Underground linkage of wells for production of coal in situ

In preparation for producing coal in situ two or more production wells are linked together through the coal seam by burned channels created by one or more blind hole burns.

BACKGROUND OF INVENTION 
This invention relates to production of coal in situ wherein vertical wells 
are drilled into an underground coal seam, the walls are linked together 
through the coal to form reaction zones and the coal is produced as gases 
and liquids. The invention more particularly is directed to methods of 
accomplishing the linkage channels through the coal. 
It is well known in the art how to produce coal in situ, the most common 
method being to set the coal afire underground, with the fire sustained by 
continuous injection of an oxidizer. By proper control of the oxidizer, a 
reducing environment is established in the reaction zone in the coal with 
the resultant generation of combustible gases. If air is used as the 
oxidizer, produced combustible gases generally range from about 80 to 200 
BTU per standard cubic foot. 
In the early experiments with burning coal in situ, shafts were excavated 
from the surface of the earth to the bottom of the underground coal seam. 
Channels were then dug through the coal to provide communication with at 
least two shafts. Workmen ignited the coal face and then evacuated to the 
surface. The fire was propagated by injecting an oxidizer such as air into 
one shaft and removing the products of reaction from the second shaft. In 
this manner a low BTU gas was generated with a heat content in the order 
of 150 BTU per standard cubic foot. As the burning proceeded and the 
linkage channel became larger, the heat content of the generated gases 
would become lower and lower due to oxygen bypass of the burning face. A 
part of the injected oxidizer would be consumed in the fire and a part 
would proceed to the exit shaft where the hot low BTU gas would be further 
burned. In severe cases the resulting flue gases would have a heat content 
too low for combustion and were therefore useless as a fuel gas. 
One of the prime objectives of early experiments in producing coal in situ 
was to minimize the time workmen were required underground. After many 
years of experimentation it became apparent that underground workmen would 
not be required if wells were drilled into the coal seam. This raised the 
problem of how to link the wells together with a communication passage 
through the seam. Through the years various linkage schemes were tried 
including hydraulic fracturing, directional drilling, explosive 
fracturing, electro-linking using electrical current, various methods of 
burning the channel and the like. 
More experimental work on linkage has been performed in Russia than the 
combined experimental work done in the other countries of the world. The 
Russian technicians have perfected a reliable method of linkage using a 
reverse burn between two or more vertical wells. A detailed description of 
the successful linking procedure may be found in U.S. Pat. No. 4,036,298 
of Kreinin et al. In its elementory form the Russian procedure provides 
for two wells drilled to the bottom of the coal seam. High pressure air is 
injected into a first well and hot ignition material is placed into a 
second well. The air injected into the first well will migrate radially 
outward and a portion of the air will reach the second well, causing 
ignition of the coal seam and propagation of the underground fire through 
the coal seam towards the on coming oxygen supply. The air passing through 
the coal seam proceeds through paths of least resistance, a path that is 
unknown to the operator except in the most general sort of way. Thus the 
channel burned as the fire proceeds from the ignition well to the injector 
well is always something other than a straight line, and often is a path 
quite circuitous in nature. As long as the burned channel remains near the 
bottom of a flat coal seam, straightness of the path is not a critical 
consideration. Should the burned channel have significant deviations in a 
vertical direction, difficult operating problems will arise later in the 
production cycle due to flame override. 
Linked vertical wells using the Russian procedures work exceptionally well 
when there is a thin parting in the coal near the bottom of the seam. In 
this case the oxidizer release point is established in the coal below the 
parting and the burned channel is thus restrained from migrating upward. 
Once the reaction zone is well established from the burned channel, the 
parting is broken by generated heat and roof fall, and the seam is 
consumed from the bottom up. 
In the Russian procedure the linkage burn proceeds as a reverse burn, that 
is, the burn moves in an opposite direction from the direction of flow of 
the oxidizer. Once the channel burns through to the oxidizer injection 
well, permeability to the flow of gases is greatly increased, injection 
pressure drops significantly and the burn reverses itself and proceeds as 
a forward burn away from the injection well. In this manner a reaction 
zone is established in the coal with an oxidizer injected into one well 
and the products of reaction withdrawn from a second well. 
