Explosive placement for explosive expansion toward spaced apart voids

A subterranean formation containing oil shale is prepared for in situ retorting by initially excavating a pair of spaced apart voids, leaving an intervening zone of unfragmented formation between the voids. The intervening zone has substantially parallel free faces adjoining the void. A plurality of elongated blasting holes are formed in the intervening zone of unfragmented formation, the longitudinal axis of each blasting hole being substantially perpendicular to the parallel free faces of the intervening zone. At least two deck loads of explosives are placed in each blasting hole, with each load being longitudinally spaced apart from each adjacent load by stemming. The loads of explosive are then detonated in a single round of explosions with a time delay between adjacent loads for expanding formation in the intervening zones toward both voids. The fragmented mass of formation particles is then retorted to recover shale oil from the oil shale.

BACKGROUND OF THE INVENTION 
This invention relates to the recovery of constituents from subterranean 
formations, and more particularly to an in situ method of recovery that is 
particularly effective for the protection of shale oil from oil shale in 
an in situ retort. 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 formation 
comprising marlstone deposit containing an organic material called 
"kerogen" which upon heating decomposes to produce carbonaceous liquid and 
gaseous products. It is the formation containing kerogen that is called 
"oil shale" herein, and the liquid product is called "shale oil." 
The recovery of liquid and gaseous products from oil shale deposits has 
been described in several patents, one of which is U.S. Pat. No. 
3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the 
assignee of this application, and incorporated herein by reference. This 
patent describes in situ recovery of liquid and gaseous carbonaceous 
materials from a subterranean formation containing oil shale by mining out 
a portion of the subterranean formation. Then explosive charges dispersed 
through a portion of the remaining formation are detonated to fragment and 
expand the portion of the remaining formation to form a stationary, 
fragmented, permeable mass of formation particles containing oil shale, 
referred to herein as an insitu oil shale retort. Hot retorting gases are 
passed through the in situ oil shale retort to convert kerogen contained 
in the oil shale to liquid and gaseous products. 
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 establishment of a combustion zone in the retort and introduction 
of an oxygen supplying combustion zone feed into the retort on the 
trailing size of the combustion zone to advance the combustion zone 
through the fragmented mass. In the combustion zone oxygen in the gaseous 
feed mixture is depleted by reaction with hot carbonaceous materials to 
produce heat and combustion gas. By the continued introduction of the 
oxygen supplying feed into the combustion zone, the combustion zone is 
advanced through the fragmented mass. The effluent gas from the combustion 
zone passes through the retort on the advancing side of the combustion 
zone to heat the oil shale in a retorting zone to a temperature sufficient 
to produce kerogen decomposition, called retorting, in the oil shale to 
gaseous and liquid products and a residue of solid carbonaceous material. 
The resulting liquid and gaseous products pass to the bottom of the retort 
for collection. 
It is desirable that the retort contain a reasonably uniformly fragmented, 
reasonably uniformly permeable mass of formation particles having a 
reasonably uniformly distributed void volume or void fraction so gases can 
flow uniformly through the retort and result in maximum conversion of 
kerogen to shale oil. A uniformly distributed void fraction in the 
direction perpendicular to the direction of advancement of the combustion 
zone is important to avoid channeling of gas flow in the retort. The 
creation of a mass of particles of uniform void volume distribution 
prevents the formation of over-sized voids or channels which hinder total 
recovery of shale oil and also provides a uniform pressure drop through 
the entire mass of particles. In preparation for the described retorting 
process, it is important that the formation be fragmented and displaced, 
rather than simply fractured, in order to create high permeability; 
otherwise, too much pressure differential is required to pass gas through 
the retort. It is important that the retort contain a substantially 
uniformly fragmented mass of particles so uniform conversion of kerogen to 
liquid and gaseous products occurs during retorting. A wide distribution 
of particle size can adversely affect the efficiency of retorting because 
small particles can be completely retorted long before completion of 
retorting the core of large particles. 
It has been proposed that oil shale be prepared for in situ recovery by 
first undercutting a portion of the formation to remove from about 5% to 
about 25% of the total volume of the in situ retort being formed. The 
overlying formation is then expanded by detonating explosives placed in 
the formation to fill the void created by the undercut. 
The general art of blasting rock formations is discussed in The Blaster's 
Handbook, 15th Edition, published by E.I. DuPont de Nemours & Company, 
Wilmington, Del. 
One method of explosive expansion is the so-called "V-cut" method, 
described at pp. 246-7 of The Blasters' Handbook, in which explosive 
charges are arranged within the formation and detonated in sequence so the 
formation is expanded in concentric sequential steps moving radially 
outwardly and upwardly within the formation generating a conical free face 
which propagates upwardly through the formation in accordance with the 
time delays between the explosive charges. A free face is the exposed 
surface of a mass of rock such as a surface in the vicinity of a shothole 
at which rock is free to move under the force of an explosion. A purpose 
of the V-cut method of expansion is to produce particles of relatively 
small size; but it has the disadvantage of tending to create a radially 
nonuniform void volume distribution throughout the expanded mass. 
Rather than using the V-cut method of expansion, it has been proposed to 
use a plurality of concentrated charges uniformly distributed throughout 
the formation to be expanded to produce a uniformly fragmented mass of 
formation particles. U.S. Pat. No. 3,434,757 issued to Prats teaches 
sequential detonation of a series of explosive in oil shale to form a 
permeable zone in the oil shale. However, it is both time consuming and 
expensive to place a large number of explosive charges throughout the 
formation. 
