A New Approach to Borehole Trajectory Optimisation for Increased Hole Stability

Abstract

Abstract Borehole instability due to stuck pipe, hole pack-off, and lost hole during drilling has been a major problem, which causes the industry to lose millions of dollars. In the last two decades, various rock mechanics principles have been tried, and experience has been shared in the literature as to the efficacy of these principles to minimize the problem. The commercial software, rich in such analytical and numerical models, however, have not mitigated the problem. We have found that this failure is attributable to:lack of full utilization of the classical Kirsch equations that represent the state of stresses around a borehole;negligence/ignorance of the complete in-situ stress field and the Biot's constant; and improper calibration of the rock mechanics parameters; andlack of translating the findings of mathematical models in an appropriate form that can be readily used for field deployment. We have addressed these shortcomings by utilizing more relevant solutions to the Kirsch equations by incorporating poro-elastic behavior of rock. Further, we have utilized hitherto neglected aspects of the geomechanical stability equations, and presented graphical solutions that can be used by field engineers having a minimal knowledge of rock mechanics. One of the important findings of these computer generated, graphical solutions is that the optimized trajectory can be vertical, inclined, or horizontal. This depends on whether the region is tectonically quiet or active. It also depends on whether or not, the prevailing insitu stress regime is normal, overthrust/reverse, or strike-slip type. Thus, the principal insitu stresses, in both magnitude and directions, are a fundamental data set that is needed in designing the safe borehole trajectory. For the deviated hole, further optimization is obtained by determining the best azimuthal direction to reach the target zone. What is more interesting, the optimized trajectory requires a much less mud weight to drill a stable borehole, thereby reducing drilling cost significantly. We have applied our method 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. Introduction Review of published literature in the last two decades (McLean & Addis, 1990; McLellan & Hawkes, 1998; Svennekjaer & Bratli, 1998; Guo et al., 2000; Onaisi, et al., 2000) shows that borehole instability due to stuck pipe, hole pack-off, and lost hole during drilling are still common, although various rock mechanics principles have been tried, and experience has been shared in order to minimize these major problems.