Note: Descriptions are shown in the official language in which they were submitted.
PATENT
2~59~ 90950l
Puri/Stein
METHANE PRODUCTION FROM CARBONACEOUS
SUBTERRANEAN FORMATIONS
RELATED APPLICATIONS
This application is related to Canadian Application
No. 605,297.
FIELD OF THE INVENTION
The present invention is a method of producing
methane from a solid carbonaceous subterranean formation.
More specifically, the invention is a method of producing
methane from a solid carbonaceous subterranean formation
by injecting an inert gas through an injection well into
the solid carbonaceous subterranean formation to strip
methane from the carbonaceous materials in the formation
and sweep the produced gases into a production well.
BACKGROUND OF THE INVENTIOM
During the conversion of peat to coal, methane
gas is produced as a result of thermal and biogenic proc-
esses. Because of the mutual attraction between the coal
surface and the methane molecules, a large amount of meth-
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ane can remain trapped in-situ as gas adhered to the
organic matter (i.e., carbonaceous materials) in the for-
mation. The reserves of such ~methane~ in the United
States and around the world are huge. Most of the
5 reserves are found in coal, but significant reserves are
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found in gas shales and other solid carbonaceous subterra-
nean formations.
Conventional methane recovery methods are based
on reservoir pressure depletion strategy; that is, methane
5 is desorbed from the carbonaceous surfac~s by reducing the
reservoir pressure. While this method of methane pro-
duction is simple, it is not efficient. Loss of reservoir
pressure deprives the pressure depletion process of the
driving orce necessary to flow methane gas to the well-
10 bores. Consequently, the gas production rate from a wellis adversely affected by the reduction in reservoir pres-
sure.
Another method of recovering methane is by
injecting into the solid carbonaceous subterranean forma-
15 tion a gas, such as CO2, having a higher affinity for coalor other carbonaceous material than the adsorbed methane,
thereby establishing a competitive adsorption/desorption
process. In this process, the CO2 displaces methane from
the surface of coal, thereby freeing the methane so that
20 ît can flow to a wellbore and be recovered. This method
is disclosed in the reference by A. A. Reznik,
P. K. Singh, and W. L. Foley, "An Analysis of the Effect
f C2 Injection on the Recovery of In-Situ Methane from
Bituminous Coal: An Experimental Simulation," Society of
25 Petroleum Engineers Journal, October lg84. The problem
with this method is the larse volume of CO2 that must be
injected into the solid carbonaceous subterranean forma-
tion in order to exchange sites with methane. In most
instances, such an amount would be uneconomical. This
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reference reports that mixing even small amounts of nitro-
gen gas with CO2 significantly reduces the effectiveness
of displacement desorption of methane by CO2.
There is a need for a method of producing meth-
5 ane from coal and other solid carbonaceous subterraneanformations that accelerates the production rate and
improves recoverable gas reserves economically.
SUMMARY OF TEIE INVENTION
The present invention overcomes the foregoing
10 deficiencies and meets the above-described needs. The
present invention is a method for producing methane from a
solid carbonaceous subterranean formation penetrated by at
least one producing well. The method comprises injecting
an inert gas through the injection well and into the solid
15 carbonaceous subterranean formation, and producing the
inert gas and the methane from the production well.
Coalbed methane recovery is accelerated and substantial
improvement is made in the net recoverable reserves.
BRIEF DESCRIPTION OF DRAWINGS
FIGURE 1 is a graphical representation of a
sorption isotherm illustrating the relationship between
the reservoir pressure of a coal seam and the gas content
of the coal. ~he sorption isotherm is a representation of
the maximum methane holding capacity of coal as a function
25 of pressure at a fixed temperature.
FIGURE 2 is a graphical representation of a
sorption isotherm of a coal sample in the presence of an
inert gas.
FIGURE 3 is a top view of a four-spot repeating
well patt~rn described in the Example.
FIGURE 4 is a graphical representation of the
methane production rate versus time for the four spot
5 repeating well pattern.
FIGURE 5 is a graphical representation of the
original gas in place recovered versus time for the four
spot repeating well pattern.
FI&URE 6 is a graphical representation of the
10 mole percent of gas produced versus time for the four spot
repeating well pattern.