Another method for preparing formations for in situ recovery is described 
in U.S. Pat. No. 4,043,597, assigned to the assignee of this invention, 
and incorporated herein by this reference. According to this patent, two 
voids vertically spaced apart from each other are excavated in the 
subterranean formation. This leaves a zone of unfragmented formation 
between the voids. Vertical blasting holes are formed in the intervening 
zone. Explosive is placed in the blasting holes and detonated to expand 
formation in the intervening zone toward both voids. 
BRIEF SUMMARY OF THE INVENTION 
Thus, there is provided in practice of this invention in one embodiment a 
method for fragmenting a subterranean formation by first excavating an 
upper void and a lower void vertically spaced apart from each other in the 
subterranean formation, thereby leaving an intervening zone of 
unfragmented formation between the voids. The zone of unfragmented 
formation has an upper substantially horizontal free face adjacent the 
upper void and a lower substantially horizontal free face adjacent the 
lower void. Explosive is placed in an upper zone of the unfragmented 
formation between the upper and lower voids. Explosive is also placed in a 
lower zone of the unfragmented formation between the upper and lower 
voids, where the lower zone is below the upper zone. The explosives placed 
in the upper and lower zones are detonated in a single round with a time 
delay between detonation of explosive in the upper zone and detonation of 
explosive in the lower zone for explosively expanding formation between 
the upper and lower voids toward both the upper and lower voids. 
The explosive can be placed in the formation by forming a plurality of 
substantially vertical blasting holes in the intervening zone of 
unfragmented formation and placing at least two deck loads of explosive in 
such a blasting hole. The loads in such blasting hole are vertically 
spaced apart from each adjacent load by a mass of stemming. The loads of 
explosive are detonated in a single round of explosions with a time delay 
between adjacent loads to expand formation in the intervening zone toward 
both voids. 
In one version of this invention, the loads are detonated sequentially 
toward the vertical center of mass of the intervening zone for expanding 
the formation uniformly toward both free faces. This is effected by 
detonating each such load in a blasting hole no later than detonation of 
any of the loads in the same blasting hole between such load and the 
vertical center of mass of the portion of the formation being fragmented. 
Sequential detonation allows creation of a new, substantially horizontally 
extending free face by detonation of loads, thereby providing a new free 
face for explosive expansion of formation in the intervening zone by 
subsequent detonation of loads. 
In another version of this invention, each load is detonated no earlier 
than detonation of any of the loads between such load and one of the free 
faces of the formation being fragmented. This expands formation 
preferentially toward that free face. 
Preferably the time between detonation of each load and an adjacent load in 
the same blasting hole is more than the time required for creation of a 
free face by explosive expansion of formation by detonation of the first 
of the adjacent loads to be detonated. 
Also, preferably the time between detonation of the first load to be 
detonated and the last load to be detonated is less than the time required 
for expanding formation beyond a selected void fraction by detonation of 
the first load to be detonated. This permits formation of a retort 
containing a reasonably uniformly permeable mass of particles. 
To avoid excessive seismic effect from detonation of the loads of 
explosive, preferably the time between detonation of loads detonated 
successively is sufficient for the seismic wave produced by detonation of 
the first load to be detonated to pass the second load to be detonated.

DETAILED DESCRIPTION 
A. General Discussion 
FIGS. 1-3 illustrate a subterranean formation 10, such as a subterranean 
formation containing oil shale, which is in an intermediate stage of 
preparation for in situ recovery of carbonaceous values such as shale oil 
and hydrocarbon gaseous products. Generally speaking, in situ recovery is 
carried out by initially excavating formation from a portion of the 
subterranean formation and then explosively expanding a remaining portion 
of the formation to produce a fragmented permeable mass of formation 
particles containing oil shale. The present invention is described in the 
context of a method for ultimately producing a subterranean retort 
comprising an approximately rectangularly prismatic retort cavity, or room 
12 (illustrated in phantom lines in FIGS. 1-3) containing a reasonably 
uniformly fragmented, reasonably uniformly permeable mass of expanded 
formation particles having a reasonably uniformly distributed void 
fraction for economical retorting operations. In the illustrated 
embodiment, the in situ retort being formed is square in horizontal 
cross-section having a vertical dimension or height which is greater than 
its maximum lateral dimension or width. The height of the retort can be 
less than or the same as the width of the retort. 
Referring to FIG. 2, access to the portion of the subterranean formation 
containing oil shale to be expanded is established by forming a horizontal 
tunnel, drift or adit 14 extending to the bottom of the volume to be 
expanded. From the drift 14, the formation is undercut and a volume of 
formation is removed to form a lower void 20 at the bottom of the 
subterranean retort 12 to be formed. The material excavated from the lower 
void is hauled away through the drift 14 for removal to the surface via a 
shaft or adit (not shown). 
The lower void 20 can be continuous across the width of the volume to be 
expanded, so formation overlying the lower void is completely unsupported 
and defines a horizontal free face 21 of the formation immediately above 
the lower void. If desired one or more pillars 49 of unfragmented 
formation can be left in the lower void to help support overlying 
formation as described in greater detail hereafter. The floor plan or 
horizontal cross section of the lower void 20 can be generally square, 
although the void, and also the in situ retort to be formed, can be of 
other horizontal cross-section such as rectangular, without departing from 
the scope of the invention. The floor 22 of the lower void is inclined 
downwardly in the direction of the drift 14 to facilitate the flow of 
shale oil in the direction of the drift during subsequent retorting 
operations. 
