Analysis of coal permeability model and influencing parameters based on volume strain

Abstract

To investigate the evolution of coal permeability during the process of gas pressure change, four different forms of permeability models were derived based on the correlation between mesoscale parameters of coal representative elementary volume (REV) and strain-related characteristics. The reliability of the models was verified using experimental data, and the variation law of strain caused by effective stress and adsorption strain was analyzed and compared with classical permeability models (P-M model, S-D model, Connell model). The sensitivity of model parameters was discussed, the relationship between REV mesoscale parameters and fracture porosity and permeability was clarified, and the influence mechanism of adsorption strain coefficient was analyzed. Furthermore, the extreme values of permeability of major coal mines in China were studied. Results show that for the four permeability models derived in this paper, the REV volume strain satisfies a linear relationship with its fracture volume strain, and the correlation coefficient is the initial fracture porosity. The bulk modulus and adsorption strain coefficient control the influence of effective stress and adsorption strain on permeability, respectively. Under three different experimental conditions of constant confining pressure, constant pore pressure, and constant differential pressure, the four permeability models have shown a better matching effect on the experimental data compared with the classical permeability models. However, compared with single-parameter fitting, the parameter error in the double-parameter fitting (bulk modulus and adsorption strain coefficient) is larger. From the sensitivity analysis of parameters, it is observed that when the parameters change within 10%, the predicted permeability deviates significantly from the true value. Therefore, for the double or multiple-parameter fitting of permeability models, the parameter fitting errors could lead to serious distortion of the predicted permeability results. Taking the experimental data of coal samples from the Qinshui Basin as an example, the mesoscopic scale parameters a0 and b0 of REV were calculated to range from 1.73 to 46.31 μm and from 0.06 to 0.49 μm, respectively. Under constant confining pressure and constant differential pressure conditions, the adsorption strain coefficient was related to differential pressure, gas types, and initial fracture porosity. Furthermore, the extreme values of CO2 gas and CH4 gas in major coal mines in China are within the ranges of 0.94−5.33 MPa and 1.06−6.94 MPa, respectively.

Keywords

• permeability
• rev
• multi-experimental condition model
• parameter sensitivity
• adsorption strain coefficient