In and around the reaction zone three significant environments are 
established. At the fire face the environment is highly oxidizing, down 
stream away from the fire a shortage of oxygen establishes a reducing 
environment, and the coal adjacent to the fire is subjected to a 
pyrolyzing environment. In the oxidizing environment coal is consumed and 
converted into carbon dioxide, sulfur dioxide and water vapor, gases that 
have little use except for their sensible heat. At these gases proceed 
down stream into the reducing environment the carbon dioxide is converted 
to carbon monoxide and the sulfur dioxide is converted into hydrogen 
sulfide, with further enrichment by the gases of pyrolysis. 
There are obvious limits of effectiveness in the Russian system of linkage. 
A practical limit is established in maximum well spacing due to the 
requirement of initially injecting the oxidizer in all directions from the 
injection well. A distant second well might never receive enough oxygen 
for ignition. Should the path of least resistance between the wells happen 
to be a path near the top of the seam, flame override and all of its 
attendant problems are sure to occur. Also in wet coal seams the path of 
least resistance to air flow normally will be above the water, a situation 
that sets the stage for flame override. 
When a coal seam is an aquifer of significance, it is necessary to lower 
the water table in the coal. Percolation of water through the coal is 
quite slow and lowering the water table in a uniform manner is virtually 
impossible when using pumps to withdraw the water. By placing pumps in 
sumps below the coal seam the water table can be lowered to the bottom of 
the seam in the immediate vicinity of the well bore. Water will remain at 
an angle of repose away from the well bore, and at a point some distance 
from the well bore, the localized water table can be several feet above 
the bottom of the coal seam. 
In this case of residual water residing in an uneven water table, the path 
of least resistance to air flow normally is a path that overrides the 
water. In attempting linkage between two wells using the reverse burn 
procedure, the resultant linkage channel will stray considerably from the 
bottom of the seam. 
It is possible to substantially remove the free water in a coal seam using 
procedures as described in U.S. Pat. No. 2,973,811 of Rogers. The methods 
of Rogers provide for injecting gas such as air into the aquifer under 
such pressure as necessary to drive the water out of the area of 
influence. Such pressures are considerable higher than those used in the 
Russian procedures of linkage, although a certain amount of water 
displacement occurs in the Russian procedure. 
A reasonable amount of free water remaining in a coal seam is beneficial to 
the reactions of coal gasification, therefore driving all of the free 
water out of the coal to be gasified is not desirable. Water reacts with 
hot coal to form carbon monoxide and hydrogen, two desirable gases with 
heat contents exceeding 300 BTU per standard cubic feet. Water driven out 
of a coal seam can be made to return by slacking off on pressure. The rate 
of return, however, is generally too slow to be of commercial interest. 
Thus it is preferable to leave most of the water in the seam provided 
linkage can be accomplished at or near the bottom of the seam. 
Another method of linkage that is independent of the water content of coal 
is described in U.S. Pat. No. 4,062,404 of Pasini et al. A well is drilled 
some distance away from the intended reaction zone and the well is 
deviated until the bore encounters the underground coal in a direction 
substantially parallel to the seam. Directional drilling continues along 
the bottom of the seam for the desired distance planned for the reaction 
zone. The circuit is completed by drilling a vertical well to intercept 
the bottom of the deviated hole. Such an arrangement provides a channel at 
or near the bottom of the seam, but has the disadvantage of difficult and 
costly drilling procedures. 
Still another method of linkage is described in U.K. Pat. No. 756,852 of 
Montagnon which provides for establishing a permeable channel with a flow 
of electric current between two points in the coal seam. The flow of 
electric current is somewhat analogous to the flow of air, in that the 
current will flow through the path of least electrical resistance. Coal, 
being a non-homogeneous rock, has unpredictable paths of electrical 
circuits. Over long distances between electrodes the likelihood increases 
for the path to stray substantially above the bottom of the coal, 
resulting in a path that promotes flame override. 
