Subterranean in situ oil shale retort and method for making and operating same

An in situ oil shale retort is formed in a subterranean formation containing oil shale. The retort contains a fragmented permeable mass of particles containing oil shale. An open base of operation is excavated in the formation at an elevation above the fragmented mass to be formed, and an access drift is excavated to provide access to the bottom of the retort site. Formation is explosively expanded to form the fragmented mass between the access drift and an elevation spaced below the bottom of the base of operation, leaving a horizontal sill pillar of unfragmented formation between the top of the fragmented mass and the bottom of the base of operation. The sill pillar provides a safe base of operation above the fragmented mass after it is formed. The fragmented mass is formed by, among other steps, drilling blasting holes from the base of operation down through the sill pillar and then detonating explosive in such holes to form the fragmented mass of particles in the retort below the sill pillar. During retorting, gas is introduced into the fragmented mass through such blasting holes for establishing a combustion zone in the fragmented mass and for advancing the combustion zone through the fragmented mass. The blasting holes have separate valves located in the base of operation for use in controlling gas flow through selected regions of the fragmented mass.

BACKGROUND OF THE INVENTION 
This application is related to U.S. Patent Application Ser. No. 603,704 
entitled "In Situ Recovery of Shale Oil", filed Aug. 11, 1975 by Gordon B. 
French, now U.S. Pat. No. 4,043,595, to U.S. Pat. Application Ser. No. 
603,705 entitled "Forming Shale Oil Recovery Retort Into Slot-Shaped 
Columnar Void", filed Aug. 11, 1975, by Richard D. Ridley, now U.S. Pat. 
No. 4,043,596, and to U.S. Patent Application Ser. No. 790,350 entitled 
"In Situ Oil Shale Retort With a Horizontal Sill Pillar" filed Apr. 25, 
1977, by Ned M. Hutchins. All three of these applications are assigned to 
the assignee of the present application and are incorporated herein by 
this reference. 
This invention relates to recovery of liquid and gaseous products from oil 
shale. The term "oil shale" as used in the industry is in fact a misnomer; 
it is neither shale, nor does it contain oil. It is a sedimentary 
formation comprising marlstone deposit with layers containing an organic 
polymer called "kerogen" which upon heating decomposes to produce 
hydrocarbon liquid and gaseous products. The formation containing kerogen 
is called "oil shale" herein, and the hydrocarbon liquid product is called 
"shale oil". 
One technique for recovering shale oil includes forming an in situ oil 
shale retort in a subterranean formation containing oil shale. At least a 
portion of the formation within the boundaries of the in situ oil shale 
retort is explosively expanded to form a fragmented permeable mass of 
particles containing oil shale. The fragmented mass is ignited near the 
top of the retort to establish a combustion zone. An oxygen-containing gas 
is introduced into the top of the retort to sustain the combustion zone 
and cause it to move downwardly through the fragmented permeable mass of 
particles in the retort. As burning proceeds, the heat of combustion is 
transferred to the fragmented mass of particles below the combustion zone 
to release shale oil and gaseous products therefrom in a retorting and 
vaporization zone. Vaporized constituuents of shale oil, water vapor and 
the like may condense on cooler oil shale in the retort below the 
retorting zone. The retorting zone moves from top to bottom of the retort 
ahead of the combustion zone, and the resulting shale oil and gaseous 
products pass to the bottom of the retort for collection and removal. 
Recovery of liquid and gaseous products from oil shale deposits is 
described in greater detail in U.S. Pat. No. 3,661,423, to Donald E. 
Garrett, assigned to the assignee of this application. 
In preparing for the retorting process the formation containing oil shale 
should be fragmented rather than simply fractured to create good and 
uniform permeability so that undue pressures are not required to pass the 
gas through the retort, and so that valuable deposits of oil shale are not 
bypassed owing to non-uniform permeability. The aforementioned patent 
applications disclose techniques for fragmenting a substantial volume of 
formation in a retort site to form a fragmented mass of particles in an in 
situ oil shale retort. The in situ retort is formed by excavating a void 
in the retort site, drilling blasting holes into the remaining portion of 
the formation in the retort site, loading explosive into the blasting 
holes, and detonating the explosive to expand the formation toward the 
void. 
