Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 95/33122 PCT/tJS95/06450
' METHOD FOR ENHANCED RECOVERY OF COAL BED METHANE
FIELD OF THE INVENTION
The present invention relates to methods for increasing the methane
recovery rate from a coal seam. More specifically, the present invention
relates to
1 0 methods which utilize the stimulation of a coal seam from which a
substantial
percentage of the original methane-in-place available to the wellbore has been
feCOHerBd.
BACKCROL1ND OF THE INVENTION
1 5 Coal seams contain significant quantities of natural gas. This natural gas
is
composed primarily of methane. The rate of recovery of methane from coal
seams typically depends on the rate at which gas can flow through the coal
seam
to a production well. The gas flow rate through a coal seam is affected by
many
factors including the matrix porosity of the coal, the permeability of the
coal seam,
2 0 the extent of the fracture system which exists within the coal seam, and
the stress
within the coal seam.
An unstimulated coal seam has a natural system of fractures, the smaller
and most common ones being referred to as "cleats" or collectively as a "cleat
system". To reach a wellbore, the methane must desorb from a sorption site on
or
2 5 within the coal matrix and diffuse to the cleat system. The methane
travels along
the cleat system and other fractures present within the coal seam to the
wellbore
where it is recovered.
Typically, the natural system of fractures within a coal seam does not
provide for an acceptable methane recovery rate. In general, coal seams must
be
3 0 stimulated to enhance the recovery of methane from the seams. Typically,
the
stimulation is completed prior to placing a production well on-line to a gas
. gathering system.
Various techniques have been developed to stimulate coal seams. One
example of a technique for stimulating the production of methane from a coal
3 5 seam is to complete the production wellbore with an open-hole cavity. In
this
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2
technique, a wellbore is drilled to a location above the coal seam to be
stimulated.
The wellbore is cased and the casing is cemented in place using a conventional
'
drilling rig. A modified drilling rig is then used to drill an "open-hole"
interval
within the coal seam. An open-hots interval is an interval within the coal
seam
which has no casing set.
The open-hole interval can be completed by various methods. One
method utilizes injection/blowdown cycles to create a cavity within the open-
hole
interval. In this method, air is injected into the open-hole interval and then
released rapidly through a surface valve. Once a suitable cavity has been
1 0 created, the modified drilling tig is removed from the wellbore and the
production
well is put into service. A metal liner, which has holes, may be placed in the
open-hole interval if desired. The coal seam will be dewatered if necessary to
improve the desorption of methane from the coal seam.
Generally, once a coal seam has been dewatered and a sufficient methane
1 5 recovery rate is maintained from the production well, very little is done
to the
production wells or the coal seam other than to pertorm routine and
preventative
maintenance on the production equipment.
As used herein, the following terms shall have the following meanings:
(a) "coal seams" are carbonaceous formation which typically
2 0 contain between 50 and 100 percent organic material by weight;
(b) "cleats" or "cleat system" is the natural system of fractures
within a coal seam;
(c) "formation parting pressure" and "parting pressure" mean
the pressure needed to open a coal seam and propagate an induced fracture
2 5 through the coal seam;
(d) "reservoir pressure" means the pressure of a coal seam near
a woll during shut-in of that welt;
(e) "recovery" means a controlled collection and/or disposition
of a gas, such as storing the gas in a tank or distributing the gas through a
3 0 pipeline. "Recovering" specifically excludes venting the gas into the
atmosphere;
(f) "sorption" refers to a process by which a gas is held by a .
carbonaceous material, such as coal, which contains micropores. The gas
typically is held on the coal in a condensed or liquid-like phase within the
3 5 micropores, or the gas may be chemically bound to the coal;
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(g) "original methane-in-place" means the quantity of methane
' sorbed to the carbonaceous material of the coal seam available to be drained
by
a wellbore penetrating the seam. The original methane-in-place is measured
prior' to the initial recovery of methane from the coal seam; and
(h) "pore pressure cracking" is shear failure which is induced in
weak formation, such as coal seams, by rapidly changing the pressure which is
present within the micropores and the macropores of the carbonaceous matrix of
the coal seam. Such failure will usually be accompanied by an increase in
permeability of the coal seam.
SUMMARY OF THE INVENTION
It has been surprisingly discovered that the recovery rate of methane from a
coal seam can be greatly increased by stimulating the coal seam after
recovering
a substantial percentage of the original methane-in-place. The substantial
percentage of methane can be recovered by standard pressure depletion
techniques or by injecting desorbing fluids such as n'ttrogen, air, carbon
dioxide,
or flue gas into the coal seam to desorb methane from the coal seam and cause
it
to move toward a production well where it can be recovered. Methods which
utilize injected desorbing fluids to enhance the recovery of methane from a
coal
2 0 seam are sometimes hereinafter referred to as "enhanced coalbed methane
recovery techniques." In the preferred embodiment of the invention, cavitation
of
the croal seam surrounding a production wellbore is carried out after a
substantial
percentage of the original methane-in-place available to the production
wellbore
has been removed from the coal seam.
2 5 It is believed that the removal of a substantial percentage of the
original
methane-in-place will allow tensile and shear failure to be more readily
created
within the coal seam. The additional failure which is created within the coal
seam
will increase the permeability of the coal seam and increase the methane
recovery rate from the coal seam. In tests performed in the field, on
production
3 0 wells which were already producing methane at very high rates, it was
surprisingly discovered that it is possible to recavitate a wellbore that was
originally completed using an open-hole cavity technique, and that the
recavitation was capable of providing an increased methane recovery rate of
more than three times the pre-recavitation methane recovery rate.
