ABSTRACT
Quantitative characterization of the change in the cleat and pore structures and fractal dimensions in anthracite after electrochemical modification is crucial for better understanding of the modification effect. Thus, lump anthracite samples were electrochemically modified in our manufactured device with 0, 0.5, 1, and 2 V/cm potential gradients. The changes in heterogeneity and porosity after modification were tested and analyzed by mercury intrusion porosimetry (MIP) and fractal theory. The results indicated that the total volume of the pores increased after electrochemical treatment and continuously increased with increasing potential gradient during the treatment process. After modification, the number of pores or fractures with a pore size between 6 and 20 µm in coal after modification increases significantly. According to the intrusion pressure, three stages were defined as lower (P M < 0.1 MPa), intermediate (0.1 ≤ P M < 10 MPa), and higher regions (P M ≥ 10 MPa), which are characterized by fractal dimensions D 1, D 2, and compression stages, respectively. After modification, the fractal dimension D 1 showed an increasing trend, while the fractal dimension D 2 showed a decreasing trend, indicating that the fracture system became more complicated and that the pore system became more regular after electrochemical treatment. The evolution mechanism of heterogeneity and porosity and their fractal dimensions were explained by the dissolution of minerals, change in pH values, and dynamics of temperatures during the process of modification. The results obtained in this work are of important guiding significance for coalbed methane (CBM) extraction via in situ modification by electrochemical treatment.
ABSTRACT
The application of cyclical microwave modification for accelerating the extraction of coalbed methane (CBM) from anthracite is limited. In this study, the apparent permeability of anthracite samples before and after each microwave treatment (three in total) for 120 s was measured by a self-built permeability-testing platform. Microcomputed tomography (micro-CT) technology and image-processing technology were employed to analyze the 3D micron-scale pore structures, especially the quantitative characterization of connected pores and throats. After modification, the average apparent permeability increased from 0.6 to 5.8 × 10-3 µm2. The generation, expansion, and connection of micron-scale pores and fractures became more obvious with each treatment. The total porosity increased from 3.5 to 6.2%, the connected porosity increased from 0.9 to 4.8%, and the porosity of isolated pores decreased from 2.5 to 1.4% after three cycles. The number, volume, and surface area of the connected pores as well as the number, radius, and surface area of the throats were significantly increased. In addition, the release of alkyl side chains from the anthracite surface reduced the capacity of the anthracite to adsorb CH4 and the decomposition of minerals promoted the development and connectivity of pores. As a result, the gas seepage channels have been greatly improved. This work provides a basis for micron-scale pore characterization after cyclical microwave modification and contributes to CBM extraction.
ABSTRACT
The application of electrochemical modification for accelerating methane extraction in lean coal seams is limited due to the lack of experimental and theoretical research studies. Therefore, electrochemical modification with different electric potential gradient values was selected to modify lean coals in this study; meanwhile, the amount of methane adsorption and the methane desorption ratio were tested and analyzed. The results showed that the maximum amount of methane adsorption in coal samples decreased after electrochemical modification and the decrease in methane adsorption increased with an increase in electric potential gradient. The methane desorption ratio increased from 83.20% up to 87.84 and 86.90% at the anode and cathode zone, respectively, after electrochemical modification using a 4 V/cm electric potential gradient. A higher electric potential gradient performs better in the electrochemical modification. The mechanism of electrochemical modification using different electric potential gradients was revealed based on the measurements of Fourier transform infrared spectroscopy and liquid nitrogen adsorption. It is due to an increase in acid groups in coal molecular structure and the change of the specific surface area of coal after modification. The results obtained from this work contribute to the methane extraction via the electrochemical method in lean coal seams.
ABSTRACT
The adsorption of CO2 and CO2/CH4 mixtures on kaolinite was calculated by grand canonical Monte Carlo (GCMC) simulations with different temperatures (283.15, 293.15, and 313.15 K) up to 40 MPa. The simulation results show that the adsorption amount of CO2 followed the Langmuir model and decreased with an increasing temperature. The excess adsorption of CO2 increased with an increasing pressure until the pressure reached 3 MPa and then decreased at different temperatures. The S C O 2 / C H 4 decreased logarithmically with increasing pressure, and the S C O 2 / C H 4 was lower with a higher temperature at the same pressure. The interaction energy between CO2 and kaolinite was much higher than that between CH4 and kaolinite at the same pressure. The interaction energy between the adsorbent and adsorbate was dominant, and that between CO2 and CO2 and between CH4 and CH4 accounted for less than 20% of the total interaction energy. The isothermal adsorption heat of CO2 was higher than that of CH4, indicating that the affinity of kaolinite to CO2 was higher than that of CH4. The strong adsorption sites of carbon dioxide on kaolinite were hydrogen, oxygen, and silicon atoms, respectively. CO2 was not only physically adsorbed on kaolinite, but also exhibited chemical adsorption. In gas-bearing reservoirs, a CO2 injection to displace CH4 and enhance CO2 sequestration and enhanced gas recovery (CS-EGR) should be implemented at a low temperature.
