Coalbed methane modeling: Multicomponent effects

Field data from coalbed methane wells indicate a significant variation in composition as a function of flow rate and time. Prediction of gas quality is critical to the economic evaluation of any coalbed methane project. Previous models have forecast only pressure and production profiles, ignoring compositional variations.; A multicomponent model that incorporates binary isotherms for methane and carbon dioxide has been developed. An extended Langmuir Isotherm represents adsorption, and a constant relative separation factor describes the phase distribution of the species. This separation factor depends on the surface characteristics of the coal and on the adsorbing species. The model equations were cast in dimensionless form and solved for pressures and compositions.; Solution of the equations was accomplished with a public-domain computer package for solution of stiff sets of ordinary differential equations, LSODE. The nonlinear equations were linearized with a three-point central finite difference scheme.; Pressure profiles for primary depletion of a single phase gas reservoir are accurately forecast. The model also correctly predicts the qualitative trends of increasing CO{dollar}sb2{dollar} concentration in produced gas.; Variation of CO{dollar}sb2{dollar} concentration in the produced gas is a function of the separation factor, the initial CO{dollar}sb2{dollar} concentration in the reservoir, and the pressure depletion range. The model shows that the fraction of CO{dollar}sb2{dollar} in the produced gas increases with dimensionless time. Dimensionless time is proportional to time and coalbed permeability and inversely proportional to gas viscosity, coal porosity and reservoir length squared. A comprehensive sensitivity study was performed to identify the effects of important parameters such as isotherm variables and reservoir characteristics on primary depletion pressure and composition profiles.; Validation of the model was accomplished by comparison of the implicit LSODE pressure solution to pressure profiles from an explicit and a fully implicit Newton-Raphson solution of the pressure equation.; No good models currently exist for validation of compositional profiles. However, a commercial reservoir simulator was adapted to provide pressure and composition information, with K values based on the same constant separation factor. The results from the simulator and the multicomponent model have been compared and show similar trends.