Response Mechanism Of Physical Properties And Structure Of Shale Gas Reservoir Treated By Supercritical CO₂

The shale gas resources in our country are abundant,with estimated technically recoverable shale gas reserves of 21.8×10¹² m³.The efficient development of shale gas resources in our country can alleviate the problem of domestic natural gas energy supply.Currently,the technology of shale gas exploitation mainly includes hydraulic fracturing and anhydrous fracturing.Supercritical CO₂（SC-CO₂）fracturing,as an anhydrous fracturing technology,has attracted considerable attention in recent years,which can not only avoid a series of problems（such as huge water resource consumption,serious environmental pollution and shale reservoir damage）caused by conventional hydraulic fracturing,but also can achieve the purpose of CO₂ sequestration and enhanced shale gas recovery（CS-ESGR）.In view of the special physical and chemical properties of SC-CO₂,the injection of SC-CO₂ into shale reservoirs will change the original physical and chemical characteristics,adsorption characteristics,and crack propagation law of shale,which will affect the effect of shale gas recovery and CO₂ sequestration.Based on previous studies,the influence of SC-CO₂ treatment on the physical properties and structure of shale reservoir were systematically and deeply studied on shale collected from the Longmaxi formation in Sichuan Basin（marine shale in Changning County,Yibin City,Sichuan Province）and the Chang-7 member of the Mesozoic Yanchang Formation in Ordos Basin（Continental shale in Yanchang County,Yan’an City,Shaanxi Province）in this paper.The experimental and theoretical studies on the effect of SC-CO₂ on the mineral components and pore structure of shale,the influence of SC-CO₂ on wettability of shale,the impact of SC-CO₂ on the adsorption characteristics of CH₄,CO₂ and their binary mixtures of shale,and the crack initiation and propagation mechanism of SC-CO₂ fracturing shale were carried out,respectively.The main innovations are as following:（1）The mechanism of the effect of SC-CO₂ on mineral composition and pore structure of shale was explained.After SC-CO₂ treatment,the total organic carbon（TOC）content,carbonate minerals,clay minerals,and specific surface area of shale decreased,while the average pore size increased.The main reason for the change of the petrophysical properties of shale is that some organic matter and carbonate minerals in the shale were dissolved and the clay minerals were dehydrated in the process of SC-CO₂-shale interaction,resulting in a transformation of some microporous and mesopores in the shale into macropores and an increase in the number of macropores.（2）Variations of wettability of shale reservoir after SC-CO₂ injection were revealed.Shale samples of Longmaxi Formation and Yanchang Formation are hydrophilic rocks.After SC-CO₂ treatment,the changes of shale-water contact angles of the two formations were basically the same,and the overall trend was increasing.Among them,the influences of SC-CO₂ treatment time and pressure were obvious,while the effect of temperature was not apparent.The main reason for the weakening of shale water wettability after SC-CO₂ treatment resulted from a reduction of clay minerals and CaCO₃ content in the shale,and the dissolution degree of clay minerals and CaCO₃ in shale by CO₂-water-shale interaction was various under different treatment time,pressure and temperature.After SC-CO₂ treatment,the weakening of the water wettability of shale will lead to a decrease of interaction force between shale and water molecules,and the flow resistance of water in the shale minimal pores and fracture will decrease accordingly.（3）The mechanism of the replacement of CH₄ by CO₂ after the injection of SC-CO₂ into shale reservoir is described.Before and after SC-CO₂ treatment,the adsorption capacity of shale to CH₄,CO₂ and their binary mixtures increased with the increase of CO₂ concentration in the mixed gas.The adsorption capacity of shale for CH₄,CO₂ and their binary mixtures reduced after SC-CO₂ treatment and is mainly attributable to decreases in the TOC,clay mineral and specific surface area of shale.The selective adsorption coefficients of CO₂ over CH₄(S_CO2/CH4)were slightly reduced after SC-CO₂ treatment.S_CO2/CH4 initially decreased and then tended to be stable with increasing adsorption pressure,and slightly decreased with the increase of CH₄concentration in the mixed gas.It is therefore suggested that the reservoir pressure should be appropriately reduced before the injection of CO₂ into shale reservoir,and the efficiency and engineering cost of CS-ESGR should be considered comprehensively for putting forward a reasonable mole ratio of CH₄/CO₂.（4）The mechanism of crack initiation and propagation in the process of SC-CO₂fracturing shale was revealed.Using a self-developed physical simulation system equipped with acoustic emission（AE）and computed tomography（CT）system,the physical simulation experiment of SC-CO₂ fracturing shale under conventional triaxial conditions was implemented.A simplified fracture initiation pressure model was established according to the crack initiation criterion of SC-CO₂ fracturing model,and the theoretical fracture initiation pressure of SC-CO₂ fracturing shale was obtained,which was basically consistent with the experimental value.The fracture initiation pressure increased with the increase of reservoir buried depth,and the pre-existing fracture decreased the fracture initiation pressure.Under conventional triaxial stress conditions,the initiation position and the propagation direction of cracks were random in the process of the SC-CO₂ fracturing of shale and were along the direction of low-strength direction or pre-existing fractures.The cumulative acoustic emission hits increased step by step in the stages of shale crack initiation,propagation and damage,and the numbers of the stepped-jumping of the cumulative hits were equal to the number of shale damage.The research results of this paper provide theoretical support for the efficient exploitation of shale gas by SC-CO₂,and further enrich and perfect the fundamental theory of shale gas exploitation by SC-CO₂,which supplys important scientific significance and engineering practical value for the efficient exploitation of shale gas in China.