Restoring Integrity and Production on a Well with Compromised Barriers: Use of Crosslinked Polymer Gel to Prevent Fluid Loss Post Well Kill to Enable the Safe Repair of Compromised Christmas Tree Valves

Abstract

Abstract Operating Oil and Gas wells safely requires two tested and independent barriers are in place at all times. If a barrier is lost, all efforts should be focused on restoring the lost barrier. This requires a detailed risk assessment is carried out and an approved deviation in place. The objective in this case was to ensure the only barrier, kill fluid, is not lost to the formation and ensure zero damage to the reservoir. In this paper, the detailed planning and execution of a well in the Eastern Delta (Ukos-21L) with compromised tree valves due to a blown-up Wireline Retrievable Surface Controlled Sub-Surface Safety Valve (WRSCSSV) that got stuck across the tree valves are presented. With a reservoir pressure of 4,000psi, this posed serious HSE risks. The intervention involved establishing injectivity through the kill wing valve. The well was bull-headed to 0psi with appropriate kill brine. A crosslinked polymer gel was then pumped to prevent fluid loss of the only barrier (kill brine) and to ensure well remained stable at 0psi throughout the barrier restoration One of the key project deliverables was to ensure the reservoir was not impaired and the well productivity unchanged during the barrier restoration activity and this was confirmed after the well was successfully put back to production post restoration operation. The crosslinked gel placed across the perforated interval prevented fluid loss without penetrating the formation but due to the introduction of internal chemical breaker, the uncross-linking of the polymer gel was done in-situ after the specified design period elapsed as determined through laboratory test done based on the formation downhole temperature. The well continued to produce ca.880bopd post the integrity restoration. The restored valves were all operable. It was also observed that the installed WRSCSSV was not dressed with the right size of packings, making it possible for the valve to be blown out of its profile. The use of polymer gel is a safe means of preventing fluid loss under a one barrier condition (fluid barrier). The pre-determined breaking period for the cross-linked polymer gel was considered and very critical to the successful execution of the activity. The execution was achieved four days ahead of the planned time with zero non productive time, NPT recorded. Hence, coiled tubing, CT was deployed to nitrogen lift the well to production post integrity restoration. Production was restored three days ahead of schedule.