This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 28484, “Learnings From the High-Temperature Water-Based-Mud Application in Offshore Sarawak,” by M. Abshar M. Nor, SPE, M. Solehuddin A. Razak, and Lingges Devadass, Petronas, prepared for the 2018 Offshore Technology Conference Asia, Kuala Lumpur, 20–23 March. The paper has not been peer reviewed. Copyright 2018 Offshore Technology Conference. Reproduced by permission.
The reservoir section of a gas field offshore Sarawak, Malaysia, consists of two massive pinnacle carbonate structures with heterogeneous porosity and permeability and many vugs and faults. Severe to total loss of circulation was expected along with a high bottomhole temperature (BHT). Considering the high risk of lost circulation, the drilling-fluid strategy involved designing and testing a high-temperature (HT) -tolerant water-based mud (WBM). While the main hole was drilled successfully with the HT WBM, the sidetrack experienced a significant reduction in rate of penetration (ROP).
Well Design and Execution Challenges
During the design stage, determination of the severity of losses in the carbonate formation was met with high uncertainty. Consequently, a WBM system was selected to drill the 8½-in. reservoir section. A shale formation was predicted to be above the carbonate section, and a 95/8-in. casing was planned to be set as close as possible at the top of the carbonate to isolate the shale. With the 95/8-in. casing in place, the WBM was to be used to drill the shoe track and the remaining small shale section before entering the carbonate reservoir.
The BHT of the reservoir was predicted to be 370°F, which was extrapolated from data from offset wells. In addition, the pore pressure in the reservoir was overpressured approximately 14 lbm/gal because of a gas column in the carbonate. The high BHT coupled with the high mud-weight requirement of up to 14.5 lbm/gal led to the design of an unconventional HT WBM, which was a first for the operator. Special attention also was given to the corrosion risk on the drillstring from expected hydrogen sulfide levels up to 150 ppm and carbon dioxide levels up to 20% in the reservoir.
Predrill Laboratory Study
Laboratory tests were conducted to address major well challenges and overall performance of the system under extreme borehole temperatures. To ensure the mud system was able to withstand temperatures up to 400°F, a synthetic polymer was introduced to the mud design. Several mud formulations were tested and fine-tuned to achieve well-specific mud specifications for optimal hydraulic capability.
Laboratory tests in accordance with American Petroleum Institute Recommended Practice 13B were conducted to evaluate the mud properties with various concentrations of additives. Four samples—Samples A, B, C, and D—were prepared and submitted to the same test procedure to understand the mud rheological behavior upon exposure to the HT environment.
