Integrated Approach for Overpressure Prediction in an HP/HT Well Offshore Malaysia

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 18909, “An Integrated Approach for Overpressure Prediction in a Wildcat High-Pressure, High-Temperature (HP/HT) Exploration Well Offshore Sarawak, Malaysia,” by Avirup Chatterjee, Amitava Ghosh, and Sanjeev Bordoloi, Baker Hughes, and Anifadora Bt. Mustapha, Petronas, prepared for the 2016 International Petroleum Technology Conference, Bangkok, Thailand, 14–16 November. The paper has not been peer reviewed. Copyright 2016 International Petroleum Technology Conference. Reproduced by permission. Pore-pressure prediction plays an important role in well planning as exploration targets shift to deeper, overpressured reservoirs. Pressure-related problems in such zones are mainly associated with narrow operating windows, resulting in severe well-control incidents, sometimes even leading to early abandonment. Uncertainties in prediction models arise from input data, assumptions used in the work flow, and the complexity of the geological or structural conditions. It is important to analyze these uncertainties and develop an understanding of them before drilling. Introduction Central Luconia is a geological province of the Sarawak Basin characterized by extensive development of Miocene to Holocene carbonate buildups. The buildups reveal complex seismic geomorphologies, facies, and structural setting. There have been several discoveries in the shallow-water (approximately 90-m depth) area of central Luconia province offshore Sarawak. A predrill pore-pressure- and fracture-gradient-prediction study was initiated for a planned high-pressure/high-temperature (HP/HT) well with three offset wells (Offsets 1, 2, and 3) drilled in the vicinity of the study area. The objective of the study was to help in well design to reach desired deeper target reservoirs. The prospect is situated in a structure characterized by a northwest/southeast normal fault. It was formed on the foot wall block for the shallow (post-carbonate clastic) reservoirs and crossing the fault to the hanging wall block to hit the main targets in deeper (precarbonate clastic) cycles. The lithology for the shallow cycles is interbedded fine-grained siliciclastic shale and sand with limestone streaks. The deeper cycles consist of predominantly sandstones with interbedded shales. Two offset wells (Offsets 2 and 3) penetrated a thick carbonate sequence, and the other offset well (Offset 1) encountered a thick sand/shale sequence with interbedded carbonate stringers, which is similar to the prognosed-well result. Therefore, in the predrill modeling, there was only one relevant offset well (Offset 1) available in the study area that penetrated similar lithostratigraphic units. Well Issues A detailed analysis of all the drilling problems encountered in the offset wells was catalogued. This was achieved by reviewing the daily drilling reports, final well reports, and mud-logging reports and documenting all the events on a daily basis. Overall, all the offset wells analyzed during the study indicated several drilling issues related to pore pressure, including internal blowout with violent gas flow from the bell nipple, influxes, gas-cut mud, high gases, connection gases, pumps-off gas, and cavings. In addition, wells showed significant challenging drilling conditions such as equivalent static- and circulating-density contrast because of high temperature, wellbore breathing caused by a narrow mud window, and several hole-stability issues such as tight hole, lost circulation, and stuck pipe, eventually leading to multiple sidetracks.