Study On Microscopic Deformation Mechanism In Sandstone Core In-Situ Loading Experiment

Due to the stress sensitivity of oil and gas reservoirs,when the effective stress on the rock of the reservoir changes,the microstructures such as pores,throats and cracks in the rock are deformed,resulting in a decrease in the porosity and permeability of the rock.The macro-scale deformation of rock is closely related to its microstructure,so it is particularly important to study the deformation and failure mechanism of rock at micro-scale.In this paper,in-situ compression scanning electron microscopy experiment and three-point bending scanning electron microscopy experiment were carried out on the berea sandstone samples using field emission scanning electron microscope and in-situ tensile rig.Then,the digital image correlation method was used to quantitatively analyze the degree of sandstone deformation.The results show that the strain concentration of sandstone under uniaxial compression mainly appears in the larger interdetrital pores and throats.As the load increases,new microcracks parallel to the loading direction are generated around the interdetrital pores.Some vertical cracks or throats communicating with the larger interdetrital pores are gradually closed under the action of horizontal loads.In the sandstone three-point bending experiment,the length of the fracture process zone gradually increased with the increase of the load,and the maximum length of the fracture process zone reached 1504.74 ?m before the peak load.When the load reaches the peak load,macro-cracks begin to form at the prefabricated crack opening,the fracture process zone moves forward as a whole,its length gradually decreases,and the macrocracks accelerate to expand forward until the sample is unstable and destroyed.The crack mouth opening displacement gradually increases with the increase of the load.The crack mouth opening displacement at the peak load is the critical crack mouth opening displacement,and its size is 4.62 ?m.