Experimental study on the damage of organic-rich shale during water-shale interaction

Abstract

Water-shale interaction remains an unsolved problem because of the complexity involved in the physical processes and the heterogeneity in the chemical composition and pore structure of rocks. By considering the basic physical properties of Longmaxi shale and performing a series of physical experiments representing the water-shale interaction, the relationship between the mineral composition and water-shale interaction, which is the process responsible for causing structural damage when water and shale interact, is analysed. The ion exchange of clay minerals occurs when shale makes contact with water and different kinds of cations experience different degrees of overflow in water-shale interaction. The charge of clay mineral changes during water-shale interaction, resulting in a change of the gravitational and repulsive forces between the particles. This leads to the passivation of the contour edges of clay minerals and changes in the mechanical properties. In a relatively short time, illite can produce a large hydration stress with a small expansion value, but the hydration speed of Na-montmorillonite (Na-MMT) is relatively slow. Uneven stress caused by the hydration of different clay minerals being soaked in different aqueous solutions can cause local stress concentration, further promote the expansion and increase of original micro-cracks in shale, and then appear as disordered macro-cracks. The macroscopic cracks also provide a channel for the continuous entry of working fluid. More water molecules enter the shale faster and make contact with clay particles, weakening the interaction and cementation between particles. Macroscopically, they are manifested as a decrease in the rock cohesion, internal friction angle, and compressive strength as well as a failure of the structural integrity. Different inorganic salt solutions have different inhibitory effects on reducing the hydration degrees of illite and Na-MMT. Therefore, the water-shale interaction of organic-rich shale is a process in which the microscopic damage of water to rock gradually evolves into macroscopic damage, and results in the local continuity loss of rock on the basis of surface hydration, ion hydration, and osmotic hydration of clay minerals. The higher the clay mineral content, the more likely that hydration will occur, resulting in more serious shale structure damage and a shorter time for damage to occur.