Note: Descriptions are shown in the official language in which they were submitted.
CA 02441640 2003-09-19
Baclc~round of the Invention
1~
1. Field of the Invention
This invention relates to the stimulation of coal seams for methane recovery.
Coal seam
Qas production is primarily dependent on the permeability of the coal seam.
Coal seam
20 reservoirs are fractured reservoirs wherein the fractures intervene between
essentially
impermeable blocks of coal matrix. As the fractures are the only effective
pathways for
movement of methane through a reservoir to a well-bore for recovery, the size,
intensity
and connectedness of the fractures determine the permeability and, hence,
producibility
CA 02441640 2003-09-19
of the reservoir. No known method exists for artificially creating fracturing
within a coal
seam on a reservoir wide scale, so techniques and methods for stimulating coal
seam
permeability are limited to enhancing the near well bore environment with the
goal of
effectively connecting the well bore to the existing natural fracture system.
S
Underground coal seams contain large amounts of natural gas, a significant
amount of
which is methane. The methane exists in a sorbed state in the coal and several
techniques
exist for increasing permeability near the wellbore and, hence, methane
production.
Several challenges exist in creating an open pathway from the coals natural
fracture
system to the wellbore for the recovery of methane. Firstly, coals are known
to possess
stress dependent permeability, meaning that the application of additional
effective stress
upon a coal causes deformation which often closes the fracture system, thus
degrading
permeability and methane production. A critical challenge in stimulating
coalbed
methane wells is to design a completion technique that allows the fracture
system to
maintain an open posture throughout production, thereby minimizing the effect
of stress
dependence on permeability.
Secondly, the nature of prior art fracture treatment on coals also serves to
damage and
degrade coal permeability in some regions of the reservoir while
simultaneously
enhancing it in others. The intent of the prior art is that the enhanced
regions of
permeability outweigh the damaged areas, with a net positive effect to the
near well-bore
environment. In a prior art fracture treatment, proppant is entrained within a
foam or
water-based slurry into newly created and/or existing fractures within the
coal seam. As
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these fractures are opened, stress is redistributed and other fractures which
may be
surrounding the opened fracture become pressed shut. This effect degrades the
reservoir
by diminishing the amount of reservoir effectively connected to the hydraulic
fracture
treatment. The object of the present invention minimizes this degradation by
shrinking
the coal matrix prior to the introduction of the propped fracture.
Prior art discloses three main techniques for stimulating coalbed methane
wells. All of
these techniques disclose injection of a treatment comprising various gases,
fluids and/or
proppants into the coal seam to fracture the coal.
The first technique is a chemical bath stimulation in which the coals are
subjected to
various aqueous-borne chemicals to either increase cleat or fracture
development within
the coals or to change a chemical characteristic of the coals. U.S patent
5,249,627
discloses a method for stimulating methane production from coal seams by
treating the
coals with various chemicals to improve the removal of water. No disclosures
for either
gas injection or proppant placement are made. U.S patent 5,470,823 discloses a
method
for stimulating methane production from coal seams by treating the coals with
an
aqueous acid solution. No disclosures for either gas injection or proppant
placement are
made. U.S patent 5,669,444 discloses a method for stimulating methane
production from
coal seams by treating the coals with various aqueous chemicals to increase
fracture
development. No disclosures for either gas injection or proppant placement are
made.
U.S patents 5,669,444 and 5,964,290 disclose methods for stimulating methane
production from coal seams by treating the coals with various aqueous
chemicals to
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increase fracture development. No disclosures for either gas injection or
proppant
placement are made. None of these disclosed techniques follow the chemical
treatments
with a proppant.
The second type of stimulation is a one-stage completion technique relying on
artificially
initiating a fracture and/or propagating an existing fracture through the
coals through the
application of high pressure injection of gas and/or water based fracturing
fluids.
Hydraulically fracturing coals is difficult given the plastic behavior of many
coals, which
tend to fracture at a greater treating pressure than the surrounding strata.
In a one-stage completion, the completion process is one continuous
application of
pressured fluid, with or without proppant, and the well is placed on
production thereafter.
No attempts are made to induce shrinkage in the coal. U.S. patent 3,384,416
discloses a
method where a refrigerant fluid containing a proppant is injected into a coal
seam to
create fracturing. The inducement of matrix shrinkage prior to proppant
placement is
undisclosed. U.S. Patent 6,412,559 B1 discloses method for recovering methane
and/or
sequestering fluids in coals whereby a gas more strongly adsorbing than
methane with or
without a proppant is injected into a coal seam, shut in for a period of time
from hours to
days and released. The inducement of matrix shrinkage prior to proppant
placement is
undisclosed.
The third type of completion is a cycled gas completion, in which gases are
repeatedly
injected and allowed to flow back, with the intention of causing in-situ
failure of the coals
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and thereby inducing fracturing. U.S. patent 5,014,788 discloses a method for
injecting
a gas into a coal seam which is intended to swell the coals and increase
stress within the
coal. A rapid depressurization of the coals caused by suddenly releasing the
pressurized
gas to surface shrinks the coal, removing the induced stress, and mechanically
failing the
S coal. This process is designed to be repeated as many times as necessary. No
disclosure
is made for following this process with a propped fracture treatment. U.S.
