The void space present in-:the initial spherical cavity becomes distributed as porosity within the broken oil shale, and this porosity or permeability isian important factor in the in situ" recovery of shale oil therefrom by retorting.

In retorting the nuclear chimney, 700-retorting under high temperature and pressure is forced through the rubble, the gas being introduced adjacent the top of 250and being withdrawn adjacent the bottom of the chimney. The permeable or open" nature of the rubble within the chimney not only permits the hot retorting gas to flow downwardly through the chimney to uniformly heat the rubble, but it permits the shale oil to flow to the bottom of the chimney from where it may be pumped to the surface. For maximum shale oil recovery it is therefore important thatzthe permeability of the rubble be maintained at a substantially uniform level (to minimize channeling and bypassing) and that crushing and compaction of the rubble be minimized.

Retorting a chimney for a considerable length of time with a gas at high temperature. (e.g., between about 700F. and l000F.) may weaken the arches of shale supporting the overburden above the chimney and cause the roof to collapse, thereby possibly crushing and reducing the permeability of the rubble and reducing the recovery of shale oil.

The present process is intended to insulate the roof of the nuclear chimney, therebyreducing the likelihood of a premature collapse of the roof and a compaction of the oil shale rubble which would reduce the recovery of shale oil.

SUMMARY'OP THE INVENTION The invention is an improvement in the process for retorting cycling of a retorting gas under a high temperature, for example, 800F., and a high pressure, for example, 100 p.s.i.g. through the rubble preferably by introducing this gas adjacent the top of the cavity and withdrawing it adjacent the bottom of the cavity. The process further includes injecting into the cavity adjacent the top thereof a quantity of a second gas having a significantly lower molecular weight than the average molecular weight of the retorting gas. The temperature of the second gas at the time of introduction is preferably substantially lower than that of the retorting gas, and the difference in molecular weight is such that even when the gas is cool (i.e., at the injecting temperature), it will be substantially less dense than the hot gases being used to retort the chimney. It is preferred that the difference between the molecularweights of the retorting gas and the insulating second gas be as great as possible, with best results being obtained when the insulating gas is no greater than two-thirds the molecular weight of the retorting gas. It is also highly desirable that the insulating gas be relatively inert, 112.. relatively nonreactive with respect to the contents of the cavity or chimney and with respect to the retorting gas under the retorting conditions.

By using a gas whose molecular weight is significantly lower than the other gases contained in the nuclear chimney the insulating gas will remain adjacent the roof of the chimney and prevent natural convection currents of the other gases in the cavity from transporting heat to the roof.

A BRIEF DESCRIPTION OF'THE DRAWING The FIG. is a schematic illustration of a subterranean cavity filled with oil shale rubble and of the equipment for retorting the chimney and for carrying out the process of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the FIG. there is-illustrated a nuclear chimney 12 formed in a strata of oil shale 14. The chimney has been formed by a nuclear explosion which initially produced a generally spherical cavity in the shale strata. The weakened shale above the original cavity then sloughed into the cavity resulting not only in the filling of the cavity but in an increase in the height of the cavity, until the final cylindrical cavity 16 was formed, this final cavity being substantially completely filled with the sloughed oil shale rubble 14a. The final cavity I6 is approximately the diameter of the initial cavity formed by the explosion, and the height of the cavity 16 is determined by the porosity of the broken oil shale within the cavity. The volume of the original spherical cavity is distributed as voids or porosity in the final cylindrical cavity 16. The nuclear chimney 12 thus comprises the final cylindrical cavity 16 and the column of oil shale rubble 14a within the cavity.

It will be noted that the roof or top portion 16a of the chimney cavity 16 is arched or substantially dome-shaped. As a result of this arching roof the weight of the overburden 18 above the chimney 12 is transferred to the oil shale l4 sur rounding the chimney. The roof is also supported to some extent by the broken oil shale 14a within the cavity 16.

The following table gives the characteristics for a typical nuclear chimney 12 formed by a 50 kiloton nuclear explosion set off at a level of 3000 feet below the surface:

TABLE I Chimney Characteristics for n SO-Kiloton Nuclear Explosion at 3,000 Feet ltadius-133 feet.

Height-655 feet.

Bulk porosity (fractional).267.

Yofid 'olume (volume of original ca\'it v)9. i5 l0 eet.

Total volume (of final cavity) 36.90 10 feet.