Flame override can be a serious detriment to successful production of coal 
in situ. The natural tendency of a fire is to burn upward as long as there 
is a source of fuel in that direction. The worst case in the reverse burn 
procedure for linkage occurs when the injected air migrates to the top of 
the seam and persists in that location until it nears the location of the 
lower pressure in the ignition well. The burned channel, for the most 
part, will lie at the top of the seam. Upon burn through and the 
establishment of a reaction zone, the two wells will appear initially to 
be performing satisfactorily, with produced gases containing approximately 
170 BTU per standard cubic foot. The first sign of trouble is signalled by 
a steady drop in the BTU content of produced gas. The reaction zone, with 
no fuel above it, is gradually becoming engulfed in its own ashes. A 
partial remedy can be applied by significantly increasing the velocity of 
the gases through the reaction zone, thus picking up the ashes into the 
flue gas for removal above ground. Such a procedure defeats one of the 
purposes of in situ gasification of coal; that is, to leave the ash 
content of the coal underground. Increased velocities of the oxidizer also 
aggrevates the oxygen by pass problem where combustible gases are 
subjected to unplanned burning underground with the resultant destruction 
of combustible gases. Also, attempting to burn an underground fire 
downward is something other than a rewarding task. 
From the foregoing it is apparent that successful gasification of coal in 
situ requires reaction zones that begin at the bottom of the coal seam. In 
this mode the fire has the preponderence of the fuel supply above it and 
the ashes fall out of the path of the fire as it seeks new fuel. Also from 
the foregoing it is apparent that a lengthy reaction zone is desirable 
because the reducing environment portion of the underground channel 
provides the setting for generation and recovery of combustible gases. In 
the Russian procedures for linkage and establishment of reaction zones, 
well spacing is generally limited to short distances in the order of 70 
feet. Greater distances between wells is desirable from an economic point 
of view as well as the desirability of having a longer distance for a 
reducing environment in the underground channel. Well spacings greater 
than that of the Russian procedures would provide more favorable economics 
and provide a setting for improved performance of the in situ reactions. 
Such lengthened spacing requires a correspondingly effective linkage 
procedure. 
In U.S. Pat. No. 4,010,801 of the present inventor, methods are taught 
wherein a blind hole burn in coal creates underground channels and 
reaction zones for the production of coal in situ. The procedures of the 
present invention extend the teachings of U.S. Pat. No. 4,010,801 to 
include methods of linking two or more wells by burning channels along the 
bottom of the coal seam. 
SUMMARY OF THE INVENTION 
Two wells are drilled from the surface of the earth into and through a coal 
seam. The wells are hermetically sealed and an oxidizer injection tubing 
is lowered into each well together with a whipstock. The whipstock is 
capable of making a 90.degree. bend in the oxidizer injection tubing. The 
whipstock is set in each case so that the oxidizer injection tubing 
emerging from the whipstock is aligned toward the opposite well. The coal 
is set afire and the fire is propagated by an oxidizer injected through 
the oxidizer injection tubing. The oxidizer is tempered with water vapor 
to control maximum temperatures of the fire and to provide cooling to the 
oxidizer injection tubing. Additional oxidizer tubing is inserted in each 
well as the channel is lengthened through the coal. Linkage between the 
two wells is thus attained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, two wells 10 and 12 are drilled from the surface of 
the earth 11 through overburden 14, through coal seam 16 and forming sumps 
27 and 29 in the underburden. The wells are hermetically sealed, for 
example by setting a casing to the top of the coal seam 16. A suitable 
closure 15 is affixed to the well casing. Into well 10 an oxidizer 
injection tubing 18 is inserted with whipstock 26 emplanted in sump 27 so 
that the oxidizer injection tubing 18 is bent at an appropriate angle, for 
example 90.degree., and the portion of oxidizer injection tubing 18 
emerging from whipstock 26 is pointing toward well 12. Initially oxidizer 
injection tubing will emerge from whipstock 26 only a short distance, for 
example 2 inches, while the illustration of FIG. 1 shows the oxidizer 
injection tubing near the final stages of the linkage procedure. Oxidizer 
injection tubing 18 contains valve 19 for regulation of flow of the 
oxidizer. Well 10 has fluid withdrawal pipe 22 with valve 23, which 
permits the products of reactions to be withdrawn from the underground 
reaction zone and provides a means of applying back pressure control. 