A horizontal access tunnel or drift 30 is excavated at an elevation above 
the elevation of the bottom void 20. From the horizontal access drift 30, 
formation is excavated from the volume to be expanded to form an 
intermediate void 32 at an elevation above the elevation of the lower void 
20. The floor plan or horizontal cross-section of the intermediate void 32 
substantially matches the horizontal cross-section and area of the lower 
void 20 and the in situ retort 12 to be formed. Thus, the intermediate 
void can be square or rectangular in shape, and preferably is 
substantially directly above the lower void so the outer edges of the two 
voids lie in common vertical planes. Pillars 49 can be left in the 
intermediate void to support the overlying formation. Alternatively, the 
intermediate void can be continuous across the width of the room 12 so 
that the overlying portion of the formation is completely unsupported. The 
formation adjacent the intermediate void defines a pair of vertically 
spaced apart, bottom and top horizontal free faces 34 and 36, 
respectively, adjoining the intermediate void 32. The two voids 20 and 32 
also define a lower intervening zone 37 of unfragmented formation 
containing oil shale left within the boundaries of the subterranean retort 
12 between the substantially parallel horizontal free faces 21 and 34. 
After the intermediate void 32 is formed, or concurrently therewith, a 
horizontal tunnel or drift 42 is excavated at an elevation above the 
elevation of the intermediate void 32. Formation is removed from within 
the boundaries of the retort 12 being formed through the drift 42 to form 
an upper void 44 at an elevation above the elevation of the intermediate 
void 32. The floor plan or horizontal cross-section of the upper void 44 
is substantially similar to the cross-section of the lower and 
intermediate voids of the retort 12. The upper void preferably is aligned 
with the voids below it so that the outer edges of the upper void lie in 
common vertical planes with the outer edges of the voids below. The upper 
void 44 is approximately the same height as the intermediate void 32 and 
can be continuous across the width of retort 12. Thus, the portion of the 
formation above it can be completely unsupported. If desired one or more 
pillars 49 of unfragmented formation can be left in the upper void to help 
support the overlying formation as shown in FIGS. 2 and 3 (not shown in 
FIG. 1). When pillars are used in any of the voids, preferably they are at 
least as wide as they are high to maximize the stability of the overlying 
formaton. The proportion of formation extracted from the void and 
proportion temporarily left in the form of pillars of unfragmented 
formation depends on many factors such as rock properties, depth of 
overburden, height of the void, time the void must remain open and the 
like. The size and location of pillars is readily determined by 
conventional techniques by one skilled in mining. 
The upper void defines a pair of vertically spaced apart bottom and top 
horizontal free faces 48 and 50, respectively, of the unfragmented 
formation adjoining the void. The two voids 32 and 44 also define a zone 
51 of unfragmented formation left between the free faces 36 and 48. An 
intact zone 52 of unfragmented formation within the boundaries of the 
retort being formed is also left above the uppermost free face 50. 
The technique for expanding oil shale illustrated in the drawings has one 
intermediate void between the upper void and the lower void. In other 
techniques according to this invention, there can be no intervening void, 
or there can be two or more intermediate voids one above another. The 
total number of voids used depends upon the height of the formation to be 
expanded. The greater the height of the formation to be expanded, the more 
voids required. 
Multiple intermediate voids can be useful where the height of the retort 
being formed is very much larger than its width. One or two intermediate 
voids can be excavated between the top and bottom voids so that the in 
situ retort can have a substantial height without need for expanding 
excessively thick zones of formation between adjacent voids. 
Conventional underground mining techniques and equipment are used for 
excavating the voids and access drifts. 
After the spaced apart voids have been excavated in the formation, the 
intervening zones 37, 51 of unfragmented formation and the intact zone 52 
above the upper void 44 are prepared for explosive expansion and 
subsequent retorting operations. A plurality of vertical blasting holes 53 
are drilled in the lower intervening zone 37 upwardly from the lower void 
20 or downwardly from the intermediate void 32. Similarly, a plurality of 
vertical blasting holes 54 are drilled in the upper intervening zone 51 
from the intermediate void 32 or the upper void 44, and a plurality of 
vertical blasting holes 55 are drilled upwardly from the upper void 44 
into the zone 52 of unfragmented formation above the upper void. The 
blasting holes 53, 54, 55 extend longitudinally through the formation and 
are substantially perpendicular to the free faces of the zones of 
unfragmented formation. One of each such vertical blasting hole 53, 54 and 
55 is shown in FIG. 2. In order to show placement of explosive and 
stemming in these blasting holes, they are shown out of proportion in FIG. 
2, i.e., the diameter of the vertical blasting holes is much smaller in 
relation to the dimensions of the retort 12 than shown in FIG. 2. If 
pillars such as the pillars 49 in the upper void 44 have been left within 
the voids, horizontally extending blasting holes are drilled in them for 
their explosive expansion. 
The blasting holes are then loaded with generally cylindrical column loads 
of explosive and stemming. The loads of explosive are distributed in the 
blasting holes 53, 54 in the intervening zones 37, 51, respectively, of 
unfragmented formation using a variation of deck loading. Deck loading is 
described in The Blasters' Handbook at pages 220 and 229. In the method of 
deck loading, two or more loads of explosive are placed in a blasting hole 
spaced apart from each other. Each load is completely separated from an 
adjacent load by a mass or segment of stemming material such as sand, 
gravel or drill cuttings. Each load is separately primed, either 
electrically or with detonating cord. 
Deck loading has been used to enable the explosive to be distributed 
according to the hardness of the rock and for distributing a charge of 
explosive through a blasting hole preferentially toward the bottom of the 
hole to provide more energy for breaking the burden near the bottom of the 
blasting hole than compared to the energy provided for breaking the burden 
nearer the free face. 
According to this invention, deck loading is used to explosively expand 
formation toward two free faces to form a substantially uniformly 
fragmented, substantially uniformly permeable mass of formation particles. 
