Abstract
Wells plug & abandonment was carried out in a deepwater field (Field C) offshore West Africa. There were 15 deepwater subsea wells, in this field. Thirteen of the wells were completed with Open Water Vertical Xmas Tree (OXT) while remaining two were completed with Enhanced Vertical Xmas Tree (EVXT). All the wells were permanently abandoned with permanent barriers established in accordance to Norsok D-010, rev. 2013. This involved establishing well barriers which would both horizontally and vertically effective.
The operator evaluated several methods in establishing well barriers for the permanent abandonment purpose of these 15 wells. The barriers placement for areas with good annulus cement were straightforward as an internal cement plug could be spotted as a permanent barrier. For this particular field, it is noted that all of the 15 wells have hydrocarbon bearing and permeable formations above the top of expected annulus lead cement which is designed basically to manage ECD during cementing job rather than giving good annular isolation. Due to this, it is important for the operator to employ the best technique that could establish a permanent cement barrier both inside the casing and in the annulus at the same time. Operator learned that Section Milling was the widely accepted method to establish barrier in cases where there is poor cement or the annulus cement is not present particularly in land and offshore wells. However, this technique poses severe operational risk especially in a deepwater operation mode.
Upon review, it was determined that Perforate, Wash and Cement (PWC) technology has been proven and it would become a valuable technique in establishing well barrier which complies with abandonment requirement as per the industry guidelines. High risk activities for deepwater operations such as section milling could be avoided by employing this method. Nonetheless, the Perforate, Wash and Cement (PWC) technology has its own set of risks especially with its method of establishing the well barrier immediately upon setting it. This involves drilling and re-logging the well immediately after setting the cement in annulus to prove the barrier. Upon discussion with PWC contractor, it was agreed that the verification technique may create additional hazards as it could jeopardize the integrity of the installed barrier in the annulus. The additional requirement for cement verification process will also prolong the project duration, hence is an additional cost to the project too.
Operator worked together with the Perforate Wash and Cement (PWC) Contractor and Well Examiner to determine the best verification method for PWC plugs with the objective to comply with regulation on cement verification while at the same time, resulting in most cost-efficient and time-efficient operation. The establishment of Element Acceptance Criteria as an alternative Barrier Verification Process for PWC technology is necessary to improve the reliability of the system as well as reducing associated risks. In total, 18 PWC operations were successfully carried out throughout the campaign. An extensive after action review was conducted at the completion of each job to capture the valuable lessons learnt. These lessons learnt are shared in this paper as well which could add value for future operations with similar settings.
It is expected that this paper will serve as a reference in the establishment of Element Acceptance Criteria as well as its implementation during the P&A campaign. It is hoped that the lessons learnt shared in the paper could assist other Operators on planning for similar campaigns in the future.
