Reinventing Well Integrity and Zonal Isolation in Well Abandonment Through Prudent Subsurface Data Integration and Novel Dual Casings Cement Bond Assessment Technology

Abstract

Recurring safety incidents due to unexpected gas leaks from downhole and sustained annulus pressure issue have become major catastrophic events which warrant an integrated solution to determine the leakage pathways, the root causes and the remedial job required. The unexpected gas migration has impacted the production leading to permanent well abandonment leaving a significant gap to meet the production target. Hence, a proactive diagnostic and remedial solution are required in place as an agile response to the uprising crises. This paper will highlight few case studies from well abandonment in offshore Malaysia involving gas migration and the application of emerging technology to diagnose the root cause and source of the unwanted gas. Major sources of gas migration can include the shallow gas accumulation and gas channeling from deeper reservoirs either through tubing leak or behind casing seepage through the poorly bonded cement across the cased interval. A novel technology combining fiber optic, acoustic and ultrasonic measurement principles were deployed to simultaneously evaluate cement bond quality and zonal isolation in both single and dual casing layers. Compressional sonic measurement gives indication of shallow gas presence in the far field due to the deeper depth of investigation while fiber optic data will reveal the near wellbore effects behind the casing. In addition fluid samples from pre and post production wells were also analysed for leakage source determination. Results indicate that cement bond quality across the target cap rock intervals are mostly poor and do not qualify as barrier in well abandonment. This finding agrees well with compressional sonic logging and fiber optic run in the shallower well section where absence of cement has caused intense casing ringing and impacts the log data quality accordingly. The novel dual casings cement bond logging tool also indicates poor cement isolation when run across the double casing layers. The observation has been validated with independent cement bond logging run in the outer casing after the inner casing was cut and pulled. Another case study confirms presence of good cement isolation behind the casing after log validation run post tubing retrieval. Comparison of hydrocarbon composition between pre and post production fluid samples taken in similar reservoir group also revealed that the source of gas migration is coming from the deeper reservoirs through the upper tubing leak as confirmed by tubing integrity check. In conclusion, deployment of dual casings cement bond logging technology has successfully provided critical insights in real time for further decision on the remedial job. Operational efficiency, cost and time have been significantly enhanced by minimizing online rig activities and reducing equipment standby period. The case study represents an innovative and integrated solution combining both near and far field perspectives for safe well abandonment. Particularly for dual casings cement integrity evaluation special care needs to be taken during job planning, monitoring and interpretation of the results due to limited database and reference data sets.