This is effected by detonating the deck loads of explosive in a blasting 
hole in a single round of explosions with a time delay between adjacent 
loads to stagger detonation of the loads. 
With reference to FIG. 2, the blasting holes 53 in the lower intervening 
zone 37 of unfragmented formation each contain three cylindrical column 
loads, an upper or top load 101, a middle or intermediate load 102, and a 
lower or bottom load 103. Each load is separated from one or more adjacent 
loads by stemming. That is, there is a segment or mass 104 of stemming 
between the upper load 101 and the intermediate 102 load, and there is a 
segment 105 of stemming between the intermediate load 102 and the bottom 
load 103. A purpose of the segments of stemming between adjacent loads is 
to allow time delay between detonation of adjacent loads. Without the 
segments of stemming, detonation of one load could unavoidably lead to 
detonation of an adjacent load. There is also a segment 106 of stemming 
above the upper load 101 and a segment 107 of stemming below the lower 
load 103 to confine these loads to maximize efficiency of blasting. 
Similarly, each blasting hole 54 in the upper intervening zone 51 contains 
three loads, a top load 111, a middle or intervening load 112, and a 
bottom load 113. Between the top and the intermediate load is a segment 
114 of stemming, and between the intermediate and the bottom load is a 
segment 115 of stemming. A segment 116 of stemming is above the top load 
111 and a segment 117 of stemming is below the bottom load 113. 
Because the zone 52 of unfragmented formation above the upper void 44 is 
explosively expanded toward only one void, the upper void, deck loading is 
not required in the blasting holes 55. Thus, there is only one load 121 of 
explosive in each blasting hole 55. Below each load 121 there is a segment 
122 of stemming. If desired, deck loading also can be used in the blasting 
holes 55 in the zone 52 above the upper void 44. 
It should be understood that in the preferred version of this invention 
there are a plurality of vertical blasting holes 53, 54, 55 in the zones 
of 37, 51, and 52, respectively, of unfragmented formation, where each 
blasting hole is loaded with explosive and stemming substantially as shown 
in FIG. 2. The size and total number of blasting holes used is that which 
provides sufficient total explosive energy to expand and fragment the 
formation being blasted. FIG. 3 shows an arrangement which can be used for 
placement of the blasting holes 54 in the upper intervening zone 51 of 
unfragmented formation. Many variations are also useful. 
In practice of this invention, there are at least two deck loads of 
explosive in a blasting hole in order to obtain explosive expansion of 
formation toward two spaced apart voids. As already discussed, blasting 
holes 53 and 54 each contain three loads of explosive. FIG. 4 shows a 
vertically extending blasting hole 130 in a zone 132 of unfragmented 
formation between two substantially parallel vertically spaced apart voids 
20 and 32, in which the blasting hole contains five loads of explosive. 
The loads of explosive are numbered in FIG. 4 from top to bottom as 141, 
142, 143, 144 and 145. Each load is separated from an adjacent load by a 
segment 146 of stemming. A segment 147 of stemming is provided above the 
top load 141 and a segment 148 of stemming is provided below the bottom 
load 145. 
Sufficient stemming is provided between adjacent loads that detonation of 
one load does not interfere with subsequent detonation of an adjacent load 
and does not cause premature detonation of an adjacent load. 
Use of deck loading with staggered detonation of the loads in a blasting 
hole can yield fragmented formation having a particle size approaching 
that achieved by using a plurality of independent, concentrated, spherical 
charges. This can be effected without incurring the cost of having a 
separate blasting hole for each spherical charge. For example, five 
individual deck loads are provided in the one blasting hole of FIG. 4. 
This is significantly less expensive than drilling five blasting holes for 
five individual charges. 
Another advantage of staggering the detonation of the deck loads in a 
blasting hole is that more effective fragmentation is achieved compared to 
detonating all the loads at one time. This occurs due to a preconditioning 
effect, where detonation of a first charge preconditions adjacent 
formation by creating small cracks and fissures in the adjacent formation. 
Thus, when a subsequent load is detonated in the adjacent formation, the 
fissured formation is more readily fragmented. 
Control of time delay between detonation of the deck loads in the blasting 
holes is important for obtaining a retort containing a uniformly 
fragmented mass of particles. There are three constraints on the time 
delay between detonation of the deck loads. 
B. Constraints on Time Delay 
1. Constraint I 
According to the first constraint, to obtain effective fragmentation, each 
deck load is not detonated until the charge is sufficiently close to a 
free face that intervening burden is free to move due to the force of 
explosion of the deck load. For example, intermediate loads 102 and 112 in 
blasting holes 53, 54, respectively, are not detonated until the loads are 
adjacent a free face. For the intermediate load 102 of explosive to be 
adjacent a free face, it is necessary that either the upper deck load 101 
or the lower deck load 103 in the blasting hole 53 be detonated to 
explosively expand formation toward an original free face 34 or 21, 
respectively, to create a new free face extending substantially parallel 
to the original free faces. Thus, to obtain effective expansion of 
fragmented formation toward two voids, the time between detonation of each 
intermediate load and an adjacent load between such intermediate load and 
a free face must be more than the time required for creation of a new free 
face by explosive expansion of formation by detonation of the first of the 
adjacent deck loads to be detonated. Only minimal expansion is needed to 
create the new free face; the formation is not completely expanded at the 
time of creation of the new free face. 
Sufficient expansion is required that the primary compression resulting 
from detonation of a load is at least partly reflected at the adjacent 
free face. If there is inadequate expansion, elastic deformation of 
formation at the free face can bridge the gap resulting in transmission of 
the primary compression wave across the free face with little, if any, 
reflection. Reflection of the primary compression wave is important 
because it sets up a tension wave in the formation which contributes 
greatly to fragmentation of the formation. 
