Method for forming an in situ oil shale retort

An in situ oil shale retort is formed in a subterranean formation containing oil shale, a horizontally extending void is excavated within the boundaries of the retort site leaving a zone of unfragmented formation above and/or below such a void. A crack is propagated in at least one of the zones of unfragmented formation along the side boundaries of the retort site and thereafter the zone of unfragmented formation is explosively expanded towards such a void for forming a fragmented permeable mass of formation particles in the retort. Such a fragmented permeable mass is retorted in situ to produce shale oil.

FIELD OF THE INVENTION 
This invention relates to the formation of a fragmented permeablized mass 
of formation particles in an in situ oil shale retort. 
BACKGROUND OF THE INVENTION 
The invention relates to a technique for forming a fragmented permeable 
mass of particles in an in situ oil shale retort. More particularly, this 
invention relates to technique for explosive expansion of unfragmented 
formation into voids excavated within the retort site which technique 
minimizes the occurrence of a relatively higher void fraction region along 
the side boundaries of the retort and a low void fraction region near the 
center of the retort. 
The presence of large deposits of oil shale in the semi-arid high plateau 
region of the Western United States has given rise to extensive efforts to 
develop methods for recovering shale oil from kerogen in the oil shale 
deposits. It should be noted that 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 liquid and gaseous products. It is the formation 
containing kerogen that is called "oil shale" herein and the liquid 
hydrocarbon product is called "shale oil." 
A number of methods have been proposed for producing shale oil from oil 
shale; these generally involve either mining the kerogen-bearing shale and 
removing it to the surface for processing into shale oil or rubblization 
and processing of the shale in situ. The latter approach is preferable 
from the standpoint of environmental impact since the treated shale 
remains in place, reducing the chance of surface contamination and the 
requirement for disposal of large quantities of solid wastes. 
The recovery of liquid and gaseous products from oil shale deposits has 
been described in several patents such as U.S. Pat. Nos. 3,661,423; 
4,043,597; 4,043,598; and 4,153,298, as well as pending applications 
including U.S. patent application Ser. No. 929,250, filed July 31, 1978, 
by Thomas E. Ricketts, now U.S. Pat. No. 4,192,554, and U.S. patent 
application Ser. No. 070,319, filed Aug. 27, 1979, by Chang Yul Cha, 
entitled TWO-LEVEL HORIZONTAL FREE FACE MINING SYSTEM FOR IN SITU OIL 
SHALE RETORTS now abandoned. Each of these patents and applications is 
assigned to Occidental Oil Shale, Inc., assignee of this application, and 
each is incorporated herein by this reference. 
These patents and applications describe in situ recovery of liquid and 
gaseous hydrocarbon materials from a subterranean formation containing oil 
shale, wherein the formation is explosively expanded to form an in situ 
fragmented permeable mass of formation particles containing oil shale, 
referred to herein as a "retort" or as an "in situ oil shale retort". 
Retorting gases are passed through the fragmented mass to convert kerogen 
contained in the oil shale to liquid and gaseous products, thereby 
producing retorted oil shale. One method of supplying hot retorting gases 
used for converting kerogen contained in the oil shale as described in 
U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the 
retort and introducing an oxygen-supplying retort inlet mixture into the 
retort to advance the combustion zone through the fragmented mass. In the 
combustion zone oxygen from the retort inlet mixture is depleted by 
reaction with hot carbonaceous materials to produce heat, combustion gas 
and combusted oil shale. By the continued introduction of the retort inlet 
mixture into the fragmented mass, the combustion zone is advanced through 
the fragmented mass in the retort. 
The combustion gas and that portion of the retort inlet mixture which does 
not take part in the combustion process pass through the fragmented mass 
on the advancing side of the combustion zone to heat the oil shale to a 
temperature sufficient to produce kerogen decomposition; this process, 
called "retorting," takes place in a retorting zone. Such decomposition of 
the oil shale in the retorting zone produces gaseous and liquid products, 
including gaseous and liquid hydrocarbons, and a residual carbonaceous 
material. 
The liquid products and the gaseous products are cooled by the cooler oil 
shale fragments in the retort on the advancing side of the retorting zone. 
These products, together with water produced in or added to the retort, 
collect at the bottom of the retort and are withdrawn. 