0
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RRIFF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graphical representation of the stresses associated with the '
failure of coal.
FIG. 2 is a graphical representation of the stresses associated with the
failure of coal and the effect that carbon dioxide has on the failure of the
coal.
FIG. 3 is a graph of the average daily total gas recovery rate from a
wellbore which penetrates a coal seam which has been recavitated using the
current invention.
FIG. 4 is a graph of the average daily total gas recovery rate from another
1 0 wellbore which penetrates a coal seam which has been recavitated using the
current invention.
FIG. 5 is a graph of the average daily total gas recovery rate from a third
wellbore which penetrates a coat seam which has been recavitated using the
current invention.
1 5 FIG. 6 is a graph of the average daily total gas recovery rate from a
fourth
wellbore which penetrates a coal seam which has been recavitated using the
current invention.
FIG. 7 is a graph of the average daily total gas recovery rate from a fifth
wellbore which penetrates a coal seam which has been recavitated using the
2 0 current invention.
FIG. 8 is a graph of the average daily total gas recovery rate from a sixth
wellbore which penetrates a coal seam which has been recavitated using the
current invention.
FIG. 9 is a graph of the average daily total gas recovery rate from a seventh
2 5 wellbore which penetrates a coal seam which has been recavitated using the
current invention.
r)FSGRIPTION OF THE EMBODIMENTS
tt has been surprisingly discovered that the methane recovery rate from a
3 0 production well which is in fluid communication with a coal seam can be
greatly
increased by cavitating the coal seam surrounding the wellbore after
recovering a
substantial percentage of the original methane-in-place from the coal seam. ,
Preferably, from 2 to 70 percent of the original methane-in-place available to
the
wellbore should be desorbed and recovered from the coal seam prior to
3 5 cavitation; more preferably, from 7 to 50 percent of the original methane-
in-place;
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WO 95/33822 PCT/US95/06450
most preferably, from 15 to 30 percent of the original methane-in-place.
It has
a
been further surprisingly discovered that the method is capable
of greatly
increasing the methane recovery rate from production wells
that have been
' completed with open-hole cavitation techniques and that are
already produang at
5 a rate of greater than 28 thousand cubic meters of methane
per day. Open-hole
cavity completion wells which are producing greater than 28
thousand cubic
meters of methane per day are considered very good wells which
in the past
would not be candidates for additional stimulation.
While it is not known why removing a substantial percentage
of the original
1 0 methane-in-place available to a wellbore prior to cavitating
the coal seam
surrounding the wellbore increases the methane recovery rate
so dramatically, it
is believed that it is at least in part a result of the matrix
shrinkage which results
when methane is desorbed from the matrix. It is believed that
matrix shrinkage
will facilitate pore pressure cracking within the coal seam
during the practice of
1 5 the invention. Since coal seams are typically very heterogeneous,
the shrinkage
which occurs within the coal seam may be very uneven. The
uneven shrinkage
can exacerbate the cracking within the coal seam. This cracking
can increase the
permeability of the coal seam and may facilitate the creation
of shear and tensile
failure within the coal seam during cavitation.
2 0 Additionally, as methane is removed from the coal seam, the
material
properties of the coal, such as cohesion strength, may change.
It is believed that
the cohesion strength of the coal is reduced as methane is
removed from the
matrix. Furthermore, other volatiles, such as ethane and propane,
together with
water, are typically removed from the coal together with the
methane. It is
2 5 believed that the removal of these compounds from the coal
also tends to reduce
the cohesion strength of the coal which in turn makes the
coal more friable. This
reduction in cohesion strength of the coal will facilitate
the creation of tensile and
shear failure within the coal seam during cavitation of the
coal seam surrounding
the wellbore. As discussed earlier, tensile failure and shear
failure created within
3 0 a coal seam will increase the methane recovery rate from the
well.
While this invention is susceptible of embodiment in many
different forms,
there will herein be described in detail, specific embodiments
of the invention. It
should be understood, however, that the present disclosure
is to be considered
t an exemplification of the principles of the invention and
is not intended to limit the
3 5 invention to the specific embodiments illustrated.
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WO 95/33122 PCT/US95/06450
6
.
F3emoval of Methane From the Coal Seam
Coal seams are comprised of carbonaceous material which includes a
matrix having an extensive system of micropores, and a system of fractures,
which
penetrate the matrix, commonly referred to as "cleats." The majority of the
methane contained in a typical coal seam is sorbed within the micropores of
the
coal. To remove the methane from the coal seam, several methods may be
utilized.
One method useful for removing methane from a coal seam utilizes primary
1 0 depletion to recover methane from the seam. In this method, as the
reservoir
pressure of the coal seam is lowered, the partial pressure of methane within
the
cleats decreases. This causes methane to desorb from the methane sorption
sites and diffuse to the cleats. Once within the cleat system, the methane
flows to
a production well where it is recovered. The reservoir pressure continually
1 5 decreases over time as methane is recovered from the coal seam. Also, the
methane recovery rate tends to decrease over time as methane is recovered from
the seam.
As discussed earlier, the carbonaceous matrix shrinks as methane is
removed from the coal seam. This shrinkage will lower the stress within the
coal
20 seam and if the shrinkage is uneven may cause cracking within the coal
seam.
Also, it is believed that as the stress within the coal seam is reduced, the
formation
parting pressure of the coal seam is reduced. A reduction in formation parting
pressure will allow tensile failures to be propagated more easily through the
coal
seam at a lower pressure. tt is preferable to reduce the stress within the
formation
25 by a sufficient amount to lower the formation parting pressure by at least
20
percent prior to cavitating the coal seam; more preferably by at least 50
percent;
and most preferably by at least 70 percent.
