Stresses around a circular opening in an elastoplastic porous medium subjected to repeated hydraulic loading

Abstract

Stresses near a circular borehole under repeated interior loading are analyzed. In a porous material which is capable of plastic deformation, the stresses and strains adjacent to a borehole depend on the loading history. A solution for stresses and strains can only be determined once the stress paths and history to reach this condition are defined; therefore, to obtain meaningful solutions in poroplastic media, loading paths must be specified. In this paper, varying loadings inside a circular opening are imposed to simulate the non-monotonic pressure changes that arise during drilling and injection in a borehole. It is assumed that stresses under such loadings may be determined by considering the rocks as Mohr—Coulomb materials. A constitutive model is used that incorporates strain-weakening as a sudden strength loss after peak strength is reached, with perfect plastic behavior after weakening has occured. Plastic yielding related to different yielding modes, i.e. active ( σ r ′ < σ θ ′) and passive ( σ r ′ > σ gq ′) stress states, is explored. Stages in the development of such stress states are analyzed, and the stress states in different zones under various circumstances are calculated separately and summed. The solutions presented can be used to understand stress development around boreholes, and can also be used to interpret hollow cylinder test results. They may be of use as parametric analysis tools to aid the drilling engineer analyze borehole stability, sand production and blow-out risks.