Water absorption characteristics and their influence on the mechanical properties of Longmaxi Formation shale in northeastern Chongqing

A certain amount of casing deformation occurs during the shale reservoir fracturing. There is a certain amount of clay minerals in shale, and after contacting with water-based fracturing fluid, it will show the same expansion characteristics and strength weakening effect as mudstone, which will increase the casing external load. Aiming at the Longmaxi Formation shale in the northeast of Chongqing, the self-absorption and swelling characteristics of shale under different solution systems are studied by physical experiments to analyze the effect of shale hydration on its rock mechanical properties. The results show that the water absorption of shale has a logarithmic increasing relationship with time. In terms of water absorption of different solution systems, it is expressed as distilled water>formation water>slippery water. The slippery water system containing a small concentration of drag reducing agent is more conducive to weakening the water absorption capacity of shale. The expansion rate and time of shale gradually increase. The clay mineral content, confining pressure, and solution system have a significant effect on the shale expansion rate. Compared to the clay content, the confining pressure has a more significant effect on the expansion rate. The expansion of shale in the solution system performance is shown as ion water>formation water>slippery water. Both the compressive strength and elastic modulus of shale decrease linearly with the increase of hydration time. The research results will have certain guiding significance for casing strength design during completion.