Summary
The natural fracture network of a dual-porosity coalbed reservoir is made up
of two sets of orthogonal, and usually subvertically oriented, cleats. Coalbed
permeability has been shown to vary exponentially with changes in the effective
horizontal stress acting across the cleats through the cleat-volume
compressibility, which is analogous to pore compressibility in porous rocks. A
formulation for changes in the effective horizontal stress of coalbeds during
primary methane recovery, which includes a Langmuir type curve shrinkage term,
has been proposed previously. This paper presents a new version of the stress
formulation by making a direct link between the volumetric matrix strain and
the amount of gas desorbed. The resulting permeability model can be extended
readily to account for adsorption-induced matrix swelling as well as matrix
shrinkage during enhanced methane recovery involving the injection of an inert
gas or gas mixture into the seams. The permeability model is validated against
a recently published pressure-dependent permeability multiplier curve
representative of the San Juan basin coalbeds at post-dewatering production
stages. The extended permeability model is then applied successfully to history
matching a micropilot test involving the injection of flue gas (consisting
mainly of CO2 and N2) at the Fenn Big Valley, Alberta, Canada.
Introduction
Over the past 2 decades, coalbed methane (CBM) has become an important
source of the (unconventional) natural gas supply in the U.S. On the basis of
this experience, CBM has attracted worldwide attention in recent years as a
potential clean energy source. Current commercial CBM production occurs almost
exclusively through reservoir-pressure depletion, which is simple but
considered to be rather inefficient, with an estimated total recovery of
generally around 50% (this figure appears to be pessimistic; mature coal plays
in the U.S. have now seen recovery of 60 to 80%) of the gas in place. In recent
years, enhanced CBM (ECBM) recovery techniques have been proposed as a more
efficient means for the recovery of a larger fraction of methane in
place.
There are two principal variants of ECBM recovery, namely N2 and CO2
injection, which use two distinct mechanisms to enhance methane desorption and
production. Unlike the primary recovery method, ECBM allows the maintenance of
reservoir pressure. The mechanism used in N2 injection is somewhat similar to
inert gas stripping because nitrogen is less adsorbing than methane. Injection
of nitrogen reduces the partial pressure of methane in the reservoir, thus
promoting methane desorption without lowering the total reservoir pressure. On
the other hand, CO2 injection works on a different mechanism because it is more
adsorbing on coal compared with methane. Carbon dioxide ECBM recovery thus has
an added benefit that a potentially large volume of greenhouse gas can be
sequestrated in deep coal seams globally.
© 2005. Society of Petroleum Engineers
View full textPDF
(
558 KB
)
History
- Original manuscript received:
3 September 2003
- Revised manuscript received:
13 January 2005
- Manuscript approved:
31 May 2005
- Version of record:
15 August 2005
View Cart
