Stress sensitivity characteristics of deep coal reservoirs and its influence on coalbed methane productivity

Deep coal reservoirs are in a high in-situ stress environment, and their permeability change characteristics are quite different from those of shallow ones. In order to study the effect of the effective stress on the permeability difference of deep coal reservoirs and the characteristics of stress sensitivity anisotropy, the samples from 1 200 m to 1 700 m of the No.15 coal seam in the Hengling block in the Qinshui Basin are taken as the research objects. The overburden porosity and permeability experiments are used to study the permeability changes of parallel and vertical bedding samples under different effective stress conditions to explore their stress sensitivity characteristics and the impact on CBM productivity. The results show that the permeability decreases in a power exponential function with the increase of effective stress. The permeability of the parallel bedding surface in the study area is generally higher than that of the vertical bedding surface, and the permeability changes in the two directions show a positive correlation. The three parameters of reservoir pore and fracture compressibility, permeability stress damage rate, and permeability curvature are selected as the stress sensitivity evaluation indexes of coal reservoirs. Among them, the pore and fracture compressibility coefficient shows a positive correlation and a negative correlation with the increase of the effective stress, with 5 MPa as the limit. The permeability damage rate and permeability curvature increase and decrease exponentially with the effective stress. Based on the study of stress sensitivity parameters, the CBM well productivity model was derived. It was found that the productivity of gas wells without stress sensitivity was higher than that with stress sensitivity. That is to say, under the same production pressure of 5 MPa, the production reduction rate of the gas well production shows an overall increasing trend with the increase of the stress sensitivity coefficient. According to the phase division, the target coal seam in the study area should adopt a small-medium-large displacement scheme to control the production flow during the CBM extraction process.