Analytical and experimental investigation of pore pressure induced strain softening around boreholes

Changes in effective stress around a borehole due to pressure drop can cause formation instabilities leading to issues such as sand/solid production and wellbore stability. Analytical studies have been conducted in the past to gain insights into physical mechanisms involved in this complex process. The developed analytical solutions for such hydro-mechanical instability often consider perfectly plastic behaviour for the rock, whereas the field observations suggest strong strain softening as a result of pore pressure drop. In addition, these analytical solutions are not verified with experimental data in many instances. In this study, we developed a series of closed form solutions to predict the stress and strain changes around a borehole within poro-elasto-softening-plastic concept using Mohr Coulomb failure criterion. For the purpose of describing the pore pressure induced softening behaviour; an exponential function was used to relate the evolution of yield surface to plastic shear strain. In a further step, a series of hollow cylinder experiments were conducted on sandstone samples where different hydrostatic confinements at elevated pore pressures were applied. The hollow cylindrical samples were imaged before and after the experiments using 3D X-ray micro Computed Tomography technique (micro-CT) to precisely identify the collapsed-damaged and plastic zones formed around the borehole. Finally, the results from the experiments and micro-CT analysis were compared with the model prediction and a good agreement was observed.