Fracture Initiation And Propagation Of Tight Sandstone When Fractured By Supercritical CO₂

Supercritical CO₂（SC-CO₂）fracturing has a promising application prospect in the development of unconventional oil and gas reservoirs,which is beneficial to alleviating water resources consumption,reducing reservoir and environmental pollution,decreasing rock initiation pressure,and achieving partial carbon storage.Since SC-CO₂has many unique physical and chemical properties,its microscopic interaction mechanism with rock,fracture initiation mechanism,and propagation characteristics in the fracturing process need to be further studied.In this research,the key issues with respect to the influence of SC-CO₂ on rock properties,initiation and propagation of fractures are investigated by combining theoretical analysis,laboratory experiment and numerical simulation.The main research work and results are as follows:The interaction experiments of SC-CO₂ with reservoir fluid and tight sandstone were carried out,and the changes in rock mineral composition,microscopic morphology,mechanical properties,porosity and permeability were analyzed before and after the SC-CO₂ interaction,the microscopic interaction mechanism between SC-CO₂ and rock are revealed as well.The results show that the relative contents of feldspar minerals and carbonate minerals decrease,while the contents of quartz and clay minerals increase after interacted by SC-CO₂;The rock strength is deteriorated,the rock porosity decreases,and the permeability increases;Compared to dry conditions,the existence of water and formation brine will aggravate the changes of rock property;The microscopic interaction mechanisms between SC-CO₂ and tight sandstone are of rock mineral dissolution,induction of new mineral precipitation,and rock expansion.A series of SC-CO₂ fracturing experiment with tight sandstone were carried out,and a set of fracture quantitative characterization methods were proposed based on the CT scanning images of fractured rock samples.The quantitative properties of fractures induced by SC-CO₂ were studied,and the influences of in-situ stress and bedding plane on fracture initiation were analyzed.The results show that compared with the water-based fluid,the SC-CO₂ can reduce the initiation pressure of the rock by nearly 62%.And the average fracture width induced by SC-CO₂ is 0.304 mm,which is slightly lower than that of the water fracturing.The roughness,complexity,volume fraction and other parameters of fractures are higher than those of water fracturing;Futhermore,the fracture initiation pressure is approximately linear with the minimum principal stress.With the increase of the principal stress difference,the complexity of fractures induced by the SC-CO₂ is reduced;When the bedding existing,the fractures are mainly initiated along the direction of the original bedding planes.The fracture propagation characteristics in SC-CO₂ fracturing were systematically studied based on the true-triaxial fracturing experimental system.The visual experimental results show that the fluid phase transition after fracture initiation is the main mechanism for inducing complex fractures in the process of SC-CO₂ fracturing.With the increasing of horizontal stress difference,the breakdown pressure of the rock gradually decreases,and the fracture morphology induced by SC-CO₂ becomes simpler;As the improvement of formation temperature,the breakdown pressure of the rock will decrease,and the more complex fractures are induced as well.Based on the phase-field method,a thermal-hydro-mechanical coupling fracturing model was established by considering the physical properties of SC-CO₂,heat transfer,and rock damage.The simulation results verify that SC-CO₂can reduce the breakdown pressure of the fromation and induce the more complex fractures.During the CO₂injection process,a temperature transition zone will be formed near the wellbore.Although the formation displacement produced by SC-CO₂ fracturing is less than that of water,the porosity and permeability of formation induced by SC-CO₂ fracturing are higher than that of water.With the increase of in-situ stress difference and fluid temperature,the conductivity of fractures induced by SC-CO₂ is weakened,while with the increase of fluid pumping rate,the conductivity of fractures induced by SC-CO₂ is increased.Based on the fracture quantification analysis method,the initiation mechanism and propagation characteristics of fracture in SC-CO₂fracturing on tight sandstone are revealed in the research.And the research results can provide a theoretical basis and reference for the parameter design of SC-CO₂ fracturing operations.