Optimization of Mud Cap Design for Tripping Operations and its Application in Challenging HPHT MPD Wells: Case Histories from Pakistan

Growing reliance and new technologies have significantly optimized the drilling operations; un-drillable wells become drillable now. We strive to increase drilling efficiency, whether managed pressure drilling (MPD) or conventional drilling operations. Designing an MPD operation inherits multiple challenges and requires the evaluation of numerous parameters. These include bottom hole pressure management, tripping, cementing, logging & completion operations. However, this paper only describes how the tripping processes can be optimized using effective mud cap design in very narrow window HP/HT MPD wells. Constant bottom hole MPD (CBH-MPD) is one of the extensively used variants of MPD around the globe. In CBH-MPD, Surface backpressure is the differentiating factor added to keep the bottom hole pressure (BHP) constant in static and dynamic conditions. A close-loop is necessary to attain this control on the annulus pressure with specialized equipment, including effective sealing around the drill string and choking the return flow. The sealing is only effective against the smooth surfaces; hence, one of the most significant challenges in MPD operations arises: having a constant BHP in case of pull out to the surface. If the downhole isolation valve (DIV) is deployed, the well can be shut on it. However, if a DIV is not available, the well can be balanced by designing an effective mud cap keeping the available trip margins. Planning and executing a mud cap for MPD is challenging and can be overlooked in the planning phase. Designing a mud cap is complicated as many factors need to be incorporated. This paper describes, in detail, how these challenges were identified, planned for, and overcome. Multiple parameters were analyzed in sequences for subjected wells to optimize the mud cap weight and spotting depths. These parameters include the available tripping window, bottom hole temperature, circulation pressures, mud additives, mud rheology, surge, and swab pressures. Residing time of the mud cap in the hole is also a key factor considered in the design as it may also disturb the CBH pressure. Other design considerations included rig compatibility and volume handling capacity, equipment limitations downhole and surface, availability of chemicals, and effective rollover plan. The effective mud cap design for these narrow HP/HT wells mentioned in these case studies was an arduous and challenging task. This paper also discusses the aforementioned mud cap design considerations and their effects on selecting an appropriate mud cap. Practical examples are shared from challenging case studies, elaborating the detailed design and execution aspects.