Micro-fractures in coal induced by high pressure CO2 gas fracturing

Effects of Liquid CO2 Phase Transition Fracturing on Methane Adsorption of Coal

“…Liquid CO 2 phase transition fracturing (LCPTF) is a kind of novel waterless fracturing technology with the rapid gasification of liquid CO 2 , generating gas expansion energy for coal damage so as to improve the connectivity of the transport channel and ability of CBM. − In recent years, LCPTF technology has received extensive attention. A series of field application studies have been achieved in improving gas drainage rate, indicating that LCPTF has unique technical advantages for raw materials with a stable source and low cost and the high safety performance as a physical reaction without generating sparks. − However, there is still a lack in comprehensive understanding of the mechanism of LCPTF for ECBM. CBM is mainly adsorbed in the coal matrix and nanopores, and the gas adsorption capacity of coal is one of the most critical factors in CBM exploration and development. ,− The coal pores can be mainly classified with micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm) according to the International Union of Pure and Applied Chemistry as a widely used classification scheme …”

“…Inspired by these studies, recent investigations are devoted to probing the effect of LCPTF on the structure of nanopores in coal. LCPTF can cause the variation of coal by generating fresh pores and fractures. − A scanning electron microscopy (SEM) measurement was employed to examine the variation of microfractures, and the variations of the pore structure after the LCPTF treatment were quantitatively analyzed by mainly adopting the MIP measurement. ,− However, the current studies are focused on exploring the transformed effect of LCPTF on the nanopore structure in coal while its effect on gas adsorption capacity is rarely reported, impeding comprehensively uncovering the mechanism of LCPTF for enhancing CBM recovery and constraining the effect evaluation and field application of LCPTF. Recently, we have found that LCPTF mainly affects the mesopore (2–50 nm) and macropore (>50 nm) of coal and has no obvious transformation effect on the micropore (<2 nm). , Therefore, further work is highly desirable for examining the effects of the structure alterations of mesopores and macropores of LCPTF on methane adsorption of coal.…”

Effects of Liquid CO2 Phase Transition Fracturing on Mesopores and Micropores in Coal

“…In this study, we conducted the L-CO 2 -PTF experiments by adopting the same experimental apparatus and procedures as the study of Cao et al 29 Figure 3 displays the L-CO 2 -PTF experiment system, consisting of a fracturing pipe, an inflation valve, a jet valve, a pressurecontrolled bursting disc, a heater, liquid CO 2 , a simulated coal seam drilling hole device, and a coal sample test holder. The fracturing pipe is a high-strength hollow steel pipe, and its two ends hermetically connect with the inflation valve and the air jet valve.…”

“…In addition, it is realized that the pore structure in coal is complex and diverse with different nanoscales, − directly influencing the properties of storage and transport for CBM. − Accordingly, the current studies focused on comprehensively understanding the mechanism, especially the effect of L-CO 2 -PTF on nanopores in coal and CBM transport. After L-CO 2 -PTF treatment, the microstructure of pores and fractures was altered as the paths and channels for CBM desorption and transport. , A scanning electron microscope was employed to examine the structure variations for microfractures and macropores in coal induced by L-CO 2 -PTF treatment. , Also, mercury intrusion porosimetry (MIP) measurement was used to evaluate the variations of pore structure in coal with L-CO 2 -PTF treatment, and the result indicated the significant influence on macropores (>100 nm) rather than transition pores. − A recent investigation by employing the MIP measurement and its fractal theory demonstrated that the coal pore structure (>10 nm) becomes more homogeneous because the pore fractal dimension is reduced in coal with L-CO 2 -PTF treatment . However, it presents an apparent diversity in the distribution for micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm) in coals with different metamorphism and deformation degrees, ,− based on the widely used IUPAC classification method. , Theoretically, the MIP method can measure the pores of 3.5–10 6 nm.…”

“…14,28 A scanning electron microscope was employed to examine the structure variations for microfractures and macropores in coal induced by L-CO 2 -PTF treatment. 29,30 Also, mercury intrusion porosimetry (MIP) measurement was used to evaluate the variations of pore structure in coal with L-CO 2 -PTF treatment, and the result indicated the significant influence on macropores (>100 nm) rather than transition pores. 31−33 A recent investigation by employing the MIP measurement and its fractal theory demonstrated that the coal pore structure (>10 nm) becomes more homogeneous because the pore fractal dimension is reduced in coal with L-CO 2 -PTF treatment.…”