Method for in situ coal gasification operations

An in situ coal gasification process adapted for large scale commercial projects is provided. Techniques are provided to insure establishment of a gasification front over the full seam thickness as each successive injection well in the array is brought on line. This is accomplished by controlling the oxidant introduction in a prescribed manner during the early stages of injection after pneumatic communication between well pairs has been established. Also provided are techniques and standards for avoiding or controlling subsidence and for conducting gasification operations in free water laden seams and in coal seams subject to spontaneous combustion.

BACKGROUND OF THE INVENTION 
This invention relates generally to the in situ gasification of coal to 
produce a combustible gas product. 
More specifically, this invention relates to methods and techniques for 
conducting in situ coal gasification on a practical and commercial scale. 
Attempts to develop in situ coal gasification technology have occurred 
around the world during the last 60 years. Large research efforts have 
been undertaken in the U.S., the USSR, the U.K., France, Poland, 
Czechoslovakia, Canada, the Federal Republic of Germany and Belgium. Only 
in the USSR has the technology operated at commercial scale. 
Impending shortages of natural gas and petroleum liquids together with 
sharply increasing prices for those commodities has focussed renewed 
interest on all processes which hold promise for the practical conversion 
of coal into gaseous and liquid forms. In situ coal gasification is one of 
the more highly developed techniques but commercial practicability in this 
country has yet to be convincingly demonstrated. 
Successful application of in situ coal gasification technology results in 
recovery of gaseous products and liquid byproducts from coal resources 
which cannot be recovered using conventional coal mining techniques. 
Either low- or intermediate- Btu gas can be obtained from the process 
depending upon whether air or oxygen is the injected oxidant, 
respectively. The process has several apparent advantages over 
surface-based coal gasification operations in that the coal need not be 
mined, no coal transportation or preparation is required, the need for 
surface pressure vessels for gasification is eliminated, and solid waste 
disposal requirements are greatly reduced since the great majority of the 
ash is left underground. Less apparent but important advantages over 
surface-based gasification processes include: Increased thermal efficiency 
since the in situ gasifier can be operated at higher temperatures because 
concerns about corrosional and erosional effects of components in the 
product gas are reduced; lower high quality water requirements since water 
of any quality present in the coal seam or adjacent aquifers can serve as 
the hydrogen source required for gasification thereby lowering steam 
injection requirements; and, less sensitivity to economics of scale since 
the in situ gasification production facility consists of adding process 
wells to increase output with the cost of each well being roughly the same 
whether 100 or 1,000 wells are required. 
The process is basically a simple one involving the following steps: 
Drilling and completing wells using conventional techniques in order to 
access the coal seam; enhancing the natural permeability of the coal seam 
in order to allow injection of sufficient oxidant to achieve efficient 
gasification conditions; and, gasification of the coal seam between 
successive pairs of process wells over a large area to provide the desired 
quantity of product output. Experiments have been conducted in the USSR on 
coals ranging in rank from lignites to anthracite in seams of variable 
thickness with dip angles from 0.degree. to near 90.degree. from 
horizontal. 
The U.S. patent literature is replete with various in situ methods for 
recovering energy from coal. In spite of this plethora of prior art, there 
is lacking an appreciation of the practical economic and technical limits 
imposed by in situ operations and of the need for a method amenable to 
large scale systematic expansion of the process. 
One common thread that runs explicitly or implicitly through much of the 
technical literature on in situ gasification is the criticality of the 
linkage path location between wells; that the linkage path must be located 
near the bottom of the coal seam to achieve a successful operation. 
Experimental support for this conclusion appears to be substantially based 
on the highly successful test burn at Hanna, Wyo., in 1976. Downhole 
instrumentation showed that the reverse combustion linkage path was 
located about 5 feet above the bottom of the 30-foot coal seam being 
gasified. 
Later experimental tests have shown that linkage path location at or near 
the bottom of the seam does not guarantee success. The first of these 
tests, conducted at a site near Gillette, Wyo., in 1977, resulted in 
formation by reverse combustion of a linkage path 8 feet off the bottom of 
the 25-foot thick coal seam being gasified. The results were still 
disappointing during the subsequent gasification phase. These lower than 
expected results were due to unsuitable site characteristics rather than 
to the location of the linkage path. The lower than expected results have 
been explained by the conducting organization as the result of combustion 
zone override to the top of the seam due to blockage of the linkage path 
by roof collapse. 
In the second test, also conducted near Gillette, Wyo., in 1979, 
directional drilling was utilized to place a small-diameter pathway in the 
lower 1/2 of the same 25-foot thick coal seam. After vertical wells were 
drilled and connected to the drilled pathway, reverse combustion was 
utilized to enlarge the drilled pathway. Again, the results were not up to 
expectations due to unsuitable site characteristics. 
Conversely, location of the linkage pathway at or near the top of the seam 
does not preclude successful operations. The first test conducted at a 
site near Hanna, Wyo., in 1973 and early 1974 was successful even though 
later drilling of the affected area clearly showed that linkages created 
by reverse combustion were located in the top few feet of the 30-foot 
thick coal seam being used. 
The inventors herein have found that the emphasis accorded linkage path 
location by the prior art has been misplaced; that, in fact, location of 
the linkage path is of no importance in the successful conduct of 
large-scale in situ gasification operations. 
SUMMARY OF THE INVENTION 
It has been found that in a properly selected site a successful in situ 
coal gasification process requires the initial establishment of the 
gasification front over the full seam thickness as each successive well in 
the well array pattern becomes an injection well. A full seam gasification 
front is established by controlling the manner of oxidant injection to 
cause the gasification zone to slowly expand outward from the bottom of 
the injection wellbore and thereafter expand upward around the wellbore 
until full seam thickness is utilized. Thereafter, the gasification zone 
becomes stable and self-propagating over the entire seam thickness and the 
linkage path serves only as a conduit for product gas flow to the 
producing well. 
Hence, it is an objective of this invention to provide an in situ coal 
gasification process amenable to systematic large scale expansion of the 
burn front over the entire seam face. 
It is another object of this invention to provide a method for establishing 
a full seam gasification front at an injection wellbore. 
Yet another object of this invention is to ptovide operating criteria for 
the successful operation of large-scale in situ gasification projects. 
Other objects, advantages and novel features of the invention will become 
apparent from the following discussion and description of the invention.

