Abstract:

Shale oil and gas resources are abundant in China, and hydraulic fracturing to stimulate reservoir is a significant way to efficiently develop these resources. Brittleness is a key parameter for reservoir stimulation and a core indicator for identifying engineering sweet spots. Taking the shale reservoir in the Dongying sag as an example, the rock mechanical properties and brittleness characteristics of the shale reservoir were analyzed through uniaxial, triaxial, and high-temperature, high-pressure (HTHP) compressive tests. Based on the rock energy balance theory and brittleness characteristics, as well as the energy evolution behaviors before and after rock failure, a new method for evaluating shale brittleness was proposed. The research results show that under uniaxial conditions, the shale exhibits significant brittle failure with multiple cracks, which is beneficial for reservoir stimulation. In HTHP conditions, the synergistic effect of temperature and confining pressure suppresses rock brittle fracture but strengthens rock ductility, leading to a significant reduction in brittleness. Based on the proposed brittleness evaluation method, the primary factors controlling shale brittleness were identified. It is found that the rock physical parameters (porosity, density, and acoustic travel time) is weakly correlated with brittleness, while mineral composition and elastic parameters are more effective in assessing brittleness. The effects of temperature and pressure cannot be ignored. The research results are conductive to identifying engineering sweet spots in shale reservoirs and provide a theoretical foundation for efficient reservoir stimulation.

Key words: shale reservoir, brittleness evaluation, rock energy, HTHP, mechanical property, engineering sweet spot