Study On The Mechanism Of Cryogenic-induced Fractures In Shale

The permeability of shale reservoirs is exceedingly low,necessitating the artificial creation of complex fractures for the extraction of shale oil and gas.Utilizing the ultra-low temperature properties of liquid nitrogen（LN₂）to induce branching fractures in reservoir shale is a new method for enhancing the efficiency of fracturing stimulation.However,the continental shale exhibits a widespread development of bedding and weak planes,along with significant anisotropic characteristics.The mechanism of cryogenically induced fractures in shale formation is still unclear.This paper introduces innovative experimental equipments and methods for conducting uniaxial compression experiments and visualization of fracture propagation in cryogenic environments.The study focuses on the formation mechanism of cryogenic-induced fractures in shale and examines the feasibility of using LN₂ to induce complex fractures in formation shale.（1）The influence of LN₂ freezing rate on shale thermal damage was studied through shale mechanical property tests and numerical simulation of heat transfer.The compressive strength and elastic modulus of shale present a nonlinear relationship with freezing rate,and the empirical formulas was obtained.The main mechanism by which freezing rate affects the thermal damage of shale has been clarified.A method is proposed to characterize the thermal damage of shale by using the average values of the peaks of thermal stress gradients at various points inside the shale,providing a reference for studying the damage and failure characteristics of rock materials induced by thermal stress.（2）The visualization experiments of continuous propagation of cryogenic-induced fractures were designed and carried out,monitoring the continuous propagation process of cryogenic-induced fractures for three modes:S-mode（splitter）,A-mode（arrester）,and D-mode（divider）.Comparative analysis was conducted on the differences in the expansion ability of three fracture propagation modes.The transverse isotropic shale thermal-mechanical coupling numerical simulation models were established,clarifying the mechanical mechanisms and criterion differences of fracture propagation for three modes induced by cryogenic temperature.The research provides a basis for accurately describing and predicting the propagation behaviors of cryogenic-induced fractures in shale.（3）A visualization experimental device for 2D fracture propagation has been developed,and visualization experiments have been conducted on the formation of cryogenic-induced fracture networks under confining pressure conditions.A characterization method for complex fracture networks based on DIC strain was proposed.The influence and mechanism of multiple factors such as cyclic cryogenic treatments,bedding planes,confining pressure,pore water and its phase transition on the morphology of cryogenically induced fractures in shale have been clarified.The failure mechanism of cryogenic-induced fractures was studied based on AE parameters in shale.This provides a basis for studying the complex fracture network induced by cryogenic temperature of LN₂ in formation shale.（4）The indoor experiments were conducted on the generation of fractures induced by cryogenic LN₂.It was found that liquid nitrogen can induce the formation of a leaf venation-like fracture network.The propagation ranges of the complex fracture networks induced by cryogenic temperature have been determined.A mixed fracturing method combining conventional fracturing and LN₂ fracturing was proposed.Based on the geological and mechanical conditions of reservoir shale in the Ordos Basin,the propagation lengths of branch fractures induced by cryogenic temperature in shale at different depths was calculated,and the critical burial depths of different types of branch fractures induced by LN₂ was obtained.The research results provide a basis for the design of LN₂ fracturing.