Borehole Instability Investigation, Challenges and Learning from Experience – A Case Study of Appraisal/Development Well X

Abstract Borehole instability constitutes a major drilling risk in the Niger Delta, especially across the Shale intervals. It can lead to significant value erosion resulting from non-productive rig time, increased well execution costs, well incidents and total loss of well/abandonment. The appraisal/development well case study highlights the drilling experience and challenge faced by the integrated subsurface and wells team, during the investigation of the wellbore instability issues in failed development sidetracks XST1 & XST2, and the learnings picked up upon re-entry, that led to a successful drilling of XST3. The development sidetrack XST1 encountered borehole instability issues, and was abandoned after several failed attempts to retrieve the stuck drill pipe. XST2 was drilled with a higher mud weight but still experienced borehole instability challenges (confirmed from cavings) leading to borehole collapse and lost hole. Correlations/mud logs revealed that shear failure was mostly across seemingly unstable shale beds within depth interval of 7100 – 8500ft.tvd. During the investigation, a geomechanical borehole stability model (based on Mohr Coulomb failure criterion) was developed using the magnitude and orientation of principal stresses, pore pressure gradients, and elastic rock properties, which were estimated using compressional slowness, shear slowness and volume of shale logs in offset wellbores. The model was calibrated against offset well data to determine the optimum mud weight required to maintain the right overbalance, while preventing shear failure and tensile fracture. Model results showed XST1 was drilled with a suboptimal MW, while XST2 though drilled with a higher recommended mud weight, likely failed due to additional complications from slippage of weak bedding planes across the trouble zone, as seen from indications on sonic logs, UCS and poisson ratio trends. Upon re-entry, the wellbore trajectory for XST3 was further optimized by reducing the inclination within vicinity of the suspected weak bedding planes to minimize risk of slippage, while maintaining mud weight threshold recommended by the field borehole stability model. The XST3 wellbore was successfully drilled to target depth, with minimal borehole issues, section cased off and completed, demonstrating the value of integrating geomechanical modeling studies and bedding plane slippage risk, in mud weight prediction and trajectory optimization for deviated/high angle wells.