Fracture Propagation Behavior And Microscopic Mechanism Of CO₂ Fracturing In Shale Reservoirs

The initiation pressure of CO₂ fracturing is low,which can form the relatively complex fracture network and reduce reservoir damage.CO₂ fracturing is technically feasible for reservoir stimulation in Chang 7 shale reservoir with low permeability,low pressure and strong water sensitivity.The fracture formation mechanism of CO₂fracturing in shale reservoirs is complex.During the fracturing process,the interaction of CO₂-fluid-rock damages the microscopic pore-throat structure,and the mechanical properties of the rock are deteriorated,causing the fracture to expand.Therefore,it is of great significance to clarify the fracture formation and propagation mechanism of CO₂fracturing in shale reservoir and the microscopic mechanism of CO₂-fluid-shale reservoir interaction for efficient fracturing of continental shale reservoirs.The geological background of the formation of Chang 7 shale reservoirs is complex,and the characteristics of different types of shale reservoirs are diverse.The characteristics of core samples from different types of shale reservoirs are classified and evaluated by X ray diffraction analysis,casting thin sections（CTS）,field emission scanning electron microscopy（FE-SEM）,triaxial compression experiments,acoustic testing,and Brazilian splitting experiments.The cores are selected to carry out triaxial simulation fracturing experiments to analyze the formation and propagation mechanism of CO₂ fracturing shale reservoirs.The microscopic mechanism of CO₂-fluid-shale reservoir interaction is systematically studied by NMR technology.Combined with the deterioration degree of rock mechanical properties,the key factors controlling the formation of complex fracture networks in CO₂ fracturing-shale reservoirs are revealed.The main conclusions are as follows:（1）The initiation pressure of liquid CO₂ fracturing in shale reservoirs is low.After fracture initiation,shale bedding diverts the direction of fracture propagation and forms branch fractures along the bedding plane.The natural fractures of shale communicate with artificial fractures to form a complex fracture network.The initiation pressure of SC-CO₂ fracturing in shale oil reservoirs is low,the scale of fractures formed is limited,and the formation of secondary fractures is only induced at the end of the main fracture expansion.CO₂ fracturing in shale reservoirs can communicate shale bedding planes,but the inter-fracture interference caused by shale bedding restricts the propagation behavior of fractures.（2）The interaction of CO₂-fluid-shale reservoirs damages the microscopic pore-throat-fracture system of the rock,and the damage degree is between 0.92%and11.93%.The damage of low temperature CO₂ is more obvious,especially for the larger pore throats,the damage degree are between 0.11%and 8.70%.The supercritical CO₂-fluid-shale reservoir action on smaller pore throats increases the damage degree.Shale with a high degree of natural microfractures reacts with supercritical CO₂ and fluid,and the microfractures with a distribution range of 132.19-613.59 ms expand.（3）After CO₂ fracturing in shale reservoirs,the elastic modulus,compressive strength and tensile strength of rock all decrease,and the loss rate of compressive strength and tensile strength can reach 21.59%and 20.82%at the highest.The strength of the rock near the main fracture after CO₂ fracturing is lower than that of the rock near the branch fracture.The damage degree of CO₂-fluid-shale reservoir interaction on rock is more obvious than that of CO₂ fracturing.The physical and chemical interaction between CO₂ and shale changes the mineral composition,grain skeleton and pore throat structure of the rock,and the mechanical properties of the rock are seriously damaged.（4）For type I and type II shale reservoirs with good physical properties and high development of pore throats,the deterioration of rock pore throat structure and mechanical properties caused by CO₂-fluid-shale reaction is the key controlling factor for the formation of complex fracture networks by CO₂ fracturing.The fracturing effect can be improved by increasing the supercritical CO₂ fracturing temperature and prolonging the CO₂-fluid-rock interaction time.Type III and type IV shale reservoirs with poor physical properties and low pore throat development can form complex fracture networks under the combined action of liquid CO₂ fracturing and liquid CO₂-fluid-shale reservoirs.The displacement and pressure can be increased to improve the fracturing effect of liquid CO₂ fracturing.