Study on the Mechanism of Carbonate Formation Strength Degradation Induced by Water-Rock Interaction

The strength deterioration of the wellbore rock induced by water-rock interactions would be an important cause of the delayed instability in the Leikoupo formation located in Western Sichuan Basin, Sichuan Province, China. Taking the carbonate rock sample obtained from the Leikoupo formation as the research object, this study systematically analyzed the composition and the microstructural characteristics of the rock firstly. Then, a series of rock mechanics experiments before and after immersion in deionized water, pH=9, pH=11, and water-based drilling fluid (pH=11.5) was conducted. In order to further analyze the microscopic mechanism, the change in the ion concentrations of the immersed fluid and rock microstructures were analyzed. The results indicated that the Leikoupo formation carbonate rock was mainly limestone with developed micro-fractures. After immersion in alkaline solution, the elastic modulus and compressive strength of the rock decreased obviously. The friction coefficient of the fracture surface decreased as well. The degree of the deterioration became more significant with the increasing solution pH value and the prolonged immersion time. The analysis suggests that the dolomite in the carbonate rock undergone de-dolomitization reaction in high-temperature alkaline solution. It dissolved the micro-protrusions of dolomite on the fracture surface, so as to reduce the surface roughness. The research findings preliminarily reveal that the dissolution-crystallization-expansion mechanism of de-dolomitization reaction was the driving mechanism for the deterioration of carbonate rock due to water-rock interactions, providing a theoretical basis for the development of drilling fluid technology for stable wellbore in fractured carbonate rock formations.