Experimental Study on the Expansion Mechanism of Secondary Fractures Formed by Shale Hydration in Fracturing Development

Shale reservoirs are usually combined with volumetric fracturing to realize economic development. After different types of fracturing fluids (guar gum, slick water, etc.) enter the reservoirs, hydration effects occur in the shale reservoir and secondary fracture expansion. Currently, there are fewer studies on fracturing fluids. This paper clarifies the hydration characteristics of shale reservoirs on the expansion of hydration characteristics of different media, initial pore pressure experiments, and long-term pore pressure experiments; combined with microscopic experiments such as acoustic emission, online NMR, and CT scanning, it elucidates the mechanism of the dynamic expansion of the secondary fractures of shale hydration with different fracturing fluids and quantitatively characterizes the fracture parameters. The results show that at the early stage of hydration, the slick water expansion is the largest, the pore pressure decreases greatly from 12 to 4 MPa, and the porosity is the largest for 0.0882%, while the guar gum has the smallest decrease in the early stage of hydration, from 12 to 2 MPa, and the smallest porosity is only 0.0515%; during the long-term hydration, the slick water expansion is smooth, the pore pressure tends to be stabilized, guar gum rises rapidly in the late stage, the overall morphology is large, fracture porosity rises the most, 280.6%, and the final porosity is 0.196%. The reason is that the early stage of slick water is affected by a surfactant and clay expansion and rapid fracture initiation, but the fluid distribution range is small and the hydration scale is small, while guar gum formation has a lag, but the overall distribution range is wide and the expansion of secondary fracture is mainly in the late stage of hydration. The research results provide important physical parameters and a theoretical understanding for the efficient development of shale oil and gas.