Oil Shale rubble volu1ne27.05 10 fee-L Oil Shale rubble mus -4.86% 10" tons.

Volume shale oil (kerogun) contained in rubble (assumed 25 gal./ton)-1.1U 10 bbls.

After the chimney 12 has been formed a first well or input shaft 20 is drilled through the roof 16a of the cavity 16, and a conduit such as the casing 22 of this shaft is extended downwardly to below the level of the arching dome or roof 16a. A second well or input shaft 24 extending from the surface through the roof 16a of the cavity has a conduit or casing 26, the end of which preferably terminates within the arched or domed area of the roof 16a. A third well or output shaft 28 extends between the surface and the bottom of the cavity 16 and has a conduit or casing 30 which extends into the bottom of the cavity 16. An instrument well shaft 32 has a casing 34 which extends downwardly through the roof 16a of the cavity 16, preferably to approximately the level of the lower end of the liner 22 of the shaft 20. a

A hot retorting gas is pumped into the cavity 16 below the level of the arched roof 1611 through the casing 22 of the first input shaft 20, and this gas is drawn downwardly through the shale oil rubble 14a and then upwardly to the surface through the conduit or liner of the output well 28. The retorting gas may, for example, comprise a major amount (i.e., over 50 percent) of nitrogen and a minor amount (i.e.. less than 50 percent) of other gases, such as carbon dioxide and water vapor.

A part of the gas exiting from the casing 30 is vented to the atmosphere, and the remainder of the gas at a preselected pressure is compressed in a compressor and then pumped into a combustion chamber 36 where air from a compressor 37 and auxiliary fuel from an injection system 38 may be added if and as needed. The heated and pressurized retorting gas is then recycled through the casing 22 to the chimney 12. A pump 40 in the lower end of the casing 30 pumps the shale oil from the-bottom of the chimney 12 to the surface. Temperature measurements within the chimney 12 are made through the instrument well 32 by means of a thermocouple 41 or other suitable temperature measuring means.

A typical set of operating parameters for the retorting of oil shale rubble within a nuclear chimney of the type set forth in Table I may be as follows:

TABLE II for Combustion Proce s Process Parameters Gas recycle it is apparent that the recovery of shale oil by the in situ retorting of shale oil rubble within a nuclear chimney is dependent upon the llow of large volumes of gases through the shale oil rubble within the nuclear chimney. The initial permeability of the oil shale rubble may decrease as the oil shale is heated due to compaction and plasticity. It is, however, desirable to maintain a high level of permeability for the rubble, and one means for accomplishing this is to minimize the amount of the compressive load upon the rubble, thereby preventing the rubble from becoming excessively crushed and compacted.

It has been found that the compressive strength of shale oil decreases with the richness of the shale oil. lt has also been found that the compressive strength of the shale decreases at least during the initial stages of retorting, with the strength of the richer shales exhibiting a greater dependence upon temperature. In other words, the richer shales not only have lower compressive strength and are more easily compacted, but they are more susceptible to being adversely affected by the retorting temperature.

Compression and compaction of the shale may result from an excessive and premature collapse of the roof 16a of the cavity 16 causing brittle failure of the oil shale rubble 14a under the overburden rubble load. Such premature roof collapse may occur if the roof of the nuclear chimney is subjected to the retorting temperatures and the arches of shale supporting the overburden above the nuclear chimney become weakened. This collapse would transfer a larger portion of the overburden weight to the oil shale rubble and perhaps crush it, resulting in a compaction of the shale and preventing adequate gas flow to the bottom of the nuclear chimney.

Not only will compaction of the oil shale in a nuclear chimney result in possible reduction in the amount of oil recovered, but it could also result in a weakening of the overburden above the nuclear chimney, particularly if a minimum overburden existed initially. Compaction could also serve to damage injection, production, and instrument wells.

ln order to maintain the strength of the arched roof 16a of the cavity 16 during retorting, particularly in the richer shales more affected by temperature. it is very desirable that this roof area be kept "cool" and protected from contact with the hot retorting gases within the cavity 16. For this purpose, a gas. whose temperature and molecular'wcight are both significantly lower than the gas contained in the nuclear chimney. is injected into the top 16:: of the cavity 16 of the nuclear chimney. This light and relatively cool blanket or protection gas must remain at the top of the chimney to protect the roof of the cavity 16a from the natural convection currents which would otherwise efficiently transport heat to the roof.

