Liquid seal for production level bulkhead for in situ oil shale retort

An in situ oil shale retort is formed in a subterranean formation containing oil shale. The retort contains a fragmented permeable mass of formation particles containing oil shale. During retorting, oxygen-supplying gas is introduced into an upper level of the fragmented mass for establishing a combustion zone and for advancing the combustion zone through the fragmented mass. Liquid and gaseous products, including shale oil and off gas, are withdrawn from a sealed portion of a production level drift which extends laterally away from the lower level of the fragmented mass. A bulkhead sealed across the drift inhibits passage of off gas from behind the bulkhead to the portion of the drift on the side of the bulkhead opposite the fragmented mass. Off gas, shale oil and water are separately withdrawn from behind the bulkhead. A liquid level is maintained against the surface of the bulkhead facing the fragmented mass to seal the lower edge of the bulkhead and the drift floor adjacent the bulkhead against the passage of off gas through any rock fissures which can be present in the floor of the drift.

BACKGROUND 
This invention relates to recovery of liquid and gaseous products from 
subterranean formations containing oil shale, and more particularly, to 
techniques for providing a gas seal in a lower production level of an in 
situ oil shale retort. 
The presence of large deposits of oil shale in the Rocky Mountain region of 
the United States has given rise to extensive efforts to develop methods 
for recovering shale oil from kerogen in the oil shale deposits. 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 which have been proposed for processing oil shale 
involve either first mining the kerogen-bearing shale and processing the 
shale on the ground surface, or processing 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 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; 
4,043,595; 4,043,596; 4,043,597; and 4,043,598, which are incorporated 
herein by this reference. These patents describe in situ recovery of 
liquid and gaseous hydrocarbon materials from a subterranean formation 
containing oil shale wherein such formation is fragmented by explosive 
expansion techniques to form a stationary fragmented permeable mass of 
formation particles containing oil shale within the formation, referred to 
herein as an in situ oil shale retort. 
In forming such a fragmented mass, at least one void is excavated from 
formation within the retort site, leaving a remaining portion of 
unfragmented formation within the retort site adjacent the void. Explosive 
is loaded into blasting holes drilled in the remaining portion of the 
unfragmented formation. The explosive is detonated for explosively 
expanding the remaining portion of unfragmented formation toward the free 
face of formation adjacent the void for forming a fragmented permeable 
mass of formation particles containing oil shale in an in situ oil shale 
retort. 
During retorting, hot retorting gases are passed downwardly 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 
suppling 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 near the top of the fragmented mass and introducing an 
oxygen-supplying gaseous combustion zone feed into the fragmented mass to 
advance the combustion zone downwardly through the fragmented mass. In the 
combustion zone, oxygen in the combustion zone feed is depleted by 
reaction with hot carbonaceous materials to produce heat, combustion gas 
and combusted oil shale. By continued introduction of the combustion zone 
feed into the fragmented mass, the combustion zone is advanced through the 
fragmented mass. 
The combustion gas and the portion of the combustion zone feed that does 
not take part in the combustion process pass through the fragmented mass 
on the advancing side of the combustion zone. This heats the oil shale in 
a retorting zone to a temperature sufficient to produce kerogen 
decomposition, called retorting, in the oil shale. The kerogen decomposes 
into gaseous and liquid products, including gaseous and liquid hydrocarbon 
products, and to a residual solid carbonaceous material. 
The liquid products and gaseous products are cooled by the cooler oil shale 
fragments in the retort on the advancing side of the retorting zone. The 
liquid hydrocarbon products, together with water produced in or added to 
the retort, are collected at the bottom of the retort. An off gas also is 
withdrawn from the bottom of the retort. The off gas contains combustion 
gas, including carbon dioxide generated in the combustion zone, gaseous 
products produced in the retorting zone, carbon dioxide from carbonate 
decomposition, and any gaseous retort inlet mixture that does not take 
part in the combustion process. 
During retorting the liquid products and a process off gas containing 
gaseous products pass to a lower level of the fragmented mass. The liquid 
products include water and shale oil which can accumulate in the bottom of 
a production level drift at a lower level of the fragmented mass. The 
water and shale oil can be separately withdrawn through the production 
level drift. The process off gas also is withdrawn through the production 
level drift. The off gas can contain nitrogen, hydrogen, carbon monoxide, 
carbon dioxide, water vapor, methane and other hydrocarbons and sulfur 
compounds, such as hydrogen sulfide. Hydrogen sulfide and carbon monoxide 
are extremely toxic gases. These and other constituents are combustible. 
