Quantitative Characterization of Pore–Fracture Structures in Coal Reservoirs by Using Mercury Injection–Removal Curves and Permeability Variation under Their Constraints

Abstract

Pore and fracture structure heterogeneity is the basis for coalbed methane production capacity. In this paper, high-pressure mercury intrusion test curves of 16 coal samples from the Taiyuan Formation in the Linxing area are studied. Based on the fractal dimension values of mercury intrusion and retreat curves, the correlation between the two different fractal parameters is studied. Then, the permeability variation of different types of coal samples is studied using overlying pressure pore permeability tests. The correlation between the permeability variation of coal samples and dimension values is explored, and the results are as follows. (1) Based on porosity and mercury removal efficiency, all coal samples can be divided into three types, that is, types A, B, and C. Among them, Type A samples are characterized by lower total pore volume, with pore volume percentages ranging from 1000 to 10,000 nm not exceeding 15%. (2) During the mercury injection stage, both the M-model and S-model can reflect the heterogeneity of seepage pore distribution. In the mercury removal stage, the M-model cannot characterize the heterogeneity of pore size distribution in each stage, which is slightly different from the mercury injection stage. (3) The permeability of Type A samples is most sensitive to pressure, with a permeability loss rate of up to 96%. The original pore and fracture structure of this type of coal sample is relatively developed, resulting in a high initial permeability. (4) There is no significant relationship between compressibility and fractal dimension of mercury injection and mercury removal, which may be due to the comprehensive influence of pore structure on the compressibility of the sample.