To promote maximum uniformity of particle size and permeability of the 
fragmented mass, and to minimize the quantity of explosives, the blasting 
holes should be reasonably accurately located with respect to each other, 
and with respect to the void toward which expansion occurs during the 
explosion. Oil shale formations in the western United States are often 
between 50 to about 500 feet thick or even more, and are covered by a 
non-productive overburden, which may be thousands of feet deep, thus often 
making it difficult to drill from the surface and accurately locate 
blasting holes in the oil shale formation. 
In one embodiment disclosed in application Ser. No. 790,350, entitled "In 
Situ Oil Shale Retort With a Horizontal Sill Pillar", an open base of 
operation is excavated in the new formation at a working level near the 
top of an in situ retort to be formed, which may be a thousand feet, or 
more, below the ground surface. A substantially horizontal access drift is 
excavated at a production level below the base of operation to provide 
access to a lower portion of the retort site. A void is excavated above 
the access drift so the void opens into the access drift and terminates 
below the base of operation at the top of the fragmented mass being 
formed. This leaves a substantially horizontal portion of unfragmented 
formation between the top of the void and the bottom of the base of 
operation. Blasting holes for explosive for expanding formation are 
drilled from the base of operation into a portion of the formation within 
the boundaries of the retort being formed. Inasmuch as the working level 
is much closer to the top of the retort being formed than the distance 
from the retort through the overburden to the ground surface, this permits 
more accurate and rapid drilling of blasting holes from the base of 
operation than from the ground surface. This, in turn, facilitates 
explosive expansion to form the fragmented mass of oil shale particles in 
the retort. Explosive is loaded into such blasting holes and detonated for 
explosively expanding formation towards such a void for forming a 
fragmented permeable mass of particles containing oil shale in the retort. 
In an embodiment disclosed in Application Ser. No. 790,350 entitled "In 
Situ Oil Shale Retort With a Horizontal Sill Pillar", a horizontal sill 
pillar of unfragmented formation remains between the top of the fragmented 
mass in the retort and the bottom of the base of operation. The sill 
pillar has a number of bore holes through it after formation of the 
fragmented mass. Such bore holes include the upper ends of blasting holes 
drilled from the base of operation. Such bore holes can be used for access 
from the base of operation for establishing and sustaining a combustion 
zone in the fragmented mass below the sill pillar. 
U.S. Pat. No. 3,661,423 to Garrett discloses an in situ oil shale retort in 
which communication is established with the top of an expanded oil shale 
deposit by drilling a plurality of communicating conduits to the top of 
the expanded shale. A source of oxygen from a compressor is then provided 
to the conduits. To establish a flow of oil from the shale, the upper 
level of the expanded shale deposit is ignited using an initial supply of 
fuel and air to the top of the shale deposit through the conduits. A 
source of oxygen is supplied at a pressure sufficient to overcome the 
inherent pressure drop through the conduits and the shale deposit to 
establish a positive downward flow of hot gases. 
U.S. Patent Application Ser. No. 716,583, entitled "Method For In Situ 
Recovery of Liquid and Gaseous Products From Oil Shale Deposits", filed on 
Aug. 23, 1976, by Gordon B. French, and assigned to the assignee of this 
application, discloses an in situ oil shale retort in which a plurality of 
air supply holes, or passages, are drilled from a tunnel to distributed 
locations at the top retort. One of the air supply holes can extend 
directly down from the tunnel to the center of the top portion of the 
fragmented mass in the retort. The other air inlet holes slope from the 
overlying tunnel to the top portion of the retort near the corners. The 
air supply holes have diameters of 4 to 7 feet for minimizing pressure 
losses. The effective sizes of the holes overlying paths through the 
retort can be selectively changed as by adjusting louvers within such 
holes. 