Likewise well 12 contains oxidizer injection tubing 20 containing valve 21 
with whipstock 28 emplaced in sump 29. Whipstock 28 is set so that tubing 
20 is pointed toward well 10 as it emerges from the whipstock. Well 12 has 
fluid withdrawal pipe 24 which contains valve 25. 
Prior to initiating the linkage procedure it is preferred that water 
withdrawal pumps (not shown) be temporarily installed in sumps 27 and 29 
and that the water table be lowered to the bottom of the coal seam in the 
vicinity of the production wells 10 and 12. When the water table is thus 
lowered the boundary of the water table 17 is distorted from its normal 
position. Coal 16A is substantially dry of free water and Coal 16B retains 
a considerable amount of free water within its void spaces. Such free 
water in Coal 16B provides a reasonably effective barrier to the migration 
of gases through the coal. Should linkage between wells 10 and 12 be 
attempted using the reverse burn technique, the linkage channel tends to 
occur in Coal 16A above the water table boundary 17. Such a linkage 
channel deviating a considerable distance above the bottom of the seam 
considerably reduces the overall efficiency of the underground burn. 
After the water table has been lowered in the vicinity of wells 10 and 12 
and the oxidizer injection tubings 18 and 20 have been positioned into 
whipstocks 26 and 28 as previously described, the linkage procedure of the 
present invention can be initiated. The procedure begins in well 10 by 
placing suitable ignition material in the lower portion of well 10, for 
example by opening closure 15 and dropping incandescent charcoal 
briquettes into the hole. Closure 15 is then returned to its sealed 
position and oxidizer injection is begun through oxidizer injection tubing 
18. While any convenient ignition procedure may be used in the practice of 
the present invention, by way of example hot charcoal briquettes are used 
in sufficient quantity to contact the coal seam adjacent to the lower end 
of tubing 18. By continuing the injection of oxidizer, for example air, 
through tubing 18, coal 16 will reach its ignition temperature at a 
location in the path of the oxidizer blast in a relatively short time, for 
example approximately two to five minutes. Once the coal seam is ignited 
in a localized area, a channel through the coal is initiated. The channel 
30 away from well 10 is lengthened by continuing injection of oxidizer 
through tubing 18, and by periodically inserting more length to tubing 18 
so that the bottom end of tubing 18 remains in reasonable proximity to the 
burning end 40 of channel 30. In this manner channel 30 may be lengthened 
from the well bore of well 10 along the bottom of coal 16 for considerable 
distance, for example as much as several hundred feet. In some cases it 
may be practical to terminate channel 30 at or near the well bore of well 
12, and thus preclude the necessity of initiating a second channel from 
well 12. Preferably, however, channel 30 is propagated to a point near the 
midpoint between wells 10 and 12. 
In a like manner channel 32 is propagated toward well 10 from well 12 by 
igniting the coal at the well bore of well 12 and injecting oxidizer 
through tubing 20. Tubing 20 is lengthened into well 12 as channel 32 is 
burned toward well 10 and the lower end of tubing 20 is kept in reasonable 
proximity of burning end 42 of channel 32. Preferably channel 32 is 
propagated to a point near the midpoint between wells 12 and 10. 
It is desirable that channel 30 and channel 32 be propagated until they 
merge, however it is not necessary that their paths be aligned so 
precisely. As illustrated in FIG. 2, channels 30 and 32 were imperfectly 
aligned. As a practical matter the channels may be aligned so that they do 
not intersect, yet the channels may be joined by an alternate procedure. 