FIG. 1 is a graph of the failure envelope for a typical San Juan Basin coal.
Shear stress is represented on the y-axis and effective normal stress is
3 0 represented on the x-axis. The effective stresses are simply the stresses
present
within the coal minus the pore pressure (Pp) present within the coal. The
cohesion strength of the coal seam can be determined from the point at which
the
lower bound of the failure envelope crosses the y-axis. The lower bound of the
failure envelope is described by two lines 21 and 23. Lines 21 and 23 are used
to .
3 5 describe the failure envelope due to the uncertainty in determining the
lower
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WO 95/33122 PGT/US95I06450
7
bound of the failure envelope. Coals subjected to stresses
which place them at or
' above the lower bound of the failure envelope are prone to
failure. Also
displayed on FIG. 1 are two Mohr circles 25 and 27 which
graphically depict the
stresses acting on the carbonaceous material of the coal
seam. The first circle 25
depicts the stresses which act on the carbonaceous material
of the coal seam
before methane has been recovered from the coal seam. The
second circle 27
depicts the stresses which act on the carbonaceous material
of the coal seam
after the reservoir pressure has been reduced by 3.578,379
Pascals (Pa).
For Mohr circles, the right foot-point corresponds to effective
overburden
1 0 stress, Sv-Pp. The left foot-points of the Mohr circles
corresponds
to effective
minimum horizontal stress, Smin-Pp. As methane is removed
from the coal seam,
reservoir pressure and pore pressure are decreased. Therefore,
since the
overburden stress is not changing, the right foot-point 29
of Mohr circle 27 is
shifted to the right compared to the right foot-point 31
of Mohr arcle 25. The left
1 5 foot-point 33 of Mohr circle 27 is believed to be shifted
to the left compared to the
left foot-point 35 of Mohr circle 25 because the minimum
horizontal stress is
reduced by the matrix shrinkage which occurs within the carbonaceous
material
as methane is desorbed from the matrix, and because for most
coals the effective
minimum horizontal stress will be reduced more by the shrinkage
than it is
20 increased by the decrease in pore pressure, as methane is
desorbed from the
matrix. As can be seen from FIG. 1, as methane is desorbed
and pore pressure is
redcced, a Mohr circle which represents the stresses acting
on the coal moves
closer to the failure envelope of the coal. This is represented
on FIG. 1 by Mohr
circle 27 being shifted up toward the failure envelope 21
as compared to Mohr
2 5 circle 25. Failure is likely to occur once the Mohr circle
touches or intersects the
failure envelope. Even if the Mohr circle is close to the
failure envelope but
doesn't touch or intersect the failure envelope, the additional
rapid change in
pressure which ocarrs within the coal seam during cavitation
and the stresses this
change creates can cause failure within the coal seam.
3 0 The effective minimum horizontal stress can be approximated
from the
welibore pressure measured at shut-in of the wellbore during
fracturing of the
coal. The approximation becomes more accurate as the fracture
produced
becomes smaller. Therefore, minifrac tests, which are known
to one of ordinary
skill in the art, are believed to be accurate predictors
of effective minimum
3 5 horizontal stress.
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WO 95/33122 PCT/US95/06450
8
As discussed above, when a Mohr circle plotted for a given coal touches or
crosses the failure envelope, 'rt means that the conditions are such that the
coal is
prone to failure. In accordance with the current invention, after a
substantial
percentage of the methane has been removed from the coal seam, a cavitation
process is used to rapidly change the pressure and exacerbate the failure
within
the coal surrounding the wellbore to create failure within the coal seam.
The relative amount of carbon dioxide sorbed to a coal matrix is believed to
effect the amount of failure which occurs within a coal seam during the
practice of
the current invention. tt is believed the greater the matrix shrinkage which
occurs
1 0 for a given reservoir pressure reduction and thereby pore pressure
reduction, the
higher the chance of failure occurring within the coal seam during the
practice of
the invention. Coal which contains carbon dioxide sorbed to the matrix will
exhibit
greater matrix shrinkage during the removal of gases from the coal than coal
which doss not contain carbon dioxide.
1 5 Turning now to FIG. 2, the tower edge of the failure envelope is bounded
by
lines 37 and 38. Lines 37 and 38 are plotted due to the uncertainty of
determining
the lower edge of the failure envelope. As discussed earlier, coals which are
subjected to stresses which place them at or above the lower bound of the
failure
envelope are prone to failure. Mohr circle 39 graphically depicts the stresses
2 0 acting on a coal which contains a known quantity of original gas-in-place
and a
known initial pressure. Mohr circle 40 graphically depicts the stresses which
will
result within the coal if 100 percent by volume methane is withdrawn from the
coal
to reduce the pressure by 1,034,214 Pa. Mohr circle 41 graphically depicts the
stresses which will result within the coal if an effluent is withdrawn from
the coal
2 5 which contains 90 percent by volume methane and 10 percent by volume
carbon
dioxide to reduce the pressure acting on the coal by 1,034,214 Pa. As can be
seen from FIG. ~ 2, for a given reduction in pore pressure, a coal seam which
contains carbon dioxide and methane will be more prone to failure by pore
pressure cracking than a coal seam which experiences a similar pore pressure
3 0 reduction but which contains less carbon dioxide sorbed to the matrix.