With reference to the blasting hole 54 in the upper zone 51 of the 
unfragmented formation, each of the following sequences for detonation of 
deck loads satisfies this first constraint: 111, 112, 113; 111, 113, 112; 
113, 112, 111; and 113, 111, 112. Any sequence of detonation starting with 
the middle load 112 violates this constraint. 
Referring to FIG. 4, any sequence of detonation starting with any of the 
intermediate deck loads 142, 143 or 144, violates the first constraint. 
Exemplary of sequences which satisfy the first constraint are the 
following: 
141, 145, 142, 144, 143 
141, 142, 145, 143, 144 
141, 142, 143, 145, 144 
141, 142, 143, 144, 145 
Exemplary of sequences which violate the first constraint are the 
following: 
141, 145, 143, 142, 144 
145, 142, 141, 144, 143 
145, 141, 143, 144, 142 
Expansion of formation is required to create a new free face. The time 
required for creation of a new free face by expansion of formation by 
detonation of an explosive deck load it is from about 4 to about 6 times 
the transit time of the primary compression wave formed from detonation of 
the load relative to the nearest substantially horizontal free face. As 
used herein, transit time refers to the round trip time of the primary 
compression wave from the load to the nearest free face and back to the 
load. Thus, according to this principle, if load 103 is detonated before 
load 102 in blasting hole 53, then the time between detonation of load 103 
and detonation of load 102 is at least equal to from about 4 to about 6 
times the round trip time of the primary compression wave from detonation 
of load 103 to the upper free face 21 of the lower void 20 and back to 
load 102. A delay of at least about 4 to about 6 transit times allows 
formation of a new free face by explosive expansion of formation in the 
zone of formation in which the load 103 is placed. The delay can be 
greater than 6 transit times, subject to the second constraint described 
below. 
The primary compression wave is the highest magnitude compression wave 
produced by detonation of a load of explosive. It is the first wave 
resulting from such detonation. The transit time of the primary 
compression wave depends upon the distance between the load and the 
closest free face as well as the speed of propagation of the compression 
wave through the formation. The speed of the compression wave can depend 
on the type of formation being fragmented. For example, in a formation 
containing oil shale having a Fischer Assay of 30 gallons per ton, the 
primary compression wave from detonation of explosive travels through the 
formation perpendicular to the bedding plane at a velocity of from about 
8,000 to 11,000 feet per second. Velocities of about 5,500 to about 7,500 
feet per second are realized when detonating explosive in formation 
containing oil shale having a Fischer Assay of 18 gallons per ton. 
2. Constraint II 
The second constraint on the time of detonation of the deck loads in a zone 
of unfragmented formation is that the time between detonation of the first 
load and the last load to be detonated is less than the time required for 
expanding formation beyond a selected void fraction by detonation of the 
first load to be detonated. The purpose of this constraint is to have all 
the formation expanding before any portion of the formation is 
overexpanded beyond the selected void fraction for creation of a 
substantially uniform permeable mass of formation particles throughout the 
retort being formed. 
As a specific example of this principle, if the deck loads in the blasting 
hole 53 are detonated in the sequence of 103, 101, 102, then the middle 
load 102 is detonated before formation expanded by detonation of load 103 
has expanded beyond a selected void fraction. If formation is permitted to 
overexpand beyond the selected void fraction, it is impossible to 
economically reduce the void fraction of the overexpanded formation. 
Overexansion of a portion of the formation is undesirable because it can 
result in another portion of the fragmented mass of particles having a 
void fraction substantially below the desired void fraction. For example, 
if a fragmented formation is to have an average void fraction of 15% and 
about 80% of the formation expands to a void fraction of about 30%, then 
the remaining 20% of the formation has no room available for expansion. 
Because there is a time delay between detonation of a load and expansion of 
formation adjacent that load, this time lag is considered when staggering 
the detonation of load in a zone of unfragmented formation. For example, 
if the maximum desired void fraction in a retort is about 30%, then the 
last load to be detonated should be detonated before formation expanding 
due to detonation of the first load to be detonated has expanded beyond a 
void fraction of about 25%. Thus, the "selected void fraction" is 25%. The 
purpose of the extra 5% of leeway is to accommodate the lag between the 
detonation of the last load to be detonated and expansion of formation due 
to detonation of the last load. 
3. Constraint III 
As the third constraint, preferably the time between detonation of deck 
loads in the same blasting hole is sufficient for the primary seismic wave 
produced by the detonation of the first load to pass the second load to be 
detonated. The primary seismic wave is the seismic wave of maximum 
amplitude produced in formation due to detonation of explosive. This delay 
is provided to avoid damage to structures and equipment which can occur if 
the primary seismic wave of two loads superimpose to yield an overly large 
primary siesmic wave. 
The time required for the primary seismic wave from one load to pass 
another load depends upon the distance between the two loads, the 
detonation velocity of the explosive, the length of the column of 
explosive being detonated, and the propagation velocity of the wave 
through the formation. IT can be as little as one millisecond. 
C. Sequence of Detonation 
According to this invention all of the explosive in a zone of unfragmented 
formation between vertically spaced apart voids is detonated in a single 
round. All loads of explosive at the same elevation in a zone of 
unfragmented formation can be detonated simultaneously. Thus, all of the 
top deck loads 111 in the blasting holes 54 in the upper intervening zone 
51 of unfragmented formation can be detonated simultaneously. Likewise, 
all of the bottom loads 113 can be detonated simultaneously and all of the 
intermediate loads 112 can be detonated simultaneously. Detonating a 
plurality of loads at the same elevation in a zone of unfragmented 
formation creates a new free face extending substantially parallel to the 
original free faces. 