DISCUSSION AND DESCRIPTION OF THE INVENTION 
The successful operation of an in situ coal gasification project requires, 
firstly, the selection of a proper site and, secondly, the proper 
establishment, control and propagation of the gasification front over the 
full seam thickness. More particularly, the necessary steps required for a 
successful operation may be generally stated as site selection, site 
characterization, process design and process operation. 
Site Selection 
Site selection consists of identifying a suitable seam or seams for the 
process on a property area sufficiently large for the term of planned 
operations. Major considerations are seam thickness and depth; coal 
quality; lithology of overburden and floor rock and assessment of 
overburden competence; general geologic characteristics such as degree of 
faulting, occurrence of aquifers, and continuity of lithology over the 
identified property; and, current land and water use patterns in the area. 
Without extensive field work involving drilling, geophysical surveys, and 
hydrologic characterization, this step is only a screening effort to 
eliminate areas for obvious reasons of unsuitability for the process. 
Examples of obvious reasons for not selecting an area are the presence of 
unconsolidated overburden such as sand from the coal seam to near the 
surface with a coincident high risk of product gas leakage to the surface 
or the presence of a prolific aquifer either within the coal seam or in 
the near overburden which, if interconnected to the gasification zone, 
could flood the zone leading to a serious drop in process efficiency. If 
the water in the aquifer is of high quality, serious environmental 
concerns could also prevent regulatory bodies from granting the permits 
required prior to operation. Thus, the process cannot be applied at any 
location just because sufficient coal resource has been identified. 
The following general site selection criteria, based in part on current 
economic conditions, have been established: 
1. Seam thickness and depth. 
The minimum acceptable seam thickness is on the order of 6 feet. This 
minimum thickness results because of increased heat losses to surrounding 
strata from seams thinner than this figure thus lowering the gross heating 
value of the product gas below acceptable levels. Injection of oxygen or 
oxygen-enriched air can overcome the low gas heating value but economic 
limitations on product selling price may preclude these options. In 
theory, no maximum seam thickness limitation exists. Practically, a thick 
seam at a shallow depth may not be acceptable due to the high risk of 
subsidence to the surface. Therefore, the ratio of depth to seam thickness 
is a measure of suitability with the minimum seam thickness listed above 
being a further limit. The acceptable values of this ratio are a maximum 
of 50 for shallow seams (greater than 200 feet to less than 500 feet) to a 
maximum of 60 for seams at depths greater than 500 feet under current 
economic conditions. Seams of greater than 6 feet thickness at depths of 
less than 200 feet are not considered suitable due to the potential for 
subsidence. As the price of energy increases, the above maximum values 
could increase substantially and are given here only as examples. 
The presence of partings in the seam must also be considered. Their 
occurrence does not preclude suitability of a coal seam. As an example, a 
coal seam might have an aggregate thickness of 10 feet with single or 
multiple partings accounting for 4 feet of that aggregate thickness. This 
could still be a suitable coal seam if it meets the depth to thickness 
ratio stated previously. No parting should be of a thickness greater than 
the thickest coal seam within the total aggregate thickness being 
assessed, e.g., multiple thin seams (2 to 3 feet thick) separated by 
numerous partings of greater than 3 feet are usually not suitable. 
Previous investigators have alluded to the beneficial nature of partings 
for maintaining a linkage path low in the coal seam as an essential 
feature of in situ coal gasification. The location of partings within a 
coal seam has been found to be irrelevant to successful operation on a 
large scale. 
2. Coal rank 
All ranks of coal can be gasified in situ as has been demonstrated 
experimentally in the USSR. The primary problem which must be overcome is 
enhancement of the natural permeability for high free swelling index 
bituminous coals, as well as for semi-anthracite and anthracite varieties. 
This is not normally a problem for lower rank coals. 
3. Lithology 
The strata overlying the coal seam to be gasified must be sufficiently 
competent to minimize the potential for subsidence to the surface. Thus, 
materials such as sand or loose aggregate are unacceptable. In addition, 
the presence of such unconsolidated materials even at large distances of 
separation above the coal seam could preclude suitability of a specific 
site since subsurface subsidence could progress to such a height above the 
coal seam that these strata are intersected further increasing the degree 
of subsurface subsidence to the extent that subsidence might propagate to 
the surface. No reliable method for predicting subsidence has yet been 
developed in the art. Only experience in the technology can be relied upon 
for judgment at this time. Development of reliable subsidence prediction 
models would offer an important tool to the site selection process, but 
any model must be capable of incorporating thermal effects on the near 
overburden to determine how the physical strengths of these strata change 
as a function of temperature. In addition, no specific criteria can be 
established for the floor rock since no reliable technique for predicting 
floor heaving has been developed. Floor heaving could be important if, for 
example, it resulted in communication of the gasification zone with an 
aquifer system below the target coal seam. In general, the overburden 
should be sufficiently competent after exposure to high temperatures to 
allow the formation of semi-stable or stable arches after removal of coal 
to preclude surface subsidence and should consist of competent sedimentary 
rocks such as limestones, shales and sandstones. 
4. Permeability distribution 
The primary criterion is that the target coal seam should be immediately 
overlain and underlain by materials of significantly lower permeability 
than the coal such that these adjacent strata will not be the path of 
least resistance to oxidant flow during permeability enhancement 
operations. In addition, higher permeability of adjacent strata relative 
to the coal seam could result in excessive gas loss rates during 
gasification operations. 
Permeability distribution within the coal seam is not critical to the 
process other than for permeability enhancement operations. If the 
permeability is extremely low, it can be overcome by high pressure oxidant 
injection and reverse combustion, for example, so long as such operations 
do not result in fracturing the overburden to such a degree as to create 
higher permeability in the overburden than in the coal seam. In addition, 
other permeability enhancement methods including, for example, hydraulic 
fracturing or stimulation, explosive fracturing, directional drilling, 
injection of components to dissolve coal to form a pathway between wells, 
and use of lasers to form a linkage pathway between wells, and a 
combination of firing projectiles from the bottom of the wellbore to form 
an initial small diameter pathway followed by reverse combustion to 
enlarge the pathway have been suggested in the art. 
