Study On Properties Of Reservoir Sandstone Under True Triaxial Stress

Reservoir rocks are under true triaxial stress??₁>?₂>?₃?conditions as tectonic stresses exist.At present,the study of rock deformation and failure process is based on the conventional triaxial stress state??₁>?₂=?₃?,but in the true triaxial stress environment the deformation and failure characteristics of reservoir rock are different from the conventional tiaxial stress environment.Therefore,the study of rock deformation and failure process under true triaxial stress has important reference significance for the fracturing technology of low permeability reservoirs.In this paper,reservoir sandstone is considered to be research object,experiments and theoretical analysis are combined to systematically analyze the mechanical properties of reservoir rock under true triaxial stress state.A novel multi-functional electric-hydraulic servo-controlled rock true triaxial machine was designed.The apparatus is used to study mechanical properties of artificial homogeneous similar materials and reservoir sandstone under conventional triaxial and true triaxial stresses.The effects of the intermediate principal stress and minimum principal stress on the strength and deformation characteristics of reservoir rocks are discussed.Based on the true triaxial test results,the failure modes of specimens are studied in detail,and based on the rock strength theory,the strength characteristics of rocks are analyzed.The main results are as follows:?1?The multi-functional electric-hydraulic servo-controlled rock true triaxial machine was developed independently.The core device of the machine is the electro-hydraulic servo system.It can keep the deformation speed constant through the failure and deformation of the specimen during the operation of the testing machine,and the device can exert three orthogonal principal stresses??₁,?₂ and?₃?independently on the cubic specimen?100×100×100 mm ³?.?2?Strength and deformation characteristics of artificial homogeneous similar materials and reservoir sandstone under conventional triaxial and true triaxial stress conditions were tested.With the increase of the minimum principal stress,the compressive strength of reservoir rocks increases gradually,which indicates that the minimum principal stress plays a protective role in rock failure;With the increase of the intermediate principal stress,the compressive strength of most specimens increases gradually,while the compressive strength of a small number of specimens increases firstly and then decreases,which indicates that intermediate principal stress plays a protective role firstly and then plays a destructive role.The minimum principal stress restrains the deformation of rock;the intermediate principal stress restrains the deformation of most of the specimens,while the test results of a small part of the specimens show that the intermediate principal stress restrains the deformation of rock firstly and then destroys it.?3?Failure mode analysis of specimens.The analysis of fractured specimens of artificial homogeneous similar materials and sandstone shows that the specimen is mainly shear failure under conventional triaxial stress conditions,and there is a fracture surface in the specimen,and the shape of the fracture surface is related to the confining pressure.With the increase of confining pressure,the fracture surface tends to the middle of the specimen;Under the true triaxial stress conditions,there are two fracture surfaces which strike along the direction of?₂and have a certain angle with?₃ direction in the rock specimen.When the failure characteristics of the specimens are brittle,the shear failure occurs,and the two fracture planes converge at the end of the specimens;When the failure characteristics of the specimens are plastic,the tensile failure occurs,and the two fracture planes are basically parallel.?4?The rock strength theory is used to analyze the rock strength characteristics.The results show:the three strength criteria of Mohr-Coulomb,Drucker-Prager and Mogi-Coulomb have good applicability to the sandstone under the conventional triaxial stress.The Drucker-Prager strength criterion can better describe the rock strength characteristics under the true triaxial stress state.