Optimization of hydraulically fractured well configuration in anisotropic coal-bed methane reservoirs

Abstract

Most coal seams are characterized by an anisotropic network of cleat system in which continuous face cleats have much more permeability than discontinuous butt cleats. In hydraulically fractured and anisotropic CBM reservoirs, pressure propagation and resulting drainage area is in elliptical geometry. The determination of optimal well pattern and well spacing in such a system is the key parameter for higher well productivity and optimized CBM field development plan. In this study, well pattern and well spacing in anisotropic CBM reservoirs with fractured wells are optimized based on the concept of balanced depressurization which is assumed that the reservoir pressure should be declined uniformly in all directions of drainage area. The mathematical relationship for optimal well spacing is first developed and is verified by numerical simulation. It is concluded that, based on the requirement of balanced depressurization, the rectangular or diamond shaped pattern is the best pattern for hydraulically-fractured anisotropic coal seams, and the optimal well spacing is determined by both permeability ratio and hydraulic fracture length. Furthermore, a procedure for optimization of CBM well pattern and spacing is presented for making development strategy.