Therefore,
when choosing wellbores to cavitate using the current invention, it is
preferable to
choose wells which are producing an effluent which contains greater than five
percent by volume carbon dioxide; more preferably, greater than nine percent
by
volume carbon dioxide, most preferably greater than ten percent by volume
3 5 carbon dioxide. This preference for wells that produce an effluent
containing
SUBST1T~TE S ~E~T (R'J.E 2~)
WO 95/33122 PCT/US95/06450
9
carbon dioxide is applicable to wellbores that are being produced using
primary
depletion techniques and enhanced coalbed methane recovery techniques which
utilize inert gases such as nitrogen.
The percentage of original methane-in-place which remains within a coal
seam is related to the isotherm for the coal and the change in reservoir
pressure
which has occurred since methane recovery was initiated. tt has been found
that
before a well is stimulated in accordance with the invention, the reservoir
pressure near the well should be preferably reduced to from 20 to 80 percent
of
the initial reservoir pressure which existed prior to methane being recovered
from
1 0 the coal seam; more preferably, from 30 to 75 percent of the initial
reservoir
pressure: and most preferably, from 36 to 59 percent of the initial reservoir
pressure. This reduction in pressure and the associated recovery of methane
from the coal seam will facilitate failure within the coal seam during
cavitation of
the coal seam surrounding the wellbore.
1 5 As discussed earner, it is believed that the cohesion strength of the coal
seam may be reduced by the removal of methane from the coal seam. This
reduction in cohesion strength as it occurs, will result in the failure
envelope
moving toward to the Mohr circle, thereby making the carbonaceous material
more prone to failure during the practice of the invention.
2 0 A discussion of a method which may be utilized to determine a failure
envelope for coal is contained in "Experimental Observations of Hydraulic
Fracture Propagation Through Coal Blocks", SPE 21289, by H. H. Abass et al., a
paper presented at the Society of Petroleum Engineers Eastern Regional
Meeting, Columbus, Ohio, October 31 through November 2, 1990.
2 5 It has been determined that the current invention is most effective when
utilized on wells which have been producing greater than 2.8 thousand
standarcl
cubic meters of methane per day (MCMD) in the months prior to cavltation in
accordance with the invention; preferably, greater than 14.2 MCMD; more
preferably, greater than 28 thousand standard cubic meters of methane per day;
3 0 and most preferably, greater than 56.6 MCMD.
Another method which can be useful for desorbing methane from a coal
seam utilizes the injection of a desorbing fluid, such as nitrogen, into a
solid
carbonaceous subterranean formation to enhance the recovery of methane from
the formation. Such a method is described in U. S. Patent Number 5,014,785 to
3 5 Puri, et al.
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The injection of a desorbing fluid into the coal seam will lower the partial
pressure of methane within the cleats of the coal seam and thereby cause '
methane to be desorbed from the coat seam. The desorbed methane will travel to
a production well where it can be recovered. Studies have shown that one
5 nitrogen molecule can sorb to the matrix for about every 2 to 2.5 methane
molecules that desorb from the matrix. Therefore, the coal matrix will shrink
as
nitrogen displaces methane from the coal. It is believed a desorbing fluid,
which
contains components which will tend to swell the matrix, will still cause the
matrix
to shrink overall if the percentage of components that swell the matrix is not
too
1 0 large.
ft is believed that the shrinkage that occurs, as a result of nitrogen
injection,
will faalitate the failure of the coal for reasons which are similar to those
listed
above for the recovery of methane by primary pressure depletion. Additionally,
it
is believed that methane recovery by the injection of desorbing fluid may
change
1 5 the material properties of the coal more than methane recovery by primary
pressure depletion. This may result because of the drying of the coal which
can
resuh from the injection of desorbing fluid into the coal seam. Specifically,
it is
believed that the cohesion strength of the coal will be reduced. The lower
cohesion strength which results, should make the coal more prone to failure
2 0 during the practice of the current invention.
As with primary depletion, a substantial percentage of the original
methane-in-place should be recovered from the coal seam prior to cavitating
the
coat seam surrounding the wellbore. Preferably, between 2 to 70 percent of the
original methane-in-place available to the wellbore should be desorbed and
25 removed from the coal seam surrounding the wellbore; more preferably,
between
30 to 70 percent of the original methane in place; most preferably, between 30
to
50 percent.
By recovering a larger percentage of the original methane-in-place than
was recovered using primary depletion; the benefits of the nitrogen injection
and
3 0 the increased recovery rate which results from the stimulation of the coal
seam
have been fully utilized.
A third method which can be useful for desorbing methane from a coal .
seam utilizes the injection of a desorbing fluid, which contains at least
fifty percent
by volume carbon dioxide, into the coal seam. ,
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11
It is believed that coal seams which have undergone enhanced
recovery
' using carbon dioxide containing fluids are also likely to
have had their material
properties altered. Specifically, it is believed that the
cohesion strength of the
coal may be markedly reduced. This reduction in the cohesion
strength will make
it easier to create tensile and shear failure within a coal
seam during the practice
of the current invention as already discussed above. Also,
fluids which contain
carbon dioxide tend to cause carbonaceous materials, such
as coal, to swell as
methane is desorbed from the matrix and carbon dioxide is
sorbed to the matrix.
This swelling may be uneven and therefore may cause cracking
within the coal.
As with enhanced recovery using nitrogen, when carbon dioxide
containing fluids are utilized to recover methane, it is preferable
to recover from 2
to 70 percent of the original methane-in-place available to
the wellbore prior to
cavitating the coal seam surrounding the wellbore in accordance
with the current
invention; more preferably, from 30 to 70 percent of the original
methane-in-
1 5 place; most preferably, from 30 to 50 percent of the original
methane-in-place.
