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Narrow margin procedures incorporating MPD techniques were developed to tackle the challenges of drilling a deepwater well DW-X, offshore Malaysia. The subsurface prognosis had a ‘most likely’ case with a narrow pore pressure – fracture gradient (PPFG) window of between 0.5ppg and 1.0 ppg, all the way from seabed to well total depth (TD). The prognosed high case did not even permit the well to be drilled to TD. The narrow margin approach was used in both pre-drill and drilling phases. The pre-drill phase included the overall well design, section TD decision trees, communication protocol, mud rollover plans, MPD plans and hydraulic and kick tolerance simulation. The drilling phase involved real-time updates to the data, with ongoing simulation updates, MPD utilisation decision trees, and specific operational procedures, such as finger printing, MPD static flowchecks and MPD leak off tests. Since this was an exploration well with limited offset data, there was high PPFG uncertainty in every hole section. Therefore, narrow margin procedures and processes were implemented in each hole section after the BOP had been landed, until well TD had been reached. The pre-drill plan focused on minimising ECD, through optimal casing design, BHA design and mud and cement design. Hydraulic simulations were run with mud hydrostatically at balance with the pore pressure, using applied surface back pressure from MPD to keep bottom hole pressure constant and equalt to the ECD pressure, at all times. Kick tolerance was simulated with a multiphase gas model to be as realistic as possible, and also to be not overly conservative as would be the case for a standard single bubble model. During drilling, the simulations were updated real-time with the actual data from the well, to optimisation casing shoe placement for each section. This led to elimination of the 11-3/4" and 7" liners, providing significant cost and time savings to the well. By optimising the operational procedures and reviewing the MPD utilisation for each section, the well was safely drilled to TD, only using MPD in those sections that required it, even though MPD had originally been planned for all sections. The combination of finding slightly wider PPFG windows than predicted, and finding MPD surface-line back pressures slightly higher than modelled, meant that MPD was not beneficial in all hole sections. The well was successfully drilled to TD, below cost and time compared to the original AFE, in spite of extending TD by more than 150m to cope with deeper formation tops than prognosis. Thus all of the drilling objectives were successfully delivered in the well. The approach described in this paper can be applied to future narrow margin deepwater wells, with valuable lessons learned on MPD setup and utilisation for each hole section. |
