Thermal Expansion and Adsorption Characteristics of Coal Based on Temperature Change and Its Influence on the Slippage Effect

Abstract

The slippage effect is an important phenomenon of coalbed methane (CBM) migration. As the exploitation of CBM, the slippage effect becomes more and more obvious. For exploring the evolution of the slippage effect as CBM extraction by heat injection, the thermal expansion test and the temperature increasing desorption test under different stress states, as well as the seepage test under variable temperature conditions, were conducted by operating the high-temperature multifunctional triaxial test system. The effect rules for temperature and pore pressure on the slippage effect were analyzed, and then the evolution mechanism of the slippage factor at the united action of pore pressure and temperature was comprehensively analyzed based on thermal expansion deformation and adsorption characteristics. The results showed that: first, the influence of pore pressure on the slippage effect is mainly caused by the molecular mean free path and effective stress when pore pressure is greater than 0.8 MPa in the range of 30–150 °C. With the pore pressure increase, the slippage effect decreases. Second, the temperature influence on the slippage effect is mostly induced by the variation of cracks in coal due to thermal expansion deformation created by temperature. With the increase in temperature, the slippage factor average increases first from 30 to 60 °C is 19 times and then decreases from 60 to 90 °C is 36%. Finally, at the combined influence of temperature and pore pressure, the slippage effect is mainly influenced by thermal expansion deformation, desorption deformation, and effective stress. At 30–60 °C, the slippage factor is principally affected by thermal expansion deformation. At 60–90 °C, the slippage factor is mostly influenced by desorption deformation and effective stress. The research results provide a foundation for the technology of CBM exploitation by heat injection.