5. Occurrence and effects of groundwater 
Since all coal gasification requires a hydrogen source, the presence of 
some free water in the coal seam or adjacent strata is beneficial to the 
process to reduce the quantity of steam which might otherwise need to be 
injected. Only the presence of excessive amounts of groundwater requires 
attention. Under optimum in situ gasification conditions, about 0.1 to 0.3 
pounds of water are consumed per pound of coal gasified. Water influx 
rates resulting in excess water availability will adversely affect the 
efficiency of the process by lowering the in situ temperatures due to 
vaporization of the excess water. Water influx rates can only be partially 
controlled by the adjustment of reservoir pressure to higher values. In 
theory, water influx can be controlled solely by increasing reservoir 
pressure. Practically, since the coal seam is not a totally confined 
reservoir, this theoretical control cannot be achieved. If an aquifer 
overlying the coal seam becomes interconnected to the gasification zone 
due to subsurface subsidence, the available water will enter the 
gasification zone in an uncontrolled manner unless the reservoir pressure 
is raised to levels well above the hydrostatic pressure in the 
interconnected aquifer. If the pressure is raised to such levels, gas 
leakage must result adversely affecting over-all process economics. 
Therefore, the presence of aquifers having the capacity to provide 
sufficient water to adversely affect the process must either be avoided to 
means to dewater such aquifers must be employed. Methods to achieve 
dewatering of aquifers overlying the target coal seam are generally 
confined to pumping excess groundwater from wells completed into the 
overlying aquifer using various well pattern arrays as has been described 
in the art. 
An entirely different problem is presented, however, when the coal seam 
constitutes an aquifer and is itself the source of excess groundwater. In 
this circumstance, it has been found that such a seam can be dewatered by 
placing an array of wells at or near the boundary established for each 
production module. Linkage is established between adjacent wells as by 
reverse combustion techniques and a boundary cavity around the whole 
production module is then created by gasifying between the linked wells. 
After the gasified area has cooled by influxing ground-water from the coal 
seam, pumping is initiated and is continued until the production module 
has been dewatered sufficiently to allow efficient gasification of the 
coal within the area outlined by the boundary cavity. Under any 
circumstances, aquifers containing water of high quality must be separated 
by sufficient distance from the target coal seam to preclude their 
becoming interconnected to the gasification zone in order to avoid 
unacceptable environmental costs. 
The coal seam need not be an aquifer for successful in situ gasification 
operations. This conflicts with previous investigators who have indicated 
that in situ coal gasification operations should be conducted in seams 
containing free water such that the available groundwater acts as a gas 
seal. This criterion can only apply for small-scale operations since roof 
falls are in integral part of any large-scale in situ operations resulting 
in pathways for gas flow to strata overlying the coal seam thus precluding 
an effective seal either by water in the coal seam or water in an 
overlying aquifer as previously discussed above. A dry coal seam can be 
utilized by adjusting the reservoir pressure to low values to minimize gas 
losses while still maintaining process control. Steam or carbon dioxide 
are then injected with either air or oxygen to maximize the production of 
hydrogen and/or carbon monoxide at acceptable concentrations according to 
the following reactions: 
EQU C+H.sub.2 O+Heat=H.sub.2 +CO (1) 
EQU C+CO.sub.2 +Heat=2 CO (2) 
Reaction (2) will only proceed to any significant degree after reactions 
(1) has utilized the available water vapor since both kinetics and 
thermodynamics favor reaction (1). Thus, injection of both CO.sub.2 and 
water vapor simultaneously with air or oxygen is of little, if any, 
benefit. In addition, injection of CO.sub.2 into a wet coal seam will also 
be of little benefit. But, injection of CO.sub.2 along with air or oxygen 
into a dry or near-dry coal seam (dry referring to the absence of any free 
water) is beneficial to increase the concentration of CO in the product 
gas and offers a means for recycling a portion of the CO.sub.2 removed 
from the product gas during surface processing. 
6. Presence of faulting and coal seam discontinuities 
The presence of large-scale (greater than seam thickness) faulting or seam 
discontinuities within the target area is of importance due to the 
detrimental effects they can have on process control and efficiency. If 
the locations of major faults, sand channels, or pinchouts are known, 
design considerations can be given to minimize process upsets which can 
result due to these features. If their locations are unknown, these 
features may provide unexpected paths for abnormal influx of groundwater, 
leakage of product gas, and potential process interruptions. Small-scale 
faulting (less than seam thickness displacement) cannot, in most cases, be 
detected or avoided, and, for large-scale operations, is of minor 
significance since only a small percentage of the production will be 
affected. 
7. Presence of other mineral recovery activities in the area 
The presence of active or abandoned oil and gas recovery wells or mining 
activities at a location being considered for in situ coal gasification 
may preclude use of significant portions of the area. This is due to the 
increased potential for leakage up along active or abandoned oil and gas 
wellbores where the cement bond may no longer be competent or due to gas 
leakage to mine workings. In addition, the casing in oil and gas wells may 
be damaged due to thermal stress or subsurface subsidence caused by the 
process resulting in rupturing of the casings. Although their presence can 
be overcome, the in situ operation must be designed to work around these 
features if they are present or must be conducted at distances sufficient 
from them to minimize the problems which could result. 
Other factors may require consideration depending upon the site, but these 
are the minimum criteria which must be assessed prior to selecting an area 
for in situ coal gasification operations. 
SITE CHARACTERIZATION 
Based on the site selection criteria described in the preceding section, it 
is evident that significant amounts of characterization work must be 
performed before a final site choice can be made. This work can be 
arranged in any logical sequence but must, at a minimum, consist of the 
following: 
1. Evaluation wells 
Drilling and downhole logging of a sufficient number of evaluation wells to 
determine coal seam continuity and to obtain cores of overburden, coal, 
and floor rock for analyses and physical properties determination are 
necessary. The great majority of these wells can later be used as process 
wells. 
2. Coal analyses 
Analyses of numerous coal samples obtained from the evaluation wells is 
necessary to determine variations in coal quality over the area to be 
gasified. These analyses should, as a minimum, include ultimate and 
proximate analyses; determination of as-received heating value; 
determination of sulfur forms (pyritic, organic, and sulfate sulfur); 
elemental composition of the ash; and, Fischer assays at 900.degree. C. to 
determine the total amount of volatile gases and concentration of 
individual gases (CO, H.sub.2 O vapor, CO.sub.2, CH.sub.4, C.sub.2 
-C.sub.4 's, H.sub.2) in the volatile gases per pound of coal as well as 
the total amount of light oils and tars volatilized per pound of coal. 
These data, used with appropriate mathematical models, allow prediction of 
product gas compositions during commercial operations. 
3. Lithologic characterization 
Analyses of overburden and floor rock samples obtained from recovered cores 
for tensile and compressive strength, bulking properties as a function of 
temperature, and permeability allow assessment of the subsidence potential 
for a specific site and indicate where potential gas loss zones are 
located relative to the coal seam. 
