U.S. patent number 3,775,073 [Application Number 05/175,602] was granted by the patent office on 1973-11-27 for in situ gasification of coal by gas fracturing.
This patent grant is currently assigned to Cities Service Oil Company. Invention is credited to Vaughan W. Rhoades.
United States Patent |
3,775,073 |
Rhoades |
November 27, 1973 |
IN SITU GASIFICATION OF COAL BY GAS FRACTURING
Abstract
Two or more wells are drilled into a coal seam. The wells are
completed so as to isolate all other strata from the coal seam and
a radially extended horizontal fracture is directed by introduction
of a first combustion supporting gas under hydraulic pressure so as
to connect the wells communitively. A horizontally and vertically
directed fracture network is formed within the coal system by
ignition of the first combustion supporting gas. A second
combustion supporting gas is subsequently injected through at least
one well to form a combustion front which may be propagated through
the fracture network to produce combustible gases and coal tar
liquids.
Inventors: |
Rhoades; Vaughan W. (Tulsa,
OK) |
Assignee: |
Cities Service Oil Company
(Tulsa, OK)
|
Family
ID: |
22640882 |
Appl.
No.: |
05/175,602 |
Filed: |
August 27, 1971 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
50790 |
Jun 29, 1970 |
|
|
|
|
Current U.S.
Class: |
48/210; 48/DIG.6;
166/256; 166/259; 166/257 |
Current CPC
Class: |
E21B
43/247 (20130101); Y10S 48/06 (20130101) |
Current International
Class: |
E21B
43/16 (20060101); E21B 43/247 (20060101); E21b
043/24 (); E21b 043/26 () |
Field of
Search: |
;48/210,DIG.6
;166/256,257,259 ;299/2 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wolk; Morris O.
Assistant Examiner: Serwin; R. E.
Parent Case Text
This is a division of application Ser. No. 50,790, filed June 29,
1970, now abandoned.
Claims
I claim:
1. A process for the combustion of coal in subterranean deposits
which comprises:
a. completing two or more wells within the coal deposit;
b. inducing a radially extended horizontal fracture in the coal
deposit so as to inter-connect the wells;
c. subjecting the coal deposit to an excess of a first combustion
supporting gas at a pressure greater than the overburden
pressure;
d. igniting the coal deposit while simultaneously preventing any
production therefrom so as to form a network of crumbled coal
within the coal deposit;
e. injecting a second combustion supporting gas into one or more
wells to form a combustion front; and
f. producing combustible gas and coal tar liquids from one or more
wells.
2. The process of claim 1 in which:
a. the first combustion supporting gas is oxygen; and
b. the second combustion supporting gas is air.
3. The process of claim 2 in which the radially extended horizontal
fracture is induced by hydraulic fracturing.
4. The process of claim 1 further comprising the simultaneous
injection of water with the combustion supporting gas so that a
water-gas shift reaction is provided to increase the calorific
content of the produced gas.
5. The process of claim 1 further comprising introducing a
combustion supporting gas into at least one production well
subsequent to the injection of the second combustion supporting gas
into the ignited wells to cause a reverse combustion and flow of
injected gas, produced gas, and coal tar liquids to the previously
ignited wells.
Description
BACKGROUND OF THE INVENTION
This invention relates to the in situ combustion of a coal seam for
the recovery of flammable gases and coal tar liquids. More
particularly, it relates to a novel method for producing a fracture
network through which combustion supporting gas and combustion
products may pass during the in situ combustion of a coal seam.
The production of coal energy by the use of wells through
underground mining has been a continual subject of interest in the
field of energy production. Coal gasification by use of above
ground retorting is an old art, one of the better known methods
being the Lurgi process developed in Germany prior to World War II.
By this method, oxygen and steam are simultaneously injected into a
field retort and upon combustion a gas having a calorific value
sufficient for commercial usage and coal tar liquids are
produced.
The idea of underground gasification then is not new, however, many
of the technological advances are. Efforts were mostly confined to
the advancement of theory until substantial work and testing was
done in Russia. Most of the Russian work involved considerable
underground mining and construction in an effort to provide a
passageway for air through the coal. Some efforts even involved
breaking up the coal underground to provide adequate air passages.
The amount of excavation encountered in this process is tremendous.
The state of the art then progressed to drilling holes into the
coal seam and charging with dynamite. As the burning front
progressed through the stratum the charges were automatically set
off in an effort to break up and crush the coal and render a
segment of the bed more permeable. This resulted in irregularities
too great to sustain continued gas flow and the gas produced
contained large amounts of air, which considerably lowered the
heating value of the produced gas. As this process is far too
expensive and limited in scope, shaft and borehole mining
combinations were devised which employed steeply sloping seams near
outcrops. In addition to being limited exclusively to steeply
pitched beds, this process also required a large amount of
excavation and mining.
The chief problem, therefore, confronting the spectrum of
investigators was the low gasification rate, that is, the rate of
air injection which directly affects the amount of gas produced.
