关键词: 高温高压, 岩石脆性, 裂纹扩展, 脆-塑性破碎, 起裂应力水平

Abstract: The current lack of understanding regarding the mechanical properties and fracturing mechanisms of rock formations at the bottoms of ultra-deep wells in China significantly constrains the efficiency of oil and gas development. To address this issue, this paper investigates crack propagation patterns under high-temperature and high-pressure conditions, analyzes rock brittleness using a brittleness index based on energy partitioning model calculations, examines rock brittle-plastic failure, and discusses the strong correlation between rock brittleness and crack propagation. Concurrently, rock samples from four distinct stratigraphic units (including outcrops and downhole cores) were subjected to coupled high-temperature and high-pressure rock mechanics experiments. The specific crack propagation process was investigated through PFC simulation modulus analysis. The research findings indicate:(1) Rock brittleness is influenced not only by the rock's mineral composition and initial defects but also by its temperature and confining pressure. Higher temperatures increase brittleness, while greater confining pressure reduces it. (2) The rock's initiation stress level reflects its heterogeneity and structural variations; a lower initiation stress level indicates stronger rock heterogeneity. The initiation stress level is influenced by multiple factors, including but not limited to:the rock's structure, composition, mineral constituents, and grain size. Additionally, confining pressure and temperature exert effects on the initiation stress level, with higher temperatures yielding lower initiation levels and greater confining pressures producing higher initiation levels. (3) Under high temperature and high pressure, cracks are widespread throughout the rock, with a tortuous and branched distribution. The cracks not only expand in all directions but also concentrate at the weak points of the rock, with the concentration points mainly at both ends of the rock. Macroscopically, the failure is manifested as damage caused by plastic flow. At room temperature, the initiation of cracks is concentrated on the internal start of the rock, then gradually expands towards the macroscopic failure surface, and also expands in all directions. The cracks do not have obvious concentration points but rather concentrate on the weak faces. As the confining pressure increases, the number of cracks in the rock increases, and the macroscopic failure of the rock gradually changes from brittle sudden failure to plastic tensile-shear failure.

Key words: high-temperature and high-pressure, rock brittleness, crack propagation, brittle-plastic failure, initiation stress level