4. Hydrologic characterization 
Hydrologic characterization and analyses of the groundwater within each 
aquifer located above, within, or within a reasonable distance below the 
target coal seam may be conducted using some of the evaluation wells. The 
hydrologic characteristics, such as location of the piezometric surface, 
hydrostatic pressure, transmissivity, storage coefficient, hydraulic 
gradient, and recharge rate, should be determined for each aquifer. In 
addition, such things as mobility, hydraulic conductivity, and isotropy or 
anisotropy may be determined. These characteristics can be used to 
determine the direction of and rate of groundwater movement for each 
aquifer. The productivity of each aquifer must be determined by pumping 
tests. Monitoring of aquifers during the necessary data gathering steps 
allows for determination of the degree of interconnection between aquifers 
identified. The presence of faults can be inferred from analyses of the 
hydrologic data and the need for dewatering operations can be determined. 
Required analyses of water samples from each aquifer are set by State or 
Federal law and need not be described here. Methods for ensuring the 
gathering of representative samples and for sampling multiple aquifers 
from the same wellbore are well known and need not be described here. 
5. Geophysical data 
Geophysical surveys aid in determining the presence and extent of faulting 
and other coal seam discontinuities. The effectiveness of techniques used 
to make these determinations will be dependent upon the depth and seam 
thickness. These techniques are well known and have been in use in the 
minerals industry for extended periods of time. Data gathered here in 
conjunction with the results of drilling and logging better indicate how 
the coal seam must be blocked out to avoid the detrimental effects of 
identified areas of faulting and other coal seam discontinuities. 
6. Acceptance testing 
Air acceptance testing serves to determine whether reverse combustion 
linking can be used to enhance seam permeability or whether other 
permeability enhancement methods are more suitable. Testing may utilize 
several evaluation wells completed into the coal seam. Testing is 
conducted by injecting air at a central well at a maximum pressure of 
about 1 psig per foot of depth to the bottom of the coal seam and 
measuring production rates at surrounding wells. These data will be used 
to determine the allowed spacing between wells for reverse combustion 
linking. 
Many previous investigators have indicated that this step is not required, 
but is has been found to be far more reliable than analyses of oriented 
cores for determining predominant flow directions. However, its usefulness 
for large areas requires conducting testing at several locations. 
Considering the large investment inherent to commercial operations, its 
reliability overshadows the cost. The conduct of such air acceptance 
testing is less expensive than taking oriented cores over a large area 
followed by laboratory analyses to determine directional permeability and 
provides better data for use in orienting the well pattern within 
production modules to take advantage of the predominant flow direction as 
determined under field conditions. The results of this acceptance testing 
will determine the orientation of the well pattern in the production 
modules to take advantage of the predominant flow direction determined in 
the field, which may vary from place to place within the total area to be 
used during the lifetime of the plant. 
If major features detected during site characterization show high 
permeability, the need for permeability enhancement may be obviated in 
certain areas. The well pattern may be situated along these features in 
such a manner as to use them advantageously thereby allowing gasification 
without linking the wells along these features. 
PROCESS DESIGN 
Assuming the site characterization results have not precluded further 
consideration of the location, process design can then proceed. The major 
factors to be determined are the method of permeability enhancement, well 
pattern layout and spacing, operating pressure, injection rate, production 
rate, product gas composition, composition of injection stream, targeted 
coal recovery efficiency, blocking out of the area from which the coal 
will be extracted, the number of modules needed, the number of modules to 
be prepared in advance, and the total area required for the life of the 
operation. These factors are determined in the following manner: 
1. Method of permeability enhancement 
As has been described previously, numerous permeability enhancement 
techniques have been proposed for use with in situ coal gasification. Only 
three have been proven during field operations. This proof has been 
described in the Soviet literature. The three methods are reverse 
combustion linking, directional drilling, and hydraulic stimulation (U.S. 
Pat. No. 3,990,514). For the purpose of this invention, directional 
drilling is defined as any technique where drilling is initiated from the 
surface and conducted in such a manner as to result in a drilled pathway, 
the last several hundred feet of which is generally parallel to the upper 
and lower boundaries of the coal seam to be gasified. This drilled pathway 
then serves as the conduit for gas flow between wellbores. Directional 
drilling is not meant to include slant drilling which is commonly employed 
in the oil and gas industry and has been developed to a high degree of 
sophistication. 
The first two of these three methods have been applied during testing in 
U.S. The method of hydraulic stimulation described in U.S. Pat. No. 
3,990,514 may only be applicable to a limited range of geologic 
conditions. Directional drilling has been developed to a high degree of 
reliability in the USSR specifically for application to in situ coal 
gasification but is in its infancy in the U.S. and is expensive for each 
foot of usable hole within the target coal seam. In addition, it does not 
offer any significant advantage over reverse combustion linking. 
Reverse combustion linking does not always result in the linking of all 
wells within the process well pattern since the fluid flow through the 
coal seam is controlled by the natural fracture distribution. The Soviets 
have reported that greater than 20% of the wells within a production 
module were not successfully linked, but the large-scale operation of in 
situ coal gasification was still successfully completed. This success was 
due to the large-scale operation where the failure of a significant 
portion of the linkages was minimized by the presence of a large number of 
active gasification channels and flow of the gases to the available 
linkage paths. As the gasification zone proceeded along a broad front, the 
wells which were not linked were eventually connected to the gasification 
zone and were then used as injection wells as the gasification zone was 
relayed through the well pattern. 
The primary advantage of reverse combustion is its low cost. It is thus the 
preferred method of permeability enhancement, but this invention is not 
limited by the method of permeability enhancement. This method has been 
successfully used on small-scale field tests in the U.S. over distances up 
to 100 feet in subbituminous coal. It is projected that it could be 
successfully employed over greater distances (up to 250 feet). Linkages 
over larger (greater than 250 feet) distances may be achieved. The reason 
these greater distances may not be practical is the increasingly greater 
risk of not being able to complete a high percentage of these links and 
the lower resource recovery which may result. The primary consideration is 
that sufficient air or oxygen percolate from the point of injection to the 
well to be linked to sustain combustion. Recovery rates as low as 5% of 
injected air at the well to be linked have been successfully employed 
during small field tests in the U.S. 
The air acceptance testing outlined during the previous description of site 
characterization tasks provides the data necessary to determine the well 
spacing which can effectively be utilized for any given coal seam. In 
general, the most effective range of spacings for this linking technique 
is on the order of 75 to 125 feet. Spacings in this range ensure a high 
percentage of linkage completion such that the process can be conducted in 
an efficient manner. 
