Investigating the effect of sorption time on coalbed methane recovery through numerical simulation

The objective of this work is to study the effect of non-equilibrium sorption time on the gas production rate in coalbed methane (CBM) reservoirs. Numerical simulation is employed to investigate this phenomenon in coal seams with single-phase flow of methane and two-phase flow of methane and water. Radial and rectangular models with vertical and horizontal wells are considered. A multi-layered model is also generated with properties similar to the Horseshoe Canyon (HSCN) formation in Alberta. The results indicate that the sorption time affects the production rate in the early production phase, namely a few months to a few years depending on how slow the desorption/diffusion process is, but this depends on the magnitude of the sorption time. This is valid both for dry and initially water saturated coalbed methane reservoirs. However, in the latter case, the effect lasts longer since the dewatering must occur first for desorption/diffusion process to start. The type of wellbore also influences the dynamics of sorption/diffusion effects. For smaller diffusion coefficients (larger sorption times), the gas decline rate in horizontal wells is larger relative to vertical wells. The results of the multi-layer study indicate that when sorption time is smaller than 10 days, the effect of sorption/diffusion phenomena on total commingled production rate is negligible. In general, we recommend non-equilibrium models for early-time production when diffusion flow from matrix to fracture is still in transient state. For late-time production, when steady-state diffusion flow has been established between matrix and fracture, equilibrium models can be used.