U.S. Pat. No. 2,481,051 to Uren discloses a plurality of vertical pipes 
installed through broken shale extending from a lower level to an upper 
level in an in situ oil shale retort. At the upper end of each pipe a 
lateral connection is provided for the introduction of compressed air. 
Compressed air is introduced through an inlet line and then through the 
lateral connections to the upper end of each of the pipes. 
It can be desirable to use the base of operation as a working level from 
which to control formation of a fragmented mass in an in situ oil shale 
retort and from which to subsequently regulate the flow of gas through the 
fragmented mass. By providing separate gas flow control valves connected 
to bore holes extending through a sill pillar, the relative amounts of gas 
supplied to selected regions of the fragmented mass can be controlled. 
Such control can inhibit non-uniform advancement of a combustion zone 
through the fragmented mass. By providing a relatively uniform advancement 
of the combustion zone, more effective retorting of the fragmented mass 
can be provided, which can result in a greater yield from the retort or 
more economical operation. 
SUMMARY OF THE INVENTION 
According to one practice of the invention, an in situ oil shale retort 
comprising a fragmented permeable mass of formation particles containing 
oil shale is formed in a subterranean formation containing oil shale. An 
open base of operation is excavated in an upper level of the formation 
above the fragmented mass, leaving a horizontal sill pillar of 
unfragmented formation between the bottom of the base of operation and the 
top of the fragmented mass. A plurality of bore holes are formed through 
the sill pillar so that they are distributed across the horizontal 
cross-section of the fragmented mass and open into the base of operation. 
A top portion of the fragmented mass below the sill pillar is ignited to 
establish a combustion zone for recovering liquid and gaseous products 
from the fragmented mass. An oxygen-containing gas is introduced to the 
fragmented mass through the bore holes for advancing the combustion zone 
through the fragmented mass. Gas flow through at least a portion of the 
bore holes is separately controlled from a location within the base of 
operation to control advancement of the combustion zone through the 
fragmented mass. 
A separate casing can be sealed in each of a plurality of the bore holes. 
The upper end of each casing opens into the base of operation, and a 
control valve and a check valve are provided at the upper end of each 
casing for control from within the base of operation. 
For operating the retort, gas pressure in a lower region of the fragmented 
mass is made lower than in the base of operation to draw air from the base 
of operation down through the bore holes and through the permeable mass, 
to advance the combustion zone downwardly through the fragmented mass.

DESCRIPTION 
General Description of Retort Forming 
An in situ oil shale retort has a base of operation formed on a working 
level in a subterranean formation. This working level is at an upper 
elevation near the top of a retort being formed. A fragmented permeable 
mass of particles containing oil shale is formed below the base of 
operation by exlosive expansion of formation toward an excavated void. The 
bottom of the base of operation is separated from the top boundary of the 
fragmented mass by a horizontal sill pillar of unfragmented formation. The 
horizontal sill pillar is sufficiently thick that it withstands stresses 
imposed by explosive expansion, as well as geologic stresses, to provide a 
safe base of operation after formation of the fragmented mass. This 
permits men and equipment to enter the base of operation over the top of 
the fragmented mass after explosive expansion. The base of operation on 
the working level can have a horizontal extent that permits effective 
access over substantially the entire horizontal cross-section of the 
fragmented mass, which is of great assistance in forming and operating an 
in situ retort. 
After explosive expansion the base of operation is convenient as a location 
from which to ignite an upper portion of the fragmented mass and to 
control gas flow through the fragmented mass so as to establish a 
combustion zone in the fragmented mass. 
In one method of forming an in situ oil shale retort in a formation 
containing oil shale, a portion of the formation is excavated to form a 
base of operation on an upper working level. A drift or similar means of 
access is excavated through formation at a lower production level to a 
location underlying the base of operation at or below the bottom of the in 
situ retort. 
In preparing such a retort, at least one void is excavated from within the 
boundaries of the fragmented mass being formed, the void being connected 
to the access drift on the production level underlying the base of 
operation. This leaves another portion of the formation within the 
boundaries of the retort being formed which is to be fragmented by 
explosive expansion toward such a void. Such a void is excavated only to 
an elevation above the access drift that leaves a horizontal sill pillar 
of intact formation between the top of the void and the bottom of the base 
of operation. The surface of the formation defining the void provides at 
least one free face which extends through the formation, and the remaining 
portion of the formation within the boundaries of the retort being formed 
is explosively expanded toward such a free face. 