2. Well pattern layout and spacing 
The conduct of large-scale operations of the technology requires a well 
pattern layout that offers ease of relaying the process over a large area. 
Thus, a square or rectangular pattern of wells within any given module is 
ordinarily better than either a random layout or a pattern based on the 
traditional 5-spot utilized in the oil and gas industry. As outlined in 1. 
above, the spacing of the wells parallel to the direction of gasification 
front movement can be determined by air acceptance testing. The spacing 
perpendicular to this direction of movement must be determined by the 
desired coal recovery efficiency balanced against the potential for 
subsidence at any given site. If a high percentage of coal recovery is 
feasible, the spacing perpendicular to the direction of gasification front 
movement is about 2/3 of the spacing parallel to the direction of 
gasification front movement. This results in overlap of the gasification 
zones propagating from the injection wells to the production wells in any 
given line of wells. Field testing in the U.S. has confirmed thsi formula 
for single well pair operations. Approximately 80% of the coal can be 
recovered in this manner. 
If site conditions are such that subsidence might occur leading to 
significant process upsets, then the spacing perpendicular to the 
direction of gasification front movement is increased to at least the same 
spacing as that parallel to the direction of gasification front movement. 
If the pattern is laid out in a square arrangement, approximately 60% of 
the coal is recovered. The remaining 40% offers roof support to delay and 
minimize subsurface subsidence thereby reducing the potential for process 
upset. 
If the spacing perpendicular to the direction of gasification front 
movement is increased to values greater than that parallel to the 
direction of gasification front movement, correspondingly lower 
percentages of in place resource are recovered but a correspondingly 
greater resistance to subsidence is obtained. The ratio of well spacing 
perpendicular to the direction of gasification front movement to the 
spacing parallel to the direction of gasification front movement should 
not exceed 2:1. Spacings at values greater than 2:1 may not be economic. 
As may be appreciated, each site will ordinarily have different 
requirements and appropriate spacings may vary from module to module 
within the same site because of the changing conditions over the area to 
be gasified. 
These variable spacings from module to module can be determined during site 
characterization or during operation. Modules may vary in size also due to 
the presence of faulting or seam discontinuities detected during site 
characterization thus establishing boundaries for individual modules, 
i.e., gasification would not be conducted across these established 
boundaries but only up to or parallel to them. Thus, for example, numerous 
modules ranging in size from 200 feet wide by 500 feet long to as much as 
1000 feet wide by 2000 feet long may be blocked out prior to well pattern 
installation. The modules blocked out may have spacings ranging from 75 
feet to 125 feet between wells arrayed parallel to the direction of 
gasification front movement and ranging from 50 feet to 250 feet between 
wells arrayed perpendicular to the direction of gasification front 
movement. Ordinarily though, the spacings within each module will be the 
same throughout that individual module. 
3. Operating pressure 
If reverse combustion is the chosen method of permeability enhancement, the 
injection pressure used during reverse combustion operations will be about 
1 psig per foot of depth to the bottom of the target coal seam. The 
controlling factor will be the amount of recovery of injected oxidant at 
the well or wells to be linked. If recovery is too low at this pressure, 
the pressure can be increased to a level where the recovery is sufficient 
but should not exceed a pressure of 1.4 to 1.5 psig per foot of depth to 
the bottom of the target coal seam due to the potential for creating high 
permeability zones in the overburden which might be detrimental to future 
gasification operations. 
During gasification, the operating pressure should be approximately equal 
to or less than the hydrostatic pressure within the coal seam if it is an 
aquifer. This allows water to influx into the gasification zone providing 
the necessary hydrogen source for efficient operations and provides 
containment of contaminants formed during pyrolysis and gasification such 
that their dispersement through the groundwater regime of the coal seam is 
minimized. Lower pressures than hydrostatic may be required if gas loss 
rates become excessive due to interconnection of adjacent strata to the 
coal seam as a result of subsurface subsidence. For dry seams, the 
operating pressure will be established by gas loss rates and will be a 
function of coal seam depth and permeability of adjacent strata as a 
function of pressure. In general, the operating pressure for dry seams 
should be held at the lowest allowable level sufficient to allow injection 
of the required amount of oxidant necessary for efficient gasification 
rates. 
4. Injection rates 
The injection rates required during permeability enhancement will be set by 
the percentage recovery at the well to be linked necessary to sustain 
combustion in the case of reverse combustion. An upper limit will be set 
by the desire to avoid too high an oxygen flux rate, i.e., exceeding the 
critical flux for reverse combustion, at the combustion focus such that 
reverse combustion is precluded. A lower rate also exists such that 
sufficient heat of combustion is available to permit the propagation of 
reverse combustion to form the linkage pathway. These upper and lower 
limits may be established through laboratory experimentation prior to 
initiation of field operations using coal samples obtained during site 
characterization. 
During gasification, the maximum injection rate is determined by well 
spacing, seam thickness, and coal analyses. This maximum value can be 
calculated. As an example, for a 30-foot thick subbituminous coal seam 
containing 33% fixed carbon and at a well spacing of 75 feet, the maximum 
air injection rate per injection well should be about 5000 scfm. For the 
same seam thickness and well spacing but with 44% fixed carbon, the 
maximum injection rate should be about 6700 scfm to maintain the same 
efficiency of gasification. 
Due to limited experience in large-scale operations, laboratory testing 
using coal samples gathered during site characterization should be 
conducted to determine the effect of various oxygen flux rates. This 
testing, conducted in a sealed chamber, should include injection of 
oxidants of differing compositions at pressures up to lithostatic to 
determine the optimum flux. The optimum value can then be compared to the 
calculated value and adjustments to the calculated value made if 
necessary. 
5. Production rates 
The production rate is a function of oxidant injection rate, gas loss rate 
and water influx rate. Laboratory tests outlined previously provide data 
amenable to mathematical process modeling for the calculation of 
production rates and total production per well after selection of well 
spacing for a specific target coal seam. 
6. Composition of injection stream 
Using the coal seam analyses obtained during site characterization, the 
availability of groundwater for influx to the coal seam, and the Fischer 
assay data, the proper mix for the injection stream can be determined. The 
available mixtures for consideration are air, air-steam, air-CO.sub.2, 
oxygen-enriched air, oxygen-steam, oxygen, oxygen-CO.sub.2, air-inert gas, 
and oxygen-inert gas. The choice of one of these over the others will 
depend upon the particular coal seam and the desired product. 
