An experimental investigation on the poroelastic response of a water-saturated limestone to hydrostatic compression

Abstract

This contribution addresses an experimental study on the poroelastic behavior of a water-saturated limestone. Samples come from Sarvak limestone, a major hydrocarbon-producing reservoir in Iran. Three back-saturated intact specimens were subjected to hydrostatic compression with different boundary pressure conditions. A coherent workflow from testing to data interpretation was established to efficiently deal with possible sources of uncertainty and provide accurate results. Obtained results revealed that drained and undrained bulk moduli, as well as the Biot and Skempton coefficients, strongly depend on the applied Terzaghi effective stress. Conversely, calculated values for the unjacketed solid modulus \( {K}_s^{\prime } \) were found to be constant in the range of applied stresses. Variations of the poroelastic moduli with the effective stress were modeled and used to extract depth-dependent poroelastic properties of the studied lithotype along a well. Besides, evaluated poroelastic moduli were utilized to verify the linear theory of poroelasticity. The indirectly calculated unjacketed pore modulus \( {K}_s^{\prime \prime } \) appeared to be highly variable with changes in the effective stress and possesses even negative values when the Skempton coefficient is lower than a critical value. A comparison between unjacketed bulk moduli and the bulk deformation modulus of the main mineral constituents Ks also disclosed that they are basically uncorrelated, and the assumption of ideal porous rock, \( {K}_s^{\prime \prime }={K}_s^{\prime }={K}_s \), does not hold in general for the examined rock samples. Nevertheless, consideration of the ideality assumption in the theory of poroelasticity may result in reliable estimates for the Skempton coefficient in certain circumstances, particularly at high effective stresses.