U.S. Pat. Nos. 4,043,597 and 4,043,598, and 4,192,554, disclose methods for 
explosively expanding formation containing oil shale toward horizontal 
free faces to form a fragmented mass in an in situ oil shale retort. 
According to such a method a plurality of vertically spaced apart voids of 
similar horizontal cross section are initially excavated one above another 
within the retort site. At least one zone of unfragmented formation is 
temporarily left between the voids. Explosive is placed in each of the 
unfragmented zones and detonated to explosively expand each unfragmented 
zone upwardly and/or downwardly towards the void or voids above and/or 
below it to form a fragmented mass having an average void volume about 
equal to the void volume of the initial voids. Retorting of the fragmented 
mass is then carried out as described above to recover shale oil from the 
oil shale. 
U.S. Pat. No. 4,153,298 describes a method for forming a retort by 
excavating at least one horizontally extending void adjacent a zone of 
unfragmented formation to be expanded. At least one support pillar of 
unfragmented formation is left in the void for supporting overburden. 
Explosive is placed in the zone of unfragmented formation and in such a 
support pillar. Explosive in such a pillar and in the zone of unfragmented 
formation is detonated in a single round of explosions with a time delay 
between detonation of explosive in such a pillar and detonation of 
explosive in the zone of unfragmented formation for first expanding such a 
pillar toward the void and then expanding unfragmented formation toward 
the void. The time delay is sufficient for creation of a free face at the 
juncture of such a pillar and the zone of unfragmented formation. The time 
delay is short enough that explosive in the zone of unfragmented formation 
is detonated before particles formed by explosive expansion of the pillars 
have come to rest on the floor of the void. 
Recovery of the shale oil resource is directly related to the distribution 
of the void fraction in the fragmented mass of oil shale particles. It is 
desirable to have a uniformly distributed void fraction in the fragmented 
mass so that there is generally uniform permeability, both horizontally 
across the retort and vertically along the length of the retort. With a 
uniformly distributed void fraction oxygen supplying gas and combustion 
gas can flow reasonably uniformly through the fragmented mass during 
retorting operations. A fragmented mass having generally uniform 
permeability prevents the retorting gas from bypassing portions of the 
fragmented mass as can occur if there is gas channelling through a portion 
of the mass due to non-uniform permeability. 
It was found upon forming a retort generally in accordance with the 
description in U.S. Pat. No. 4,192,554 that the fragmented mass of 
particles had a relatively high void volume fraction region along the side 
boundaries of the retort and a low void fraction region nearer the center 
of the retort. It is theorized that during explosive expansion there is a 
tendency for oil shale to be expanded preferentially away from the walls 
of the retort due to the force balance of expanding gas generated by the 
explosion. Expanding gas from explosive in the central portion of the 
retort, according to this theory, encounters reasonably uniform resistance 
so that the net direction of expansion is essentially vertical. Near the 
side boundaries, however, gas pressure extending laterally toward the 
boundary is resisted by the unfragmented formation that will form the 
walls of the retort and the resultant force balance thus has a component 
directed away from the walls. This tends to cause preferential expansion 
away from the walls and results in a high void fraction region adjacent 
the walls. Another theory for understanding the high void fraction region 
near the walls assumes that particles in the central region of the retort 
can expand vertically without substantial rotation due to adjacent 
particles which are also expanding vertically. Particles adjacent the 
walls encounter the unfragmented walls and the resultant friction causes 
partial rotation. Such rotation, as contrasted with the non-rotation in 
the central region of the retort, can account for the relatively higher 
void fraction adjacent the walls of the retort. It is also possible that 
either or both of these phenomena may be occurring. 
BRIEF SUMMARY OF THE INVENTION 
A method is provided for forming an in situ oil shale retort in a 
subterranean formation containing oil shale. According to the method at 
least one either horizontally or vertically or horizontally and vertically 
void is formed within the boundaries of an in situ oil shale retort side, 
leaving a zone of unfragmented formation adjacent such a void. A first 
group of substantially vertically extending shot holes is drilled into the 
zone of unfragmented formation adjacent the boundaries of the retort. A 
second group of substantially parallel, substantially vertically extending 
shot holes is drilled into the zone of unfragmented formation within the 
boundaries of the retort. The spacing between shot holes in the first 
group of shot holes is preferably closer than the spacing between shot 
holes in the second group of shot holes. Both groups of shot holes are 
loaded with explosive; preferably the average explosive charge in each of 
the shot holes in the first group of explosives is less than the average 
explosive charge in the shot holes of the second group. The explosive in 
both groups of shot holes are preferably shot in a single round with the 
explosive in the first group being shot slightly in advance of the 
explosive in the second group. 