In a preferred embodiment, the horizontal extent of the base of operation 
over the fragmented mass in the in situ retort is sufficient to provide 
effective access to substantially the entire horizontal cross-section of 
the fragmented mass. This does not require that there be an open 
evcavation over the entire horizontal extent of the fragmented mass. 
Roof-supporting pillars can be left on the working level in a portion of 
the area directly above the fragmented mass. The size and arrangement of 
such working level pillars leaves an open base of operation having a 
sufficient horizontal extent to provide access to substantially the entire 
horizontal cross-section of the retort site. Such a base of operation 
facilitates excavation operations for forming a void for drilling and 
explosive loading for explosive expansion of formation toward such a void, 
and introduction of oxygen containing gas into the top of the fragmented 
mass below the horizontal sill pillar. 
In one embodiment a plurality of vertically extending bore holes are 
drilled through the sill pillar into formation remaining below the sill 
pillar. The bore holes are sometimes referred to herein as "blasting 
holes" inasmuch as they are used to hold explosive for blasting the 
formation to form the fragmented permeable mass of particles containing 
oil shale. Such blasting holes can be ten inches or more in diameter. 
Smaller bore holes can also be present through the sill pillar. Explosive 
is loaded into such blasting holes from the base of operation up to a 
level about the same as the bottom of the horizontal sill pillar, which is 
to remain unfragmented. Such explosive is detonated for explosively 
expanding subterranean formation toward such a void below the sill pillar 
and forming a fragmented mass of formation particles in the retort while 
leaving unfragmented formation forming the sill pillar. 
The base of operation can be used as a location from which to initiate and 
control advancement of the combustion zone through the retort. 
DETAILED DESCRIPTION 
Referring to FIGS. 1 and 2, a fragmented permeable mass 10 of formation 
particles containing oil shale is in an in situ oil shale retort 12 in a 
subterranean formation containing oil shale. The fragmented permeable mass 
has vertical side boundaries 14 substantially perpendicular to each other 
to give the retort a rectangular horizontal cross-section. The lower 
boundary 16 of the fragmented permeable mass slopes downwardly and 
inwardly (see FIG. 2) at an angle of about 45.degree. and opens into the 
top of an elongated, substantially horizontal access drift 18 at the 
bottom of the retort 12. The access drift 18 has a gradual slope 
downwardly from the center of the bottom of the retort toward a sump 52 
for recovering liquid products of retorting at the production level. The 
fragmented permeable mass also fills the portion of the access drift 
beneath the retort. 
A horizontal sill pillar 22 of unfragmented formation forms the upper 
boundary 23 of the fragmented permeable mass in the retort. The top of the 
sill pillar 22 forms the floor 24 of an open base of operation 25 spaced 
above the fragmented mass by a distance equal to the thickness of the sill 
pillar. In this embodiment the base of operation 25 is an excavation 12 to 
14 feet high at a working level above the retort. It extends over 
substantially the entire horizontal cross-section of the fragmented mass 
and opens at the left (as viewed in FIG. 1) to other excavations at the 
working level used for exploiting the oil shale deposit. Such underground 
workings open to a vertical shaft or horizontal adit (not shown). 
A plurality of vertical blasting holes 30 extend through the sill pillar. 
The blasting holes remain in the sill pillar after the blasting which 
formed the fragmented mass in the retort. The blasting holes are 
approximately uniformly distributed over the area of the sill pillar 22. 
In a working embodiment, the horizontal cross-section of the fragmented 
permeable mass is square, each side being about 120 feet long; and ten 
inch diameter blasting holes are located at intervals of about 25 feet and 
about 30 feet in a rectangular grid over essentially the entire horizontal 
cross section of the fragmented mass. Formaion of such an in situ oil 
shale retort is described in detail in U.S. Patent Application Ser. No. 
