Brittleness of deep sandstone in central Junggar Basin and its influence on fracture modes of rocks

Abstract

The tight sandstone reservoirs are deeply buried and strongly influenced by diagenetic processes such as mechanical compaction， dissolution， and metasomatism. Therefore， their lithological characteristics are complex. As the exploration and development of oil and gas resources go towards deeper reservoirs， the high ground stress and high confining pressure in deep layers have a significant impact on the brittleness and plasticity of reservoirs， which affects the hydraulic fracturing effect and oil and gas production. The deep tight sandstone reservoir in central Junggar Basin has abundant reserves. However， traditional oil and gas extraction methods fail to meet production needs due to the deep burial depth， poor physical properties， and abnormally high confining pressure of the reservoir， as well as limited understanding of the brittleness variation law of its in-situ geomechanical core. To this end， through physical experiments and numerical simulation techniques， this paper studies the differences in rock mechanics characteristics， brittleness and plasticity transformation， fracture modes， and acoustic emission characteristics of deep rock cores in the central Junggar Basin. The results show that as the confining pressure increases， the rock transitions from brittle failure to plastic failure， and the fracture mode changes from splitting failure to shear failure， weakening the brittleness of sandstone. The overall degree of sandstone brittleness reflected by the six brittleness indices based on the energy balance method and normalization treatment decreases， but the numerical ranges of different brittleness indices and their correlation with brittleness vary. The triaxial compression test experiments of core models with different brittle values show that when the content of brittle minerals is low， the rock exhibits single oblique shear failure. As the content of brittle minerals increases， rock exhibits a composite failure mode with multiple fracture surfaces. There is a correlation between the acoustic emission characteristics during rock fracture and its brittleness， involving three modes： swarm type， foreshock–main shock–aftershock type， and main shock type.

Keywords

• deep sandstone
• high ground stress
• brittleness
• fracture mode
• acoustic emission characteristics