For this reason, the production level drift is sealed against the passage 
of off gas from the portion of the drift where the gas collects, so that 
workers in adjacent underground workings at the production level are 
isolated from the off gas collected in the production level drift. A 
bulkhead placed across the production level drift can provide such a gas 
seal. 
Cracks or fissures can be present in the walls of unfragmented formation 
adjacent the production level drift. Such cracks or fissures can result 
from shock caused when explosive is detonated for excavating the 
production level drift. 
It is desirable to seal the floor of the production level drift adjacent 
the production level bulkhead against the passage of off gas through any 
cracks or fissures which can be present in the drift floor. Such a gas 
seal can inhibit passage of toxic off gas into other underground workings. 
SUMMARY OF THE INVENTION 
This invention provides a gas seal for a production level of an in situ oil 
shale retort containing a fragmented permeable mass of formation particles 
containing oil shale. A production level drift is excavated laterally away 
from a lower production level of the fragmented mass. A bulkhead is placed 
across the production level drift for inhibiting gas flow from an inside 
portion of the drift adjacent the fragmented mass to an outside portion of 
the drift on the side of the bulkhead opposite the fragmented mass. A pool 
of liquid on the floor of the drift is maintained at a sufficient liquid 
level against the bulkhead for providing a gas seal across a lower inside 
edge of the bulkhead and along an inside portion of the drift floor 
adjacent the bulkhead. The liquid is impervious to gas flow and inhibits 
the passage of gas from the inside portion of the drift to the outside 
portion of the drift through any portion of the drift floor covered by 
liquid.

DETAILED DESCRIPTION 
The drawing shows an in situ oil shale retort formed in a subterranean 
formation 10 containing oil shale. The in situ oil shale retort includes a 
fragmented permeable mass 12 of formation particles containing oil shale. 
The fragmented mass 12 can be formed by conventional explosive expansion 
techniques wherein at least one void (not shown) is excavated from 
formation within the retort site, leaving a remaining portion of 
unfragmented formation adjacent such a void. Blasting holes (not shown) 
are then drilled in such remaining portion of unfragmented formation 
adjacent the void, and the blasting holes are loaded with explosive which 
is detonated for explosively expanding such remaining portion of formation 
toward such a void for forming the fragmented mass 12. 
The fragmented mass 12 shown in the drawing is rectangular in horizontal 
cross-section and has a top boundary 14, four vertically extending side 
boundaries 16, and a lower boundary 18 which can taper narrower toward the 
bottom of the fragmented mass 12 for forming a bottom portion of the 
fragmented mass smaller in horizontal cross-section than the principal 
portion of the fragmented mass. 
A drift 20 at a production level provides a means for access to the lower 
boundary of the fragmented mass 12. The production level drift 20 extends 
generally horizontally away from the bottom of the fragmented mass. During 
formation of the fragmented mass, formation particles 22 from the 
fragmented mass fall under gravity into an inside portion of the 
production level drift 20 at the bottom of the fragmented mass being 
formed. 
The in situ oil shale retort can include an open base of operation 24 
excavated on an upper working level. The floor of the base of operation 24 
is spaced above the upper boundary 14 of the fragmented mass, leaving a 
horizontal sill pillar 26 of unfragmented formation between the bottom of 
the base of operation and the top boundary 14 of the fragmented mass. The 
base of operation 24 can provide effective access to substantially the 
entire horizontal cross-section of the fragmented mass. Such a base of 
operation provides an upper level means for access for excavating 
operations for forming a void and for drilling and explosive loading for 
explosively expanding formation toward such a void when forming the 
fragmented mass 12. The base of operation 24 also facilitates introduction 
of oxygen-supplying gas into the top of the fragmented mass 12 during 
retorting operations. 
During retorting the fragmented formation particles at the top of the 
fragmented mass are ignited to establish a combustion zone in the top of 
the fragmented mass. Air or other oxygen-supplying gas can be supplied to 
the combustion zone from the base of operation 24 through vertical air 
passages 28 drilled downwardly from the base of operation through the sill 
pillar 26 to the top of the fragmented mass. Conduits can be installed in 
the vertical air passages 28 and gas flow control valves (not shown) in 
the base of operation 24 can be used for controlling the flow of 
oxygen-supplying gas through the respective conduits to the fragmented 
mass. Air or other oxygen-supplying gas introduced to the fragmented mass 
through such conduits maintain the combustion zone and advance it 
downwardly through the fragmented mass 12. Hot gas from the combustion 
zone flows through the fragmented mass on the advancing side of the 
combustion zone to form a retorting zone where kerogen in the fragmented 
mass is converted into liquid and gaseous products. As the retorting zone 
moves down through the fragmented mass, liquid and gaseous products are 
released from the fragmented formation particles. Liquid products, 
primarily shale oil 30 and liquid 32 containing water, produced during 
operation of the retort collect below the fragmented mass 12 in a lower 
portion of the production level drift 20. The liquid 32 containing water 
can include an emulsion of water and shale oil and is referred to below as 
water, for simplicity. 
