Evolution of pore-fracture and permeability prediction in soft coal rock under variable-pressure water saturation

Abstract

As a critical reservoir property, permeability governs both fluid transport dynamics in coal seams and consequent coalbed methane recovery rates. The pore-fracture structure evolution of soft, low-permeability coal under stress disturbance is complex, making permeability prediction challenging. There is an urgent need to establish a permeability prediction model for such coal. Based on nuclear magnetic resonance characterization of pore-fracture structures and permeability testing of variable-pressure saturated coal rock, fractal permeability prediction models (Sw-FPPM) K(Sw)T-C and K(Sw)SDR incorporating variable water saturation (Sw) were established to reveal the evolution characteristics of pore-fracture structures and permeability under variable-pressure saturated water conditions. The results indicate that variable-pressure water saturation significantly enhances the permeability of coal seams. At the maximum pressure of 9 MPa, the permeability increased by 7.2–51.38 times compared to the initial coal samples. Softer coal samples with lower firmness coefficients exhibited higher initial permeability but showed smaller permeability increments under higher water pressures. The fractal dimension (Ds) of seepage pores approached 3, indicating strong heterogeneity in the coal's pore-fracture structure. Ds slightly decreased with increasing water saturation pressure, while permeability significantly increased as Ds decreased. Additionally, pore shrinkage effects inhibit seepage, and the control exerted by small-aperture pores on permeability is commonly underestimated. The improved models K(Sw)T-C and K(Sw)SDR, incorporating Sw and Ds, demonstrated significantly enhanced accuracy by 61.09% and 53.84%, respectively. This provides an effective method for predicting permeability in soft, low-permeability coal seams.

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