A New Approach to Borehole Trajectory Optimisation for Increased Hole Stability

Analytical model based on linear elastic behavior of rock is an excellent tool for designing and optimizing the trajectory of a borehole such that instability due to compressive failure, especially in shaly zones, is minimized. An indirect method was incorporated to upgrade the model from conventional linear isotropy to transversely anisotropic case by improving the computation of the building block parameters, such as the in situ stress magnitudes and Biot's constant. The importance of modeling the overburden stress component was emphasized, failing which the stress regime in a region could be improperly modeled, leading to significant errors in the borehole trajectory design and higher critical mud weight. The method was applied in a case study in an offshore field, where frequent borehole collapse has been reported in drilling through shale/sand stringers. The successful application of the proposed method would not only save millions by preventing borehole instabilities, but also cut drilling cost significantly by replacing the expensive and environmentally unsafe oil-based mud.