Study of fracture toughness and weakening mechanisms in gypsum interlayers in corrosive environments

Abstract

China's rock salt deposits, which are mostly located in the Yunying area, are typically characterized by their interlayers. In the construction and operation of salt caverns, gypsum interlayers are soaked in brine and oil. To investigate the fracture toughness and weakening mechanisms of gypsum interlayers in corrosive environments, a series of laboratory tests, including cracked chevron notched Brazilian disc (CCNBD) tests and micro computed tomography (MCT) scanning, was performed. Ninety CCNBD specimens and 15 small specimens, with dimensions of 5 mm × 5 mm × 5 mm, were prepared and then soaked in 4 types of liquid and at 3 different temperatures. The results indicate that the fracture toughness of gypsum decreases significantly with increasing temperature when soaked in a given liquid. By contrast, the fracture toughness of gypsum remains approximately identical after treatment with water, half-saturated brine, and saturated brine at the same temperature. With increasing time and temperature, the porosity of the gypsum increases. The rate of porosity change (slope) increases with increasing temperature, but it remains stable with increasing brine concentration. Water and brine have more severe corrosive effects on gypsum than does acidic oil. The weakening of gypsum soaked in water and brine is attributed to the combined effects of water, temperature, and chlorine ions. Notably, the water-temperature effect contributes to a severe weakening of gypsum interlayers, but the chlorine ions have little effect. The mechanism of the weakening of gypsum soaked in acidic oil is chemical reactions and dehydration. Petroleum acid (naphthenic acid) reacts with gypsum to produce calcium naphthenate, which can be dissolved in the oil phase. During this process, the dissolution of calcium naphthenate in the oil phase promotes the forward reaction. Consequently, the study has significance for the evaluation of the leakage risk of storage caverns in bedded salt deposits.