790,350, filed Apr. 25, 1977, and entitled "In Situ Oil Shale Retort With 
a Horizontal Sill Pillar". 
During operation of the retort, gas used for retorting of the oil shale is 
passed downwardly through the fragmented mass. An oxygen containing gas is 
introduced into an upper portion of the fragmented permeable mass from the 
base of operation for sustaining a combustion zone in the fragmented mass 
and advancing the combustion zone through the fragmented mass. Heat from 
the combustion zone, carried by flowing gas advances a retorting zone 
through the fragmented mass on the advancing side of the combustion zone. 
Liquid and gaseous products are retorted from oil shale in the retorting 
zone. The production level drift 18 provides a means for collecting and 
recovering liquid products and withdrawing off gas containing gaseous 
products from retorting oil shale in the retort 10. A variety of retorting 
techniques can be used, some of which are set forth in the prior art, so 
no further description of them is set forth herein. 
FIG. 3 is a horizontal cross-section at the working level viewing the open 
base of operation 25 from above. The base of operation 25 is generally 
E-shaped and has a central drift 70 and a separate side drift 72 on each 
side of the central drift. The two side drifts are similar to each other 
in size and shape. Elongated roof-supporting pillars 74 of unfragmented 
formation separate the side drifts 72 from the central drift 70. Short 
crosscuts 76 interconnect the side drifts 72 and the central drift 70 to 
form a generally E-shaped excavation. Other arrays of drifts and 
roof-suppoting pillars also can be used. A branch drift 78 provides access 
to the base of operation 25 from underground workings (not shown) at the 
level of the base of operation. 
After the base of operation is formed, a void in the shape of a vertically 
extending slot (not shown) is formed between the production level access 
drift 18 and an elevation spaced below the bottom of the base of 
operation. Blasting holes or shot holes 90 are drilled downwardly from the 
central drift 70 of the base of operation 25. In a working embodiment 
these blasting holes are about 35/8 inches in diameter. Such blasting 
holes are loaded with explosive which is detonated to ultimately form the 
slot shaped void. Particles of formation from forming the slot are 
excavated from the production level access drift 18. In forming the slot, 
the blasting holes 90 are loaded only to an elevation spaced about 40 feet 
below the bottom of the base of operation. In a working embodiment the 
thickness of the horizontal sill pillar 22 left unfragmented between the 
top of the slot and the bottom of the base of operation is about 40 feet. 
The tops of the holes 90 are stemmed to inhibit breakage into the sill 
pillar 22. The side walls of the void formed by the slot provide 
vertically extending free faces within the side boundaries of the 
fragmented permeable mass of particles to be formed in the in situ oil 
shale retort site. 
After the slot is excavated, a remaining portion of the formation within 
the retort site is explosively expanded toward the void formed by such a 
slot. A plurality of blasting holes are drilled downwardly in the 
formation within the retort site from the side drifts 72 of the base of 
operation 25 on the working level. In the illustrated embodiment, five 
such blasting holes, each about 10 inches in diameter, are in each of two 
rows parallel to the large side walls of the slot 78. The pattern of ten 
blasting holes on each side of the slot is similar to the pattern on the 
other side. The first or inner row of blasting holes 91 is along the 
roof-supporting pillar 74 on the opposite side thereof from the central 
drift 70 of the base of operation 25. An outer row of blasting holes 92 is 
drilled downwardly along a side boundary of the fragmented permeable mass 
of particles to be formed in the retort site. 
Explosive is then loaded into each blasting hole 91 and 92 and is detonated 
in all of the blasting holes in a single round to form the fragmented 
permeable mass 10 shown in FIGS. 1 and 2. The blasting holes 91 and 92 are 
stemmed with inert material over the explosive to minimize overbreak of 
formation above the level of the explosive. Thus, in the illustrated 
embodiment, the blasting holes are stemmed from about 40 feet below the 
floor of the base of operation 25. Detonation of the explosive in the 
blasting holes for expanding formation toward the slot thereby leaves 
unfragmented formation as a 40-foot thick horizontal sill pillar 2 
between the fragmented permeable mass so formed and the base of operation. 