The optimum choice can be estimated through use of mathematical process 
modeling using the above mentioned data as inputs to the model. As a check 
of the model, laboratory simulation can be conducted on coal samples using 
the mixtures identified to better determine their effectiveness and a 
final choice can be made based on weighing improvements in gas heating 
value versus any increased costs necessary for each individual injection 
stream composition. 
7. Product gas composition 
The product gas composition is, of course, highly dependent upon the 
particular injection stream used and the characteristics of the coal seam. 
Process modeling can be used with a high degree of reliability for 
predicting product gas composition especially if modeling is conducted 
conjointly with laboratory simulation. 
8. Targeted coal recovery efficiency 
Complete gasification of a coal seam within the project boundaries is not 
generally feasible and is often undesirable because of subsidence 
considerations discussed more fully in section 2 above. Coal utilization 
generally ranges from about 50% to 80% at a suitable site with properly 
applied techniques. 
9. Blocking out the resource to be extracted 
Seam discontinuities and the like serve to establish boundaries for process 
modules or for areas where operations cannot be successfully conducted. 
The number of process modules needed at any given time is established by 
the plant size, the number of wells within each process module, the well 
spacing within each process module, the coal seam thickness and quality, 
and the lifetime of each module. The number required can thus be 
calculated. The number of modules to be prepared ahead of time such that 
new process modules can be brought into production as needed can also be 
calculated such that process interruptions are eliminated. 
After completion of these tasks, all of the information needed to perform 
the final engineering design is available to enable construction of the 
plant. 
PROCESS OPERATIONS 
Operation of the process itself will be described in relation to well pairs 
and to the propagation of a gasification front through a single process 
module. After the predetermined well array within a module has been 
accomplished with the wells drilled and completed into the coal seam, 
pneumatic communication between well pairs must be established. 
Thereafter, a full-seam gasification front is established and is 
systematically advanced through the module. 
Referring now to FIG. 1, there is shown in plan view a well pair, one 
injection well 11 and one production well 12, at an intermediate stage of 
gasification. Oxidant is injected at well 11 to produce a gasified or 
depleted area 13 defined by gasification front or zone boundary 14 
expanding generally radially to injection well 11. Linkage path 15 is 
established prior to beginning gasification and serves to conduct product 
gases from the gasification area to the producing well. 
Turning now to FIG. 2, there is shown a detailed view of a portion of 
gasification front 14. Because of the low thermal conductivity of coal, 
the total thickness of the gasification reaction front is ordinarily only 
a few feet. Combustion zone 21 is adjacent to previously gasified or 
depleted area 13. The primary reaction occurring in this zone is the 
combination of oxygen with carbon to produce carbon dioxide and heat. 
Preceeding in order, there follows gasification zone 22, pyrolysis zone 
23, drying zone 24 and unmodified coal 25. 
Two primary reactions occur in the gasification zone. They are: 
EQU C+H.sub.2 O+Heat=CO+H.sub.2 (1) 
EQU C+CO.sub.2 +Heat=2CO (2) 
Field data gathered to date show that reaction (2) is of minor significance 
if the gasification zone contains even small amounts of free water. Coal 
volatiles are driven off by heat in the pyrolysis zone leaving a char 
residue while a lower level of heat violatilizes water in the drying zone 
leaving dry coal. 
Temperature within combustion zone 21 is estimated to range from 
1800.degree. to 2700.degree. F. depending on oxygen flux rate, injection 
stream composition, and volume of influxing groundwater. Heat produced 
within this zone provides the energy necessary to drive the endothermic 
reactions occurring in the other three zones. Conduction of heat into the 
coal face weakens the coal structure and greatly increases its 
permeability due to removal of volatiles and water. As many Western coals 
contain as much as 35% volatile matter and 30% water depending upon coal 
rank, the extent of permeability enhancement can be readily appreciated. 
Thus, the face is continually prepared for the advance of the gasification 
front over a thin shell at the boundary of the gasification area. The rate 
of movement is greatest at the edges of the gasification front nearest the 
linkage path, lower at the sides and lowest behind the injection well 
because of fluid flow distribution imposed by the pressure differential 
along the line of least resistance from the injection well to the inlet to 
the linkage path. 
Because of the thermal effects described and the flow distribution within 
the previously gasified area, the influence of the linkage path location 
is minimal. Only if the gasification front is not initially established 
over full seam thickness will the process suffer. Previous researchers 
have interpreted failures as being due to linkage path location when in 
fact this cannot be an explanation. The pressure drop through the linkage 
path is too small (less than 5 psig in small-scale field tests) for it to 
be a significant factor affecting fluid flow distribution to the extent 
necessary to yield the results obtained. 
Establishment of the gasification front over the full seam thickness at an 
injection well requires careful control of the manner of oxidant 
introduction. As has been discussed previously, calculation of a maximum 
injection rate is a necessary step in the design of a gasification 
project. It varies as a function of seam thickness, well spacing, fixed 
carbon content of the coal, and oxygen content of the injection stream. 
The calculated maximum injection rate is defined as follows: 
EQU Calculated Maximum Injection Rate (in scfm)=kSW(FC)C 
where 
k=A constant varying as a function of coal rank For Wyoming subbituminous 
coal, a value of about 8.4 ft/min has been determined 
S=Seam thickness in feet 
W=Well spacing in the direction of gasification front propagation in feet 
FC=Fixed carbon content of the coal expressed as the decimal less than one 
C=The ratio of O.sub.2 concentration in air to the O.sub.2 concentration in 
the injection stream chosen 
This calculation only serves as a guide to operations. The intent is to 
operate the process at relatively constant oxygen flux. 
Control of oxidant gas introduction is accomplished by limiting the 
injection rate to a minor fraction of the calculated maximum injection 
rate for a period of time sufficient to allow the gasification zone to 
slowly expand outward from the bottom of the injection wellbore and then 
expand downward (at a much slower rate) to the bottom of the coal seam and 
upward around the wellbore until full seam thickness is utilized. 
Thereafter, the injection rate is slowly and progressively increased, 
until the maximum injection rate has been attained. As a general rule, the 
initial injection rate should not exceed 1/3 of the calculated maximum 
injection rate during approximately the first 15% of the calculated well 
pair life. (The calculated well pair life is equal to the well spacing in 
feet divided by the average rate of gasification front movement in 
feet/day. The average rate of front movement observed during small-scale 
field tests in the U.S. has been about 1.5 feet/day. This value serves 
only as a guide and can vary depending upon conditions.) The injection 
rate is then increased to the calculated maximum injected rate over the 
next 15% to 30 % and preferably over a period not greater than 25% of the 
calculated well pair life and maintained at or near this level for the 
remainder of the calculated well pair life. 
