Evolution of Coal Microstructure and Mechanical Properties during High-Temperature CO2 Displacement

Abstract

Coalbed methane (CBM) is an important unconventional natural gas resource, but its efficient recovery is commonly restricted by the low permeability and strong gas adsorption capacity of coal reservoirs. CO2 injection can promote CH4 displacement and provide a potential pathway for geological CO2 storage. However, most previous CBM/ECBM studies have focused on relatively moderate temperatures or pulverized coal samples, while integrated observations of gas displacement, pore–fracture evolution, and mechanical response in intact coal cores at temperatures above 70 °C remain limited. Compared with pulverized coal, intact coal cores preserve the original bedding, cleat system, pore–fracture connectivity, and core-scale mechanical integrity, allowing the coupled response of coal to elevated-temperature supercritical CO2 treatment to be evaluated in a more structurally representative system. In this study, supercritical CO2–CH4 displacement experiments were conducted on intact coal specimens at 35, 80, 120, and 150 °C under a constant injection pressure of 8 MPa. Low-field nuclear magnetic resonance, high-pressure NMR imaging, CT-based three-dimensional pore–fracture reconstruction, and triaxial compression testing were combined to evaluate CH4 displacement, CO2 retention, structural modification, and mechanical response over an extended temperature range. The CH4 displacement efficiency increased from approximately 63.5% at 35 °C to 82.0% at 150 °C, whereas the measured CO2 retention decreased from 82.3 to 58.7 mL/g, indicating a possible trade-off between enhanced methane recovery and CO2 storage capacity. The specimens treated at 120 and 150 °C exhibited more pronounced pore–fracture modification than those treated at 35 and 80 °C. At 120 °C, porosity increased from 5.7% to 16.7%, corresponding to an absolute increase of 11.0 percentage points and a relative increase of approximately 193%; at 150 °C, porosity increased from 5.5% to 18.6%, corresponding to an absolute increase of 13.1 percentage points and a relative increase of approximately 238%. The specimens subjected to elevated-temperature SC–CO2 treatment also exhibited lower compressive strength and elastic modulus. These observations extend previously reported CO2–coal responses to temperatures up to 150 °C and highlight the potential balance between improved gas-transport pathway availability and reduced mechanical stability. Because only one independent specimen was examined at each temperature, the intertemperature comparisons should be regarded as preliminary observations under the prescribed laboratory conditions rather than statistically validated general relationships. The 150 °C condition was used as an upper-bound laboratory case and should not be interpreted as a directly recommended field injection temperature.