Fracture Propagation Modes of Lower Cambrian Shale Filled with Different Quartz Contents under Seepage-Stress Coupling

Abstract

The content and spatial distribution of brittle minerals, such as quartz, are important factors in determining the fracture initiation mechanism induced by hydraulic fracturing in shale reservoirs. To further research the impact of quartz content in shales of the Lower Cambrian Niutitang Formation in northern Guizhou on the fracture expansion of its reservoir, 7 groups of randomly filling shale models with different quartz contents were established using rock failure process analysis (RFPA2D-flow) code for numerical test studies under seepage-stress coupling, and 5 samples were also subjected to uniaxial compression tests using the INSTRON 1346 electrohydraulic servo-controlled material testing machine (200T). The results show that the average growth rate of the compressive strength and the fracture proportion for a quartz content of 50% to 65% are 4.22 and 1.15 times higher than those for 35% to 50%, respectively. Fractures sprout, expand, and breakdown in the shale matrix or at the junctions of the shale matrix and quartz grains. The mechanical properties and pattern of the fracture extension of the shale in the physical tests are similar to those in the numerical tests, indicating the reliability of the numerical simulations. The fractal dimension curves at different stress levels are divided into three stages: flattening, increasing, and surging, and the fractal dimension value for a quartz content of 50%~65% at a 100% stress level is 1.02 times higher than that for 35%~50%. The high degree of natural fracture development in high quartz content formations in shale gas reservoirs is of some reference value for logging data. The research results provide a reference value for the content and spatial distribution of brittle minerals for the initiation mechanism and fracture propagation of hydraulic fracturing in shale reservoirs.