Since carbon dioxide causes the carbonaceous matrix of coal
to swell, it is
preferable to desorb some of the carbon dioxide from the coal
prior to cavitating
the coal seam surrounding the wellbore. This can be effectively
done by relieving
the pressure within the coal seam through the wellbore. It
is believed that the
pressure, preferably, should be relieved at.a rate essentially
equivalent to the
maximum flow rate permitted by the wellbore and wellbore equipment.
tt should
be noted that the wellbore and wellbore equipment utilized
to carry out the
invention may provide a higher fluid flow rate than. that
achievable when the
wellbore is configured to send gas to commercial sales. By
desorbing some of
2 5 the carbon dioxide from the coal surrounding the wellbore,
the amount of swelling
caused by the carbon dioxide can be reduced.' tt is believed
that this will assist in
creating failure within the coal seam during the practice
of the invention.
Additionally, uneven shrinkage is believed to occur within
the carbonaceous
matrix of the coal seam as carbon dioxide is desorbed from
the matrix. This
3 0 uneven shrinking may cause cracking within the matrix which
will make it easier
to create tensile and shear failure within the coal seam during
the cavitation of the
coal seam surrounding the wellbore.
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The Wellbore and Cavitation of the Coal Seam Surrounding the Wellbore
In one aspect of the invention, the wellbore which is cavitated after a
substantial percentage of the original methane-in-place has been recovered, is
the same wellbore which was originally completed into the methane producing
coal interval. 'Same wellbore" means that the wellbore has not been
sidetracked
or redrilled at a nearby location. The cost effectiveness of the invention is
greatly
enhanced by using the same wellbore. It is also believed that in most
arcumstances, the highest methane recovery rate can be achieved by using the
same wellbore.
1 0 In another aspect of the invention, the wellbore which is cavitated after
a
substantial percentage of the original methane is recovered from the coal seam
may be a sidetracked wellbore or may be a newly drilled well which is closely
located to the original wellbore. This may be done when it is impracticable to
use
the original wellbore. For example, if the formation directly adjacent to the
original
1 5 wellbore was greatly damaged by the original completion technique used, it
would be preferable to sidetrack to create a new wellbore in the region of the
coal
seam or to drill a new well. Even if a new well or a sidetracked wellbore is
utilized, it is believed that the wellbore should be located close enough to
the
original wellbore so that a substantial percentage of the original methane-in-
place
2 0 will have been recovered from the region of the coal seam which is to be
drained
by the new wellbore.
The cavitation may be accomplished by a variety of methods. For example,
the cavitation can be effected by introducing a gaseous fluid, such as air,
nitrogen,
flue gas, or carbon dioxide into the coal seam in a series of
injection/blowdown
2 5 cycles which will tend to destabilize the coal seam and cause carbonaceous
material to be released into the wellbore during blowdown. Additional shear
failure will occur within the coal seam during blowdown. The failure will
usually
result in increased permeability within the formation adjacent the wellbore.
The
increase in permeability is believed to be greatest next to the wellbore and
will
3 0 taper off as one gets farther away from the wellbore. In an alternative
method for
cavitating the coal seam surrounding the wellbore, the wellbore is shut-in to
allow
the pressure within the wellbore to build-up. Once the wellbore pressure has
reached a desired level, the wellbore is allowed to blowdown to the surface
with
minimal restriction. The differential pressure which is created during this
type of
3 5 blowdown will also cause shear failure within the coal seam. In general
both
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WO 95133122 PCT/US95/06450
13
injection/blowdown cycles and wellbore shut-ins are utilized in a typical
cavitation
procedure utilized by the current invention.
In another method which can be utilized to cavitate the coal seam, a first
fluid, which sorbs to the coal, is introduced into the coal seam and allowed
to sorb
to the coal prior to a second fluid being introduced into the coal seam. The
second fluid is introduced into the coal seam at a pressure greater than the
formation parting pressure of the coal seam. After the second fluid is
introduced
into the coal seam, the pressure within the coal seam is relieved to create
shear
failure within the coal seam. This procedure can be utilized to cavitate the
coal
1 0 seam surrounding wellbore intervals completed with cased-hole techniques
and
open-hole techniques.
When utilizing injection/blowdown cycles to cavitate a coal seam
surrounding the wellbore, the fluid is typically injected for about 2 to 3
hours. As
fluid is injected, the pressure within the formation increases, rapidly and
then
1 5 begins to level off. It is believed that the leveling off of the pressure
during
injection occurs as the formation parting pressure is reached. It is believed
that
tensile failure is created within the coal seam as injection is continued at
or above
the formation parting pressure. It is believed that the formation parting
pressure
will be approximately 689,476 to 1,378,951 Pa above the effective minimum
2 0 horizontal stress present within the formation. Therefore, as methane is
desorbed
from the coal seam and minimum stress is reduced, the formation parting
pressure will decrease. It is believed that the minimum stress can be further
reduced by failure which is induced within the coal seam by each cavitation
cycle.
A reduced formation parting pressure can be advantageous because less
2 5 compression is required to cavitate the coal seam. This reduced
compression
requirement should lower the costs associated with cavitating the coal seam
surrounding the wellbore.
