Experimental study on mechanical properties of rock degraded by hydraulic fracturing

The accurate determination of the degree of mechanical property degradation of rock layers after hydraulic fracturing is the basis for revealing the pressure relief mechanism of hydraulic fracturing and evaluating the fracturing effect. To reveal the degradation law of rock hydraulic fracturing mechanical properties, a two-stage scheme of true triaxial hydraulic fracturing physical model experiment and rock mechanics testing experiment was designed separately In order to study the degradation law of mechanical properties of rock layers before and after hydraulic fracturing, a two-stage plan of true triaxial hydraulic fracturing physical model experiment and rock mechanics testing experiment was designed. CT scanning was used to reconstruct the three-dimensional morphology of fractures. Through true triaxial hydraulic fracturing physical model experiment, rock specimens containing hydraulic fracturing fractures were prepared; Uniaxial and triaxial compression tests, as well as Brazilian splitting tests, were conducted on rock specimens containing hydraulic fracturing fractures. The research results indicate that grooving can effectively reduce the pressure of crack rupture, increase the width of the groove, and improve the probability of crack initiation. However, if the groove spacing is too small, cracks are prone to merge; The higher the viscosity of the fracturing fluid, the faster the pumping pressure rises, and the higher the fracture pressure; The lower the displacement, the greater the proportion of liquid filtration, and the lower the crack rupture pressure. After the displacement increases, the crack initiation pressure significantly increases, and the time required for initiation is shorter. After hydraulic fracturing, the reduction coefficients of internal friction angle, cohesion, and tensile strength of the rock are 0.896, 0.996, and 0.295, respectively; The relationship between the reduction coefficient of elastic modulus was obtained. With the increase of confining pressure, the elastic modulus of both samples showed an increasing trend, and the growth rate gradually slowed down. However, the reduction coefficient showed a decreasing trend with the increase of confining pressure, but the rate of decrease slowed down; The tensile strength of the sample decreased by 70.49% after fracturing, which was 2.39 times that of the sample after fracturing. The significant decrease in tensile strength of the sample indicates that fracturing causes cracks and damage inside the sample. The standard deviation before and after fracturing increased by 1.87 times, indicating that the fracturing process generates a large number of unevenly distributed cracks inside the sample, and there are significant differences in the number and distribution characteristics of micro cracks, resulting in significant mechanical differences in the sample.