Study on the Thermal Expansion Characteristics of Coal during CO2 Adsorption

Abstract

The adsorption of CO2 fracturing fluid into coal reservoirs causes the expansion of the coal matrix volume, resulting in changes in the fracture opening, which alters the permeability of the coal reservoir. However, it is not yet clear whether thermal expansion during CO2 adsorption on coal is the main cause of coal adsorption expansion. Therefore, by testing the thermal properties, expansion coefficient, and adsorption heat of the three coal samples, the adsorption thermal expansion characteristics of coal and their impact on the permeability of coal reservoirs are clarified. The results reveal the following: (1) Under the same conditions, the adsorption heat increases with increasing pressure, while it decreases with increasing temperature. The relationship between adsorption heat and pressure conforms to the Langmuir equation before 40 °C, and it follows a second-order equation beyond 40 °C. At 100 °C, the adsorption heat of coal samples to CO2 is primarily determined by temperature. (2) The maximum temperature variation in coal samples from Xinjiang, Liulin, and Zhaozhuang during CO2 adsorption is 95.767 °C, 87.463 °C, and 97.8 °C, respectively. The maximum thermal expansion rates are 12.66%, 5.74%, and 14.37%, and the maximum permeability loss rates are 16.16%, 7.51%, and 18.24%, respectively, indicating that thermal expansion is the main reason for coal adsorption expansion. (3) This research can elucidate the impact of CO2 fracturing fluid on coal reservoirs and its potential application value, thus providing theoretical support for coalbed methane development and CO2 geological storage.