Effect of interlayer heterogeneity on multi-seam coalbed methane production: A numerical study using a gray lattice Boltzmann model

Multi-seam or mull-zone commingled production of coalbed methane (CBM) in a single wellbore would substantially increase the cumulative gas production. In general, CBM reservoir is highly heterogeneous and anisotropic. To understand the impact of interlayer heterogeneity on commingled production, numerical simulations for two-seam commingled production of CBM were carried out using a gray lattice Boltzmann model. The heterogeneous permeability fields were modeled using the Gaussian distribution function. Based on the interpolation-supplemented lattice Boltzmann method (ISLBM), the non-uniform mesh grid was used in the simulations to achieve a higher computational efficiency. The pressure and velocity distributions of mull-seam commingled production with different heterogeneity coefficient were obtained. The results show that as the coefficient of permeability heterogeneity increased, the sweep region of pressure drop and flow velocity decrease significantly in the heterogeneous seam. By introducing the coefficient B, which represents the level of production decline caused by interlayer heterogeneity, we note that the gas production appears most sensitive in the case of low heterogeneity coefficient s. When the value of s is fixed, for a greater reservoir average permeability K-mp, the effect of reservoir heterogeneity on CBM production is more significant. This work presents a simplified reservoir model that can be useful for production dynamic analysis, production system optimization and productivity prediction of heterogeneous reservoirs in mull-seam commingled production.