Concerted Approach for Annular Pressure Build-Up APB Mitigations to Safeguard Well Integrity of Subsea, High Temperature Carbonate Exploration Well

Exploration well with carbonate reservoir is a challenging well to plan for due to risk of total losses because of karst presence. It became even more challenging for a subsea well with high bottom hole temperature (BHT) and prospect of well testing. Flow of HT reservoir fluids (BHT up to 175 deg C) to surface will resulted in significant heat transfer to adjacent casing & its annulus fluids, and lead to annular pressure build-up (APB). High APB will lead to loss of well integrity via 13-3/8" intermediate casing burst and 9-7/8" production casing collapse if left unmitigated. As per The Company technical standards, two APB mitigations were required in a subsea well. The first selected mitigation is an open casing shoe. The exposed shoe will act as a natural relief valve whenever APB exceeding its fracture pressure (FP), therefore, limit the APB to its FP. However, it is challenging to keep the 9-7/8" casing top of cement (TOC) below the 13-3/8" casing shoe and fulfil the open shoe barrier requirement for this well where the open hole interval is relatively short and subject to be plugged off by barite sagging, insufficient open shoe length for safety margin of excess cement and requirement of minimum annulus cement length for shoe integrity. Extra mitigations were addressed through extensive lab tested solids-free annulus fluid to mitigate barite sagging. Open shoe interval also designed with multiple weak sands exposure and higher FP were considered for worst-case APB simulation. The second barrier is the 13-3/8" intermediate casing and 9-7/8" production casing itself. Based on WellCAT simulation, the intermediate casing unable to meet The Company standards of burst (safety factor, SF < 1.1) in the worst-case scenario whereby APB is unmitigated. The casing burst pressure rating was recalculated using API Bulletin 5C3 equation with the inputs taken from minimum actual casing wall thickness measurement and internal yield pressure from its mill certificate. Technical derogations were raised and approved once the casing passed all the load cases using the revised burst rating by minimum SF of 1.0. The well was delivered successfully with the open hole barrier for both casing was executed flawlessly despite the complex fluid train while cementing.