For example, during the burning of channel 30, oxidizer is injected into 
tubing 18 and the products of reaction are withdrawn through withdrawal 
pipe 22. Likewise during the burning of channel 32, oxidizer is injected 
through tubing 20 and the products of reaction are withdrawn through 
withdrawal pipe 24. The coal around channels 30 and 32 is at pyrolysis 
temperature as a result of the underground fires and such coal is giving 
off the gases of pyrolysis. In a shrinking coal, the permeability of the 
coal adjacent to channels 30 and 32 is significantly increased. Thus when 
channels 30 and 32 are burned to points near each other, an alternate 
procedure can be employed to complete the linkage between burning ends 40 
and 42. With the increased permeability in the coal between burning ends 
40 and 42 due to pyrolysis, linkage can be completed, for example, by 
closing valves 19 and 25 and continuing oxidizer injection through tubing 
20. Preferably the oxidizer injection pressure is increased, for example 
an increase in the range of 20% to 200%, in order to provide excess 
oxidizer. With this arrangement the burn in channel 32 will continue as a 
forward burn toward channel 30 and the burn in channel 30 will propagate 
as a reverse burn toward channel 32 until the two channels burn together, 
thus completing the linkage between wells 10 and 12. 
It is preferred that the temperatures in the reaction zones of channels 30 
and 32 be controlled to avoid severe damage to the metal parts installed 
in wells 10 and 12. Generally the temperatures should be in the range of 
above the ignition temperature of the coal, for example approximately 
800.degree. F., to a maximum range of about 1200.degree. F. The maximum 
temperature of incandescent coal is generally about 2000.degree. F. 
without flames. This temperature can be lowered to the preferred maximum 
range of about 1200.degree. F. by injecting appropriate quantities of 
water into the reaction zone. Such injection of water preferably is done 
as a mixture of water and oxidizer injected through tubing 18 and 20. Such 
injection of a mixture of water and oxidizer will keep tubing 18 and 20 
sufficiently cool to avoid significant damage to the tubing. Preferably 
tubing 18 and 20 is of relatively small diameter, for example less than 
2", so that they may be properly bent in whipstocks 26 and 28. 
Preferably oxidizer injection pressures are kept at relatively low levels, 
for example in the order of two atmospheres, although the pressures 
required will vary from site to site. For example in deep seams the 
hydraulic pressure of the water in Coal 16B may be sufficiently high that 
water encroachment into burning channels 30 and 32 becomes a problem. The 
reaction zones in channels 30 and 32 can be destroyed by quenching if 
encroachment water is permitted to enter the channels in sufficient 
volumes to reduce the temperature below that required for reaction of 
fluids with the coal. Thus control is required to limit encroachment of 
water into the reaction zones. Such control can be applied by increasing 
oxidizer injection pressures in tubing 18 and 20 while holding back 
pressure with the proper adjustment of values 23 and 25. By maintaining 
the pressure in channels 30 and 32 above that of the hydraulic head 
pressure, water can be excluded from the channels. By maintaining the 
pressure in the channels slightly below hydrostatic head pressure, free 
water in Coal 16B can be permitted to enter the channels and thus provide 
a measure of temperature control in the reaction zones. Such controlled 
water encroachment can serve as an alternate to injecting water with the 
oxidizer through tubing 18 and 20. 
The emplacement of whipstocks 26 and 28 can be done in several ways. In one 
method tubing 18 is inserted into whipstock 26 prior to lowering into well 
10, with a small length of tubing 18 emerging from the whipstock, for 
example 2" of tubing protruding outside of the whipstock. A stopper is 
inserted in the protruded end of tubing 18, such stopper serving as a 
temporary barrier to fluids entering tubing 18. The assembled unit of 
whipstock 26 and tubing 18 is lowered in well 10 until the whipstock 
reaches the bottom of sump 27. The assembled unit then is aligned so that 
the protruding tubing is pointed toward well 12. A suitable sealant, for 
example portland cement, is poured into sump 27 and allowed to set. Once 
the whipstock is thus emplaced, oxidizer is injected into tubing 18 with 
sufficient pressure to dislodge the stopper, thus permitting ignition and 
initiation of channel 30. In this method whipstock 26 becomes a permanent 
installation in well 10, and upon completion of the linkage procedure 
remains in well 10 as an expendable item. 
It is important that tubing 18 and 20 be sufficiently rigid to withstand 
the compressive forces required to insert additional lengths of tubing 
into wells 10 and 12 through whipstocks 26 and 28. It is also important 
that tubing 18 and 20 be sufficiently flexible to be capable of bending 
through whipstocks 26 and 28 without causing failure to the tubing. 