The sill pillar has a horizontal extent sufficient to provide effective 
access to essentially the entire horizontal cross-section of the 
fragmented mass formed in the retort 10. 
According to the present invention, the base of operation 25 is used as a 
location from which to control gas flow through the fragmented mass. 
Separate vertical steel casings 32 are disposed in selected blasting 
holes. A conventional external packer 34 at the lower end of each casing 
seals against the casing exterior and the adjacent portion of the 
horizontal sill pillar 22. The annular space between the casing and the 
sill pillar above the packer is filled with concrete or grout 36 commonly 
referred to as cement which anchors the casing securely in the sill 
pillar. In some situations, the casing can be adequately secured by using 
only the packer, or the cement can be replaced by drilling mud or the like 
to facilitate removal of the casing after the fragmented oil shale in the 
retort is completely treated. 
The lower end of the casing is above a level in the fragmented mass where a 
combustion zone is established. Gas in the combustion zone can include 
carbon monoxide, carbon dioxide, hydrogen sulfide and water vapor. Such 
gases can be corrosive to steel pipe, particularly at the operating 
temperatures involved. By limiting the location of the lower ends of the 
casings to a level above the combustion zone, corrosion of the casings is 
inhibited. 
Although one embodiment of in situ oil shale retort has been described and 
illustrated herein, many modifications and variations will be apparent to 
one skilled in the art. Thus, for example, the blower 61 for withdrawing 
gas from the fragmented mass has been illustrated as if located in an 
access drift, it will be apparent that such a blower can be located above 
ground and connected to underground workings for withdrawing gas from a 
plurality of in situ retorts. In such an embodiment the valves connected 
to casings in the base of operation can cooperate with or replace gas 
control systems at the production level for individual retorts. Because of 
variations such as this, it will be apparent that this invention can be 
practiced other than as specifically described. Preferably, the lower end 
of each casing is not below the bottom of the sill pillar to avoid contact 
with combustion zone gases. In a working embodiment each casing extended 
through about the top 20 feet of a bore hole through a sill pillar with a 
total thickness of about 40 feet. Such a length assures adequate support 
for the casing and sealing of the annulus between the casing and the bore 
hole. 
A casing collar 40 secures an upper section 38 of the casing to the portion 
of the casing 32 cemented in the sill pillar. A check valve 42 and a 
throttle valve 44 (shown schematically in FIG. 3) are mounted in the upper 
section of the casing. An inlet section 46 connected above the throttle 
valve admits air from the base of operation to the fragmented mass through 
the throttle valve and the check valve. 
An additive line 48 is sealed through the side of the casing below the 
check valve and throttle valve so that an additive or diluent such as 
steam, retorting off gas, auxiliary fuel, additional oxygen, particulate 
combustible matter such as coal, or the like, can be admitted through the 
casing into the top of the fragmented mass. The admission of additive or 
diluent is controlled by a valve 50 in the additive line 48. The additive 
or diluent can be used to adjust the oxygen concentration of gas flowing 
into the fragmented mass through the casings. 
Returning to FIG. 1, a sump 52 in the region of the access drift 18 beyond 
the fragmented mass collects shale oil 53 and water 54 produced during the 
operation of the retort. A water withdrawal line 56 extends from near the 
bottom of the sump out through a sealed opening (not shown) in a vertical 
barrier or bulkhead 57 sealed across the access drift. The water 
withdrawal line is connected to a water pump 60. An oil withdrawal line 58 
extends from an intermediate level in the sump out through a sealed 
opening (not shown) in the barrier and is connected to an oil pump 59. The 
oil and water pumps can be operated manually or by automatic controls (not 
shown), to remove shale oil and water separately from the sump. 
The inlet of a blower 61 is connected by a conduit 62 to an opening 63 
through the barrier 57 for withdrawing off gas from the retort. The outlet 
of the blower delivers off gas from the retort through a conduit 64 to a 
recovery or disposal system (not shown). Thus, the access drift 18 
provides means for collecting and recovering liquid and gaseous products 
from the in situ oil shale retort. A variety of collection and recovery 
techniques can be used, some of which are set forth in the prior art. 