As discussed earlier, the wellbore is rapidly blown down to reduce the
pressure within the coal seam surrounding the wellbore once the desired
quantity
3 0 of fluid has been injected into the formation. It is believed that shear
failure is
created during this blowdown. In order to maximize the shear failure which is
created within the coal seam, the pressure is relieved at a rate essentially
equivalent to the maximum flow-rate permitted by the wellbore and wellbore
control equipment. If desired, the wellbore and wellbore control equipment
3 5 utiiized~ during cavitation can be modified to increase the rate of
pressure
SUBSTITUTS S~~~T (RUSE 26)
WO 95/33122 ~ PCT/US95/06450
14
reduction which can be obtained during blowdown. Typically, the pressure
within
the coal seam surrounding the wellbore will be reduced to approximately the '
reservoir pressure in less than one minute. During this time, the pressure
within
the bottom of the wellbore will be reduced to approximately atmospheric
pressure .
plus the hydrostatic pressure within the wellbore which results from the
column of
gas within the wellbore. Coal fines, water, and methane are generally produced
during the blowdown. The blowdown is typically continued until coal fines are
no
longer produced. The coal fines may continue to be produced for between
several minutes to several days.
1 0 Periodically, a flow test which lasts approximately 2 hours should be
performed. During the cavitation procedure, the methane flow rate will
generally
continue to rise as cavitation is occurring. The flow rate, however, may vary
up or
down between subsequent cycles. Because of the var6ance in the methane flow
rate which may occur behnreen subsequent cycles, a stable methane flow rate is
1 5 preferably determined by comparing the methane flow rates from at least
three
consecutive cycles.
The cavitation is generally continued until a stable cavity is attained. When
a stable cavity is attained, coal fines should no longer be produced during
the
blowdowns or during clean out of the wellbore or the amount of fines produced
2 0 should be rapidly decreasing with subsequent blowdowns. A clean-out of the
wellbore can be accomplished by circulating fluid through the wellbore. If
required, a drillbit can also be rotated within the wellbore to aid in the
clean out of
the wellbore. In addition to attaining a stable cavity, it is also preferable
that the
methane flow rate be stabilized before ceasing to cavitate the coal seam. As
25 discussed above, a stable methane flow rate should be determined from
measuring the flow rate from three consecutive cavitation cycles. Preferably,
the
methane flow rate from three consecutive flow tests should differ no more than
5
percent from the highest rate to the lowest rate obtained from the three
consecutive tests; more preferably, no more than 1-5 percent; most preferably,
no
3 0 more than 2 percent.
Modifications to wellbore and wellbore control equipment which can be
utilized to cavitate the coal seam surrounding a wellbore are more fully
described
in SPE 24906, "Openhole Cavity Completions in Coalbed Methane Welis in the
San Juan Basin", by I. D. Palmer et. al, a paper presented at the 67th Annual
SUBSTITUTc S~I=~T ~RJ_~ !~)
WO 95/33122 ~ ' r~ PCTIUS95106450
Technical Conference and Exhibition of the Society of Petroleum Engineers held
in Washington, DC October 4-7, 1992.
Once the cavitation procedure is completed, the well can be realigned so
' that the methane produced can be recovered. Typically, the methane recovered
5 from the well will be sent to a pipeline.
Exams t~ a 1
This example shows that it is possible to more than triple the methane
recovery rate from a wellbore using the current invention.
1 0 Referring to FIG. 3, a wellbore was drilled into the fruitland formation
coals
of the San Juan Basin of New Mexico. The wellbore was initially completed
using
an open-hole cavity completion technique. The initial reservoir pressure near
the
wellbore, before methane was recovered from the wellbore, was approximately
11,031,611 Pa. During the initial completion, the water production rate was
1 5 approximately 2000 barrels per day. The high water production rate limited
the
amount of cavitation which could be performed on the well. Once the wellbore
was completed, it was aligned to recover methane from the formation by primary
pressure depletion through 6.05 centimeters (cm) diameter tubing. For
approximately three years, methane was recovered from the wellbore by primary
2 0 pressure depletion. During the three year period, approximately 10 percent
of the
original methane-in-place was recovered from the wellbore. After the three
year
period, the wellbore was taken off line and recavitated. During the
recavitation,
the water production rate had decreased substantially, indicating that the
coal
sear~n surrounding the wellbore had been significantly dewatered. During the
2 5 recavitation, the reservoir pressure was estimated to be about 6,894,757
Pa. The
recavitation was continued until a stable cavity was attained. Once a stable
cavity
was attained, the wellbore was realigned to recover methane from the formation
by primary pressure depletion through 11.43 cm diameter tubing.
FIG. 3 is a graphical representation of the total gas recovery rate from the
3 0 wellbore. The average daily total gas recovery rate is depicted for the
calendar
months preceding and following the recavitation of the wellbore. The gas
recovered from the wellbore contained approximately 90 percent by volume
methane and approximately 10 percent by volume carbon dioxide both before
and after the recavitation. For months 1 and 2 shown, the average daily total
gas
3 5 recovery-rate was approximately 127 thousand standard cubic meters per
day.
SUBSTiTUTc S~i4~ST ~RJLS 2~)
f ~' Pi:T'/US95/06450
W0 95/33122
16
The wellbore was taken off-line on about the 17th day of month three and
therefore the average daily total gas recovery rate as depicted for month
three is
reduced. The wellbore was realigned to send gas to the pipeline on about the
15th day of month four.
As can be seen from FIG. 3, by month eight, the average daily total gas
recovery rate was approximately 495.5 thousand standard cubic meters per day.
Exam~l~2
Referring to FIG. 4, a wellbore was drilled into the fruitland fomsation coals
1 0 of the San Juan Basin of New Mexico. The wellbore was initially completed
using
a cased-hole technique. An initial gas-flow rate test to the atmosphere, which
produced less than one percent of the original methane-in-place, was
unsatisfactory. A decision was made to sidetrack the original wellbore and to
create an open-hole cavity within the formation before the wellbore was put on-
1 5 line to sales. The new wellbore was also sidetracked into the fruitland
formation
coals of the San Juan Basin of New Mexico. The sidetracked wellbore was
completed using an open-hole cavity completion technique. The initial
reservoir
pressure near the sidetracked wellbore was approximately 7,928,970 Pa. During
the initial cavity completion, the completion rig was removed from the
wellbore
2 0 without determining whether a stable cavity was attained.
