Borehole Stability in Shale: Beyond Mud Weight

Abstract

Abstract Several wells in the Gulf of Mexico were analyzed to understand the source of shale instability during drilling and wireline operations. In spite of the mud weight being above the borehole collapse pressure, the wells still exhibited signs of shale instability. As such, it was evident that another mechanism was playing a role in the failure of the shales. This study investigated the drilling fluid properties that are known to affect shale stability. One fundamental property investigated was the water phase salinity, as shales can act as osmotic membranes, allowing fluid diffusion through the porous space while restricting the movement of salt ions. This establishment of an osmotic pressure can either preserve or destroy the mechanical integrity of the shale. The analysis compared the water phase salinity (WPS) and mud weight (MW) of several wells, in the context of geomechanics, to understand their relationship with the actual borehole stability condition. In one particular case, the data revealed the impact of mud salinity on shale stability, where three different boreholes penetrated the same shale: the original hole (OH) and two sidetracks. In the OH, there was no indication of shale instability during the eight days the hole was exposed, no cavings were reported, and the caliper log showed an in-gauge hole. The first sidetrack (ST1) was completely different where the salinity level was reduced by 7% while the MW remained the same as the OH. During the first ten days of shale exposure in ST1, there was no significant indication of shale sloughing. However, after ten days, it was challenging to maintain a stable wellbore, which led to a stuck pipe event, and an unsuccessful attempt to regain wellbore stability by raising the MW. In the second sidetrack (ST2), the salinity concentration and MW were the same as in the OH, and after fourteen days of exposure to the mud system, there were no signs of instability. The caliper from ST2 confirmed that an in-gauge hole was maintained the entire time of drilling and wireline operations. These observations were used to calibrate a borehole stability model, which was used to plan a subsequent infill well in the field drilled at a higher inclination and with a long open hole section through the same shales. The model successfully predicted the optimum combination of MW and WPS to maintain a stable wellbore throughout drilling and wireline operations. The results show that a scientifically-driven modeling approach based on offset wellbore observations and appropriate model selection and calibration can result in a significant improvement in drilling operational uptime by minimizing shale instability.