Evolution characteristics on coal fractures induced with CO2 phase transition fracturing based on CT scanning
This study conducted the CO2-PTF coal experiment to further reveal the fracturing transfor-mation mechanism of CO2 phase transition fracturing(CO2-PTF)coal.According to the CT scanning and 3 D fracture reconstruction,we analyzed the fracture structure parameters of coal before and after CO2-PTF,and clarified the evolution characteristics of the three-dimensional fracture structure of coal induced by CO2-PTF.The research results indicated that after CO2-PTF,the total number of fractures in the coal sample decreased,while the total volume and surface area of fractures increased.The CO2-PTF generated fracture expansion and transformation effects where the small-scale fractures were ex-panded and transformed into larger scale fractures under the CO2-PTF pressure.The number,volume,and surface area of fractures of less than 1 000 μm in length were significantly reduced,while the vol-ume and surface area of fractures of longer than 1 000 μm in length were significantly increased.The expansion and connection between fractures caused a decrease in their quantity.CO2-PTF improves the connectivity of the three-dimensional fracture in coal and is conducive to gas migration and produc-tion.This study offers new insights into and evaluation method for the effect of CO2-PTF,and could provide references for the research on fracture evolution characteristics in other unconventional natural gas reservoirs and their modifications.
CO2 phase transition fracturingCT scanningcoal reservoirthree dimensional fracture evo-lution
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