The void formed in the retort site before explosive expansion is 
proportioned relative to the formation expanded toward the void, so that 
after explosive expansion is completed, the retort is filled with 
fragmented particles containing oil shale which are packed against the 
lower surface of the horizontal sill pillar 22. This provides support for 
the bottom of the sill pillar during high temperature retorting 
operations, and minimizes any tendency of formation to slough from the 
bottom of the sill pillar. 
The blasting holes remaining through the sill pillar 22 after formation of 
the fragmented mass can be cleaned out, reamed, and/or redrilled, if 
necessary, after the fragmented mass 10 has been formed. For example, the 
blasting holes to be used for the casings 30 can be reamed out to about 
twelve inches in diameter to accomodate larger casings and/or remove a 
layer from the hole wall which can have some damage from blasting. Other 
blasting holes not to be used for gas flow to the retort are sealed, such 
as by filling with concrete. 
During retorting operations, the fragmented mass 10 is ignited through the 
blasting holes to establish a combustion zone across the top of the 
fragmented mass. Gas flow through each of the casings can be monitored 
during retorting and separately controlled from the base of operation to 
control advancement of the combustion zone through the fragmented mass. 
The combustion zone is advanced downwardly through the fragmented mass by 
introducing an oxygen containing gas to the fragmented mass through the 
casings. Hot gases flowing downwardly from the combustion zone decompose 
kerogen in a retorting zone in the fragmented mass of oil shale particles 
to produce liquid and gaseous products. The liquid products percolate 
through the fragmented mass on the advancing side of the retorting zone 
and accumulate in the sump 52 in the access drift 18, as described above. 
The oxygen containing gas introduced through the casings can be fresh air, 
or air mixed with other gases, liquids and/or particulate matter. Gas flow 
through the fragmented mass is generated by the blower 61 which produces a 
lower gas pressure in the access drift 18 then in the base of operation 
25. This draws air from the base of operation 25 into the casings and into 
the fragmented mass. Gas flows down through the fragmented mass to the 
lower access level drift 18. 
The throttle valves 44 on the separate casings provide means for separately 
controlling the flow of oxygen containing gas from the base of operation 
into selected regions of the fragmented mass to control advancement of the 
combustion zone through the fragmented mass. For example, prior to 
ignition of the fragmented mass, gas flow rate measurements can be 
conducted to determine the permeability distribution of the particles in 
the fragmented mass. Such measurements are achieved by generating gas flow 
across the horizontal cross-sectional extent of the fragmented mass from 
the top of the fragmented mass to a gas withdrawal point at the bottom of 
the fragmented mass. The blower 61 in the access drift 18 draws air from 
the base of operation 25 down through the fragmented mass and out the 
access drift 18. The rate of flow air through each casing is then 
measured, preferably by a conventional vane anemometer or hot wire 
anemometer. To simplify gas flow rate measurements, the throttle valves 44 
in the casings are preferably set at the same valve opening so that the 
cross-sectional area provided through each valve is essentially identical. 
The anemometer readings are then taken to determine the rate of flow of 
air through the corresponding casings. Inasmuch as the casings are 
distributed essentially uniformly across the face of the sill pillar, the 
flow rate measurements provide a reasonably accurate sampling of the 
permeability distribution of the formation particles essentially uniformly 
across the horizontal cross-section of the fragmented mass. A relatively 
low flow rate measurement indicates relatively low permeability in a 
portion of the fragmented mass, or possibly unfragmented formation present 
in a region of the fragmented mass. On the other hand, a relatively higher 
flow rate measurement indicates a relatively greater tendency for 
channeling in a vertically extending portion of the fragmented mass, and 
the magnitude of the flow rate will be substantially directly proportional 
to the amount of gas channeling. 