Control of oxidant gas utilization is aided and establishment of the 
gasification zone over the full seam thickness is more completely ensured 
by use of proper well completion techniques. It is highly desirable that 
the well completion technique selected produce a reliable and competent 
bonding between the well casing and the coal seam such that oxidant flow 
up the outside of the casing is minimized and chimneying up around the 
casing does not result. It is also preferred that the wellbore be cased at 
least 2/3 of the way through the coal seam or within about 5 feet of the 
bottom of the seam, whichever is closer to the bottom of the seam. 
Upon initial establishment of the gasification front over the full seam 
thickness, it thereafter becomes stable and self-propagating over the 
entire seam due to thermal effects at the gasification front. After full 
seam thickness gasification is established, the linkage path serves only 
as a conduit for product flow to the production well. 
The procedure described above is in effect a startup procedure to ensure 
establishment of a stable gasification zone over full seam thickness. It 
may not have to be repeated in this exact manner after startup depending 
upon how the well pattern within any process module has been laid out, and 
upon the step-by-step operating procedure which has been chosen. The need 
to adjust the procedure used is highlighted in the following examples: 
EXAMPLE 1 
Referring to FIG. 3, there is illustrated a schematic diagram of a portion 
of a process module showing wells laid out in columns and rows in a 
rectangular pattern. Each well is completed into the coal seam as 
previously described. In this example, it is desired to obtain maximum 
utilization of the coal resource and to achieve interconnection of the 
gasification channels. Hence, the spacing 31 between adjacent columns is 
set to be somewhat less than the spacing 32 between adjacent rows. The 
procedure is carried out as follows: 
(a) The coal seam is ignited at well 2. Ignition is accomplished in 
conventional fashion using a downhole burner, placement of a pyrophoric 
material at the bottom of the wellbore or other suitable technique. Air is 
then injected into well 2 at a pressure of about 1 psig per foot of depth 
to the bottom of the coal seam to sustain combustion. Gas samples 
collected from wells 1 and 3 may be monitored to determine when ignition 
has been achieved. Thereafter, high pressure air, or other suitable 
oxidant, is injected at wells 1 and 3 until reverse combustion linkage to 
those two injection wells has been achieved as evidenced by a substantial, 
greater than 50%, reduction in injection pressure at those wells. Well 2 
is used as the production well. 
(b) Upon completion of reverse combustion linkage, air injection at wells 1 
and 3 is limited to a rate less than 1/3 of the calculated maximum 
injection rate for up to about 15% of the calculated well pair life to 
initiate gasification from wells 1 and 3 toward well 2. The injection 
pressure is such that the pressure within the coal seam is at or below the 
hydrostatic pressure within the coal seam if the coal seam is an aquifer. 
The air injection rate is then increased to the calculated maximum over a 
period preferably not greater than 25% of the calculated well pair life. 
(c) High pressure air injection is initiated at wells 4, 5 and 6 to begin 
reverse combustion linking from Row 1 to Row 2. This step should not be 
initiated until the product gas temperature at Well 2 has reached a 
temperature of at least 300.degree. F. indicating that the permeability 
pathway between wells 1 and 2 and 2 and 3 has been heated to a temperature 
above the ignition point of the coal all along the pathway. 
(d) Upon completion of reverse combustion linking from Row 1 to Row 2 as 
indicated by a reduction of at least 50% in the injection pressure at all 
Row 2 wells, injection is initiated at each of the Row 2 wells at a rate 
less than or equal to 1/3 of the calculated maximum injection rate at a 
pressure equal to or slightly less than the seam hydrostatic pressure to 
begin gasification from Row 2 and Row 1. All wells in Row 1 are converted 
to production wells after terminating injection at wells 1 and 3 prior to 
beginning high-volume injection at the Row 2 wells. After a time not 
greater than 15% of the calculated well pair life, the injection rate is 
increased at each well in Row 2 to the calculated maximum injection rate 
over a period not longer than 25% of the calculated well pair life and 
maintained at this rate at a relatively constant level. 
(e) After the low pressure injection rate at all wells in Row 2 has been 
increased to greater than 1/3 of the calculated maximum injection rate, 
high pressure injection is initiated at all wells in Row 3 to initiate 
reverse combustion linking from Row 2 to Row 3. Because the rate of 
movement of the reverse combustion front proceeds at a rate about 4 times 
faster than the gasification front (about 6 ft/day compared to about 1.5 
ft/day), it may not be desirable to initiate this step until after a time 
of at least 40% of the calculated well pair life for gasification from Row 
2 to Row 1 has passed in order to synchronize the relaying of the 
gasification process from the area between Rows 2 and 1 to the area 
between Rows 3 and 2 to avoid any need to reignite the coal at the bottom 
of the Row 3 wells. As an alternative, after completion of reverse 
combustion linking from Row 2 to Row 3, it may be desirable to maintain 
low levels of injection at all Row 3 wells to maintain the combustion zone 
at the bottom of these wellbores until gasification from Row 2 to Row 1 
has been completed. 
(f) Upon completion of gasification from Row 2 to Row 1 and completion of 
reverse combustion linking from Row 2 to Row 3, low pressure injection is 
initiated at all wells in Row 3 at a rate not greater than 1/3 of the 
calculated maximum injection rate. All wells in Row 2 are opened to 
production and all wells in Row 1 are shut in. The injection rate at all 
Row 3 wells is maintained at a value less than or equal to 1/3 of the 
calculated maximum injection rate for a period not greater than 15% of the 
calculated well pair life. 
(g) The process then becomes a repetition of steps (e) and (f) as the area 
of the process module is gasified. 
EXAMPLE 2 
A variation of the procedure set out in Example 1 is as follows. Steps (a), 
(b), and (c) are identical to Example 1. Thereafter, the following steps 
are performed in sequence: 
(d) Upon completion of reverse combustion linking from Row 1 to Row 2, all 
wells in Row 2 are converted to production wells and high-volume, 
low-pressure air injection is begun at all wells in Row 1 to initiate 
gasification from Row 1 to Row 2. The rate of injection at each well in 
Row 1 may be greater than 75% of the calculated maximum injection rate 
because the gasification front has already been established over full seam 
thickness during step (b). 
