This paper presents a comprehensive laboratory and field study, discussing the development, formulation, and application of a wellbore strengthening mechanism, for strengthening weak formations while drilling in a deepwater high-pressure/high-temperature (HP/HT) well environment. The use of this technology has potential to eliminate nonproductive time (NPT) related to downhole losses, along with extending the drillability of sections and eliminating additional casing strings, during exploratory drilling.
During the planning phase of a sequence of deepwater and HP/HT exploration wells, the potential high-pressure case scenario drove the planned and contingency well casing designs. This led to an extensive casing program with a 16-in. sub mudline hanger casing string added to the base design, as well as the normal 36-in. conductor, 20-in. surface casing, 13 ⅜-in. intermediate casing, and 9 ⅝-in. casing, which would enable reaching total depth (TD) within a planned 8 ½-in. hole. The realistic offset well driven by the high-pressure case also required two further contingency liner strings (11 ¾ in. and 7 in.), to be included in the well design. A key enabler for the sequence of wells was that the semisubmersible rig was upgraded to include a managed pressure drilling (MPD) below tension ring (BTR) arrangement. This was enhanced by the MPD well control system and associated risk assessment, allowing working to reduced acceptable kick tolerance limits. In addition to the outlined base and contingency plans, wellbore strengthening was also to be available, as an additional contingency application, to reach TD objectives.
Thus, extensive laboratory tests were performed for wellbore strengthening design, using proprietary software, along with past established practices. Subsequent to laboratory testing and the optimal formulation, a detailed wellbore strengthening program was prepared and included in the drilling program, for potential use at any point while drilling ahead. On one well, after cementing of 13 ⅜-in. casing and performing a leakoff test (LOT), it was found that the value was insufficient for drilling through the entire planned section. A contingency 11 ¾-in. liner was being enabled before it was decided to pump the wellbore strengthening pill and strengthen the casing shoe. The pill application gave sufficient increased formation strength, leading to the well section being successfully drilled and cased with no losses, even though the high-pressure well scenario was actually encountered. This solution eliminated the time and cost implication and considerable operational challenges of the 11 ¾-in. contingency liner.
This paper presents the study of conceptualizing the wellbore strengthening mechanism and implementing this customized solution in the field. A detailed analysis is also done to identify the optimal products, compatibility with drilling fluid, formation and existing chemical permit, and cost-effectiveness and savings using wellbore strengthening practice. The paper also discusses the comprehensive pit management program and required treatment plan while drilling.