Once the sidetracked wellbore was completed, it was aligned to recover
methane from the formation by primary pressure depletion through 6.05 cm
diameter tubing. For approximately two years, methane was recovered from the
wellbore by primary pressure depletion. During the two year period,
25 approximately 12 percent of the original methane-in-place was recovered
from
the wellbore. After the two year period, the wellbore was taken off fine and
recavitated. During the recavitation, the reservoir pressure was estimated to
be
about 4,798,751 Pa. The recavitation was continued until a stable cavity was
attained. Once a stable cavity was attained, the wellbore was realigned to
3 0 recover methane from the formation by primary pressure depletion through
8.9 cm
diameter tubing.
FIG. 4 is a graphical representation of the total gas recovery rate from the
wellbore. The average daily total gas recovery rate is depicted for the
calendar
months preceding and following the recavitation of the wellbore. The gas
3 5 recovered from the wellbore contained approximately 91.5 percent by volume
SUBSTI-TUT~ Sac~T (RUSE 2~)
WO 95/33122 PCT/US95/06450
17
10
methane and approximately 9.5 percent by volume carbon dioxide both before
and after the recavitation. For months 1 and 2 shown, the average daily total
gas
recovery-rate was approximately 57 thousand standard cubic meters per day.
The wellbore was taken off-tine on about the 28th day of month three and
therefore the average daily total gas recovery rate as depicted for month
three is
reduced. The wellbore was realigned to send gas to the pipeline on about the
25th day of the month four.
As can be seen from FIG. 4, by month eight, the average daily total gas
recovery rate was approximately 113 thousand standard cubic meters per day.
Exam ~i 1e 3
Referring to FIG. 5, a weltbore was drilled into the fruitland formation coals
of the San Juan Basin of New Mexico. The wellbore was initially completed
using
an open-hole cavity completion technique. The initial reservoir pressure near
the
1 5 wellbore, before methane was recovered from the wellbore, was
approximately
7,170,547 Pa. During the initial cavity completion, the completion rig was
removed from the wellbore without determining whether a stable cavity was
attained.
Once the wellbore was completed, the wellbore was aligned to recover
20 methane from the formation by primary pressure depletion through 6.05 cm
diameter tubing. For approximately two years, methane was recovered from the
wellbore by primary pressure depletion. During the two year period,
approximately 2 percent of the original methane-in-place was recovered from
the
wellbore. After the two year period, the wellbore was taken off line and
25 recavitated. During the recavitation, the reservoir pressure was estimated
to be
about 5,240,015 Pa. The recavitation was continued until a stable cavity was
attained. Once a stable cavity was attained, the wellbore was realigned to
recover methane from the formation by primary pressure depletion through
7.32 cm diameter tubing.
3 0 FIG. 5 is a graphical representation of the total gas recovery rate from
the
wellbore. The average daily total gas recovery rate is depicted for the
calendar
months preceding and following the recavitation of the wellbore. The gas
recovered from the wellbore contained approximately 91 percent by volume
methane and approximately 9 percent by volume carbon dioxide both before and
3 5 after the recavitation. For months 1 and 2 shown, the average daily total
gas
SUBSTITIiTc S ~~ST ~RU~.E 2 i)
WO 95/33122 PCT/US95/06450
18
recovery-rate was approximately 14.2 to 17 thousand standard cubic meters per
day. The wellbore was taken off-line on about the 23th day of month three and
therefore the average daily total gas recovery rate as depicted for month
three is
reduced. The wellbore was realigned to send gas to the pipeline on about the
29th day of the month four.
As can be seen from FIG. 5,.by month ten, the average daily total gas
recovery rate was approximately 34 thousand standard cubic meters per day.
1 0 Referring to FIG. 6, a wellbore was drilled into the fru'ttland formation
coals
of the San Juan Basin of New Mexico. The wellbore was initially completed
using
an open-hole cavity completion technique. Once the wellbore was completed, it
was aligned to recover methane from the formation by primary pressure
depletion.
The wellbore was taken off line and recavitated after approximately 4 percent
of
the original methane-in-place had been recovered from the wellbore. The
recavitation was continued until a stable cavity was attained. Once a stable
cavity
was attained,. the welibore was realigned to recover methane from the
formation
by primary pressure depletion.
F1G. 6 is a graphical representation of the total gas recovery rate from the
2 0 wellbore. The average daily total gas recovery rate is depicted for the
calendar
months preceding and following the recavitation of the wel~bore. The gas
recovered from the wellbore contained approximately 91.4 percent by volume
methane and approximately 8.6 percent by volume carbon dioxide both before
and after the recavitation. For months 1 to 3 shown, the average daily total
gas
2 5 recovery-rate was approximately 79.3 thousand standard cubic meters per
day.
The wellbore was taken off-line on about the 8th day of month four and
therefore
the average daily total gas recovery rate as depicted for month four is
reduced.
The wellbore was realigned to send gas to the pipeline on about the 11th day
of
month five.
3 0 As can be seen from FIG. 6, by month eleven, the average daily total gas
recovery rate was approximately 169.9 thousand standard cubic meters per day.
