Study on mechanical properties and wellbore stability of deep sandstone rock based on variable parameter M-C criterion

With the depletion of shallow oil and gas resources and the increase in fossil energy consumption, deep oil and gas resources have become the main driving force of economic development. However, the physical and chemical properties and rock mechanics characteristics of the deep sandstone stratum are remarkable, and there is little research on the rock failure law of the deep sandstone stratum. Therefore, a batch of sandstone with similar homogeneity was selected, and several triaxial mechanical experiments with variable confining pressure were conducted for such sandstone. The mechanical constitutive model of rock samples under different confining pressures, the macroscopic failure characteristics of rock samples, and the evolution law of wave velocity and internal energy of rock samples during mechanical loading were analyzed. According to the experimental results of triaxial mechanics, a calculation model of wellbore collapse pressure in deep sandstone formation was established based on the variable parameter Mohr-Coulomb criterion. The results show that the crack propagation process of rock samples under mechanical loading can be divided into four stages. The characteristic strength, peak strain, elastic modulus, and deformation modulus of rock samples were positively correlated with confining pressure and showed obvious nonlinearity in high confining pressure areas. The failure mode of the rock sample changed from brittle failure to plastic brittle failure with the increase in confining pressure. According to the characteristics of the stress-strain curve, the variation of acoustic wave velocity during triaxial mechanical loading can be divided into three different types. The variation trend of acoustic wave amplitude of rock samples is consistent with the stress-strain curve. With the loading of axial stress, the elastic energy curve was a concave function, the curve of dissipated energy started increasing at the cracking stress of rock samples. After destroying the rock sample, the decline slope of the dissipated energy curve decreased with the increase in confining pressure. Considering the limited failure of the borehole wall, an evaluation model of borehole wall stability in deep sandstone formation based on variable parameter M-C criterion was established. The equivalent density of borehole collapse pressure predicted using the M-C strength criterion method with variable parameters was smaller than that predicted with traditional methods. The research results provide a theoretical basis for safe and efficient drilling in a deep sandstone formation.