A bulkhead 34 is sealed across the production level drift at a location 
which is spaced apart longitudinally from the portion of the drift 
occupied by the fragmented formation particles forming the bottom of the 
fragmented mass. The production level drift 20 has a closed inside portion 
36 on the side of the bulkhead 34 adjacent the fragmented mass 12 and an 
outside portion 38 on the side of the bulkhead opposite the fragmented 
mass 12. The outside portion 38 of the drift can be open to adjacent 
underground workings. The bulkhead can be steel and/or concrete and can be 
made to conform to the walls, roof, and floor of the drift, or preferably 
it can be formed in a notch or keyway extending into the unfragmented 
formation surrounding the drift. An inside edge 42 of the bulkhead extends 
across the juncture between the bulkhead and the floor 40 of the inside 
portion of the drift. The bulkhead also has an inside surface 44 facing 
toward the inside portion 36 of the production level drift. During 
retorting operations, the shale oil 30 and water 32 collect in the inside 
portion 36 of the production level drift behind the bulkhead. Off gas 
collects in an open space in the inside portion of the drift above the 
liquid level. 
According to principles of this invention, a pool of liquid is maintained 
against the inside surface 44 of the bulkhead at a liquid level at least 
above the lower inside edge 42 of the bulkhead. In one embodiment wherein 
a depth of liquid is maintained against a lower portion of the bulkhead 
above the inside edge 42 of the bulkhead, the pool of liquid extends along 
the drift floor 40 continuously from the inside surface of the bulkhead 
for at least a portion of the lengthwise extent of the drift floor. Such a 
pool of liquid provides a gas impervious seal across the lower inside edge 
42 of the bulkhead and the portions of the drift floor 40 below the liquid 
level. Such a gas impervious seal can seal against the passage of off gas 
from the inside portion 36 of the drift through any cracks or rock 
fissures present in formation forming the drift floor in the vicinity of 
the bulkhead below the liquid level. The liquid is more impervious to gas 
flow and therefore provides a more effective gas-tight seal against 
leakage of off gas than formation which is directly exposed to such off 
gas. 
In one embodiment, the gas seal is constantly provided during retorting 
operations by maintaining the level of the water in the lower portion of 
the drift above the lower inside edge 42 of the bulkhead. The water and 
the shale oil 30 are separately withdrawn from the inside portion 36 of 
the drift as they collect in the drift during retorting operations. The 
shale oil 30 and water are withdrawn so that the water is constantly 
maintained at a level at least above the lower inside edge 42 of the 
bulkhead, at least as long as off gas is present in the inside portion of 
the drift. The water is withdrawn from behind the bulkhead by a lower 
water withdrawal line 46 extending through a sealed opening in a lower 
portion of the bulkhead and an upper water withdrawal line 48 extending 
through a separate sealed opening in the bulkhead with an intake at a 
level above the intake of the lower water withdrawal line 46. Both water 
withdrawal lines 46, 48 are connected to a water pump 50. Lower and upper 
water control valves 52 and 54, are respectively used to control flow of 
water through the lower and upper water withdrawal lines 46,48 
respectively. 
Shale oil is withdrawn from behind the bulkhead by an oil withdrawal line 
55 which extends through a sealed opening in the bulkhead and has an 
intake above the intake of the upper water withdrawal line 48. The shale 
oil withdrawal line 55 is connected to an oil pump 56. An oil flow control 
valve 58 controls the flow of shale oil through the oil withdrawal line. 
The locations of the upper water withdrawal line 48 and the oil withdrawal 
line 55 are exaggerated in the drawing for clarity, since these lines are 
actually closer to the bottom of the bulkhead than shown. 
Shale oil and water are each allowed to collect at desired levels behind 
the bulkhead 34 and are periodically withdrawn from the drift, as desired. 
The oil and water pumps, as well as the water and oil flow control valves, 
can be operated manually or by automatic controls (not shown) to remove 
the shale oil and water separately from behind the bulkhead. 
