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Coalbed methane (CBM) reservoirs generally have very low permeability and requires 8 stimulation to make gas extraction economical. Hydraulic fracturing has been widely applied to enhance 9 CBM productivity, but this technology has a number of key limitations, including fractures only propagate 10 along existing joints, large amount of water usage, and potential environmental impact. Microwave 11 irradiation technology can likely overcome the above limitations. In this study, the effect of microwave 12 irradiation on the petrophysical properties of an unconstrained bituminous coal was comprehensively 13 investigated through a suite of integrated diagnostic techniques including Nuclear Magnetic Resonance and 14 X-ray Computed Tomography. A series of experiments were conducted both on centrifuged samples and on 15 samples with different water contents ranging from 1 to 15%. The mineral removal and moisture 16 evaporation due to the microwave selective heating lead to the enlargement, opening, and interconnection 17 of coal pores. The NMR-determined porosity increases linearly with the microwave power while grows 18exponentially with respect to water contents. When the water content is above 6%, the porosity increases 19 by around 98~211%. The fracture volume and coal permeability increase while the P-wave velocity 20 decreases with increasing water contents. Microwave irradiation is effective in enlarging mesopores and 21 macropores and in enhancing the pore connectivity. The significant enhancement of coal permeability and 22 pore fracture structure indicates that the microwave irradiation is effective in improving gas productivity 23 thus has the potential to become a new CBM reservoir simulation technology. 24 25

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Hot steam injection in coal seams can raise the molecular kinetic energy of methane and promote gas desorption, which provides a new idea to solve the difficulty in gas extraction in most coal seams in China. To explore the law of pore and crack expansion in coal under the action of hot steam, bituminous coal and anthracite samples were obtained from Jining Coal Mine (JN) and Linhua Coal Mine (LH), respectively. With the aid of a self-built experimental platform for stream-treated coal, a contrastive study was conducted before and after heat treatment. In terms of macrocracks, photos of cracks on the coal surface before and after hot steam treatment were captured by a high-definition camera; then, the cracks in the photos were extracted, processed, and analyzed. In terms of micropores, evolutions of micropores and cracks in coal before and after heat treatment were studied on the basis of mercury intrusion. The following results were obtained: After hot steam treatment at 140 °C for 180 min, the areas of macrocracks on the surface of the JN bituminous coal sample and the LH anthracite sample jump by 13.05 and 3.76 times, respectively, and their porosities reach 1.075% and 0.081%, respectively. The JN coal surface has more developed cracks than the LH coal surface after hot steam treatment. The cumulative volumes of mercury intrusion for JN coal and LH coal rise by 11.4% and 3.6%, respectively. The volumes of micropores and minipores in coal decrease, while those of mesopores and macropores increase. Hot steam promotes the development of pores and cracks in coal to a certain extent, and its pore expansion effect is more remarkable for JN coal than for LH coal. The mechanism of the action of hot steam on coal is concluded as follows: Uneven expansion force occurs in coal during heating due to the anisotropy of coal, the different expansion coefficients of different components in coal, and the temperature gradients. Constrained by the mutual deformation of matrix, when strength limit is exceeded, pores and cracks of coal develop and expand from weak media, so that micropores and minipores are transformed into mesopores and macropores.

Injecting steam into coal seam is an important means to accelerate gas desorption and improve gas extraction efficiency. However, the change law of pore–fracture structures of coal after high-temperature steam shock (thermal shock) is still unclear. Through this study, pore–fracture structures of coal samples before and after thermal shock were compared and analyzed based on the experimental methods of surface pore and fracture extraction, scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) spectroscopy. The results show that after thermal shock, the surface porosity, max equivalent fracture width, fracture lengths, fracture number, and probability entropy of coal samples increased significantly, and the increment of bituminous coal was greater than that of anthracite. This indicates that thermal shock can promote the development of coal pores, which is significantly better for bituminous coal than anthracite. A SEM analysis reveals that fractures tend to appear at the interface between minerals and coal matrix. The NMR analysis demonstrates that the absolute increment of micropores is the largest, followed by that of mesopores, and that of macropores is the smallest. The increase of porosity in coal shows pore enlargement and penetration, which enhance the connectivity between the pores, thus providing a smoother channel for methane migration. Heterogeneous distribution of mineral components with different thermal expansion coefficients as well as the temperature gradient is the fundamental mechanism behind thermal stress-induced porosity development. The research results provide theoretical support for enhanced gas extraction technology by high-temperature steam injection into coal seams.