Coalbed methane (CBM) is a relatively
common unconventional natural
gas, which has great exploitation value. Coal permeability is an important
parameter that affects the production and production efficiency of
CBM, which is mainly controlled by the sorption expansion/contraction
strain and effective stress. To study the seepage characteristics
of coal in the process of CBM production, we have used CH4 and CO2 as test gases separately and conducted comparative
seepage tests of different gases under constant pore pressure conditions.
At the same time, the elastic modulus reduction coefficient R
m has been introduced to characterize the sorption
strain of coal, following which the permeability models suitable for
different boundary conditions were derived according to the stress–strain
relationship. Under the two gases, the new model could not only better
reflect the law of coal sorption strain but also better reflect the
relationship among effective stress, pore pressure, and coal permeability.
Under the conditions of constant pore pressure, coal permeability
was mainly controlled by effective stress; with the increase of effective
stress, permeability decreased sharply initially and then gradually.
Under the conditions of uniaxial strain and constant external stress,
with an increase of pore pressure and R
m, the matrix sorption expansion strain increased, resulting in a
narrowing of the seepage channel, and R
m indirectly inhibited permeability. At this point, coal permeability
was mainly controlled by sorption expansion/contraction strain and
effective stress. In addition, compared with other permeability models,
the new permeability model possesses higher applicability both in
theoretical mechanism and in data matching. The general change trend
concerning coal permeability, determined by rebound pressure p
rb, was consistent with the test results, which
further verified the applicability of the model. It is believed that
the results of this study could provide a basis for subsequent research
on the stress–strain–permeability relationship and for
the study of efficient development of CBM.