Detonation of the explosive in the first group of shot holes will cause a 
crack to propagate between adjacent holes along the boundaries of the 
retort. This crack can provide a gas leak path along the boundaries which, 
in turn, can minimize the component of force of expanding gas from the 
explosive charge which tends to cause explosive expansion away from the 
boundaries. The crack can also minimize boundary friction and rotation of 
particles adjacent the boundaries of the retort. By counteracting either 
or both of these effects the occurrence of a higher void fraction portion 
along the boundaries of the retort can be minimized.

DESCRIPTION 
FIGS. 1 and 2 depict an exemplary in situ oil shale retort site after 
excavation of part of the formation to form the voids that provide the 
void volume in an in situ oil shale retort and before explosive expansion 
of formation to form such a retort. The retort site is in a subterranean 
formation 10 and has an upper boundary 11, a lower boundary 12, and side 
boundaries 13. In the illustrated embodiment, the retort has a square 
horizontal cross section, however, it will be understood that an unequal 
rectangular cross section or other cross section is suitable, and that a 
square cross section is illustrated solely for convenience. 
In this embodiment a horizontally extending access drift 14 or the like is 
excavated through subterranean formation to a side boundary of the retort 
site at its lower level. A void 16 is excavated at this lower or 
production level via the access drift. The production level void extends 
horizontally across the retort site and has side boundaries substantially 
coinciding with the side boundaries 13 of the retort. The production level 
void is on the order of about 20 to about 25% of the total height of the 
retort between the lower boundary 12 and the upper boundary 11. Overlying 
unfragmented formation above the void has a horizontal free face 15 at the 
top of the void. 
A support pillar 17 of unfragmented formation may be left within the void 
for the temporary support of overlying formation or overburden above the 
production level void. In the illustrated embodiment the pillar is 
rectangular in horizontal cross section and is somewhat wider than the 
height of the production level void. The pillar as shown occupies about 
20% of the horizontal cross-sectional area of the void; that is, at the 
production level there is an extraction ratio of about 80%. 
An upper or air level drift 18 is excavated through a side boundary of the 
retort site near its upper boundary 11. The air level drift provides 
access to the retort site for excavation of an upper or air level void 19. 
The air level void extends horizontally across the retort side and has 
side boundaries substantially coinciding with the side boundaries 13 of 
the retort. As with the production level one or more pillars may be left 
in the void for temporary support of the overlying formation. In the 
exemplary embodiment a pair of air level pillars 21 of unfragmented 
formation are left for this purpose. The air level pillars 21 are shown as 
relatively long rectangular pillars located within the air level void; 
this is desirable in that it provides effective access to substantially 
the entire horizontal cross section of the retort site for drilling and 
loading shot holes. 
A thick zone 22 of unfragmented formation is left within the boundaries of 
the retort site between the upper air level void 19 and the lower 
production level void 16. In one example the zone 22 of unfragmented 
formation can occupy about 70% of the total retort height. The top of the 
zone has a free face 20 at the floor of the air level void. 
It is understood that one or more vertically extending voids such as taught 
in U.S. Pat. No. 4,043,595, the disclosure of which is incorporated herein 
by this reference, to provide the requisite void volume for rubblization 
of the retort or the combination of one or more horizontal and one or more 
vertical voids are within the scope of the present invention. 
To form an in situ oil shale retort, the pillars 17 and 21 in both voids 
are explosively expanded and the zone 22 of unfragmented formation between 
the voids is explosively expanded toward the voids to form a fragmented 
permeable mass of particles. The volume of excavated voids provides the 
void space between particles in the fragmented mass and the average void 
fraction in the fragmented mass is substantially determined by the 
available volume of the excavated voids. Thus, for example, when the total 
excavated volume of the two voids is between about 15% to about 25% of the 
total volume of the retort site, the resulting fragmented mass has an 
average void fraction of between about 15% to about 25%. 