For example, assuming a sequence of detonation of the loads in the upper 
intervening zone 51 of 111, 113, 112, detonation of all of the top loads 
111 in an upper portion or zone 241 of the upper intervening zone 51 
expands the upper portion 241 toward the upper void 44 and creates a first 
new free face, shown schematic by dashed line 242 in FIG. 2, which is 
substantially parallel to the original free face 48 and the remaining free 
face 36 of the upper intervening zone 51. Likewise, detonation of the 
bottom loads 113 results in expansion of a lower portion 243 of the upper 
intervening zone toward the intermediate void 32 with creation of a second 
new free face shown by dashed line 244 in FIG. 2, which is substantially 
parallel to the first new free face 242. The first new free face 242 and 
the second new free face 244 are near to the top and bottom, respectively, 
of the middle charges 112 of explosive. Then, by detonating the remaining 
middle loads 112 of explosive, the remaining central portion of the upper 
intervening zone 51 is explosively expanded toward both the upper void 44 
and the lower void 32. 
To avoid excessive seismic shock and damage to above ground and below 
ground structure, loads of explosive at the same elevation in the 
formation can be sequentially detonated. Thus, all loads of explosive at 
the same elevation are not necessarily detonated simultaneously. 
When explosively expanding formation toward two substantially parallel 
voids, the sequence of detonation of the deck loads in a blasting hole, 
the detonation point of each load, the type of explosive used for each 
load, and the relative amount of explosive used for the loads can all 
affect the proportion of formation which is expanded toward each of the 
voids. How each of these factors can affect the distribution of formation 
is now discussed. 
With respect to sequence of detonation, to expand an intervening zone of 
unfragmented formation between an upper void and a lower void uniformly 
toward both voids, each load in the blasting holes is detonated no later 
than detonating any of the loads between such load and the vertical center 
of mass of the portion of the formation being fragmented. 
For example, to expand the upper intervening zone 51 uniformly toward the 
intermediate void 32 and the upper void 44, then the middle load 112 is 
the last load to be detonated. Similarly, referring to FIG. 4, to 
uniformly distribute the intervening zone 132 of unfragmented formation 
toward the upper void 32 and lower void 20, the middle load 143 is the 
last load to be detonated, the upper intermediate charge 142 is detonated 
after the top load 141, and the lower intermediate load 144 is detonated 
after the bottom load 145. 
To explosively expand formation preferentially toward one of the two 
parallel voids, then each deck load in a blasting hole is detonated no 
earlier than detonating any of the deck loads between such load and the 
void toward which a higher proportion of the formation is to be expanded. 
For example, referring to FIG. 2, to preferentially expand the lower 
intervening zone 37 of unfragmented formation toward the lower void 20, 
then the loads in the blasting holes 53 are detonated from the bottom to 
the top, i.e., the bottom loads 103 are detonated first, followed by the 
middle loads 102, and finally the top load 101. Even with this sequence of 
detonation expansion of formation toward both voids is unavoidable. 
D. Locus of Initiation 
The locus of initiation of detonation of a load affects the direction of 
expansion of formation adjacent the load. When detonation of a cylindrical 
load is initiated at one of its ends, formation tends to be preferentially 
expanded toward the end at which detonation is initiated. This results 
from the time required for the detonation wave to travel through a column 
of explosive. 
Referring to FIG. 2, explosive initiation devices, such as electric 
blasting caps used for detonating explosive loads are each represented by 
an "X" 160. For example, it can be desired to expand the upper intervening 
zone 51 uniformly toward the upper void 44 and the lower void 32, and to 
expand the lower intervening zone 37 primarily toward the lower void 20. 
Thus, in the blasting holes 54 in the upper intervening zone, detonation 
of each top load 111 is initiated substantially at its top, detonation of 
each bottom load 113 is initiated substantially at its bottom, and 
detonation of each middle load 112 is initiated substantially in the 
middle of its vertical height. To expand a higher proportion of the lower 
intervening zone 37 toward the bottom void 20, detonation of the lower 
loads 103 and the middle loads 102 in the blasting holes 53 is initiated 
substantially at thier bottom, and detonation of the top loads 101 is 
initiated substantially at their top. The sequence of detonation used is 
the bottom loads 103 first, the middle loads 102 next, and the top loads 
101 last. Preferably the bottom loads 113 in the upper intervening zone 51 
and the top loads 101 in the lower intervening zone 37 are detonated 
substantially at the same time so that formation can be substantially 
expanded from both the upper intervening zones 51 and the lower interveing 
zone 37 toward the intermediate void 32. 
E. Size of Loads and Loading Ratio 
The relative size of the loads and the relative loading ratio of the 
explosive used for deck loads affects the proportion of formation expanded 
toward each of two voids. Loading ratio refers to the quantity in tons (or 
cubic yards) of formation blasted per pound of explosive used. 
Because the middle portion of a zone of unfragmented formation is not 
adjacent to a void, it can be more difficult to fragment and expand the 
middle portion of the zone toward the voids than it is to expand the upper 
and lower portions of that zone. To overcome this, the intermediate loads 
used for expanding and fragmenting the intermediate portion of the 
formation can be larger and/or use explosive having a higher loading ratio 
than the top and bottom charges. 
F. Pillars 
If pillars of unfragmented formation are left in the voids, preferably the 
pillars are fragmented before detonating the explosive in the blasting 
holes in the intervening zones of formation so the pillars do not 
interfere with explosive expansion of the intervening zones of formation. 