(e) After the temperature of the product gas at all wells in Row 2 has 
reached a temperature greater than 300.degree. F., high pressure injection 
at all wells in Row 3 is initiated to begin reverse combustion linking 
from Row 2 to Row 3. Upon completion of reverse combustion linking from 
Row 2 to Row 3, high pressure injection is terminated at all wells in Row 
3 and all wells in Row 3 are then converted to production wells. Low 
levels (less than about 5% of individual well production capacity) of 
product gas are bled from the Row 3 wells to maintain open permeability 
pathways from Row 2 to Row 3 and to further prepare the pathways for 
subsequent gasification operations. The flow of hot production gases down 
these pathways results in further devolatilization thereby increasing the 
effective diameter of the pathways. 
(f) Upon completion of gasification from Row 1 to Row 2, all wells in Row 3 
are then opened for full production, all Row 2 wells are converted to 
injection wells, injection is terminated at all Row 1 wells, and injection 
is initiated at all Row 2 wells while all Row 1 wells are shut in. Because 
the gasification zone has already been established over full seam 
thickness between Rows 1 and 2, the injection rate at each of the Row 2 
wells may not need to be reduced to 1/3 or less of the calculated maximum 
injection rate. It usually is not necessary to reduce the injection rate 
to less than 50% of the calculated maximum injection rate. If any 
reduction is required upon initiation of injection at each of the Row 2 
wells, the injection rate should be increased to the calculated maximum 
injection rate over a period not to exceed about 15% of the calculated 
well pair life. 
(g) Steps (e) and (f) are then repeated until the coal in the process 
module has been gasified. 
The procedure set out in this Example is less preferred than that of 
Example 1 as it has the disadvantage of exposing the process wells to hot 
production gases before they are converted to injection wells. This 
exposure could result in damage to the cement bond between the well casing 
and the overburden leading to the potential for increased gas losses up 
the outside of the casing to the overburden or to the surface. 
EXAMPLE 3 
In those cases where a high potential for subsidence is determined during 
site characterization and where it is desired to avoid the effects of 
subsidence to the greatest extent possible, gasification may be carried 
out in a series of isolated gasification channels. To accomplish this, 
spacing 31 between adjacent well columns of FIG. 3 is increased to a value 
up to but not greater than twice that between adjacent well rows, or 
spacing 32. Essentially isolated gasification channels are formed along 
each column of wells, as along the column defined by wells 1, 4, 7 and 10, 
leaving an ungasified residual seam portion between the columns to provide 
support for the overburden. 
Because of the isolation between adjacent well columns, step (a) of the 
procedures set out in Examples 1 and 2 is modified as follows. The coal is 
ignited in all of the Row 1 wells and high pressure air is injected at all 
wells in Row 2 to initiate reverse combustion linking from Row 1 to Row 2. 
Thereafter, the procedure may be that set out in either Example 1 or 
Example 2, starting with step (d). 
EXAMPLE 4 
Some coal seams otherwise amenable to in situ gasification are themselves 
aquifers or lie in near proximity to aquifers, both conditions which may 
act to provide such an excess of groundwater as to preclude successful 
gasification. Dewatering of such seams must be carried out prior to 
gasification in order to achieve a reasonable degree of gasification 
efficiency. 
As is illustrated in FIG. 4 dewatering may be accomplished by creating a 
boundary cavity around a process module. Referring now to FIG. 4, there is 
illustrated a process module having a plurality of wells arranged in a 
generally rectangular grid pattern of n rows and n columns. The periphery 
or boundary of the process module is defined by the well arrays making up 
Column 1, Row n, Column n, and Row 1 as is shown by the arrow path. It is 
usually advantageous but not essential to decrease the well spacing in the 
boundary columns and rows compared to the remainder of the process module 
as is shown in the Figure. 
Isolated gasification channels or cavities are produced around the process 
module by igniting the coal in a selected boundary well and thereafter 
injecting high pressure air into an adjacent boundary well until reverse 
combustion linkage has been achieved. The adjacent boundary well is then 
converted to an injection well in the manner described in Example 1, step 
(d) with a progressive increase in injection rate as set forth therein. 
High pressure air injection in the next adjacent, or third, well is 
commenced to establish reverse combustion linkage between the second and 
third wells. The third well in turn is converted to an injection well and 
this procedure is repeated with succeeding wells until an isolated 
gasification channel is formed around the module periphery. 
Injection of air into a well for some period of time before it is ignited 
may be necessary in order to remove free water at the bottom of the 
borehold. In addition, a system for liquid removal from the wells may be 
required in order to ensure that influxing groundwater does not quench the 
ignition attempts and to remove condensed liquids which will collect 
during reverse combustion linking operations. 
Influxing groundwater will quickly cool the gasified boundary cavity. 
Pumping from the cavity is then initiated and is continued until the 
module area has been sufficiently dewatered, as evidenced by monitoring 
wells within the module, to allow efficient gasification of the coal seam. 
All of the preceding examples have illustrated gasification methods which 
use reverse combustion as a permeability enhancement, or well linking, 
technique and which use air as the oxidant. Use of other well linking 
techniques, while not preferred because of technical and economic 
considerations, require merely the substitution of another linking 
technique, such as directional drilling, for the reverse combustion 
linkage in the outlined methods. 
Substitution of oxygen-enriched air, oxygen, air-carbon dioxide, air-stream 
and similar mixtures may be made as desired without affecting the 
procedures outlined in the examples. 
In addition, it is to be noted that the previously gasified area can serve 
as a source of hydrogen-rich gas as influxing groundwater contacts hot, 
residual carbon according to the following reactions: 
EQU C+H.sub.2 O=CO+H.sub.2 
EQU CO+H.sub.2 O=CO.sub.2 +H.sub.2 
Thus, low volumes of this hydrogen-rich gas can be produced by opening the 
previously shut in wells in the previously gasified area for either 
blending with the product gas from the area undergoing gasification or for 
uses which require a hydrogen-rich gas. 
Some lignites and subbituminous coals, usually in deep seams, spontaneously 
ignite when contacted with air. In these situations, the reverse 
combustion linkage procedure must be modified in order to achieve linkage 
and maximum control of the process. This is accomplished by reducing the 
oxygen content of air by dilution with any suitable gas including carbon 
dioxide, combustion exhaust gases and the like, to a level below that at 
which spontaneous combustion occurs but not below that which will support 
combustion in the presence of an ignition source. After linkage has been 
completed, the oxygen content is increased to those levels required for 
efficient gasification. The degree of oxygen content reduction required 
may be determined by laboratory testing of core samples. 
Having now fully described the invention, it will be apparent to one of 
ordinary skill in the art that many changes and modifications can be made 
thereto without departing from the spirit or scope thereof.