The Effects of Quartz Content, Particle Size and Distribution Mode on the Mechanical Properties and Fracturing of Shale

The study of shale mechanical characteristics on the microscopic scale can provide a theoretical basis to evaluate shale's macroscopic mechanical properties and fracability. Quartz is a rigid mineral that exists extensively in shale, and quartz of various types exerts different influences on the quality of shale oil/gas reservoirs due to their different microscopic occurrence states. However, this difference lacks rigorous quantitative evaluation and mechanism explanation. Therefore, using the finite element numerical simulation method, three classes containing different models were set up based on microscopic occurrence states of quartz observed under a scanning electron microscope to solve these problems. When quartz content is high (about more than 45%), the effect of quartz grain size and occurrence mode on shale compressive strength is gradually significant, exceeding the influence of quartz content on shale compressive strength, and this provides scientific evidence that authigenic microcrystalline quartz (Qm) featured by small particle size and distributed in an aggregate state can effectively enhance shale compressive strength and thus protect the primary intergranular pores of shale. Quartz content has the most significant influence on shale brittleness, and Qm cannot greatly improve shale brittleness, which is inconsistent with previous opinions. The abundant brittle minerals rather than Qm particles are the “underlying cause” of good brittleness of shales rich in Qm. The shale containing more quartz particles of small size can form long hydraulic fractures and thus form a larger volume of complex fracture network.