Thus, preferably explosive in the upper intervening zone 51 is not 
detonated until after creation of the free face at the juncture of the 
pillars 59 in the upper void 42 and the upper intervening zone 51 by 
detonation of explosive in the pillars. 
After the pillars 49 are explosively fragmented, caving of formation 
supported by the pillars can occur. Since such caving can interfere with 
explosive expansion of the upper intervening zone 51 toward the upper void 
44, explosive in the upper intervening zone preferably is detonated before 
or at the same time as caving of the formation 52 previously supported by 
the pillars 49. To this same effect, explosive in the blasting holes 54 in 
the upper intervening zone is detonated before or at the same time as 
explosive is detonated in the blasting holes 55 in the zone 52 of 
unfragmented formation above the upper void 44. 
To obtain a uniform distribution of formation in a void containing pillars, 
preferably explosive in an unfragmented zone below and/or above the void 
is not detonated until after pillar fragments from fragmenting the pillars 
in the voids are uniformly distributed. Thus, preferably the loads of 
explosive in the upper intervening zone 51 and the loads 55 of explosive 
in the zone 52 above the upper void are not detonated until after pillar 
fragments resulting from fragmenting the pillars 53 in the upper void are 
substantially uniformly distributed in the upper void 44. In this regard 
it can be noted that caving of formation previously supported by pillars 
is time dependent, the start of caving depends on the properties of the 
formation, its depth and the unsupported span. In some cases many seconds 
can elapse between removal of pillars and caving of overlying formation. 
G. Void Fraction 
The distributed void fraction or volume of the permeable mass of particles 
in the retort, i.e., the ratio of the volume of the voids or spaces 
between particles to the total volume of the fragmented permeable mass of 
particles in the subterranean in situ retort 12, is controlled by the 
volume of the excavated voids into which the formation is expanded. 
Preferably, the total volume of the excavated voids is sufficiently small 
compared to the total volume of the retort that the expanded formation is 
capable of filling the voids and the space occupied by the expanded 
formation prior to expansion. In other words, the volume of the voids is 
sufficiently small that the retort is full of expanded formation. In 
filling the voids and the space occupied by the zones of unfragmented 
formation prior to fragmentation, the particles of the expanded formation 
become jammed and wedged together tightly so they do not shift or move 
after fragmentation has been completed. In numerical terms, the total 
volume of the voids is preferably less than about 30% of the total volume 
of the retort being formed. In one embodiment of this invention, the 
volume of the voids is preferably not greater than about 25% of the volume 
of the retort being formed, as this is found to provide a void fraction in 
the fragmented formation containing oil shale adequate for satisfactory 
retorting operation. If the void fraction is more than about 25%, an undue 
amount of excavation occurs without concomitant improvement is 
permeability. Removal of the material from the voids is costly, and 
kerogen contained therein is wasted or retorted by costly above ground 
methods. 
The total volume of the excavated voids is also sufficiently large compared 
to the total volume of the retort that substantially all of the expanded 
formation within the retort is capable of moving enough during explosive 
expansion to fragment and for the fragments to be displaced and/or 
reoriented. Such movement provides permeability in the fragmented mass to 
permit flow of gas without excessive pressure requirements for moving the 
gas. When the fragmented particles containing oil shale are retorted, they 
increase in size. Part of this size increase is temporary and results from 
thermal expansion, and part is permanent and is brought about during the 
retorting of kerogen in the shale. The void fraction of the fragmented 
permeable mass of shale particles should also be large enough for 
efficient in situ retorting as this size increase occurs. In numerical 
terms, the minimum volume of the voids in view of the above considerations 
is preferably above about 10% of the total volume of the retort. Below 
this average percentage value, an undesirable amount of power is required 
to drive the gas blowers causing retorting gas to flow through the retort. 
The above percentage values assume that all of the formation within the 
boundaries of the retort is to be fragmented; that is, there are no 
unfragmented regions left in the retort. If there are unfragmented regions 
left within the outer boundaries of the retort, e.g., for support pillars 
or the like, the percentages would be less. 
H. Examples 
In one example of practice of this invention, the total height of the in 
situ retort or room 12 is about 268 feet. The intermediate void 32 and 
lower void 20 each have a height (represented by the dimension a in FIG. 
2) of about 30 feet, and the height (represented by the dimension b) of 
the upper void is about 23 feet. Each void contains pillars comprising 
about 30% of the volume of the void. Each intervening zone of unfragmented 
formation 37 and 51 and the zone of unfragmented formation 52 above the 
top void is about 184 feet square (represented by dimension c in FIG. 2) 
in horizontal cross-section, which essentially matches the horizontal 
cross-section of the voids 20, 32 and 44, although these can be a foot or 
so wider to accommodate drilling equipment near the edges. The thickness 
(represented by the dimension d) of each intervening zone is about 76 feet 
and the thickness of the upper zone 52 above the upper void is about 33 
feet. 
Explosive is dispersed in a plurality of vertical blasting holes in the 
upper and lower interveing zones of unfragmented formation and in the zone 
of unfragmented formation above the top void substantially as shown in 
FIG. 2. Means for detonating the loads of explosive are provided and are 
placed in the load of explosive substantially as shown in FIG. 2. Deck 
load 103 is detonated first, followed by load 102 about 25 to about 50 
milliseconds later. Loads 101, 111, 113 and 121 are all detonated 
substantially simultaneously about 25 to 50 milliseconds after detonating 
load 102. Load 112 is then detonated about 25 milliseconds later. 