A first group of substantially vertical shot holes 23 are drilled 
downwardly from the air level void 19 into the zone 22 of unfragmented 
formation adjacent the side boundaries of the retort. Preferably the first 
group of shot holes 23 are located on the side boundary of the retort. A 
second group of substantially parallel, substantially vertical shot holes 
24 are also drilled downwardly from the air level void into the remainder 
of zone 22 of unfragmented formation. Shot holes 23 and 24 are shown out 
of proportion, i.e., the diameter of the shot holes is actually much 
smaller in relation to the horizontal dimensions of the retort than is 
shown in FIGS. 1 and 2. In the illustrated embodiment the shot holes 24 
are in a square array. Other arrangements or configurations are similarly 
suitable. It should also be apparent that shot holes 23 and 24 may be 
drilled downwardly from some other level in the formation above the air 
level, such as from the surface, or that they may be drilled upwardly, 
such as from the production level 16. 
As shown best in FIG. 1, the average spacing between shot holes 23 is 
closer than the average spacing between shot holes 24. In an exemplary 
embodiment the average spacing between centers of shot holes 23 is on the 
order of from about 1 foot to about 8 feet, preferably from about 1 foot 
to about 6 feet, and most preferably from about 1 foot to about 4 feet. 
The average spacing between shot holes 24 is on the order of from about 6 
feet to about 30 or more feet, preferably from about 8 feet to about 25 
feet, and most preferably on 20 foot centers. The diameter of the first 
and second group of shot holes may be the same or the first group of shot 
holes may be drilled with a smaller diameter than the second group of shot 
holes. By way of example, the first group of shot holes can have a 
diameter of from about 1 inch to about 8 inches, preferably about 2 to 
about 4 inches, and the second group of shot holes can have a diameter of 
about 8 to about 15 inches, preferably about 10 inches. 
A portion of the second group of shot holes 24 are drilled completely 
through the zone of formation between the upper and lower voids and 
through the support pillar 17 in the lower production level void. As 
shown, both groups of shot holes 23 and 24 in this exemplary embodiment 
are drilled most of the way through the intervening zone 22. Horizontal 
shot holes (not shown) are also drilled in the air level pillars 21. If 
desired, additional vertical shot holes can be drilled through the lower 
level pillar 17 or horizontal shot holes can be drilled in the lower level 
pillar. 
Explosive charges (not shown) are placed in both groups of shot holes and 
the shot holes in the pillars for explosively expanding the pillars and 
zone of formation 22 between the two voids. Preferably the first group of 
shot holes will have less of an average explosive charge than the second 
group of shot holes. By way of example, each of shot holes 23 may be 
loaded with from about 0.30 to about 20.0 pounds per foot of an explosive 
such an ANFO, preferably from about 1.1 to about 4.4 pounds per foot, and 
each of the shot holes 24 may be loaded with from about 20 to about 70 
pounds per foot of ANFO, preferably from about 25 to about 50 pounds per 
foot and most preferably about 30 pounds per foot. Moreover, it is not 
necessary for each of the shot holes in the first group to be charged with 
explosive, although in the preferred embodiment each of the first group of 
shot holes 23 will contain an explosive charge. It is also desirable for 
the top about 2 to about 3 feet of each shot hole 23 to be stemmed. 
In this exemplary embodiment, the unfragmented formation 22 between the 
upper and lower voids is explosively expanded in two stages. In a first 
stage, a lower zone 27 is explosively expanded downwardly toward the 
underlying production level void 16. In a second stage an upper zone 28 is 
explosively expanded both upwardly and downwardly. Roughly half of the 
upper zone is expanded downwardly towards the void space overlying the 
fragmented mass formed by expansion of the lower zone 27, and roughly half 
of the upper zone 28 is explosively expanded upwardly towards the 
overlying air level void 19. These two zones 27 and 28 can be explosively 
expanded in a single round of explosions, or if desired a substantial time 
interval can elapse between expansion of the lower zone and expansion of 
the upper zone 28. The latter arrangement permits loading of explosive 
charges in shot holes 23 and 24 in the upper zone and in the air level 
pillars after explosive expansion of the lower level pillar and lower 
zone. Alternatively, all such explosive charges are loaded in a single 
operation and detonated in a single round including the production level 
pillar 17, the lower zone 27, the upper zone 28 and the air level pillars 
21. 
