CO2 permeability of fractured coal subject to confining pressures and elevated temperature: Experiments and modeling

The CO2 permeability of fractured coal is of great significance to both coalbed gas extraction and CO2 storage in coal seams, but the effects of high confining pressure, high injection pressure and elevated temperature on the CO2 permeability of fractured coal with different fracture extents have not been investigated thoroughly. In this paper, the CO2 permeability of fractured coals sampled from a Pingdingshan coal mine in China and artificially fractured to a certain extent is investigated through undrained triaxial tests. The CO2 permeability is measured under the confining pressure with a range of 10-25 MPa, injection pressure with a range of 6-12 MPa and elevated temperature with a range of 25-70 degrees C. A mechanistic model is then proposed to characterize the CO2 permeability of the fractured coals. The effects of thermal expansion, temperature-induced reduction of adsorption capacity, and thermal micro-cracking on the CO2 permeability are explored. The test results show that the CO2 permeability of naturally fractured coal saliently increases with increasing injection pressure. The increase of confining pressure reduces the permeability of both naturally fractured coal and secondarily fractured coal. It is also observed that initial fracturing by external loads can enhance the permeability, but further fracturing reduces the permeability. The CO2 permeability decreases with the elevation of temperature if the temperature is lower than 44 degrees C, but the permeability increases with temperature once the temperature is beyond 44 degrees C. The mechanistic model well describes these compaction mechanisms induced by confining pressure, injection pressure and the complex effects induced by elevated temperature.