Experimental Study On Supercritical CO₂ Fracturing Enhancing Permeability In Shale

Efficient extraction of shale gas can effectively supplement and replace conventional energy consumption and is of strategic importance for ensuring energy security.Scholars have proposed a new waterless fracturing technology-supercritical CO₂-enhanced shale gas extraction with high efficiency for shale gas reservoirs with high clay content in water-scarce areas.In this paper,using the group’s self-developed and improved supercritical CO₂ fracturing Seepage test platform,we conducted an experimental study of supercritical CO₂ fracturing shale considering the influence of CO₂ injection displacement and the influence of confining pressure and an experimental study on supercritical CO₂ enhancing permeability in Shale using the outcrop shale of Longmaxi Formation in Changning,Sichuan,achieved the following conclusions.（1）Through the test of shale mechanical properties,it is confirmed that the mechanical anisotropy of shale is quite different,and the characteristic laws of the mechanical properties of two kinds of layered shale specimens are obtained.The parallel layered shale specimens are determined as the research object of this paper.（2）By comparing the pressure-time curves of the injected CO₂ fluid damaged by the fracturing of the shale specimen,it can be seen that when the injection pressure reaches 7.4MPa,the phase state of CO₂ changes,from the gaseous state to the supercritical state,and the fluid compressibility becomes smaller.The instantaneous pressurization rate increases,the curve increases sharply,the CO₂ fluid pressure reaches a peak value,and the shale starts to crack.During the fracturing process of injecting CO₂ fluid,the pressure in the open-hole section changes with time into five stages:the open-hole section full stage,the gentle pressurization stage,the phase change pressurization stage,the crack initiation,and instability stage,and the dynamic equilibrium stage.（3）Under the experimental environment condition of 45℃,the fracture initiation pressure of shale specimen increases with the increase of CO₂ fluid displacement,and the fracturing initiation pressure of shale specimen under constant injection displacement increases with the increase of confining pressure and increase.During the fracturing experiment,the sensitivity to pressure change showed a linear increasing trend with the increase of the injected CO₂ fluid displacement,and the sensitivity to the pressure change of the fracturing shale experiment under a constant injection displacement showed a linear increasing trend with the increase of the confining pressure.Taking the first group of fracturing experiments as an example,the pressure change sensitivity increased by 0.0728MPa?s^-1 and 0.0387MPa?s^-1,respectively,and the change rate of the pressure change sensitivity was 191.08%and 34.89%.（4）Supercritical CO₂ fracturing of shale specimens can lead to"V","Y"and"X"type complex fractures on the lower surface of the specimen,and multiple fractures on the lateral surface of the specimen after fracturing.With the increase of CO₂ injection,the width of the main fracture of the fractured shale specimen decreases step by step,and the number of main fractures on the side surface of the fractured specimen increases with the increase of the surrounding pressure,and the distribution of fractures becomes more complex.The surface fracture expansion pattern of fractured shale specimens affects the permeability enhancement effect of the fracturing test,and the overall N₂ permeability increases by 8-9 orders of magnitude after fracturing shale specimens,and the permeability enhancement effect of supercritical CO₂ fracturing shale is significant.There are 46 figures,20 tables and 106 references.