No abstract

Micro-structures of the pore-fracture size distribution of target sandstone samples are studied through high-pressure mercury intrusion (HPMI) experiments, and the compressibility coefficient is quantitatively described by using overlying pressure porosity−permeability tests. Then, four single and multifractal models were used to quantitatively describe the fractal characteristics of mercury intrusion curves, and the relationship between different fractal models and pore structure parameters is analyzed. Furthermore, the applicability of fractal models in characterizing pore-fracture structures was explored. The results are as follows: (1) fractal model results show that fractal dimensions of type A by using Sierpinski (D S ) and thermodynamics models (D M ) are larger than those of type B, and fractal dimensions of type A by using Menger (D M ) and multifractal models (D −10 −D 10 ) are smaller than those of type B. (2) The Menger model is used to describe heterogeneity of smaller pore size distribution, which is proportional to volume percentage of pores with the diameter smaller than 100 nm; the thermodynamic model is used to describe heterogeneity of medium pore size distribution, which is proportional to the volume percentage of pores with a diameter of 100∼1000 nm; The Sierpinski model is used to describe heterogeneity of larger pore size distribution, which is proportional to volume percentage of pores with the diameter larger than 1000 nm; (3) permeability decreases in the form of power function with the increase of confining pressure, and the compressibility coefficient and permeability variation coefficient decrease with the increase of the compressibility coefficient. There is no significant correlation between the compressibility coefficient, permeability variation coefficient, and pore structure parameters.

Ultrasonic propagation in coal seams is accompanied by heat transfer, which has the potential to increase coal seam permeability, enhance coalbed methane (CBM) recovery, and prevent coal and gas outbursts. Therefore, it is important to analyze the effects of heat transfer by ultrasonic vibration on CBM recovery and evaluate the prospects of engineering applications of ultrasonic heating technology. In this study, the factors affecting heat transfer by ultrasonic vibration were analyzed theoretically; then, CBM recovery under the condition of ultrasonic heating was simulated by establishing a coupled acoustic–thermal–mechanical–hydrological model. The correctness of acoustic–thermal model was validated by matching simulated data with experimental data. And the accuracy of the gas flow model was verified by comparing the production data from CBM extraction boreholes with the simulated data. The research results were as follows: heat transfer by ultrasonic vibration was affected by frequency and sound pressure. When the ultrasonic frequency varied from 30 to 40 kHz and the sound pressure varied from 0.1 to 0.12 MPa, the lower the frequency, the higher the sound pressure, and the better the ultrasonic vibration heat transfer effect. In addition, the thermal expansion volumetric strain of the coal matrix caused by the ultrasonic heating of coal seams was weaker than the shrinkage volumetric strain of the coal matrix caused by gas desorption, improving the porosity and permeability of the coal seams. Furthermore, the gas drainage standard area increased by 20.8 m2 after 720 days of CBM recovery when replacing conventional CBM recovery with ultrasonic-assisted CBM recovery. With a production time of 720 days, the maximum production of CBM after ultrasonic excitation at a frequency of 40 kHz and a sound pressure of 0.10 MPa increases from 3744 to 9740 m3/day compared to conventional excitation. Our fully coupled acoustic–thermal–mechanical–hydrological model can improve current understandings of heat and mass transfer in thermal simulation of ultrasonic-enhanced CBM recovery.