Gulf of Thailand 4-String Well Design Transformation Using MPD System – Cost Saving, Operational Challenges and Learnings

Abstract

Gulf of Thailand (GoT) or Gulf of Siam is mainly offshore development gas wells in Thailand. Well C and well H were originally designed with GoT 4-string standard template. However, Managed Pressure Drilling (MPD) solution was then selected to save one (1) casing string provided that the maximum expected formation pressure to be within the range of 1.46-1.50SG as per geological prognosis. Both wells were drilled to plan TD with 3-casing strings as planned to result in a cost-saving from the design transformation (4-strings to 3-strings) using the MPD system. There is always a risk of lost circulation and ballooning in intermediate/high-pressure wells (Pore Pressure (PP) > 1.45SG) when drilling conventionally with 3-casing strings (instead of GoT 4-string standard design). Required high mud weight imposes an overbalance during drilling and tripping operations that exceed formation integrity at shallower depths below the 7" casing. MPD is a proven technology for maintaining Constant Bottom Hole Pressure (CBHP) that enables precise control on wellbore pressure to mitigate ballooning and kick/loss hazards, narrow margin drilling window, and formation pressure uncertainties. The 6-1/8" hole sections of both wells were drilled with the MPD technology to mitigate the loss of circulation and ballooning while drilling. MPD system worked for this project to drill the wells to TD successfully according to the plan and without any major issue. The MPD solution enabled drilling with the lowest and safest underbalanced mud weight to allow a reduction in Bottom Hole Pressure (BHP) and consequently eliminated ballooning while drilling. Surface back pressure (SBP) on connections was increased (compared to the plan) based on connection gas reading caused by pore pressure uncertainties. MPD dynamic formation integrity test (DFIT) and static pore pressure tests (PPT) have established a baseline for heavy kill mud weight estimations and displacing before tripping out of the hole. Observations and learnings from well C confirmed the study’s preliminary conclusions that the 4-strings transformation is quite challenging for PP greater than 1.55SG, and 4-string transformation is not recommended if PP exceeds 1.60SG. The wells were completed with 3-casing strings however, no NPT related to well design and MPD was encountered. The technology was proven and allowed to drill the well with 3-strings design. The solution can save some cost in intermediate pressure wells (1.46-1.54SG PP), and it is proposed for the next trials with more focus on wellbore conditioning and tripping practices.