Biot's effective stress coefficient is a critical property for several geomechanical applications, particularly for the field scale geomechanical modeling of hydrocarbon reservoirs. Indeed this work, which focuses on its experimental evaluation, is part of a comprehensive geomechanical study of a low porosity fractured carbonate reservoir.
Two ad hoc laboratory procedures were implemented to evaluate Biot's coefficient by measuring only bulk volume variations. The first procedure was based on the evaluation of bulk compressibilities during several phases of an isotropic compression test performed at constant confining pressure or at constant pore pressure. The Biot's coefficient was, then, computed as ratio between the two corresponding compressibility coefficients. The second one, instead, was conceived in order to directly measure solid skeleton and grain compressibilities by means of two isotropic compression tests carried out on twin plugs by adopting the proper specific experimental configurations.
The observed values of Biot's coefficient were found to be largely lower than unit and the obtained results were also successfully compared with other experimental measurements available in literature. Particularly, the measured values of this study were judged to be well predicted by a phenomenological model, proposed by Bouteca et al. (1991) and Laurent et al. (1993), describing the dependence on porosity of Biot's coefficient in carbonate rocks.
This paper presents the outcomes of an experimental campaign, performed in the context of a larger geomechanical study at the scale of a field of an onshore fractured carbonate reservoir that dates back to the Cretaceous period. Reservoir rocks are mainly made up of limestones and dolomites with low primary porosity (1-2﹪) and permeability. From a geo-structural point of view, the field is composed by three main culminations that are drained by several slanted or horizontal wells. Reservoir permeability and wells productivity are linked to the fracture network that is present at various scales.