Coupled relationships between overburden stress and ultra-deep sandstone brittle deformation properties based on in situ CT scanning

Abstract

For low poroisty (tight) sandstone of the Cretaceous Bashijiqike Formation, Kuqa Depression foreland thrust belt (Tarim Basin), samples from an ultra-deep setting (ca.7–8 km depth) and from outcrop locations were collected to analyze and compare brittle deformation in triaxial tests to infer mechanical behavior. Core samples sustained protracted burial whereas outcrop samples were deeply buried then experienced late uplift and exposure. Fracture-pore changes during loading in situ uniaxial compression documented with 3D X-ray imaging show that loading results in extensive development of fractures and enhanced pore connectivity that elevates permeability from 1mD to 10mD through local tensile fracture zones and high-permeability layers. The collapse of 60% pores and 70% reduction of connected pores may lead to locally compacted low-permeability zones. These characteristics are also reflected in the high heterogeneity of ultra-deep reservoir properties—the alternating development of dense and sparse zones of fractures. Research lead to the inference that under burial and tectonic loading conditions sandstones experience concurrent collapse of pores and growth of fractures, and reservoir structure changes from pore-dominated to fracture-dominated. Under deep conditions, the reservoir rock is subjected to plastic-brittle strain, increasing porosity and permeability followed by, under ultra-deep conditions, porosity decreasing markedly and permeability increasing significantly.