Explosive charges (not shown) are loaded in the array of both groups of 
shot holes 23 and 24 in the upper half of the lower zone 27. Stemming is 
provided above explosive charges in the longer shot holes 24 in the 
production level pillar to separate such charges from charges in the upper 
half of the lower zone 27 of unfragmented formation. Stemming is also 
provided in shot holes above the explosive charges in the upper half of 
the lower zone 27. 
Another array of explosive charges is loaded in the center half of the 
upper zone 28, and the upper portions of the shot holes 23 and 24 are then 
stemmed. Thus, for example, in an embodiment where the upper zone 28 is 
about 100 feet thick, the lowermost 25 feet of the shot holes in that zone 
are stemmed; a 50 foot long explosive column is placed in the shot holes; 
and the upper 25 feet of the shot holes are stemmed. 
Each of the explosive charges is provided with a detonator and booster (not 
shown) for detonating the respective explosive charge at a selected 
moment. 
The first event in explosive expansion is detonation of explosive charges 
in the production level pillar 17 which explosively expands the pillar 
towards the side boundaries of the void. After a selected time interval 
explosive charges in the shot holes 23 in the lower zone 27 are detonated. 
At the same time or after a selected time interval explosive charges in 
shot holes 24 in the lower zone 27 are detonated for explosively expanding 
the lower zone downwardly towards the production level void. Detonation of 
the explosive charges in the pillar and in the lower zone is preferably in 
a single round, i.e., in a continuous series of explosions. It is not 
necessary that all of the explosive charges be detonated simultaneously 
and it is preferable to detonate such charges in sequence for minimizing 
the quantity of explosive detonated at any instant to reduce the blasting 
damage due to the seismic impact. In practice of this invention a time 
interval is provided between detonation of explosive in the production 
level pillar and detonation of explosive in shot holes 23 and 24 in the 
overlying zone 27 of unfragmented formation above the void. 
The time interval between detonation of explosive in the pillar and in the 
adjacent zone of unfragmented formation is preferably at least sufficient 
for a principal portion of the pillar fragments to travel to the side 
boundaries of the void. 
The next event in forming a fragmented permeable mass of particles in the 
retort involves explosive expansion of the upper zone 28 towards the air 
level void 19 and the void space over the top of the fragmented mass 
formed by explosive expansion of the production level pillar 17 and lower 
zone 27. Explosive charges in the air level pillars 21 are detonated for 
explosively expanding the pillars. After a time interval, e.g., sufficient 
for a principal portion of the pillar fragments to travel to the side 
boundaries of the void, explosive is detonated in shot holes 23 and 24 in 
the upper zone 28. This causes propagation of a crack between adjacent 
shot holes 23 in the upper zone and explosive expansion of roughly the 
lower half of this zone downwardly towards void space over the underlying 
fragmented mass and roughly the upper half of the zone towards the 
overlying air level void. 
Development of the crack between adjacent shot holes 23 can prevent the 
formation of a retort with a relatively higher void volume adjacent the 
boundaries of the retort than at the center of the retort. The crack can 
provide a gas leak path along the boundaries which, in turn, can minimize 
the component of force of expanding gas generated by the explosion of the 
charges in shot holes 24, i.e., the component of force which tends to 
cause explosive expansion away from the boundaries. The crack can also 
minimize boundary friction and rotation of particles adjacent the 
boundaries of the retort. By counteracting either or both of these effects 
the occurrence of a higher void fraction portion along the boundaries of 
the retort can be minimized. 
FIG. 3 illustrates in vertical cross section another exemplary embodiment 
of an in situ oil shale retort site after excavation of voids within the 
boundaries of the retort and before explosive expansion. A lower 
production level access drive 31 is excavated to a side boundary 32 of the 
retort site near the lower boundary 33. A horizontally extending 
production level void 34 is excavated at the lower boundary of the retort 
via the access drift 31. The side boundaries of the production level void 
substantially coincide with the side boundaries 32 of the retort. A pair 
of relatively long narrow support pillars 36 of unfragmented formation are 
left within the side boundaries of the production level void for 
supporting overlying formation. 
An intermediate level access drift 37 is excavated to a side boundary of 
the retort site approximately half-way between the lower boundary 32 and 
upper boundary 38 of the retort. An intermediate level void 39 is 
excavated via the intermediate level access drift 37. The intermediate 
level void extends horizontally across the retort site and its side 
boundaries coincide substantially with the side boundaries of the retort 
being formed. Four rectangular pillars 41 of unfragmented formation are 
left in the intermediate void 39 for temporary support of overlying 
formation. In the illustrated embodiment each of the four pillars 41 is 
centrally located in a quadrant of the intermediate level void. 