Although a coal seam contains an appreciable amount of natural
cracks and fissures, its overall permeability is quite low. This
permeability is considerably below that which is necessary to
sustain combustion at rates to be of commercial interest.
Consequently, without expensive underground construction, the
natural air passageways within the coal bed severely limit air
injection. A major effort has been concentrated on methods of
increasing air injection rates by shaftless methods.
Electrocarbonization utilizing high pressure air injection is one
of the methods which has been tested. One of the most promising
techniques involves hydraulic fracturing of the coal bed, packing
the fractures with sand, backward burning to establish better
communication within the bed, then forward combustion to gasify the
coal and release the combustible products. All of the tests proved
uneconomic and unfeasible by present production standards and have
been discontinued before being developed into a commercial process.
What is required, then, is a process which may be implemented
without utilizing previously used techniques of underground
excavation and tunneling. Also required is a method by which air
injection rates are not restricted due to low permeability of the
coal, the restrictions of fluid flow in the fracture network, and
the low permeability of the remaining unburned coal behind the
combustion front.
It is an object of our invention, therefore, to provide a novel
method for the combustion of underground coal structure.
It is another object of our invention to provide for a method by
which a network of fractures created in a coal seam may be used to
allow sufficient gas injection volumes to sustain combustion in a
coal seam.
It is a further object of our invention to provide a method by
which flammable gas and coal tar liquids are afforded a production
path by the inducement of a fracture network within a coal
seam.
With these and other objects in mind, the present invention will be
set forth in further detail with particular reference to the
following drawings and description.
SUMMARY OF THE INVENTION
The present invention comprises a method for the combustion of coal
in subterranean deposits. In the method, two or more wells are
completed within a coal deposit. A radially extended horizontal
fracture is induced through the coal deposit so as to interconnect
the wells.
The coal deposit is subjected to an excess of a first combustion
supporting gas at a pressure greater than the overburden pressure
so as to distribute the combustion supporting gas throughout the
coal network. Subsequently, the coal deposit and first combustion
supporting gas are ignited while simultaneously preventing any
fluid or gas production from the coal deposit, so as to form a
network of crumbled coal within the coal deposit. Continued
injection of a second combustion supporting gas into one or more
wells and production of combustible gas and coal tar liquids from
one or more of the wells completes the process.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is hereinafter described in further detail
with particular reference to the accompanying drawings in
which:
FIG. 1 represents an injection well, as utilized in the present
invention, completed within a subterranean coal deposit; and
FIG. 2 represents a cross-sectional view of a subterranean coal
deposit with the process of the present invention applied
thereto.
DETAILED DESCRIPTION OF THE INVENTION
The present invention describes a novel method for the production
of combustible gases and coal tar liquids by the in situ combustion
of a coal bed. The process comprises the completing of wells in a
coal seam, creating a horizontal fracture in the coal seam to
establish communication between the wells, igniting a first
combustion supporting gas within the horizontal fracture so as to
form a horizontal and vertical fractured network between the wells,
injecting a second combustion supporting gas, propagating a
combustion front through the coal about the injection well and
recovering volatile liquids and combustible gases at the production
well.
Each production well is completed in a manner so as to be in direct
communication with a maximum portion of the coal bed. Casing is
barely notched into the coal seam and cemented. The open or uncased
hole extends to the bottom of the coal bed. The significant factor
in this completion technique is that only the coal bed is left
exposed at the producing wellbore. All other strata are cemented
and sealed off from the producing wellbore in order that any
production from the wells must come through the coal bed itself.
Injection wells are cased and cemented to the center of the coal
layer to enhance the fracturing operation. Configuration of
injection and production wells is not a significant criteria of the
present invention and therefore is not treated herein. Essentially,
any well pattern combination with which horizontal communication
between the wells may be effected may be utilized with the present
invention. In addition, the completion technique used may consist
of any of the various and sundry well isolation methods as long as
the coal seam is left undamaged and remains isolated from the
overburdened strata.
Formation of a horizontal fracture within the coal bed is an
essential criterion of the invention for success of the entire
operation. The fracture should be initiated as close to the center
of the coal bed, within the injection well, as feasible and
propagated through the bed of the coal seam so as to intersect the
surrounding producing wells. The fracture is to provide a
passageway through which combustion gas and products may flow.
Fracturing out of the coal seam and into another horizon should be
avoided where adjacent strata would provide a path through which
the gasified products could escape, thereby preventing their
recovery. The fracturing of the coal stratum is, therefore, an
imperative consideration of the present invention.