After the flow rate measurements are conducted for all casings, the 
throttle valves 44 are adjusted in accordance with the flow rate 
measurements to adjust the volume of gas for retorting introduced through 
each casing. The gas flow adjustments are made by increasing throttle 
valve area in inverse proportion to the magnitude of the measured flow 
rate. Thus, in those bore holes corresponding to relatively higher gas 
flow rates, the throttle valves are adjusted to provide a relatively lower 
gas flow volume to the fragmented mass; and in those bore holes 
corresponding to relatively lower gas flow rates, the valves are adjusted 
to provide a relatively greater gas flow volume to the fragmented mass. 
The valves are adjusted in relation to one another to produce an 
essentially uniform gas flow distribution across the horizontal 
cross-section of the fragmented mass from its top to its bottom. As 
described above, this will tend to minimize the effects of channeling by 
equalizing the rate of gas flow through the fragmented mass and tend to 
produce an essentially flat and horizontal advancing combustion zone 
through the fragmented mass. 
After the throttle valves have been adjusted in accordance with the gas 
flow rate measurements, the retort is ready for inlet gas to be introduced 
through the casings for use in sustaining and advancing a combustion zone 
through the retort. Hot gases flowing downwardly from the combustion zone 
decompose kerogen in the fragmented mass of oil shale particles to produce 
liquid and gaseous products. The liquid products percolate through the 
fragmented mass on the advancing side of the retorting zone and accumulate 
in the sump 52 in the access drift 18, as described above. 
If one or more portions of the fragmented mass are detected as being more 
permeable than another portion or other portions, resulting in non-uniform 
burning in the fragmented mass, or if it is found that the locus of the 
combustion zone is undesirable, the relative flow of gas through the 
various casings can be adjusted independently of one another by using the 
separate throttle valves. This can provide a desired gas flow gradient 
through the fragmented mass which can result in producing a combustion 
zone which is substantially flat and horizontal as it advances through the 
retort. 
If a combustion zone is not properly advanced through the fragmented mass, 
the combustion zone can become skewed and/or warped. It is desirable to 
establish and maintain a combustion zone which is flat and uniformly 
transverse to the direction of its advancement to maximize yield of 
hydrocarbon products from oil shale in an in situ oil shale retort. If the 
combustion zone is skewed relative to its direction of advancement, there 
is more tendency for oxygen present in the combustion zone to migrate into 
the retorting zone, thereby oxidizing the hydrocarbon products produced in 
the retorting zone and reducing hydrocarbon yield. In addition, with a 
skewed and/or warped combustion zone, excessive cracking of hydrocarbon 
products produced in the retorting zone can result. 
By providing means for separately controlling gas flow to selected regions 
throughout the horizontal cross-section of the fragmented mass, the 
present invention facilitates advancing a combustion zone which is 
essentially flat and transverse to its direction of advancement. 
Further, by using the base of operation as a plenum chamber for admission 
of air to the fragmented mass, all of the air introduced into the 
fragmented mass is drawn from the base of operation and passageways 
connecting it to the surface, thereby adding to the fresh air supply in 
those working areas. 
Moreover, by using the blower 61 as the means for drawing gas through the 
fragmented mass from the base of operation, gas pressure within the 
fragmented mass is reduced relative to gas pressure within the base of 
operation 25 mass. This inhibits leakage of off gas from the fragmented 
mass into the base of operation and its surrounding underground workings. 
Inasmuch as off gas from the fragmented mass can contain a substantial 
amount of hydrogen sulfide and carbon monoxide, such off gas would 
otherwise pose a potential hazard to operating personnel in the base of 
operation and other underground workings. 
The check valve in each casing prevents inadvertent back flow of retorting 
gases into the base of operation if the blower 61 withdrawing gas from the 
fragment mass is temporarily shut down. There can be continued production 
of gas in the fragmented mass even when the blower 61 is not operating due 
to high temperatures which can cause further retorting. This can cause a 
gas pressure increase in the fragmented mass beneath the sill pillar 22 
which could result in a gas pressure higher than that in the base of 
operation. Reverse flow of gas is inhibited by the check valves 42 in the 
casings which permit gas to flow into the fragmented mass from the base of 
operation while preventing reverse flow.