Changes in retained fracturing fluid properties and their effect on shale mechanical properties

To understand the fluid-shale interactions and the mechanisms that produce cracks during the retention of the fracturing fluid. This paper details experiments performed in shale particle immersion and core plug imbibition, and it investigates changes in fluid properties during a shale well shut-in period at the Longmaxi formation in Southeast Chongqing, China. The results indicate that soluble salts in a shale formation are dissolved during fracturing fluid retention in a shale reservoir, causing the salinity and pH of the fracturing fluid to increase significantly and eventually resulting in the formation of a weakly alkaline solution. Furthermore, fracturing fluid retention significantly reduces the mechanical strength of shale. In the experiments, the elasticity modulus and Poisson's ratio of the shale decreased by 85% and 54%, respectively, which was immersed in the fracturing fluid for seven days. Two types of forces, repulsive hydration and electric double-layer repulsion, are shown to be the main factors that lead to the formation of a large number of microcracks. The repulsive hydration pressure force is in a short range, operating over only a few nanometers, while the electric double-layer repulsion force operates over a long distance and can act between clay mineral slices. As the concentration of the solution increases, the double-layer repulsive force increases, though the range over which it acts becomes shorter. The increase in the OH - concentration of the solution also accelerates crack propagation. These conclusions are conducive to developing reasonable rules for fracturing fluid flowback in shale gas wells.