Experimental study the impact of loading deformation characteristics and mineral composition on shale rock brittleness

In the exploitation of shale gas, the brittleness not only directly influences the fracturing effect of shale and the stability of wellbore, but also is an important index for the evaluation of the sweet pot of shale reservoir.Deformation characteristic (stress-strain curve) of shale was combined with the mineral components of shale to analyze the change rule of shale brittleness, and brittleness index obtained by three kinds of different indexes was compared.The shale deformation characteristic was obtained by uniaxial and conventional tri-axial compression tests, and the shale mineral components was determined by the XRD and XRF techniques.The results show that the brittleness index of shale decreases with the increase of confining pressure, the fracture surface presents a decreasing trend with the increase of confining pressure, and the trend of brittle to ductile transition.The brittleness index presents a linear increasing change rule with the brittle mineral content.During the reservoir fracturing layer selection and the evaluation of fraccability, the influence of in-situ stress state and mineral composition should be comprehensively considered.Brittleness indexB₃ and B₁have a better correlation than B₂ and B₁.In comparison with North America, which widely uses the content of quartz to evaluate brittleness index, the complex of mineral components in southern China Paleozoic shale, the quartz, calcite and dolomite used as brittle mineral content for brittleness evaluation is more suitable.