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- ormultimodal pore structure. This paper describes the implementation of abidisperse 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 astortuous 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 reservoir scan 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.