High-pressure methane adsorption-induced coal swelling on equilibrium moisture coal samples

ABSTRACT

The methane adsorption at the surface of coal pores can induce coal swelling. Swelling strain is not only related to adsorption capacity but also equilibrium moisture content and coal ranks. The swelling strain in confined conditions will lead to the closure of coal reservoir cleat system, which reduces coal reservoir permeability, and further impedes free gas percolation. In this work, adsorption and strain experiments on two coal samples from China with vitrinite reflectance of about 1.64 and 3.68% were performed at 30, 40, and 50°C with the pressure of 15 MPa in the corresponding moisture state. The error is compared between measurement values and theoretical values. The results indicate that coal swelling is irreversible and anisotropic, and the methane adsorption capacity and swelling strain on two coal samples have a concave parabolic curve relationship with the increase in temperature. The effects of temperature on sorption and swelling on middle-rank coal is inferior to equilibrium moisture in the temperature range of 30–40°C, and superior to it in the range of 40–50°C, however, it is the opposite on high-rank coal. The swelling strain curve shapes of different rank coals are similar no matter what temperature, pressure, and moisture content are, and also are a function of time. The whole strain process can be divided into three phases: rapid growth, slow growth, and stability with the increase in pressure. These show that swelling strain of tested coal samples is synthetically affected by moisture, temperature, and pressure.

KEYWORDS:

• Adsorption isotherm
• adsorption swelling strain
• methane
• swelling effect

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (grant numbers 41072153 and 41172141; Major Program, grant numbers 50490271 and 40672104), The National Science and Technology Pillar Program in the Twelfth Five-Year Plan Period (grant numbers 2012BAB13B01 and 2012BAC10B03), Henan Province Basic and Frontier Technology Research Projects (grant number 132300413204). We thank the reviewers and editors for their constructive suggestions for improving the manuscript.