The key mechanisms of two-phase gas-water flow through coal beds as utilized in a new three-dimensional finite-difference simulator, COMETPC 3-D, are presented. The theory for gas transport through coal matrix, transfer of gas from matrix to fracture, and the adsorption isotherm boundary condition at the matrix-fracture interface is described. Several unique features of coal beds which can affect gas productivity including stress-sensitive permeability, matrix shrinkage compressibility, and gas re-adsorption are incorporated in the model. Applications to history matching production data from coal bed methane wells, studying changes in flow' behavior in coals with gas desorption, and modeling horizontal wells are presented.
Simulation of coal bed methane reservoirs is a more complex and data intensive process than for conventional gas reservoirs. This is because the primary means of gas storage in coal beds is byadsorption of methane molecules on coal surfaces. When a coal bed is subjected to pressure drawdown, the desorbed gas must move by diffusion through the extremely low permeability coal matrix in order to reach the natural fracture (cleat) system. Once in the cleats, which normally have a high permeability relative to the matrix, gas and formation water flow according to Darcy's law.
A number of approaches have been taken to simulate these complex mechanisms in coal. The methods vary from the early work of Price and Abdalla1 on equilibrium sorption models for degassing coal mines, to the unsteady state models of Ancell et al2 and Kolesar et al3. An excellent review of numerical simulation work to date pertaining to coal bed methane has been given recently by King and Ertekin4.
The fully 3-D, multi-well model described here is based on the non-equilibrium, pseudo steady-state formulation discussed by King et al5, and as such is an extension of an earlier 2-D models6. The 3-D model was developed in conjunction with the Gas Research Institute (GRI) and a thirteen company industry consortium. It has been validated against other industry simulators, and used in numerous coal bed reservoir studies, most notably in the Black Warrior and San Juan Basins.
Coal cleat-matrix represents a well-defined dual porosity system as described by Warren and Root7. The face and butt cleats constitute a well developed set of approximately orthogonal vertical natural fractures, shown as a highly idealized schematic in Fig. 1. The coal matrix consists of a micro porous system through which gas diffuses to the fractures.
Desorption of methane is described by a Langmuir8 isotherm, which relates the coal cleat pressure, P, to the equilibrium matrix gas concentration, C(P), according to
Equation (1) (Available In Full Paper)
Where VL is the maximum amount of gas that can be adsorbed, and PL, a characteristic pressure, is a measure of the residence time for a gas molecule on the surface. Both VL and PL may be determined from laboratory adsorption isotherm measurements.
Eq. 1 provides the necessary boundary condition between cleats and the coal matrix.