This results in formation of a retort about 184 feet square having a height 
of about 268 feet filled with a reasonably uniformly fragmented, 
reasonably uniformly permeable mass of particles having an average void 
fraction of about 21.7%. As described above, this void fraction is within 
the desired range for maximizing recovery of shale oil from the volume 
being retorted and for providing for a minimal pressure drop from top to 
bottom of the vertical retort. 
In another example, two vertically spaced apart voids are excavated in a 
formation containing oil shale with the lower void having a height of 
about 30 feet and being about 184 feet square in cross-section. The upper 
void has the same cross section and is about 15 feet in height. An 
intervening zone of unfragmented formation about 96 feet thick is left 
between the lower void and the upper void. Four elongated pillars, each 16 
feet by 172 feet in horizontal cross-section, are left in the upper and 
lower voids in the pattern shown in FIG. 3. Vertical blasting holes 10 
inches in diameter on 20 .times. 20 feet centers are drilled downwardly 
into the intervening zone and each is loaded with three deck loads of 
explosive. The upper and lower loads are 13.5 feet in height and comprise 
an explosive having a loading ratio of 0.55 cubic yards of formation per 
pound of explosive. The middle loads are 40 feet in height and comprise an 
explosive having a loading ratio of 0.37 cubic yards of formation per 
pound of explosive. Between each upper load and each middle load and 
between each middle load and each lower load are 4 feet of sand stemming. 
Below each bottom load and above each upper load are 10.5 feet of 
stemming. Electrical detonators are provided for each load at about its 
vertical center of mass. After detonation of explosive in the pillars, the 
upper loads are detonated, and then 25 to 50 milliseconds later the bottom 
loads are detonated. Then the middle loads are detonated from about 75 to 
about 100 milliseconds after detonation of the upper loads, i.e. about 25 
milliseconds after detonation of the bottom loads. This results in a 
subterranean room or cavity about 141 feet high and about 184 feet square 
in a horizontal cross section. The room contains a substantially uniformly 
fragmented, substantially uniformly permeable mass of formation particles. 
The mass has an average void volume or void fraction of about 21.5%. 
Following explosive expansion of the formation, at least one gas access 
communicating with an upper level of the retort 12 is established by 
forming a horizontal tunnel 58 and several communicating vertical conduits 
60 to the top of the fragmented permeable mass of expanded formation 
contained in the room. 
I. Recovery of Product 
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 application 
Ser. No. 772,760, filed Feb. 28, 1977, now abandoned, and assigned to the 
assignee of the present application, and incorporated herein by this 
reference for one method in which an access conduit 58 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 14 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 58 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 to 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 an oxygen containing retort inlet mixture 
through the access conduit 58 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 gases from the 
retort at a lower elevation serves 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 61 and pumped to 
the surface by a pump 62 through a liquid product transfer line 64. 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 
via a gas product transfer line 66. 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. 
J. Orientation 
Many formations containing oil shale have bedding plane dips of less than 
about 5.degree., in which case the edges of the vertically spaced apart 
voids should be in a substantially vertical plane and the resulting retort 
has substantially vertical side boundaries. If the dip of the formation 
containing oil shale is more than about 5.degree., the voids can have 
their edges offset and be tilted so that the free faces of the intervening 
zone of unfragmented formation are substantially parallel to the bedding 
plane of the formation. The result would be a retort that is re-oriented 
accordingly to conform to the bedding plane so that the side boundaries of 
the retort are perpendicular to the bedding plane. This provides oil shale 
having approximately the same kerogen content across the retorting zone at 
any particular time as the retorting zone advances through the retort. 
Also, expanding formation perpendicular to the bedding plane maximizes 
fragmentation of the formation. 
The above described use of the invention for recovering carbonaceous values 
including shale oil from subterranean formation containing oil shale is 
for illustraftive purposes only, and is not considered to be a limitation 
of the scope of the invention. For example, the invention can be used in a 
variety of instances where it is desirable to prepare subterranean ore 
formation for in situ recovery where the particle size and subsequent void 
volume distribution of the ore particles are to be controlled to maximize 
the recovery of constituents from the formation. 
In addition, instead of loading upper and lower explosive loads into the 
same blasting hole, a blasting hole can contain an upper explosive load 
and another blasting hole can contain a lower explosive load, where the 
center of mass of the upper explosive load is at a higher elevation than 
the center of mass of the lower explosive load. 
For example, with reference to FIG. 5, there is shown a first blasting hole 
260 extending vertically through both an upper zone 262 and a lower zone 
264 of an intervening zone of unfragmented formation between an upper void 
20 and a lower void 32. There is a second blasting hole 267 which is 
adjacent to the first blasting hole 260 and which extends vertically from 
the upper void 20 through the upper zone 262 of unfragmented formation. 
The first blasting hole contains a lower cylindrical explosive load 266 in 
the lower zone 264. Below the lower explosive load is a short segment 268 
of stemming, and above the lower explosive load is a longer segment 270 of 
stemming filling the first blasting hole up to the upper void 20. The 
second blasting hole is loaded with an upper cylindrical explosive load 
272 in the upper zone 262, and a short segment 274 of stemming above the 
upper load. The center of mass of the upper explosive load 272 is at a 
higher elevation than the center of mass of the lower explosive load 266. 
The upper and lower explosive loads are detonated at separate times in a 
single round for explosively expanding formation between the upper and 
lower voids toward the upper and lower voids. 
A plurality of such blasting holes 260 and 267 can be used either adjacent 
to each other as shown in FIG. 5, or spaced apart from each other. In 
addition, blasting holes containing only an upper explosive load and 
blasting holes containing only a lower explosive load can be used in 
conjunction with blasting holes containing both upper and lower explosive 
loads. 
Therefore, because of variations such as these, the spirit of scope of the 
appended claims should not be limited to the versions described herein.