Experimental study on the coupling effect of pore-fracture system and permeability controlled by stress in high-rank coal

Abstract

During the coalbed methane (CBM) exploitation, the reservoir permeability can be affected by the effective stress that varies with the reservoir fluid pressure, which is a complex, dynamic and significant engineering problem. To analyze the response characteristics of the pore-fracture system by the changing stress, this work simulated reservoir and fluid pressures during the exploitation by adjusting confining pressure and displacement pressure. Stress sensitivity experiments under different effective stresses were conducted to systematically study the stage variation characteristics of porosity and permeability of coal. The results show that the permeability decreases exponentially with the increase in effective stress, consistent with previous studies. However, the porosity shows a V-shaped trend, which is different from the traditional understanding that it would decrease continuously with rising effective stress. These variation characteristics (of porosity and permeability above) therefore result in a phased porosity sensitivity of coal permeability (P_PS). Moreover, the stress sensitivity of the samples was evaluated using the permeability damage rate method (M_PDR) and the stress sensitivity coefficient method (M_CSS), both of which showed that it ranges from the degree of strong to extremely strong. When the effective stress is lower than 5–6 MPa, the stress sensitivity of the coal reservoir drops rapidly with effective stress rising; when it is higher than 5–6 MPa, the change in stress sensitivity tends to flatten out, and the stress sensitivity coefficient (C_SS) goes down slowly with rising effective stress. Finally, suggestions are proposed for the drainage scheme of CBM wells based on the experimental results.

Graphical abstract

Keywords

stress sensitivity / porosity sensitivity / permeability / high-rank coal / effective stress

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