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

Abstract

In this study, we conduct laboratory experiments on coal with liquid CO2 phase transition fracturing (L-CO2-PTF) treatment under the fracturing pressures of 120 and 185 MPa. The variations of structure and fractal characteristics for mesopores (2–50 nm) and micropores (<2 nm) are studied by employing the low-temperature N2/CO2 adsorption measurements and the fractal theory. The results indicate that the effects of pore enlarging and fractal dimension reducing of L-CO2-PTF are visible for mesopores, while these effects are underperformed for micropores. The connectivity of mesopores is improved due to the transformation of semi-open pores (type II) and thin-necked bottle pores (type III) into open pores (type I). The size of mesopores is increased, while the pore volume, pore specific area, and fractal dimension are reduced. All these performances are conducive to enhancing the capacity of coalbed methane (CBM) desorption and diffusion. It should adopt different fracturing pressures for coals with different metamorphic degrees. Primarily, it is significant to explore a novel L-CO2-PTF device with considerable energy, high fracturing pressure, and long action time for improving the nanoscale channel connectivity of CBM transport. This study further reveals the effects of L-CO2-PTF on mesopores, micropores, and CBM transport, which proposes theoretical guidance for the improvement and optimization of the L-CO2-PTF technique.

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

“…The raw coal samples ( PDS - R -1, PDS - R -2, and PDS - R -3) and the fractured coal samples ( PDS - F -1, PDS - F -2, and PDS - F -3) were selected and prepared with 60–80 mesh. The pore specific surface area can be analyzed based on the Brunauer–Emmett–Teller (BET) theory, and the pore volume and size distribution can be derived based on the Barrett–Joyner–Halenda (BJH) theory. ,, …”

“…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.…”

“…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 CS2 Solvent Extraction on Nanopores in Coal

“…Actually, micropores in coal display chemical properties, and the micropore walls are atoms in the chemical structure. − Therefore, the increase in chain spacing of the aliphatic side chains expands the pore wall, leading to the micropore-enlarged effect. An aromatic structure accounts for the majority in coal and is the main structure controlling micropores for the high-rank coal .…”

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

“…24 LCPTF can cause the pore-enlarged and fractal-dimension-reduced effects on macro-and mesopores, while it underperforms in the transformation of micropores. 25 Recently, we have revealed the effects of LCPTF on the reduction of adsorption capacity and the increase in desorption capacity as a result of alterations in the structure of nanopores. 26 However, the production of CBM is a dynamic migration process from adsorption to desorption, and the gas desorption volume and gas desorption efficiency show different variation characteristics in different desorption processes.…”