A micromechanical study of shale rock-proppant composite interface

The micromechanical properties of shale rock-sand type of composite interface were investigated by performing laboratory tests with a custom-built loading apparatus. This type of interface is of significant interest in petroleum engineering in the process of hydraulic fracturing using proppants. Two different types of quartz sands of different particle shapes were used as proppant simulants and they were sheared against rough and smooth surfaces of shale. It was shown that sand grain type and surface roughness of shale affected the contact stiffness in the normal and tangential directions because of their influence on the induced contact stresses. Based on the micromechanical shearing tests, it was found that the coefficient of friction, the initial tangential stiffness and the microslip displacement were positively correlated with the magnitude of normal load. Although clear surface damage/plowing of the shale surface caused by shearing was observed and captured by interferometry images, the effect of loading history was found to be small. Shearing tests on wet and dry shale surfaces did not show that much difference in results, but immersion of shale in water prior to shearing affected the initial tangential stiffness and coefficient of friction at certain normal loads. Smoothening of the surface had an influence on friction as long as limited plowing was triggered in the low range of normal loads, so that the influence of surface roughness of the shale would diminish at higher normal loads where plowing mechanism dominated the interface response.