Integrity Risk Management Pre-Drilling Workflow for Seabed Gas Bubbles in Gulf of Thailand (GoT) Offshore Gas Fields
Authors: Yuhun P. , Intasalo P. , Sornchangwat B. , Loux F. , Thatmali P. 
Published: February 2025

Seabed gas bubble has been identified on various GoT platforms, posing significant risks to both drilling and platform integrity. This has led to their immediate removal of the project to the drilling sequence, resulting in interruptions such as rig standby. This study prioritizes the development of a comprehensive drilling planning workflow on gas bubble wellhead platforms (WHPs) to ensure safe operations prior to the rig's arrival and to mitigate the occurrence of unexpected interruptions to the rig activities.

A comprehensive investigation covered all infill WHPs planned before 2027. The presence of seabed gas bubbles, detected via Remotely Operated Vehicle (ROV) survey, prompted gas sample collection for origin composition analysis. Compared against the produced gas zone, this distinction as Biogenic or Thermogenic aids in risk evaluation. Additionally, pulse neutron log (PNL) coupled with seismic information and drilling history pinpoints potential shallow gas in the riserless section, aiding in leak-path determination. Gas-zone identification in this section signals a high-risk scenario, necessitating specialized risk assessment for planning-mitigation. Conversely, gas zones' absence suggests a low-risk level for our well design.

Our investigation discovered gas presence at 14 WHPs on the seabed by ROV. Through composition analysis, one platform was identified as thermogenic, while the others produce biogenic gas. PNL logging on 12 biogenic platforms detected no-gas effects at the riserless section, suggesting low-risk drilling operation. The leak path is expected from randomly nonproductive reservoir gas formed by anaerobic decomposition instead. One biogenic platform lacking existing well to perform PNL was then classified as high-risk. Regarding one thermogenic platform, a comprehensive review of potential leak paths on drilling history, seismic data, and casing pressure information indicated a shallow sand break due to inner-and-outer casing communication. Despite no gas accumulation in the riserless section, the well was abandoned to seal that leak. Subsequent ROV inspection found no bubbles, reducing drilling risk to low. This pre-drilling workflow classified 13 platforms (12 biogenic + 1 thermogenic) as low risk for drilling, continuing as scheduled. The one high-risk platform was deferred from current drilling schedule (DSQ) beyond 2027 for further risk mitigation. This approach aids in evaluating drilling integrity risks from gas bubbles before rig arrival, averting unexpected rig-standby cost. Confirming drilling feasibility contributes adding at least 100 wells to infill drilling, enhancing Thailand energy security.

This project tackles the integrity hurdles encountered during well construction in the GoT gas brown field due to seabed gas bubble issues. To support well factory drilling model, which is low-cost drilling development solution for low connectivity of subsurface pay zones, implementing this pre-drilling workflow effectively screens risks from low to high before drilling commences, facilitating the seamless management of rig schedules without interruptions. This project is also a first joint development workflow to encounter seabed gas bubble WHP resulted in standardizing and applying for all PTTEP asset guideline.