Collectively the intermediate level pillars 41 occupy about 20% of the 
horizontal cross-sectional area of the retort site. Thus, the extraction 
ratio at the intermediate level void is about 80%. 
An air level access drift 42 is excavated to a side boundary of the retort 
site near the upper boundary 38. From this drift an upper horizontally 
extending void 43 is excavated with side boundaries substantially 
coinciding with side boundaries of the retort being formed. A pair of 
elongated pillars 44 of unfragmented formation are left in the air level 
void for support of overlying formation. The air level pillars 44 can be 
similar to the production level pillars 36 and are arranged in the air 
level void to provide effective access to substantially the entire 
horizontal cross-sectional area of the retort site for drilling and 
loading of shot holes and the like. Excavation of the upper, intermediate, 
and lower voids in the retort site leaves a lower zone 46 of unfragmented 
formation between the lower void and the intermediate void, and an upper 
zone 47 of unfragmented formation between the intermediate void and the 
upper void. Such zones of unfragmented formation are explosively expanded 
towards the free faces adjacent the voids for forming a fragmented mass of 
formation particles in the retort. 
As with the exemplary embodiment of FIGS. 1 and 2 two sets of shot holes 
(not shown) are drilled and loaded with explosive in preparation for 
explosive expansion of particles in the retort. The first group of shot 
holes are positioned along the periphery of the retort and the second 
group of shot holes are within the first group, spaced throughout the 
remainder of the unfragmented formation to be explosively expanded. As was 
also the case with the illustrative embodiment described in detail in 
FIGS. 1 and 2, the first group of shot holes are preferably spaced closer 
together than the second group of shot holes, the first group of shot 
holes are preferably smaller in diameter than the second group and the 
average explosive charge in the first group of shot holes is preferably 
less than the average explosive charge in the second group of shot holes. 
Horizontal shot holes are drilled in the lower level pillars 36 and upper 
level pillars 44 for explosive expansion thereof. Either vertical or 
horizontal, preferably horizontal, shot holes are drilled in the 
intermediate level pillars 41. Explosive charges are also loaded into the 
shot holes in the pillars. Preferably the explosive in the pillars and two 
zones of unfragmented formation are detonated in a single round with 
suitable short time delays within the round. Alternatively, if desired, 
the lower zone 46 of unfragmented formation can be explosively expanded 
before the upper zone 47. 
Explosive is first detonated in the lower level pillars 36 and/or 
intermediate level pillars 41 for explosively expanding such pillars 
toward the surrounding void. The upper level pillars 44 can be explosively 
expanded at the same time or somewhat later. 
After a time interval, e.g., sufficient for a principal portion of the 
pillar fragments to travel to the side boundaries of the respective void, 
explosive is detonated in the first group of shot holes in the lower zone 
46 of unfragmented formation. These explosions cause a crack to propagate 
between adjacent shot holes in the first group of shot holes. At the same 
time or after a suitable time interval explosive is detonated in the 
second group of shot holes in the lower zone 46. Explosive can also be 
detonated in the same sequence in the upper zone 47 at about the same time 
or somewhat thereafter. 
Upon detonation of the explosive and propagation of the crack along the 
side boundaries of the retort, approximately one-half 46a of the lower 
zone expands downwardly toward the production level void 34 and 
approximately one-half 46b expands upwardly toward the intermediate void 
39. Similarly about one-half of the upper zone 47a expands downwardly 
towards the intermediate void 39 and the other half 47b expands upwardly 
towards the overlying air level void 43. 
Each half of these zones of unfragmented formation can be considered a zone 
expanding towards its respective void. Thus, a zone 46a above the lower 
void is expanded downwardly toward the void. An uppermost zone 47b below 
the air level void expands upwardly towards that void. Two zones, 44b 
below the intermediate void and 47a above the intermediate void. To 
accommodate such expansion and assure reasonably uniform void fraction 
distribution in the resulting fragmented mass, the volume excavated from 
intermediate level void is approximately twice the volume excavated from 
either the upper or lower level void. 
