Combined effect of rock fabric, in-situ stress, and fluid viscosity on hydraulic fracture propagation in Chang 73 lacustrine shale from the Ordos Basin

Abstract

Shale oil is an important area for the increasing of crude oil reserves and production. Due to the tight structure and ultra-low permeability of shale oil reservoir, the industrial exploitation needs large-scale hydraulic fracturing. However, compared with marine shale reservoirs, the Chang 7₃ lacustrine shales of the Ordos Basin present large differences in mineral composition and rock fabric, resulting in strong mechanical heterogeneity. The target interval selection and effective hydraulic fracturing of lacustrine shale oil reservoir requires a thorough understanding of the mechanical behavior and hydraulic fracture propagation in shale rocks. In-situ X-ray CT mechanical tests and triaxial hydraulic fracturing tests on Chang 7₃ lacustrine shales were conducted. The effects of rock fabric, in-situ stress difference and fluid viscosity on hydraulic fracture vertical propagation were analyzed. The results show that the rock fabric significantly influences the mechanical behavior and failure process of lacustrine shales. The vertical growth of the hydraulic fracture in lacustrine shales is dictated by vertical stress difference and fluid viscosity. When the vertical stress difference is small, the intrinsic weak interfaces in black shale significantly inhibit the vertical growth of hydraulic fracture. With the increase of vertical stress difference, the fracture network volume of black shale increases linearly, however, the fracture volume of laminar shale first increases and then decreases. Increasing the fracturing fluid viscosity could weaken the obstruction effect of weak bedding interfaces and promote the vertical propagation of hydraulic fracture.