Revisiting Subsalt Exploration Drilling Challenges

The quest for hydrocarbons continues to increase and is becoming more challenging as the search pushes operators further into unexplored basins and frontiers globally. In the last 20 years, subsalt drilling has provided some new and promising prospect landscapes and fairways; however, major challenges are inherent with drilling subsalt exploration wells. One of the most problematic properties of salt is deformation or mobile behavior; in other words, salt has almost no permeability and porosity, and it creeps under differential stresses, causing hole stability issues. Historically, where global offshore and onshore exploration frontiers encountered, salt drilling posed significant challenges to drilling. Because salt behavior is also attributable to salt type and the stress state of the confining geological environment, lateral forces from the rift ridge system and evaporite seal produce high mobility in the halite subsalt accumulations. Major challenges faced while drilling this salt were stuck pipe, tripping in and out issues, wellbore enlargement, and severe bottomhole assembly (BHA) whirl leading to catastrophic lost-in-hole incidents and nonproductive time. In-depth analysis of the formation’s geomechanical attributes, mud properties, drilling practices, and casing design was fundamental to developing better defined and customized drilling solution strategies to address the unique subsalt drilling challenge. Drilling fluid selection, chemistry, and application engineering also played a fundamental role in preventing washed-out holes. Additionally, the salt creeping rate is generally a function of in-situ stress, exposure time, and temperature. Proper mud design and drilling practice help minimize potential risks of hole instability in addition to modifying the casing design to isolate the salt, which requires a different mud weight than the overlying and underlying formations. Most early exploration wells through salt were drilled as vertical wells; however, achieving the necessary directional trajectory control through a mobile salt caused concern. Therefore, potential geomechanical risks, drilling mechanic analysis, such as torque and drag, and potential vibration issues are considered while designing the directional path through a subsalt formation. This paper discusses a comprehensive geomechanical overview, recent successful experiences of drilling salt with the application of drilling fluid, drilling practices, casing design, and trajectory design perspectives in addition to a comprehensive report of lessons learned.