In-situ stress, pore pressure and stress-dependent permeability in the Southern Qinshui Basin

This study focuses on the in-situ stress, pore pressure and permeability in the Southern Qinshui Basin, one of largest coalbed methane basins in China. Well tests show that permeability in this basin is higher than other coalbed methane reservoirs. This is because it is located in an extensional basin, where the normal faulting stress regime is dominated. This in-situ stress regime is advantageous to keep coal cleats open. Hydraulic fracturing tests indicate that the fracture gradient or minimum horizontal stress is much lower than the shales in the Gulf of Mexico and other oil basins. The minimum horizontal stress model is proposed with consideration of the stress coefficient based on the uniaxial strain method. This model provides a fairly good prediction on the minimum stress. Permeability data show that the effective stress-dependent permeability is pronounced in the coalbed methane reservoir. This is significant for the dual-porosity and dual-permeability coal reservoirs, which consist of coal porous matrices and cleats. The reason is that a rapid increase in effective stress can induce the closure of cleats, which may cause a permanent loss of permeability in the cleats. This reduces the connectivity between the cleats and coal matrices, hence the coal matrices cannot deliver gas pressure to the cleats for supporting the cleat space. Therefore, slowing down the effective stress change during production (e.g. slowing reservoir drawdown) can decelerate the permeability reduction. This is particularly important for the reservoir in which the pore pressure is not significantly overpressured, such that in the Southern Qinshui Basin.