Experimental Research on Dynamic Variation of Permeability and Porosity of Low-Rank Inert-Rich Coal Under Stresses.

Abstract

The effective stress variation can cause permeability change during coalbed methane (CBM) production. At the same time, macerals have an important influence on the stress sensitivity of coal reservoirs. To investigate the low-rank inert-rich coal permeability dynamic response to effective stress during the production of CBM, eight low-rank coal samples were collected from the Huanglong coalfield, China, and the dynamic changes in helium permeability and porosity were tested under conditions in which the effective stress changed from 0, 15, to 0 MPa. Then, the permeability dynamic change, stress sensitivity control mechanism, and their influence on the development of low-rank CBM are discussed. The results show that with the increase in effective stress, the permeability and porosity of coal samples decrease in the form of negative exponents. During the process of effective stress increase, the permeability variation of low-rank coal includes three stages: rapid loss stage (0–3 MPa), slow loss stage (3–9 MPa), and stable loss stage (>9 MPa). In the process of effective stress reduction, the irreversible damage rate linearly increases. Fracture permeability is easier to recover than pore permeability during pressure relief. Coal macerals have an important influence on the initial permeability and stress sensitivity of low-rank coal. The ratio of vitrinite to inertinite of low-rank coal is positively correlated with initial permeability, but negatively correlated with the permeability loss rate and the fracture compression coefficient (Cf). Cf shows a decreasing trend of the negative index with an increase in effective stress and is highly positively correlated with the average permeability loss rate. The porosity–permeability index model and the permeability–effective stress-sensitive model of low-rank coal under effective stress are established. Research shows that inertinite is helpful to improve the stress sensitivity of low-rank coal. Accordingly, to reduce the damage of coal reservoir permeability, the pressure should steadily change either in the process of fracturing or drainage.