Since the blanket gas is to remain at the top ofthe chimney. the weight per unit volume (density) of the blanket gas within the cavity 16 must always be less than the weight per unit volume (density) of the retorting or recycling gas within the cavity. The densities of the gases, of course. will be dependent not only upon the molecular weights of the gases but also upon the temperatures of the gases. The densities of the gases will vary directly with the respective molecular weights and inver sely with the respective temperatures of the gases. ln accordance with the present invention the blanket gas, which protects the roof of the nuclear chimney from contact with the hot (e.g., 700 l0O0F.-) retorting gas, is maintained at a cooler temperature (e.g., 250500F., depending on the grade of shale). This relatively cool blanket gas will, therefore, be more dense than would be the case if it were at the same high temperature as the retorting gas; nevertheless, within the operating temperature ranges of the chimney, the blanket gas must not be as dense as the retorting gas. The relationship between the molecular weights and the temperatures may be stated as follows:

If the gas composition and other retorting parameters are those ofTable ll, the molecular weight of the recycling gas will be 28.8, the temperature of the recycling gas will be 800F. (1260R.), and the temperature of the blanket gas will be 300F. (760R.). The molecular weight of the blanket gas should, therefore, be less than or less than 17.4.

Not only should the blanket be one which has a molecular weight of less than 17.4, but it should be a gas which is stable in the operating range of the recycling process and under the recycling conditions, and it should be relatively inert with respect to the contents of the nuclear chimney. In other words, the gas should not have properties which would materially detract from' its tendency to remain in place as a relatively cool protective medium in the top of the chimney. One gas which would satisfy these conditions is methane (Cl-l having a molecular weight of 16 or a gas consisting essentially of methane with impurities such as may bring the average molecular weight to between 16 and 17. Another satisfactory gas would be hydrogen (H having a molecular weight of 2. It may be desirable to use a hydrogen-rich (i.e., greater than 50 percent H gas mixture which would also have a very low average molecular weight and would be effective in the prevention of natural convection currents to the supporting roof 16a of the cavity. A hydrogen-rich gas could be conveniently prepared from shale gas by stripping off the light hydrocarbons, water and carbon dioxide.

in retorting a nuclear chimney having the characteristics set forth in Table l, the amount of blanket gas necessary to protect the roof 16a of the cavity 16 may be calculated based upon the assumption that the roof is hemispherical. In order to completely protect the roof from the hot 800F.) gas currents, it is, of course, desirable to completely fill the hemispherical roof area with the blanket gas. The volume of a hemisphere (5; 4/3 1r r which has a radius of 133 feet is calculated to be 5.03 X l0 ft.

The blanket gas is injected into the top of the cavity 16 through the conduit 26 under the same pressure as the retorting gas (e.g., 100 p.s.i.g.) but at a much lower temperature (e.g., 300F.). Assuming that the original temperature of the gas is 60F. (520 Rankine) and the original pressure of the gas is atmospheric (l4.7 p.s.i.g.), the volume of this blanket gas required to fill the cavity at the higher temperature and pressure may be calculated from the gas laws using the expres- SlOl'l P .T V1- V X l X T2 nwhere V., P. and T. are the original volume (standard cubic feet), pressure and temperature (absolute) respectively of the blanket gas, and V.,, P and T are the Volume, Pressure and Temperature (absolute) respectively of the blanket gas within the cavity 16. The volume of the gas required to fill the hemisphere at 100 p.s.i.g. and 300F. (760 Rankine) is thus calculated to be 23.4 X SCF.

During the retorting of the nuclear chimney, it is contemplated that additional quantities of the blanket gas will have to be injected intothe top of the nuclear chimney from time to time to replace the blanket gas which may mix and be drawn downwardly with the retorting gas as well as that which may escape through fissures in the shale roof 16a. The blanket gas must be maintained against the roof 16a in sufficient quantity to prevent convective currents from warming the top of the chimney. The volume of this additional blanket gas to be used may best be determined by measurement of the temperature of the top portion of the chimney. It is contemplated that perhaps .5 percent of the total volume of the blanket gas would have to be added each day in order to make up for possible losses during the operation of the retort and in order to maintain the temperature of the gas in the area of the roof 16a within the range of about 300F. to about 500F. If the amount of blanket gas initially required to fill the hemispherical top 16a of the cavity 16 is 23.4 X 10 6 SCF, as previously calculated, .5 percent of this or 1.15 X 10 SCF of the blanket gas would have to be added each day.