DETAILED DESCRIPTION OF THE INVENTION
The desorption of methane from the carbonaceous
surface of the formation is controlled by the partial
15 pressure of methane gas rather ~han the total system pres-
sure. Therefore, methane is desorbed as a result of
reduction in methane partial pressure. The methane recov-
ery from a solid carbonaceous subterranean formation can
be accelerated and enhanced by the continuous injection of
20 an inert gas into the solid carbonaceous subterranean for-
mation. While the total reservoir pressure is maintained,
if not increased, the partial pressure of methane is
reduced. The term "inert gas" defines a gas that (i) does
not react with the coal or other carbonaceous material in
25 the formation under conditions of use (i.e., the standard
meaning for "inert") and (ii~ that does not significantly
adsorb to the coal. Carbon dioxide and gaseous mixtures,
such as flue gas, that contain carbon dioxide as a signif-
icant constituent do not meet the later criteria. It is
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known that coal has a higher affinity for carbon dioxlde
than for adsorbed methane. It is also known th~t coal has
a lower affinity for the inert gases used herein than for
adsorbed methane. See, for example, the French paper
5 "Etude de la liaison gaz-charbon" by J. Gunther, Rev. Ind~
Min. 47, 693-708 (October, 1965) and also the disclosure
in USP 4,043,395, Every (for CO2). Examples of inert
gases include nitrogen, helium, argon, air and the like.
Nitrogen is preferred based on current commercial avail-
10 ability and price. FIGURE 2 shows the equilibrium sorp-
tion isotherm of a coal sample in the presence of an inert
gas. As illustrated, 35% of the gas in place can be
recovered from coal by either reducing the total pressure
from 465 psi to 200 psi or by diluting the free methane
15 gas concentration in coal with an inert gas so as to reach
an equilibrium value of 43% methane and 57% inert gas
without any change in the total pressure.
The use of inert gas to desorb methane is eco-
nomically and technically feasible primarily because of
20 the low effective porosity of the carbonaceous formation.
For example, the permeability of coal is in the order of
1%. Injection of a relatively small amount of inert gas
into the solid carbonaceous portion of the formation
causes a large reduction in the partial pressure of free
25 methane gas in the treated carbonaceous portion of the
formation, such as the cleat system of a coalbed. Conse-
quently, methane is desorbed from the carbonaceous
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materials in the formation until a new equilibrium is
reached, as per the sorption isotherm. Th~ mixture of
methane and
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inert gas flows across and through the solid carbonaceous
subterranean formation along with water until it is recov-
ered at the surface by means of producing wells~ The pro-
duced gas is separated from water and recovered using
5 known separation methods. Methane is separated from the
inert gas also using known separation methods. The meth-
ane is then marketed, the inert gas can be recycled. Eco-
nomics of the methods are enhanced by recycling the inert
gas.
The novel inert gas stripping method of the pre-
sent invention can be further improved by heating the
inert gas before it is injected into the solid carbona-
ceous subterranean formation.
The injection pressure of the inert gas should
15 preferably-he lower than the fracture parting pressure of
the solid carbonaceous subterranean formation but should
be higher than the initial reservoir pressure. Mainte-
nance of a constant injection pressure is also desirable,
although not necessary.
The present invention requires at least one
injection well and at least one production well. The
number and location of the injection and production wells
can be varied and will usually be determined after reser-
voir engineering and economics of a specific field project
25 have been evaluated.
During the present process, the solid carbona-
ceous subterranean formation is dewatered, but reservoir
pressure is not lost. This is an important advantage
because maintenance of reservoir pressure in a methane
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field also helps reduce water migration from the surround-
ing aquifers. This is particularly advantageous in solid
carbonaceous subterranean formations with high permeabil-
ity and effective cleat porosity. Over the life of the
5 degas project, the amount of water that is recovered from
and disposed of can be reduced because of the reduced
water migration in the field.
Inert gas injection can al50 be conducted in
existing fields that have been on pressure depletion for a
10 period of time prior to such injection. In this method,
methane is produced through at least a first and second
well. Then such production is ceased in the first well
and inert gas in injected through the first well into the
solid carbonaceous subterranean formation. The inert gas
15 and methane is then produced from the second well.
EXAMPLE
Four wells are drilled in a 320 acre square in a
repeating well pattern (as shown in Figure 3) and produced
at total gas rates of approximately 1200 thousand standard
20 cubic feet per day for a period of five years (base case)
using a reservoir pressure depletion technique. At that
time, one of the wells (No. 1) is converted into an
injection well and nitrogen is injected through this well
and into the solid carbonaceous subterranean formation for
25 the next twenty years.
FIGURE 4 shows the gas production rates for the
four producing wells of the base case and for the three
producing wells during N2 injection. As shown, methane
recovery from the field increases substantially when N2
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injection is initiated. FIGURE 5 shows the percent of
original gas in place recovered for the base case and for
the three producing wells during N2 injection. As illus-
trated, the injection of inert gas in the field increases
5 the net recoverable reserves of methane gas by more than a
factor of 2. The composition of the produced gas is shown
as a function of time in FIGURE 6.
This example shows that inert gas injection in
coal is of considerable value in accelerating and enhanc-
10 ing methane recovery from coal.
The present invention h~s been described in par-
ticular relationship to the attached drawings. However,
it should be understood that further modifications, apart
from those shown or suggested herein, can be made within
15 the scope and spirit of the present invention.
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