Experimental of the influence of mineral composition and normal stress on the frictional slip of shale

Abstract

Natural fractures are widely distributed in shale reservoirs. During the hydraulic fracturing process, frictional slip occurring on natural fractures can increase the reservoir permeability and is of great significance to improve the efficiency of reservoir stimulation. Shale contains a large amount of clay and organic matter, and its frictional behavior is different from that of other previously studied lithologies. In this paper, the frictional behavior of shale is analyzed, and the results show that the frictional behavior is controlled by the content of clay and organic matter. As the content of clay and organic matter increases, the micro support type transforms from the particle support mode by hard quartz mineral to matrix support mode by plastic clay and organic matter. Accordingly, the shear strength and friction coefficient of shale both decrease, and the shear type transforms from brittle to plastic. When the content of clay and organic matter is low, the asperity of friction surfaces will break in a brittle manner and the wear degree of surfaces is low. Therefore, fractures are still featured by moderate apertures after friction. The lower the content of clay and organic matter is, the easier the asperity of crack surfaces supports themselves, and the higher the fracture residual permeability is. Thus, promoting shear slip is the main measure of reservoir stimulation. However, when the content of clay and organic matter is high, the remaining post-slip fracture aperture is small. It is difficult to increase reservoir permeability through the frictional slip of natural fractures, and in this situation, the proppant support efficiency needs to be improved.