From the foregoing it will be apparent that the roof 16a of the cavity 16 may be insulated and protected from the excessive heat of the retorting gases which could cause premature weakening and collapse of the roof and concomitant compaction of and reduced oil recovery from the shale rubble being retorted.

Although the compressive strength of the oil shale decreases during the initial stages of retorting, in subsequent stages the compressive strength increases. After it has been retorted, the shale has a relatively high compressive strength and is not as susceptible to being crushed and compacted. It may therefore be desirable to eventually permit collapse of the roof of the nuclear chimney, thereby producing additional oil shale rubble for retorting. One possible way of accomplishing this would be to permit the blanket gas to be depleted by not replenishing it. If desired the blanket gas could be evacuated from the roof toward the end of the time of retorting of the original chimney contents. Thus the roof would be heated by the hot retorting gas, and it would tend to collapse.

Although the process has been described in connection with the shale gas recycle process for retorting nuclear chimneys, it will be appreciated that it may be employed to most other retorting processes such as, for example, the natural convection process and the direct drive combustion process.

Since in general the problem of roof collapse during retorting increases with the diameter of the chimney, the present invention permits the retorting of larger chimneys than was heretofore possible.

It is to be understood that the present disclosure has been made only by way of example and that many additional modifications, changes. and various details may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed.

1 claim:

1. A process for retorting oil shale rubble in situ within a subterranean cavity, which process comprises cycling a retorting gas under high temperature through said rubble and injecting into the top of said cavity a second gas having a cooler temperature and a significantly lower molecular weight than that of the average molecular weight of the retorting gas, whereby said second gas will remain in the top of said cavity to protect the roof of the cavity from being excessively heated by the retorting gas.

2. The process of claim 1 wherein the retorting gas and the second gas are under pressure within said cavity.

3. The process of claim 2 wherein the retorting gas is introduced into the cavity at a level below the top of the cavity and it is withdrawn adjacent the bottom of the cavity; and the second gas is introduced into the top of the cavity at a point intermediate the top thereof and the level of introduction of said retorting gas.

4. The process of claim 3 wherein the cavity has an arched roof area, with the retorting gas being introduced in an upper area of the cavity just below the level of the roof area and the second gas being introduced into the roof area in sufficient quantity to substantially fill the roof area.

5. The process of claim 4 including the step of periodically injecting additional amounts of the second gas into the top of the cavity to maintain the temperature within the top of the cavity at the desired level and to maintain a sufficient quantity of the second gas within the top of the cavity to protect the top from the heating effects of the retorting gas.

6. The process of claim 1 wherein the second gas is substantially inert with respect to the oil shale rubble under the operating conditions within the cavity.

7. The process of claim 1 wherein the retorting gas comprises a major amount of nitrogen gas and minor amounts of other gases.

8. The process of claim 1 wherein the second gas is a hydrogen-rich gas.

9. The process of claim 1 wherein the second gas consists essentially of methane gas.

10. The process of claim 1 wherein the retorting gas is introduced at a temperature of between 700--l000F. and the blanket gas is introduced at a temperature of below 500F.

11. The process of claim 9 wherein the retorting gas consists essentially of a major amount of nitrogen and minor amounts of carbon dioxide and water vapor.

12. The process of claim 1 wherein the molecular weight of the insulating gas is no greater than two-thirds of the molecular weight of the retorting gas.

Patent No.

Invcntor(s) UNITED STA ES PATENT OFFIQE (5/89) WT q W ClLRTLfICATIL OF COHRECTXON 3,533, 9 Dated October 13, 1970 Harry W. Parker It is certified that error appears in the above-identificd patent and that said Letters Patent are hereby corrected as shown below:

3 Column 5, line 36,

Column 1, delete lines 31 through 3 and substitute therefor:

--In retorting the nuclear chimney, a retorting gas under high temperature and pressure is forced through the rubble, the gas being introduced adjacentthe top of the chimney and being-withdravm adjacent the bottom of the chimney. The permeable Column l, line 33, delete 0003" and insert .the folloviing:

Blanket Recycle Molecular Weightgk t Molecular weight x Column r, line 7M, delete and insert so that it reads as follows Column 5, line 12, "nwhere" should be --where-- Column 5, line 30, .5 should be -5-- 6 "l0 6" should. be ---l0 l min mi 3mm FEE:

. Museum; as.

stone:- of Patents Aueat:

EdwardMJletcherJr.

Attesting Office-r o

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