Alternative method to determine the uniaxial-strain compressibility of cohesive sandstone

Abstract

Compaction and subsidence risk related to hydrocarbon production are frequently assessed using Geertsma models. The underlying theory assumes linear poroelastic reservoir rock to compress by pore pressure depletion under uniaxial-strain conditions. Adequate mechanical testing techniques have been developed to measure the uniaxial-strain compressibility, although the execution of these methods is technically difficult and requires a significant level of experience. In this paper, a simplified technique is proposed to approximate the compressibility from triaxial strain data, which is different from earlier hydrostatic stress/uniaxial-strain conversion approaches. The method is applied to Berea outcrop sandstone and Groningen reservoir material, employing pore pressure depletion protocols under triaxial stress and uniaxial-strain boundary conditions. By interpolation of triaxial stress–strain data and adopting linear poroelastic theory, the uniaxial-strain compressibility required for the application of the Geertsma model is yielded. This does not require a priori information of elastic constants. Moreover, the interpolation technique allows for an independent check of the technical performance of the radial strain control system. The method holds for rocks that are close to linear poroelastic, subjected to stress conditions below the critical state line.