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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.

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Drilling in Kutch Saurashtra Basin of Western offshore of India is characterized by slow ROP, frequent high torque and drag, wellbore instability and challenges while running casing. The exploratory well A is one of the deepest HPHT well to be drilled through hard, abrasive and thick massive layers of Basaltic formation (Deccan Trap) before reaching the Jurassic reservoir sands. A Geomechanics aided study was conducted and executed to improve drilling performance, optimize well condition and increase ROP to minimize rig days and cost of undesirable NPT. A pre-drill 1D Mechanical Earth Model was prepared for well A based on offset wells for reservoir as well as for overburden rocks. Stable mud weight window was identified to optimize drilling mud weights. Motor with specially ordered abrasion resistant sleeves and compatible fixed cutter bit with innovative conical diamond elements (CDEs) were used to drill the harder Mesozoic formation (Jurassic to Cretaceous) inclusive of unexpected igneous intrusives, with remarkably good penetration rate. Torque & Drag reduction and tool wear were kept to minimal with optimal BHA design and drilling practices. LWD and Wireline conveyed pressure measurement tool was utilized for better understanding of reservoir complexity based on which mud weight was optimized in real time for the section. Drilling parameters were optimized without compromising the ROP and 700m of HPHT formation was drilled without stuck pipe and lost in hole incidents. More than 100% improvement in ROP in basaltic formation is observed compared to offset wells. Motor out-performed its operating hours in both runs (220 hrs. in first run & 370 hrs. in second run) without any failures. Judicious selection of drill mud weight based on estimated stable mud weight window, resulted in improved wellbore condition in the reservoir as compared to offset wells, as indicated by LWD and wireline callipers. Improved borehole condition in reservoir reduced wireline "lost seals" pressure stations to 2 out of 6 attempted. The CDE bit was field tested on PDM LWD BHA and drilled 813m at an average ROP of 1.9m/hr through the Mesozoic formation. The Volcanic dykes (Basaltic) had unconfined compressive strength as high as 33 kpsi making it vulnerable for conventional drill bits. Compared to offset wells drilled with roller cone bits, the CDE bit drilled eight fold increase in meterage at 40% higher ROP while providing good wellbore quality and came out with excellent dull condition (1-1-CT). The durability and high ROP of the CDE bit saved the operator about 15.7 rig-days of rig time and reduced cost per meter by 50%. This amounts to significant savings in monetary value. Considering the high formations pressures, tectonic stresses and fractured nature of the formations LWD logging turned out as the best choice for acquiring virgin formation response and identification of zones of interest. Moreover, improved borehole condition and minimal drilling NPTs, reduced the operation risks and ensured achieving the well objectives. Effective operational procedures and constant communication with drilling team empowered them to take quick decisions regarding the well.

Simplified analytical methods are used in 1D geomechanics workflows to predict the rock's behavior during drilling, completion and production operations. These methods are simplistic in their approach and enable us in getting a time-efficient solution, however it does lose out on accuracy. In addition, by simplifying equations, we limit our ability to predict behavior of the borehole wall only i.e. near wellbore solutions. Using 1D analytical methods, we are unable to predict full field behavior in response to drilling and production activities. For example, when developing a field wide drilling plan or preparing a field development plan for a complex subsurface setting, a simplified approach may not be accurate enough and on the contrary, can be quite misleading. A 3D numerical solution on the other hand, honours subsurface features of a field and simulates for their effect on stresses. It generates solutions which are more akin to reality. In this paper, difference between a simplified semi-quantitative well-centric approach (1D) and a full field numerical solution (3D) has been presented and discussed. The subsurface setting considered in this paper is quite complex - high dipping beds with pinch outs and low angled faults in a thrust regime. Wellbore stability and fault stability models have been constructed using well-centric approach and using a full field-wide 3D numerical solution and compared to understand the differences. In this study, it was clearly observed that field-based approach provided us with more accurate estimation of overburden stresses, variation of pore pressure across the field, changes in stress magnitudes and captured its rotation due to pinch-outs and formation dips. For example, due to variation in topography, the well-centric overburden estimates at the toe of deviated well at reservoir level is lower by 0.21gm/cc as compared to the 3D model. It is also observed that within the field itself stress regime changes from normal to strike slip laterally across the reservoir. In comparison to 1D model, considerable differences in stable mud weight window of upto 1.5ppg is observed in wells located close to faults. This is due to effect of fault on stress magnitude and azimuth. Stress state of 4 faults were assessed and all are estimated to be critically stressed with elevated risk of damaging three wells cutting through. However, a simple 1D assessment of stress state of faults at wells cutting through them, show them to be stable. Moreover, the 3D geomechanical properties that are input into the numerical simulation also play an important role on the results. The algorithms and data used to populate the properties away from the well, need to be validated and calibrated with the well data, to predict reliable results. As the subsurface was quite complex, and well data was not sampled optimally, both horizontally and vertically, the selection and optimum usage of 3D trends also became crucial. By comparing the differences between 1D and 3D solutions, importance of 3D numerical modelling over 1D models is highlighted.