Lost Circulation Assessment and Planning Program: Evolving Strategy to Control Severe Losses in Deepwater Projects

Abstract There was a time when simply dumping some mica or nut hulls down a wellbore was a standard procedure for stopping mud losses. Certainly, these materials still work - for some applications. But, many of today's drilling operations bear little resemblance to that not-so-distant past. The industry is accelerating its activities in deepwater and depleted zones, both of which present narrow operating limits, young sedimentary formations, and high degree of depletion overbalanced drilling. All of these now-common conditions are ideal for fracturing and lost circulation. Drilling through and below salt formations presents a host of technical challenges as well. The thief zone at the base of the salt can introduce severe lost circulation and well control problems, often resulting in loss of the interval or the entire well. Controlling losses in this zone has proven to be extremely difficult. Very few lost circulation remedies have been successful, especially when using invert emulsion drilling fluids. The lost time treating severe sub-salt losses can last for several weeks, with obvious cost implications, especially for deepwater drilling operations. This paper reviews the methods applied to avoid lost circulation in the sub-salt thief zone, as well as in the subsequent intervals for a Gulf of Mexico deepwater project and discusses the time and cost savings obtained. For maximum success in these situations, emphasis has been placed on assessment and planning rather than individual lost circulation products. The authors will detail the Lost Circulation Assessment and Planning process that has been employed to explore and evaluate specific lost circulation problems and link them to existing products, systems and services. The integrative pre-planning process analyzes offset histories and formation data not only to identify risk zones but also to gather information on the exact fracture and pore size as well as fracture density. Afterward, detailed interval-specific decision-tree charts are developed for stopping losses encountered while drilling or tripping in. In addition to discussing the application of this Assessment and Planning process, the authors will outline the lessons learned on the deepwater project, featuring pre-planning issues geared toward ensuring circulation is maintained throughout the wellbore. Introduction and Problem Definition The problem of lost circulation was apparent in the early history of the drilling industry and was magnified considerably when operators began drilling deeper and/or depleted formations. The industry spends millions of dollars a year to combat lost circulation and the detrimental effects it propagates, such as lost of rig time, stuck pipe, blow-outs and, frequently, the abandonment of expensive wells. Moreover, lost circulation has even been blamed for minimized production in that losses have resulted in failure to secure production tests and samples, while the plugging of production zones have led to decreased productivity.1 When dealing with induced fractures the problem is even more complicated, as the shape and structure of induced formation fractures are always subject to the nature of the formation, drilling and mechanical effects, as well as geological influences over time. One condition of paramount importance in sealing induced fractures is having the lost circulation material (LCM) reaching the tip of the fracture.2 Related to the "breathing" tendency of induced fractures (to change shape and size as per wellbore pressure changes), "pressure buffering" is another condition that has to be fulfilled for effective sealing. Ideally, to stop the breathing tendency in a robust manner, the pills should be able increase the fracture gradient at a level sufficiently high to avoid re-opening the fracture during the subsequent drilling phases.2