Influencing Drilling Decisions: Challenges and Lessons from a Success Story, Kirthar Block, Pakistan

Drilling in tectonically active thrust-fold regions is normally challenging due to pore pressure and wellbore instability problems. Gaining control of these problems can be even more difficult due to the complex geomechanical behavior of the formations to be drilled. This paper presents a case study on how the use of an iterative geomechanics approach combined with drilling fluid optimization recommendations and continuous engagement with the operator, helped to reduce drilling risks and costs in a tectonically active thrust-fold region in Pakistan. The major challenges anticipated in this region, which was drilled to explore a tight-play (at ~3 Km depth), were (i) suitability of offset wells due to lateral formation discontinuities and structural complexities, (ii) uncertainties in the stresses and rock properties and (iii) pore pressure compartmentalization. A geomechanical model was built initially with available data to start the drilling campaign. Although reasonable data was available, the region was tectonically active, raising the level of uncertainties in the model. As the first well was drilled, various observations were shared during the operations, which helped to identify time-dependent failure. During drilling of subsequent wells, various additional challenges such as faults in the region, critically stressed fractures and weak bedding failure were identified. Various opportunities arising during drilling were used to learn lessons such as; (1) why a certain limestone section was causing losses; (2) what was the most important parameter for casing depth selection; (3) the effects of ROP on hole cleaning and the eventual effect on hole stability; (4) the effects of faults in the region; and (5) the consequences of a reactive approach to changes in the mud weights. When drilling the fourth well of the campaign, key changes were proactively implemented such as (1) mud weight design (2) salinity optimization (3) silicate mud system usage and (4) casing design changes. The key influencing factors included recommending changes to mud weights and the approach to mud weight changes, fluid system and design of the system, and a continuous engagement between the geomechanics and operations teams. During the course of our interactions, the usefulness of understanding the prior assumptions, customized data acquisition, and developing trust within teams were observed and significantly contributed to the overall success of the project. The fourth well was drilled within 62 days compared to 90+ days in the first well. The iterative process adopted for continuous geomechanical model updates not only helped increase hole stability in the region but also ensured continuous improvement and learning. This approach helped to have robust control on operations and models for field development.