The recovery of shale oil and gaseous products from the oil shale in the 
retort generally involves the movement of a retorting zone through the 
fragmented permeable mass of formation particles in the retort. The 
retorting zone can be established on the advancing side of a combustion 
zone in the retort or it can be established by passing heated gas through 
the retort. It is generally preferred to advance the retorting zone from 
the top to the bottom of a vertically oriented retort, i.e., a retort 
having vertical side boundaries. With this orientation, the shale oil and 
product gases produced in the retorting zone move downwardly toward the 
base of the retort for collection and recovery aided by the force of 
gravity and gases introduced at an upper elevation. 
A combustion zone can be established at or near the upper boundary of a 
retort by any of a number of methods. Reference is made to U.S. Pat. No. 
4,147,593, and assigned to the assignee of the present application, and 
incorporated herein by this reference for one method in which an access 
conduit 50 is provided to the upper boundary of the retort and a 
combustible gaseous mixture is introduced therethrough and ignited in the 
retort. Off gas is withdrawn through an access means such as the drift 
extending to the lower boundary of the retort, thereby bringing about a 
movement of gases from top to bottom of the retort through the fragmented 
permeable mass of formation particles containing oil shale. A combustible 
gaseous mixture of a fuel, such as propane, butane, natural gas, or retort 
off gas, and air is introduced through the access conduit 50 to the upper 
boundary and is ignited to initiate a combustion zone at or near the upper 
boundary of the retort. Combustible gaseous mixtures of oxygen and other 
fuels are also suitable. The supply of combustible gaseous mixture of the 
combustion zone is maintained for a period sufficient for the oil shale at 
the upper boundary of the retort to become heated, usually to a 
temperature of greater than about 900.degree. F. so combustion can be 
sustained by the introduction of air without fuel gas into the combustion 
zone. Such a period can be from about one day to about a week in duration. 
The combustion zone is sustained and advanced through the retort toward the 
lower boundary by introducing any oxygen containing retort inlet mixture 
through the access conduit 50 to the upper boundary of the retort, and 
withdrawing gas from below the retorting zone. The inlet mixture, which 
can be a mixture of air and a diluent such as retort off gas or water 
vapor, can have an oxygen content of about 10% to 20% of its volume. The 
retort inlet mixture is introduced to the retort at a rate of about 0.5 to 
2 standard cubic feet of gas per minute per square foot of cross-sectional 
area of the retort. 
The introduction of gas at the top and the withdrawal of off gas from the 
retort at a lower elevation serve to maintain a downward pressure 
differential of gas to carry hot combustion product gases and non-oxidized 
inlet gases (such as nitrogen, for example) from the combustion zone 
downwardly through the retort. This flow of hot gas establishes a 
retorting zone on the advancing side of the combustion zone wherein 
particulate fragmented formation containing oil shale is heated. In the 
retorting zone, kerogen in the oil shale is retorted to liquid and gaseous 
products. The liquid products, including shale oil, move by gravity toward 
the base of the retort where they are collected in a sump and pumped to 
the surface. The gaseous products from the retorting zone mix with the 
gases moving downwardly through the in situ retort and are removed as 
retort off gas from a level below the retorting zone. The retort off gas 
is the gas removed from such lower level of the retort and transferred to 
the surface. The off gas includes retort inlet mixture which does not take 
part in the combustion process, combustion gas generated in the combustion 
zone, product gas generated in the retorting zone, and carbon dioxide from 
decomposition of carbonates contained in the formation. 
Although the method for forming an in situ oil shale retort has been 
described and illustrated in two embodiments, many modifications and 
variations will be apparent to one skilled in the art. Thus, other 
arrangements wherein a horizontally extending void is excavated, leaving a 
zone of unfragmented formation above and/or below such a void are 
contemplated, such as an arrangement having plural intermediate voids as 
described and illustrated in U.S. Pat. No. 4,192,554. Other arrangements 
of explosive charges for expansion of pillars and/or zones of unfragmented 
formation above and/or below such voids will be apparent. Thus, for 
example, decking of charges of explosive in a zone of unfragmented 
formation can be employed as described in U.S. Pat. No. 4,146,272. A 
variety of techniques for blasting pillars are disclosed in U.S. Pat. No. 
4,300,800, entitled METHOD OF RUBBLING A PILLAR, issued Nov. 17, 1981, by 
Thomas E. Ricketts. The patent and application are incorporated herein by 
reference. Since many such variations and modifications are contemplated, 
this invention should not be limited except as recited in the following 
claims.