Wellbore Stability Modeling and Real-Time Surveillance for Deepwater Drilling to Weak Bedding Planes and Depleted Reservoirs

Wellbore instability is the primary cause of losses in boreholes and represents a serious challenge in the drilling industry. Drilling along bedding planes and in depleted reservoirs is risky, and when a well is drilled at shallow angles to thinly bedded shales, it is often highly unstable. Rock failure can occur as a result of large anisotropy in rock strength caused by bedding-parallel weak planes. In these cases, an increased mud weight while drilling is required. However, when the reservoir immediately beneath the bedded shales is depleted, the increased mud weight can lead to lost circulation. This paper presents some of the cases we encountered in surveying offset wells in depleted reservoirs located in deepwater Gulf of Mexico for a new predrill study, where borehole instabilities resulted in a loss of a hole section and a casing shoe became massively fractured.

The key solutions are to not only improve wellbore stability modeling associated with bedding planes, rock anisotropy, and pressure depletion, but also to account for their impact on horizontal stresses. These factors are considered in the proposed model, thereby enabling calculations of wellbore failures along borehole trajectories with various drilling orientations versus bedding directions. The model has been verified by case studies. The minimum stress and fracture gradient calculations are also implemented to consider rock anisotropy and the depletion. Lab test data of rock strengths with different loading directions are analyzed. A new correlation is developed to allow for predicting anisotropic rock strength from sonic velocities.