In the drilling and stimulation operations of deep/ultra-deep oil and gas wells, high in situ stress conditions may increase the occurrence probability of rock shear failures. According to Mole-Coulomb laws, the friction coefficient becomes more significant in evaluating borehole instability, faults/natural fractures activation and hydraulic-natural fractures intersection scenarios. This paper proposes a multiscale model to calculate the rock friction coefficients based on the surface properties at different scales. The key parameters of surface properties are obtained from three-dimensional laser scan of friction planes, and the results are verified by the direct shearing and triaxial compression strength tests. For the flat or new-cutting surfaces, the computed basic friction coefficients range of 0.231–0.509, fitting well with the tested values of 0.303–0.437. It also shows that the basic friction coefficients grow in the order of shale, carbonate and tight sandstone, positively associated with the size of the rock sedimentary particles. For the roughness surfaces, the coefficients are computed based on the former basic friction values and the surface asperity dip angles distribution, and the values located in a wide range of 0.541–1.113, also matched well with the measured friction coefficients via direct shearing tests. When the normal stress increase, the rock friction coefficients generally decline, and the values of some shale and dolomite samples can decrease to 0.2, due to the existence of beddings or fracture fillings. The above outcomes may provide useful insights for wellbore instability assessment and hydraulic fracturing optimization.
