Summary
Unipore diffusion models are used widely to model gas transport in a coal
matrix in conventional dual-porosity coalbed-reservoir simulators. The unipore
models implemented in conventional coalbed-reservoir simulators assume that
there is a negligible free-gas phase in the coal matrix and that gas exists
only in an adsorbed state under hydrostatic pressure. In low-rank coals,
however, a substantial amount of free gas may exist in the macropores of the
coal matrix.
There is strong laboratory evidence that many coals exhibit bi- or
multimodal pore structure. This paper describes the implementation of a
bidisperse pore-diffusion model in a coalbed-reservoir simulator. In the
bidisperse model, gas adsorption is assumed to take place only in the
micropores, with the macropores providing storage for free gas, as well as
tortuous paths for gas transport between the micropores and cleats.
Gas-production performance from a sub-bituminous Powder River basin coalbed
reservoir has been studied using an in-house coalbed-reservoir simulator. The
implementation of the triple-porosity formulation in the simulator overcame the
reported inconsistency between field gas-production rates and predicted rates
obtained with conventional dual-porosity simulators. With the introduction of
an appropriate storage volume of free gas in the macropores, the predicted
increase in gas-production rates are consistent with the published field
data.
Introduction
Coal seams may be characterized by two distinctive porosity systems: a
well-defined and almost uniformly distributed network of natural fractures
(cleats), and matrix blocks containing a highly heterogeneous porous structure
between the cleats. The cleat system can be subdivided into the face cleat,
which is continuous throughout the reservoir, and the butt cleat, which is
discontinuous and terminates at intersections with the face cleat (Fig. 1). The
cleat spacing is very uniform and ranges from the order of millimeters to
centimeters.
Unlike conventional gas reservoirs, methane in coalbeds is stored primarily
as a sorbed gas, at near-liquid densities, on the internal surface area of the
microporous coal. The surface area of the coal on which the methane is adsorbed
is very large (20 to 200 m2/g) and, if saturated, coalbed-methane reservoirs
can have five times the volume of gas contained in a conventional sandstone gas
reservoir of comparable size.
Virgin seams are often saturated with water. During primary recovery by
pressure depletion, methane production is facilitated by dewatering the target
seams to allow desorption of the adsorbed methane, which then migrates through
the coal matrix into the cleats. The transport of gas through a coal seam is
considered a two-step process. It is generally assumed that flow of gas and
water through the cleats is laminar and obeys Darcy’s law. On the other hand,
gas transport through the porous coal matrix is controlled by diffusion.
As in a fractured conventional reservoir, the permeability of coalbeds comes
primarily from the network of natural fractures. Being normal to the bedding
plane and orthogonal to each other, the face and butt cleats in coal seams are
usually subvertically orientated. Thus, changes in the cleat permeability can
be considered to be controlled primarily by the prevailing effective horizontal
stresses that act across the cleats, rather than the effective vertical stress,
defined as the difference between the overburden stress and pore pressure.
Permeability of coal has been shown to be highly stress-dependent.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
16 February 2004
- Revised manuscript received:
31 January 2005
- Manuscript approved:
14 February 2005
- Version of record:
15 April 2005
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