patent
5,417,286 discloses a method for stimulating methane production from
carbonaceous
subterranean formations be injecting a first fluid to sorb into the formation
and
subsequently injecting a chemically different second fluid to part the
formation and
relieving the pressure to produce shear failure within the formation. No
claims are made
regarding the use of a proppant. U.S. patent 5,566,755 discloses a method for
recovering
methane from solid carbonaceous subterranean formations through the repeated
injection
and extraction of an oxygen depleted gas. No claims regarding subsequent
placement of
a proppant are made. U.S. Patent 6,412,559 B1 discloses method for recovering
methane
and/or sequestering fluids in coals whereby a gas more strongly adsorbing than
methane
and a proppant are simultaneously injected into a coal seam, shut in for a
period of time
from hours to days and released. U.S. Patent 6,450,256 discloses an enhanced
coalbed
gas production system whereby gases are injected into a coal seam and
released. The
injection of the gas increases methane production displacing water within the
cleat
system, by affecting the gas saturation within the coal and by reducing the
partial
pressure between the coal and the cleat system. No disclosure of introducing a
proppant
after release of the injected gas is made. U.S. Patent 6,571,874 B1 discloses
method for
extraction of in-situ methane from coals whereby gases are injected into a
coal seam, shut
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CA 02441640 2003-09-19
in for a period of time from days to weeks and released. The process is
repeated many
times with the purpose of creating a propped fracture utilizing fines sourced
from the
coals themselves. This prior art method contains serious drawbacks. Firstly,
fines control
is considered a major problem in coalbed methane production, as the fines are
known to
migrate and plug open fractures, rather than maintain open fractures. As well,
the low
compressive strength of coal makes it a very poor candidate for a proppant, as
the fines
would be easily crushed into a damaging powder which would cause plugging.
This prior
art method does not utilize an introduced proppant after release of the
injected gas.
None of the prior art inventions utilize matrix shrinkage in coal and all fail
to take into
account the benefits of inducing matrix shrinkage first, followed by the
placement of a
propped fracture.
2. Summary of the Invention
The realization of this object, including advantageous embodiments and
modifications of
this invention can be seen from the content of the patent claims that follow
this
description.
The goal of the present invention is to provide a method to stimulate coalbed
methane
wells. The invention improves on the previously disclosed techniques as it
uses the
reaction of the coal to the predetermined gas to induce shrinkage within the
coal matrix,
thus reducing effective stress and enhancing the fracture void volume. This
increased
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fracture void volume allows placement of proppant within the coals while
maintaining
open fractures in the regions surrounding the propped fracture, thus allowing
the
enhanced fracture system to communicate more effectively with both the natural
fracture
system and the wellbore and aiding methane recovery.
S
3. Description of the Preferred Embodiments.
The present invention has the advantage that the pressing of the predetermined
gas into
the coal seam opens the existing natural fracture system within the coal and
causes
shrinkage of the coal such that when pressure is removed from the system the
volume of
the coals is less than the volume originally in place. This shrinkage results
in additional
fracture void volume and serves to create more and better connected pathways
through
the coal and lessens effective stress.
The object of the present invention is to stimulate a coalbed methane well by
injecting a
predetermined gas into a well bore that is open to a coal seam, shutting the
well in to
induce shrinkage of the coal seam and enhancement of the intervening natural
fractures
and then placing a propped fracture stimulation into the enhanced intervening
fractures to
maintain the fractures in an open state.
During the first stage injection of the predetermined gas, the gas travels
into the fracture
network along the fracture planes where it contacts the coal matrix and begins
displacement of the methane contained within the coal matrix. This
displacement shrinks
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the contacted portions of the coal matrix, resulting in an increase in the
intervening
fracture void volume.
In the second stage, the well is shut in for a period of time to allow the
maximum amount
of methane displacement to occur. In this manner, the coal is an active
participant in the
production of induced fractures and planes of weakness, with the interaction
between the
coal and the predetermined gas creating preferential paths of weakness within
the coal.
This differs significantly from most prior art fracture treatments which treat
the coal as a
static medium and attempt to create new fractures within the coal by exceeding
it's
parting strength.
A second object of the invention is the maintenance of smaller scale fractures
in an open
posture surrounding the larger scale induced fractures. The second stage shut
in period is
critical to this effect as it allows the maximum amount of coal matrix to be
bathed in the
predetermined gas, thereby maximizing the reservoir volume treated for induced
fracturing. Once the shut-in period has ended, matrix shrinkage has produced a
similar
effect to removal of matrix by creating additional void space in the enhanced
fractures
within the coal seam. This allows the proppant placed in stage three to occupy
the
enhanced fractures without producing excessively elevated local effective
stress fields
and collapsing the smaller scale fractures that occupy the region surrounding
the propped
fracture. In this fashion the propped fracture system maintains an effectively
connected
posture with both the reservoir's natural fracture system and the well bore
and methane
collection is enhanced.
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CA 02441640 2003-09-19
Title: Method for enhancing methane production from coal seams by inducing
matrix
shrinkage and placement of a propped fracture treatment
References Cited:
U.S. Patents
6,571,874 B1 6/2003 Lovenich et
al.
6,450,256 B2 9/2002 Mones
6,412,559 7/2002 Gunter et al.
B1
6,024,171 2/2000 Montgomery et
al.
5,964,290 10/1999 Riese et al.
5,669,444 9/1997 Riese et al.
5,566,755 10/1996 Siedle et al.
S,S01,273 3/1996 Puri
5,470,823 11/1995 Williams et
al.
5,417,286 5/199 Palmer et al.
5,249,627 10/1993 Harms et al.
5,014,788 5/1991 Puri et al.
Canadian Patent
Applications
60/252,956 11/2000 Gunter et al.
Other References
Puri, R., King, G.E., and Palmer, LD., 1991, Damage to coal permeability
during
hydraulic fracturing, Proceedings of the 1991 Coalbed Methane Symposium, The
University of Alabama, Tuscaloosa, May 13 - 16, 1991.
St. George, J.D., and Barakat, M.A., 2001, The change in effective stress
associated with
shrinkage from gas desorption in coal, International Journal of Coal Geology,
45 (2001)
pp. 105 - 113.
