Sand stress analysis around a producing wellbore with a simplified capillarity model

It is well known in oil industry that water saturation changes during production can lead to sand instability, with possible sand mobilization and flux. For unconsolidated sand with little water-sensitive cementation, two mechanisms are identified as the main reasons for this phenomenon: strength reduction because of capillarity from water-oil menisci, and stress elevation due to changes of relative fluid permeabilities. In this paper the effect of both mechanisms on sand stability is evaluated in terms of stress changes and growth of a plastic radius defining a shear-yielded zone around an oil well after water breakthrough. With only two input requirements, mean particle radius and water saturation, a simplified micromechanical model is proposed to describe the capillary effects. Using this micromechanical model, a new method to calculate pore pressure in multiphase environments and a coupled elastoplastic geomechanical model describing stress redistributions with increasing water saturation are developed. The proposed models lay down a new way to analyze rock stresses in porous rock around a wellbore and can be used to help evaluate sand production risk for unconsolidated or weakly consolidated sands in multiphase fluid flow environments.