A New Approach to Borehole Trajectory Optimisation for Increased Hole Stability

摘要

Borehole instability due to stuck pipe, hole pack-off, and lostrnhole during drilling has been a major problem, which causesrnthe industry to lose millions of dollars.rnIn the last two decades, various rock mechanics principlesrnhave been tried, and experience has been shared in thernliterature as to the efficacy of these principles to minimize thernproblem. The commercial software, rich in such analyticalrnand numerical models, however, have not mitigated thernproblem.rnWe have found that this failure is attributable to:rn(1) lack of full utilization of the classical Kirsch equationsrnthat represent the state of stresses around a borehole;rn(2) negligence/ignorance of the complete in-situ stress fieldrnand the Biot's constant; and improper calibration of thernrock mechanics parameters; andrn(3) lack of translating the findings of mathematical models inrnan appropriate form that can be readily used for fieldrndeployment.rnWe have addressed these shortcomings by utilizing morernrelevant solutions to the Kirsch equations by incorporatingrnporo-elastic behavior of rock. Further, we have utilizedrnhitherto neglected aspects of the geomechanical stabilityrnequations, and presented graphical solutions that can be usedrnby field engineers having a minimal knowledge of rockrnmechanics.rnOne of the important findings of these computer generated,rngraphical solutions is that the optimized trajectory can bernvertical, inclined, or horizontal. This depends on whether thernregion is tectonically quiet or active. It also depends onrnwhether or not, the prevailing insitu stress regime is normal,rnoverthrust/reverse, or strike-slip type. Thus, the principalrninsitu stresses, in both magnitude and directions, are arnfundamental data set that is needed in designing the safernborehole trajectory. For the deviated hole, furtherrnoptimization is obtained by determining the best azimuthalrndirection to reach the target zone. What is more interesting,rnthe optimized trajectory requires a much less mud weight torndrill a stable borehole, thereby reducing drilling costrnsignificantly.rnWe have applied our method in a case study in an offshorernfield, where frequent borehole collapse has been reported inrndrilling through shale/sand stringers. The successfulrnapplication of the proposed method would not only savernmillions by preventing borehole instabilities, but also cutrndrilling cost significantly by replacing the expensive andrnenvironmentally unsafe oil-based mud.