Experimental study of effects of shearing on proppant embedment behaviour of tight gas sandstone reservoirs

Proppants play an important role in propping fractures open after pumping ceases during horizontal drilling and hydraulic fracturing. However, some proppant degradation mechanisms, mainly proppant embedment, cannot be avoided in deep underground reservoirs, and greatly reduce hydraulic conductivity and production. To make the situation even worse, rock shearing associated with fault activation during the hydraulic fracturing process inevitably generates shear stress or even causes rock shear slip, which may enhance proppant embedment. Although no study to date has focused on this area, it is vital to investigate the influence of shear stress and the corresponding movement on proppant embedment. Therefore, a series of tests was conducted for rocks with two joint roughnesses. After the experiments, rock samples were scanned and joint surface profiles were plotted in 3-D Surfer. The results show that embedment increases with horizontal displacement for both types of rock joint due to rock dilation. Furthermore, embedment increases with increasing shear stress after the stress limit is exceeded. In addition, the proppant greatly reduces rock shear resistance, including peak shear strength and shear stiffness. The addition of proppant can also lead to the reduction of peak and residual friction angles, and accelerate the reduction of dilation angle. Quantitative analysis was conducted utilizing three correlation coefficients to measure the relationship between rock shear strength and influencing factors (proppant addition, surface roughness and fluid effect). The results extend the understanding of rock shear performance and proppant behaviour optimization during horizontal drilling and hydraulic fracturing.