Holistic Annulus Investigation as a Tool to Assure Well Integrity in a Deep Water Environment
Authors: Ajayi Ayotunde , Mamman Saratu , Agbahara Chidi , Udo Nsikak , Karunwi Akinola , Ogunsina Oluseye 
Published: July 2014

The physical integrity of a well is a lynchpin for any oil and gas company to achieving it's production target safely. In a deep water field with subsea wells, ensuring the physical integrity of the wells is paramount. It is given key focus because of the high value of the investment, difficultly in accessibility and the sensitivity of the surrounding environment. Subsequently, it is important that all subsea wells in a deepwater field with observed excursions outside of it's annulus limits are investigated in systematic manner to prevent any incidents or loss of containment.

Field X is an offshore deep water asset in the Gulf of Guinea. The field is developed by subsea wells tied back and producing to an FPSO. It's subsea wells are drilled and completed in ca 3300 ft of water targeting hydrocarbon bearing reservoirs, with reservoir depths ranging from 7600 ftss to 10200 ftss. The wells were completed with the full capability to monitor the pressure, temperature and to control the xmas tree valves remotely from the FPSO. The maximum allowable annulus surface pressure (MAASP) and the minimum annulus pressure (minAP) limits are set for each well based on the stress modeling of the expected operating conditions during the life of the well and the casing and tubing stress and tensile limits. The MAASP, MAWOP and MinAP annulus pressure limits are part of the well and reservoir operating envelope (OE) which specify the operating points and the boundaries governing the safe production.

Since the start of production in the mid 2000's, the asset team has observed that several wells exceed the set annulus pressure limits in the operating envelopes. These non-routine excursions, are detected automatically on the FPSO by the DCS and reported on the Processbook and the electronic well integrity management system portal (e-WIMS). These exceptions are an indication of a potential problem or leak within the well or a valve in the subsea xmas tree. To determine the root cause of the annulus pressure excursion or the possible leak paths/points, an annulus investigation is be conducted on the well. The annulus investigation is a workflow that starts with a preliminary risk assessment of the observed excursion. Thereafter, a systematic workflow is followed, which requires a comprehensive work program or FMS to determine the root cause of the suspected leaks. After the investigation is executed in the field, the results are analyzed and a follow-up risk assessment is held to determine the mitigating actions to manage the well safely.

This paper presents the gamut of annulus problems observed in the wells in field X, the premise for executing an annulus investigation, the workflow and the results and the mitigating actions taken to minimize the risk of incident. Well integrity is an important aspect of managing an oil and gas asset. This paper demonstrates how annulus investigations are executed as a effective tool in a process to ensure the physical integrity and safety of these wells.