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• The pore structure of coal under microwave-assisted acidification was studied. • Acidification simplifies pore distribution while microwave destroys pore structure. • Microwave-assisted acidification promotes the connection and expansion of pores. • The pore surface of coal tends to be smooth and the permeability is improved. Coal reservoir usually requires stimulation to enhance coalbed methane (CBM) production. As a novel stimulation method, microwave heating contributes to methane desorption and coalbed fracturing. However, the microwave-induced fractures are easily closed under in-situ stresses. Acidic stimulation is a process of demineralisation in the coal reservoir using hydrochloric, acetic, or formic acids, resulting in opening and interconnecting pores and fractures to flow gas. However, it is difficult for acids to enter microfissures under normal pressures. Microwave-asisted acidification may offer an alternative. In this work, the evolution of the pore structure and fractal characteristics of coal under microwave-assisted acidification was evaluated using low-temperature nitrogen adsorption and Scanning electron microscope. Compared to microwave heating or acidification, microwave-assisted acidification greatly increases the pore volume of coal and extends the length and width and connectivity of fractures. In addition, the isolated pores were interconnected and the methane migration was facilitated. This work demonstrates the potential of microwave-assisted acidification for CBM recovery. [ABSTRACT FROM AUTHOR]

• Accurate characterization of porous structure and fluid mobility of coal utilizing 1H NMR. • Study of pore throat evolution processes and response to pore conversion trends. • Analyze T1-T2 spectrum to obtain development characteristics of coal pores. • Freeze-thaw cycles promote the connectivity and expansibility of coal pores. Microwave-LN 2 cyclic processing (MLCP) is a clean and high-efficiency anhydrous fracturing technology. The change in the porous structure of coal after freeze-thaw cycles significantly affects development efficiency of coalbed methane (CBM) in low porosity and permeability reservoirs, while there is a lack of in-depth macro-meso-micro studies. The damage mechanism of coal under microwave-LN 2 cold and thermal shock and the evolution of pore fluid reservoir space is based on cross-scale pore characterisation. This paper evaluates the multi-scale porous structure variations of coal before and after MLCP was assessed by hydrogen nuclear magnetic resonance (1H NNR) and field emission scanning electron microscopy (FE-SEM). The evolutionary trends in coal pore fluid fugacity space under freeze-thaw cycles was indirectly characterised by T 1 - T 2 spectra. Additionally, utilizing T 2 distribution, peak area and pore throat changes reflect the evolution trend of coal pore space. The results indicate that as the cycles increased, the coal free water and total fluid reservoir space gradually expanded beyond the cycle threshold (n > 10), the free-fluid index rose to 25.68 %, and the free fluid saturation increased by 20.39 %. Additionally, the NMR permeability of the coal increased by 98.37–1471.65 %, and the pore throat distribution increased by 28.05 %. Comprehensive analysis showed that as the cycles progressed, the free water space in the pore space increased significantly, the proportion of bound water gradually decreased, and the internal pores and fluid space of the coal were improved. This study provides fundamental support for multidimensional evaluation of the porous structure of microwave-LN 2 freeze-thaw cycles. [ABSTRACT FROM AUTHOR]

Water blocking damage is one of the most serious formation damages that restricts the development of low-permeability coalbed methane (CBM) reservoirs. Characterized by the conversion of the electromagnetic energy into heat, microwave heating may be a promising method to remove water blocking damage. In this study, contact angle tests and permeability tests were performed to investigate the effects of microwave heating on water blocking damage in CBM reservoirs. The results indicate that microwave heating can lead to hydrophobization of coal. The permeability of coal first decreases after water intrusion and then increases after microwave heating. The permeability sensitivity of the moist coal to microwave heating is much greater than that of the dry coal. Furthermore, the increase in the water content of coal promotes the microwave recovery effect of permeability. Theoretical analysis shows that the petrophysical evolution of coal during microwave heating involves gas desorption, dehydration, pore damage, fracturing and decomposition. Outcome of this study implies that microwave heating can be used as a supplemental measure of hydraulic fracturing to enhance CBM recovery by removing the water blocking damage. [ABSTRACT FROM AUTHOR]

• Coupled electromagnetic, heat transfer and multiphase porous media model was built. • Temperature rise of coal heated by microwave is characterized by "fast-slow-fast". • The saturation time of the water always lags behind that of the vapour. • The higher the microwave power, the quicker the fluid reaches the ultimate value. Microwave has gained widespread popularity in coal drying. In the current electromagnetic models, coal is always assumed as a solid continuum. The thermodynamic evolution and the fluid migration in coal cannot be accurately predicted. In this study, a fully coupled electromagnetic, heat transfer and multiphase porous media model was developed to investigate the drying kinetics of coal under microwave irradiation. Results show that the temperature rise of coal during microwave irradiation is characterized by "fast-slow-fast", and water evaporation is the key factor in thermal distribution. The water in the hot spot decreases, while the vapour increases and migrates towards the cold spot. The saturation time of the water always lags behind that of the vapour. In addition, the higher the microwave power, the quicker the fluid reaches the ultimate value. The nonuniform microwave heating enhances the thermal gradient and gas pressure gradient in coal, leading to irreversible damage. Outcomes of this study can be used to identify the drying behaviors of coal during microwave irradiation and thereby help to optimize the microwave applicators. [ABSTRACT FROM AUTHOR]

• Coal pore structure through microwave-LN 2 freeze–thaw cycling was studied. • Microwave-LN 2 freeze–thaw promotes pore connection and expansion. • Freeze-thaw penetrates isolated pores and cracks, improving permeability. • As the cycle increases, the pore structure was first destroyed then simplified. Coalbed methane (CBM) extraction has been restricted by low permeability. Liquid nitrogen fracturing has been proven to be a valid method for ameliorating the condition of coal reservoirs. Microwave-liquid nitrogen (LN 2) cyclic processing (MLCP) may be a potentially viable technology to increase the coal reservoir permeability. Therefore, this study conducted an analysis of the pore structure and surface morphology of single-mode processing coal (SPC) and MLCPC based on LT-N 2 GA and SEM. The results indicate that maximum N 2 adsorption capacity and the frequency of cycles for both meso- and macropores volume of the cycles coal samples are positively correlated, and the effects are more significant than that of the SPC. Moreover, the fractal dimension D 2 tended to increase and then decrease as the cycles deepened. This means that MLCP destroys the pore structure at the beginning of the cycle. When the frequency is exceeded the cycle threshold (n > 20), it will promote the development and fusion of micro- and mesopores to simplify the pore structure. After 30 freeze–thaw cycles, the TPV increased by 242.2%, and isolated pores were penetrated. The study provides essential theoretical support and a scientific basis for the engineering application of MLCP. [ABSTRACT FROM AUTHOR]