SUBST1TUTS S~i~i=T (RJR 2~)
WO 95133122 PCT/US95/06450
19
Referring to FIG. 7, a wellbore was drilled into the fruitland formation coals
of thus San Juan i3asin of New Mexico. The wellbore was initially completed
using
an open-hole cavity completion technique. Once the wellbore was completed, it
was aligned to recover methane from the formation by primary pressure
depletion.
The wellbore was taken off line and recavitated after approximately 19 percent
of
the original methane-in-place had been recovered from the wellbore. The
recavitation was continued until a stable cavity was attained. Once a stable
cavity
was attained, the wellbore was realigned to recover methane from the formation
1 0 by primary pressure depletion.
FIG. 7 is a graphical representation of the total gas recovery rate from the
wellbore. The average daily total gas recovery rate is depicted for the
calender
months preceding and following the recavitation of the wellbore. The gas
recovered from the wellbore contained approximately 90.4 percent by volume
1 5 methane and approximately 9.6 percent by volume carbon dioxide both before
and after the recavitation. For months 1 and 2 shown, the average daily total
gas
recovery-rate was approximately 70.8 thousand standard cubic meters per day.
The wellbore was taken off-line on about the 24th day of month three and
therefore the average daily total gas recovery rate as depicted for month
three is
2 0 reduced. The wellbore was realigned to send gas to the pipeline on about
the
11th day of month four.
As can be seen from FIG. 7, by month ten, the average daily total gas
recovery rate was approximately 101.9 thousand standard cubic meters per day.
2 5 ~xa~yle 6
Referring to FIG. 8, a wellbore was drilled into the fruitland formation coals
of the San Juan Basin of New Mexico. The wellbore was initially completed
using
an open-hole cavity completion technique. Once the welfbore was completed, it
was aligned to recover methane from the formation by primary pressure
depletion.
3 0 The wellbore was taken off line and recavitated after approximately 5
percent of
the original methane-in-place had been recovered from the wellbore. The
recavitation was continued until a stable cavity was attained. Once a stable
cavity
was attained, the wellbore was realigned to recover methane from the formation
by primary pressure depletion.
SUBSTITUTE S~;I:=T {RJ~S 2~)
WO 95/33122 PCT/US95/06450
FIG. 8 is a graphical representation of the total gas recovery rate from the
wellbore. The average daily total gas recovery rate is depicted for the
calendar
months preceding and following the recavitation of the wellbore. The gas
recovered from the wellbore contained approximately 91.7 percent by volume
5 methane and approximately 8.3 percent by volume carbon dioxide both before
and after the recavitation. For months 1 to 3 shown, the average daily total
gas
recovery-rate was approximately 116 thousand standard cubic meters per day.
The wellbore was taken off-line on about the 12th day of month four and
therefore
the average daily total gas recovery rate as depicted for month four is
reduced.
1 0 The wellbore was realigned to send gas to the pipeline on about the i 2th
day of
the fifth month.
As can be seen from FIG.B, by month eight, the average daily total gas
recovery rate was approximately 339.8 thousand standard cubic .meters per day.
1 5 ExamQle 7
Referring to FIG. 9, a wellbore was drilled into the fruitland formation coals
of the San Juan Basin of New Mexico. The wellbore was initially completed
using
an open-hole cavity completion technique. Once the wellbore was completed, it
was aligned to recover methane from the formation by primary pressure
depletion.
20 The wellbore was taken off line and recavitated after approximately 30
percent of
the original methane-in-ptace had been recovered from the wellbore. The
recavitation was continued until a stable cavity was attained. Once a stable
cavity
was attained, the wellbore was realigned to recover methane from the formation
by primary pressure depletion.
FIG. 9 is a graphical representation of the total gas recovery rate from the
wellbore. The average daily total gas recovery rate is depicted for the
calendar
months preceding and following the recavitation of the wellbore. The gas
recovered from the wellbore contained approximately 87.7 percent by volume
methane and approximately 12.3 percent by volume carbon dioxide both before
3 0 and after the recavitation. For months 1 and 2 shown, the average daily
total gas
recovery-rate was approximately 175.6 thousand standard cubic meters per day.
The wellbore was taken off-line on about the 12th day of month three and
therefore the average daily total gas recovery rate as depicted for month
three is
reduced. The wellbore was realigned to send gas to the pipeline on about the
8th
3 5 day of month four.
SUBS i; l i i~~c S'tC i (~JLE 2S~
WO 95/33122 PCTIUS95/06450
21
As can be seen from FIG. 9, by month six, the average daily total gas
recovery rate was approximately 339.8 thousand standard cubic meters per day.
From the foregoing description, it will be observed that numerous
variations, aftematives and modifications will be apparent to those skilled in
the
art. Accordingly, this description is to be construed as illustrative only and
is for
the purpose of teaching those skilled in the art the manner of carrying out
the
invention. Various changes may be made and materials may be substituted for
those described in the application. For example, it is believed that the
conditions,
parameters, and techniques described in the application can be utilized to
1 0 incr~aase the methane recovery rate from other solid carbonaceous
subterranean
formations, such as antrium, carbonaceous, and devonian shales. Also, it is
beGoved that the effectiveness of other stimulation techniques, such as
fracture
stimulation, can be enhanced by establishing the conditions and parameters
discussed in this application prior to fracture stimulating a solid
carbonaceous
1 5 subterranean formation, such as a coal seam.
Thus, it will be appreciated that various modifications, alternatives,
variations, etc., may be made without departing from the spirit and scope of
the
invention as defined in the appended claims. tt is, of course, intended that
all
such modifications are covered by the appended claims.
SUBSTIThTS S~~c T .1R~J~ S 2~)
