Mechanical characterization of continental shale in Sichuan Basin of China and its potential impact on reservoir stimulation

Abstract

Organic-rich shale in Sichuan Basin of China preserves tremendous oil and gas resources, one important type of which is the continental shale gas. Owing to the special lacustrine depositional environment, continental shale showed strong heterogeneity in longitudinal profile and had not been fully characterized.Using the cores of an exploratory well within Lower Jurassic continental shale formation, rock mechanical experiments, including tri-axial compression, Brazilian splitting and fracture toughness, were performed systematically. Basic mechanical parameters of the entire reservoir section were acquired at the first time. Reservoir brittleness was also evaluated by a newly proposed method based on complete stress-strain curve. Results showed that mineral compositions exhibited a feature of wide fluctuation in longitudinal profile, with the good gas-bearing layers (D1-3, D1-4) containing high content of clay minerals, 63.93% and 59.67%, respectively. Mechanical parameters from tri-axial compression tests also showed noteworthy difference in longitudinal section. With increasing confining pressure, the dispersion reduced to a certain extent, but was still obvious. The newly proposed brittleness index could reasonably reflect the influence of buried depth and lithology difference. Formation brittleness tended to reduce with increasing confining pressure. Layers of D1-3 and D1-4 still maintained relatively high brittleness at high confining pressure. The biggest brittleness reduction occurred in the interlayer of shelly limestone. Tensile strength and type-Ⅰ fracture toughness at various depths exhibited high dispersion. The anisotropy was also obvious, with the values in vertical direction generally higher than those in horizontal orientation. The experimental findings would provide a basis for the optimum design of reservoir stimulation in this area.