Post-Drill Pore Pressure Study For Infill Well Design To Uncover Deep Gas Potential Left Behind Primary Drilling Campaign (Gulf Of Thailand)
The area of study is located in the South-West of Pattani basin, characterized by complex structural trend of multiple North-South trending graben systems. Highly faulted and stratigraphically complex reservoirs in this area are commonly overpressured, and can exhibit significant differences in formation pressure gradients between the shallow and deep formations, adjacent fault blocks, and over some distances within individual fault blocks. The new platform wells were planned along several normal fault planes, 500m-4500m from the existing wells. The drilling program was divided in 2 batches. Project pore pressure profiles were generally defined by fault block using formation tests from the nearest platform to the North. First several wells drilled during the campaign encountered well control events due to high formation pressures, resulting in shorter TDs and considerable NPT. New formation tests indicated that some intervals had pore pressures more than 1.5ppg EMW higher than predicted, leading to underbalanced drilling. Pore pressure regimes appear to change over a distance of 1-1.5 km along delineated fault blocks within previously tested stratigraphic intervals. Starting from the first well control event the project team had to make changes to the drilling order strategy, selecting less risky wells to acquire maximum data for the rest of the program. Acquired data revealed considerably higher magnitude of overpressure and its steeper ramp, which led to change of all pre-drill pore pressure profiles after 5 first wells were drilled. Early investigation identified that elevated structure away from the offset platform acts as a considerable size gas trap with potential pressure communication across the faults. Pre-drill pressure profiles show clear difference between the fault blocks, however in this area normal faults make a complex East-West dipping fault junction and pressure trends become less predictable. Although the team expected and accounted for this communication, it was eventually observed at a wider area, extended to shallower stratigraphic intervals, and with higher overpressure. As most of new wells showed higher pay counts it was important to adequately revisit pore pressures so further wells can cover the pay window, but on-the-fly analysis and measures taken could not mitigate the risk for all the wells. Further well control events led to shortening TDs and deferring a number of wells to future infill campaign, because pore pressures encountered exceed the standard well design limit. Subsurface and economic analysis indicated feasibility of deferring high risk wells to 4-string design infill drilling project. The post-drill study is focusing on detailed structural interpretation, and integrating all the recently acquired well logs and the data from the offset platforms with seismic velocities to predict formation pressures below the depths reached during the drilling campaign. It is important to accomplish pore pressure prediction in the well control areas and adequately design 4-string wells so the remaining potential of this complex structure is uncovered and monetized.
AAPG Datapages/Search and Discovery Article #90331©2018 AAPG Asia Pacific Region GTW, Back to the Future – The Past and Future of Oil and Gas Production in the Asia Pacific Region, Bangkok, Thailand, September 26-27, 2018