Implementation of Managed Pressure Drilling in Through Tubing Rotary Drilling Operations Conducted from a Floater
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As mature fields face depletion the ability to reach smaller near- wellbore pockets of hydrocarbon resources at a low cost is paramount to prolong the economical lifetime of the field. The cost associated with drilling and completion conventionally and well engineering risks often results in classifying several fields as uneconomical to develop further. However,increasing awareness in the oil industry towards this predicament has resulted in progress in technology. Through tubing rotary drilling (TTRD) is a technology that enables access of near wellbore targets at significantly reduced drilling and completion costs by sidetracking from existing completion. Still, performing TTRD operations to increase the economical benefit of a mature field faces a number of challenges. There is an inherent risk of encountering depleted reservoir zones with narrow margins between the pore pressure and fracture pressure of the formation. It is therefore difficult to drill through this fragile formation without introducing considerable formation damage which will reduce the deliverability of the reservoir and impair the safety of the operation. Managed pressure drilling (MPD) is a general term describing technologies that allow control of the annular pressure profile downhole. By implementing MPD in TTRD operations, the problems associated with drilling in fragile formations are mitigated. Managed pressure drilling does not invite influx into the wellbore, in contrast to underbalanced drilling,instead the technology enables drilling at closed to balanced or slightly overbalanced conditions. This makes it possible to drill through fragile formations with minimal formation damage and well control issues. During this work, available MPD technologies where evaluated with the systems inherent limitations and advantages. On this background, four MPD systems were determined applicable for TTRD implementation; pressurized system with backpressure control, low riser return system (LRRS), subsea mudlift drilling (SMD) and drillstring integrated equivalent circulation density reduction tool (DI ECD RT). An analysis was realized to evaluate the selected MPD systems based on TTRD feasibility.The technologies were rated based on a grading system to get a good overview over the methods inherent weaknesses and strength with respect to TTRD integration. The result of the analysis shows a significant potential for improvement by implementing MPD technology compared to conventional drilling various aspects of TTRD operations by implementing MPD technology in TTRD operations. Furthermore a case study was realized to demonstrate the ability of the evaluated MPDmethods to compensate for the added dynamical pressure during circulation. The case is intended to reflect a realistic TTRD drilling scenario with narrow pore pressure and fracture pressure margins and small annular dimensions. It was demonstrated that conventional drilling for the selected drilling scenario would have resulted in severe overbalanced conditions and formation fracture. However, with MPD technology it is possible to produce a pressure relief to compensate for the added dynamical pressure during drilling and bring the well back into balanced conditions. Of the four evaluated MPD systems, it was suggested that the drill string integrated ECDreduction tool was the most applicable technology to implement in TTRD operations. The tool is fueled by the kinetic energy of the circulating fluid and does not require a power source. It can be integrated in the drillstring and repositioned according to operational needs.In order to successfully implement the device the current design of the tool has to be downsized to meet the specifications of the TTRD stack. Further development with respect to annular pump effect, wear resistance and tool usage is also necessary prior to TTRD implementation. In contrast with the drillstring ECD reduction tool, the pressurized system with backpressure control, subsea mud lift drilling and low riser return system offers manual management of the annular pressure profile in the well. However a higher level of complexity is associated with implementing these technologies.