Factors affecting the mechanical properties of tight sandstone and their patterns of variation in Cretaceous Bashijiqike Formation of Kuqa Depression in Tarim Basin

Abstract

To clarify the mechanical characteristics of tight sandstone in the Cretaceous Bashijiqike Formation of Kuqa Depression in Tarim Basin, and address field issues in deep and ultra-deep oil and gas exploration and development, triaxial compression experiments were used to quantitatively study the patterns of changes in rock mechanical properties influenced by confining pressure, fluid, and loading rate, with a preliminary analysis of their causes. The results showed that the maximum principal stress difference and elastic modulus of the sandstone samples increased significantly with confining pressure. The micro-reason was that the increase in confining pressure reduced the distance between particles inside the rock, enhancing the rock's cohesion and making particle dispersion less likely. Sandstone samples exhibited a progression from brittleness under low confining pressure to brittle-ductile transformation, and to ductile deformation under high confining pressure. Compared with dry sandstone samples, the reduction in the elastic modulus of samples soaked in pure water, 150 g/L solution, 250 g/L solution, and 350 g/L solution were 67.71%, 61.45%, 64.69%, and 57.32%, respectively, with pure water soaking causing the greatest reduction. Increasing fluid salinity could mitigate the weakening trend in rock mechanical parameters. Crystallization on crystal surfaces and changes in the electric double layer thickness were important controlling factors for these patterns. At lower loading rates, the values for maximum principal stress difference, elastic modulus, and Poisson's ratio of the sandstone samples were smaller, but they increased faster with increasing loading rates. When the loading rate reached a certain critical value (around 0.05 mm/min in this experiment), the rate of increase in rock mechanical parameters slowed down.

Keywords

• mechanical properties of rock
• loading rate
• fluid type
• bashijiqike formation
• cretaceous
• kuqa depression
• tarim basin