Coupled Flow Simulation in Coalbed Methane Reservoirs

Abstract Coal seams are characteristic of two distinct porosity systems: macropores and micropores, similar to fracture systems and matrix systems for fractured hydrocarbon reservoirs. Since coal is relatively impermeable, any coalbed methane (CBM) recovered usually must flow through existing fracture systems helping increase the flow rate of CBM, which may induce CBM non-darcy (N-D) flow when high CBM rate. During CBM depletion, the deformation effect of coal skeleton may also have considerable influence upon CBM flow. Although there are a number of CBM reservoir simulators that have been available for many years, to our knowledge, almost commercial-scale CBM simulator have not provided the features of N-D flow and coal skeleton deformation. In this presentation, a coupled flow model in CBM reservoir, accounting for N-D flow effect and coal skeleton deformation effect, was presented. The mathematical model was numerical solved by differential discrete and the implicit format treatment method. Based on our in-house gas-water simulator developed before, a simple coupled flow simulator in CBM reservoirs was developed. The technique uses an explicit algorithm to couple CBM flow and the changes of porosity and permeability of coal seams where flow calculations are performed every time step and the changes of porosity and permeability of coal seams are calculated only during selected time steps. Sample simulations are conducted to compare with the differences in influence of N-D flow effect and the coal skeleton deformation effect upon CBM production behavior. The results indicted the decreased in coal seams porosity and permeability can reduce the CBM rate and recovery much than N-D flow effect. Unfortunately, porosity and permeability of coal seams are constant in almost CBM simulations, as often assumed. The work recommended the alteration of coal seams porosity and permeability should be considered globally to correctly predict the CBM rate and recovery.