Looking now to well 10 as an example, once the burning of channel 30 is 
initiated, the hot gases from the reaction zone of channel 30 will 
significantly raise the temperature of whipstock 26 and tubing 18 located 
near the bottom of well 10. Such increase in temperature will facilitate 
the bending of tubing 18 through whipstock 26. Such increase in 
temperature also lessens the rigidity of tubing 18 between the whipstock 
and the well head. When the increase in temperature expected to be 
encountered within well 10 is sufficient to alter the regidity of tubing 
18 to the point that the tubing tends to buckle, an alternate procedure 
should be used in emplacing whipstock 26. 
In the alternate emplacing procedure (FIG. 3) a protective pipe 50 is 
affixed to whipstock 26, such pipe being of larger diameter then tubing 18 
so that an annulus 51 is formed between tubing 18 and the protective pipe 
50. While it is preferable that all of the tubing to be used as tubing 18 
be in one piece, the protective pipe can be in several joints. The first 
joint of the protective pipe is affixed to whipstock 26 and preferably the 
protective pipe contains perforations 52 located immediately above 
whipstock 26. Thus the assembly to be lowered into well 10 contains the 
whipstock affixed to the protective pipe, tubing 18 inserted into 
whipstock 26 with a portion of tubing 18 protruding through the whipstock. 
The assembly is lowered into the well with extra joints of the protective 
pipe being added as the assembly is lowered. Once the whipstock reaches 
the bottom of sump 27, the assembly is aligned so that protruding tubing 
18 is pointed toward well 12. The protective pipe is equipped with a water 
injection pipe 53 containing valve 54 and is hermetically sealed at the 
well head. Once channel 30 is initiated and the temperature of the 
protective pipe increases substantially, for example up to 250.degree. F., 
water is injected into the annulus between tubing 18 and the protective 
pipe with the water flowing out of the perforations in the lower end of 
the protective pipe. Water flow into the annulus preferably is controlled 
so that upon exit through the perforation it is in the vapor phase. In 
this manner the rigidity of tubing 18 can be preserved between the 
whipstock and the well head. 
Maintaining rigidity of tubing 18 between whipstock 26 and its lower end 
near burning face 40 is not a critical consideration, although some 
measure of rigidity should be maintained to assure that tubing 18 is 
capable of being lengthened as burning face 40 recedes into the coal. The 
cooling effect of the injected oxidizer, particularly when water is mixed 
with the oxidizer, is generally sufficient to maintain the required 
measure of rigidity for additional lengths of tubing 18 to be inserted 
into lengthening channel 30. A measure of flexibility of tubing 18 located 
in channel 30 is desirable in that by gravity tubing 18 will tend to 
remain close to the interface between the coal and the underburden. Thus 
by maintaining the oxygen release point at the bottom of the coal, channel 
30 will lengthen at the preferred location. By emplacing the whipstock 
using a protective pipe affixed to the whipstock, upon completion of the 
linkage procedure, the whipstock can be removed from the wall. 
Using the methods of the present procedure, two wells several hundred feet 
apart can be linked through the coal, with the linkage channel 
substantially following the bottom of the coal seam. As a practical 
matter, however, lengths of the linkage channel should be limited. While 
it is desirable to have linkage channels sufficiently long to provide an 
adequate length for a reducing environment, excessive lengths result in 
the ultimate lowering of the temperature of produced fluids to a point 
where condensible liquids accumulate in the channel. Excessive 
accumulations of condensed heavy liquids such as tars can severely 
restrict the flow of fluids through the underground channels, and in 
extreme cases the channels can become plugged. Generally the distance 
between wells should be limited to a maximum distance in the order of 300 
feet. 
Thus it may be seen that positive control may be applied in the linkage of 
two production wells with the channel through the coal being formed 
substantially at the bottom of the coal seam, that such linkage may be 
accomplished by removing only a part of the free water contained in the 
coal, and that the problem of flame override can be substantially 
eliminated by accomplishing such linkage. 
While the present invention has been described with a certain degree of 
particularly, it is understood that the present disclosure has been made 
by way of example and that changes in detail of structure may be made 
without departing from the spirit thereof.