Abstract
Subsurface lithofacies sequences encountered in the Kutch & Saurashtra Basin has its own set of challenges brought about due to its complex geological settings. These challenges are related to drilling, logging and completion and demand rigorous planning for the upcoming wells with detailed analysis of hazards associated with the overburden and reservoir rocks. In the study, these challenges are found to be linked with three prime geological sequences. Detailed integrated geomechanical analysis with inputs from drilling parameters, real-time formation experience, geophysical and geological are conducted for the improvement in borehole condition and improvising the effective drilling rate.
A customized geomechanical workflow has been adopted to construct Mechanical Earth Model (MEM, Plumb et al., 2000) for strategic wells across the basin. Wellbore stability events related to geomechanics were reproduced and analyzed. The cause of the events was established and mitigatory methods were proposed. In addition, stress orientation along the wellbore trajectory and across the basin was estimated using breakouts identified on images and multi-arm calipers. Fast shear azimuth from Dipole Shear Sonic anisotropy analysis was also integrated to provide more robust and accurate estimates.
Wells in the region are characterized by slow ROP, high torque and drag, wellbore instabilities (severe held ups, cavings, stuck pipes, string stalling etc.) and challenges while logging and running casing. The study has characterized these challenges and identified required solutions linked to the three geological sequences - weak Tertiary, Late Cretaceous Deccan Trap and Early Cretaceous to Jurassic clastic formations. The Tertiary formations are relatively weak (UCS∼300 to 1500psi) and prone to sanding and cavings due to breakouts. MEM based mud weight window estimation predicts that shear/failure hole collapse can be prevented using 10ppg to 11ppg mud weight. The formations below the Deccan Trap are locally categorized under Mesozoic sequence. The Deccan Trap and Mesozoic formations are extremely hard, tight, extremely stressed, heavily fractured and in some areas are also of HPHT nature. Rock strength shows a wide variation (UCS ∼5,000psi to 25,000psi) making bit selection a difficult task. Borehole failure is complex and cuttings analysis shows the signature of both shear and weak plane failure. Fractures on the image logs, rotation of breakouts, and fast shear azimuth support this theory. Mixing fracture sealing agents along with the use of optimal mud weights is found to be the most likely drilling solution.
The understanding developed in the region and implementation of recommended steps assisted in successful drilling of two recent wells wherein gun-barrel shape borehole condition in both Tertiary and the Mesozoic sequence was achieved. The non-productive time was reduced by nearly 40 days increasing the effective ROP by 40%. In addition, smooth borehole prevented any major issues while carrying out casing and cementing operations.
3D mechanical earth model for optimized wellbore stability, a case study from South of Iraq