The effect of temperature on the mechanical behavior of Berea sandstone under confining pressure: experiments

Abstract

For the first time, axial and circumferential (diametral) strains are measured directly on Berea sandstone cylindrical samples at different high temperatures as a function of confining pressure. The maximum compressive principal stress was in the direction perpendicular to the bedding plane. Tri-axial compression tests have been conducted under confining pressures ranging from atmospheric pressure to 12,000 psi “82.73 MPa” (gage), and the temperature was varied from room temperature (RT) to 250°F (121.11 °C). In all the experiments, specimens are dry, and no pore pressure is applied. Uniaxial axial force (100 lbs/min or 444.82 N/min) and strains in the three directions (Ɛ₁, Ɛ₂, Ɛ₃) is directly measured from the strain gages mounted on the specimen; hence the study reflects the behavior of the Berea sandstone up to the initiation of the failure only. The experimental observations involving yield, failure, transition from brittle to ductile behavior, and their dependence on temperature and confining pressure are presented. Parameters like ductility and dilatancy (reflecting volumetric behavior before failure) and their variation with temperature and confining pressure are given. The initial yield and failure of Berea sandstone are presented in terms of the generalized von-Mises criteria, i.e., variation of octahedral shear stress (τ₀) with the mean stress. The loads at yield and failure are found to decrease with increasing temperature and augment with increasing confining pressure. The ductility and dilatancy reduction are computed using measured strains with increasing temperature. With an increase in temperature, the volume decreases first, partly due to the initial closure of micro cracks and voids, then increases due to generation of additional microcracks and voids (damage). There is less volume decrease in the uniaxial test response at higher temperature as compared to the corresponding ambient temperature response. A monotonic increase in the octahedral shear stress at yield and failure is observed with increasing confining pressure.