The inlet of a blower 60 is connected by a conduit 62 to an opening sealed 
through the bulkhead 34 for withdrawing off gas which collects above the 
shale oil in the inside portion 36 of the drift. The outlet of the blower 
60 delivers off gas from behind the bulkhead through a conduit 64 to a 
recovery or disposal system (not shown). The sizes of the conduits 
illustrated are smaller than actual for ease of illustration. It will also 
be understood that the off gas blower 60 can be located outside the 
underground workings rather than in the drift adjacent the retort. 
In one method of withdrawing liquid, shale oil can be withdrawn from behind 
the bulkhead through the oil withdrawal line 55 until gas is sucked in 
through the line, indicating that the liquid level has reached the level 
of the oil withdrawal line. Shale oil and water can then be allowed to 
collect in the drift until more shale oil can be withdrawn through the oil 
withdrawal line. If water is withdrawn through the shale oil withdrawal 
line (as indicated by a densitometer (not shown) connected to the oil 
withdrawal line), then the water pump 50 is used to withdraw water from 
behind the bulkhead through either of the water withdrawal lines, until 
shale oil can be withdrawn through the oil withdrawal line. 
Water can be withdrawn from behind the bulkhead either through both the 
lower and upper water withdrawal lines 46, 48 or separately through either 
of the water withdrawal lines. The intake of the lower water withdrawal 
line 46 is at a level spaced above the lower inside edges 42 of the 
bulkhead. In one method of withdrawing liquid, the lower water withdrawal 
line 46 can be used to withdraw water from behind the bulkhead until gas 
is sucked in through the line (indicating that the liquid level has 
reached the level of the lower water withdrawal line), or until shale oil 
is withdrawn through the lower water withdrawal line (as indicated by a 
densitometer (not shown) connected to the lower water withdrawal line). 
When either gas or shale oil is drawn through the lower water withdrawal 
line, pumping operations are stopped, and more shale oil and water are 
allowed to collect above the water level until it is desired to withdraw 
more shale oil and water. This procedures constantly leaves a pool of 
water above the floor of the drift and against the inside face 44 of the 
bulkhead at least at the level of the lower water withdrawal line 46, 
which provides a constant gas seal, during retorting across the portions 
of the drift floor and the lower inside edge 42 of the bulkhead below the 
water. 
In the embodiment shown in the drawing, a pool of liquid is maintained 
above substantially the entire length of the floor 40 which extends below 
an open space in which off gas can collect behind the bulkhead. The lower 
water withdrawal line 46 is located in the bulkhead at a sufficient 
elevation above the highest level of the drift floor that a liquid level 
can be maintained above the drift floor for substantially the entire 
length of the drift floor behind the bulkhead. Since the entire portion of 
the drift floor which potentially could be exposed to off gas is sealed 
from such off gas by the liquid in the bottom of the drift, off gas 
leakage through the drift floor from the inside portion 36 of the drift to 
the outside portion 38 of the drift is inhibited. 
The lower water withdrawal line 46 is also located at a sufficient level 
above the drift floor that a liquid level can be maintained above the 
drift floor to cover a substantial portion of the width of the drift floor 
at least adjacent the bulkhead. In one embodiment, a substantial portion 
of the width of the drift floor is covered with a depth of liquid above 
essentially the entire length of the floor below an open space in which 
off gas can collect behind the bulkhead. This inhibits leakage of off gas 
through a substantial portion of the drift floor area which could 
otherwise be exposed to off gas. 
Before retorting is commenced, liquid is added to the inside portion of the 
drift to form a pool that seals the lower edge of the bulkhead. For 
example, water can be pumped in through one of the withdrawal lines to 
bring the water depth up to the desired level. Once liquid products begin 
to collect at the bottom the pool is maintained by withdrawing liquid only 
down to the selected level that maintains the gas seal. 
Thus, the present invention provides a gas seal in a lower production level 
of an in situ oil shale retort. The gas seal is provided by maintaining a 
gas impervious liquid pool behind the production level bulkhead, which 
seals against the passage of off gas through any cracks or fissures which 
can be present in the floor of the drift behind the bulkhead. The liquid 
seal can be constantly maintained during retorting operations by allowing 
liquid products of retorting, primarily shale oil and water, to collect 
behind the bulkhead and above the drift floor during retorting operations. 
Shale oil and water are intentionally withdrawn from behind the bulkhead, 
but only to the extent that a minimum depth of liquid can be constantly 
maintained against the inside face 44 of the bulkhead, above the lower 
inside edge 42 of the bulkhead and above the drift floor adjacent the 
bulkhead. The minimum depth of liquid is maintained throughout the active 
life of the retort.