The method by which horizontal fracturing of the coal seam is
achieved is to introduce a liquid into the coal bed and thereby
cause a hydraulic fracture network. To accurately describe the
fracture growth wtihin a formation, the mechanism that controls the
leak-off rate when the system is pressurized with a liquid must be
approximated. Reservoir response to liquid pressure can be grouped
to either of three main categories. The first category, which is
the dominant factor controlling the leak-off, is the viscosity of
the fracturing fluid itself. The compressibility of the fluid
within the coal bed should be considered as it is significant when
compared to the resistance to flow through a coal formation. The
third consideration in predicting leak-off is whether there exists
a substance in the fluid which will deposit on the fracture faces
and thus create a significant pressure drop between the inside of
the fracture and the formation so that most of the fracturing fluid
will remain within the fracture. Once these factors are determined,
the appropriate fracturing fluid and quantity may be chosen.
After completing the radial extending horizontal fracture, the
horizontal and vertical network is created throughout the coal
deposit by the introduction of a first combustion supporting gas
into the horizontal fracture between the wells and thereafter
igniting the coal deposit so as to form a network of crumbled coal
within the coal deposit. The method of producing the fracture
network within the formation is to subject the coal deposit, after
the introduction of a radially extended horizontal fracture, to an
excess of a first combustion supporting gas, for example oxygen.
The gas forms a highly volatile or combustible combination with the
hydrocarbons contained within the coal deposit. Subjecting the coal
deposit to a pressure greater than the overburden pressure creates
a blanket of the volatile mixture over the entire coal deposit.
Ignition of the coal deposit, while simultaneously preventing any
production, forms a crumbled network of coal in the coal deposit
for propagation of a combustion front through the coal seam by
injection of a second combustion supporting gas. The amount of
combustion movement is directly proportional to the oxygen
injection rate so that the advancement of the combustion front may
be controlled as it moves throughout the coal deposit. The
requirement of creating the combustion front is alleviated in that
the combustion front is already initiated during the fracture
network formation and propagated by the subsequent injection of a
second combustion front throughout the coal deposit. It is
understood that by any of the processes described, a reverse
combustion drive may be induced by reversing injection of the
combustible gas into the production wells.
To more fully understand the application of the present process,
referral to FIG. 1 is suggested. In FIG. 1, an injection well 110
is completed from the earth's surface 101, through overburden 102,
through coal bed 103 and is spudded into underlaying formation 105.
The well 110 is sealed by a wellbore liner 109 and casing 108 to a
point midway in the coal bed 103. The lower portion of the coal bed
103 and underlaying formation 105 being left as an open hole 106. A
wellhead 112 is connected to the wellbore liner 109 so that fluids
may be introduced into well 110. Retrievable isolation means, for
example a packer 111, are placed in the well 110 at a point
slightly below the mid-section of the coal bed 103 by shoe 107 such
that the subsequent introduction of a hydraulic fluid at pressures
higher than the overburden pressure create a radially extending
horizontal fracture 104 through the coal bed 103 to subsequent
offsetting wells.
FIG. 2 typifies the resultant crumbled coal bed 228 produced by the
process for forming a fracture network as described previously. The
injection well 210 and offset production well 220 are completed in
similar fashion with respective cement 208 and 218 and casing 209
and 219 as described for FIG. 1. It is noted, however, that
production well 220 is completed to the top of the coal bed 203.
Compressor 213, having inlet 214, through which a combustible gas
is supplied, is connected to wellhead 212 such that combustible gas
is introduced into the formation and propagates a combustion front
226 through the crumbled coal bed 228 leaving behind charred
residual 227. Combustible gas and coal tar liquids are produced
from wellhead 222 and thereby introduced into scrubber-separator
223 from which dry combustible gas 224 is produced overhead as
liquid coal tar products 225 are produced from the bottom.
It is conceivable that pure oxygen may be utilized in the
fracturing process, whereas it is generally not thought to be safe
or economic, in that the depth of most coal seams subjected to this
method is generally not more than several hundred feet, thereby
requiring very little safety equipment and a lesser tendency for
pre-ignition of materials within the wellbore while oxygen is being
loaded into the formation.
A preferred method for controlling the temperature of the flame
front, but more particularly to adjust the calorific value of the
produced gas, is by the simultaneous injection of water with the
combustion supporting gas. A water-gas shift reaction is then
obtained at the site of the combustion front which yields a
considerably enhanced calorific content produced gas and lowers the
temperature of the combustion front. This temperature lowering
results in a decreased loss of heat to the surrounding strata and a
decrease in the destructive degradation of coal tar liquids. The
particular reaction of the water-gas shift reaction is presented
below:
C + H.sub.2 O .sup.heat H.sub.2 + CO
The increased hydrogen content of the produced gas yields a high
energy content energy source gas.
When the present invention is applied to the art of in situ
combustion of coal seams it provides an effective means for the
combustion and reclamation of coal tar liquids and produced gas in
order that a greater area extent of the coal seam may be contacted.
The invention enhances the art of in situ combustion of
subterranean coal deposits by allowing an economic and facile
method for the combustion and reclamation of energy from these
deposits.
The present invention has been described herein with respect to the
particular embodiments thereof, it will be appreciated by those
skilled in the art, however, that various changes and modifications
can be made without departing from the scope of the invention as
presented.
* * * * *