Abstract
This paper highlights the difficulties and limitations faced along with the respective solutions proposed and executed during the zonal isolation of a highly permeable, naturally fractured and acidized carbonate reservoir during its exploration phase after having performed an extended pressure buildup (PBU) test on a remote field in Saudi Arabia.
To accurately compute the hydrocarbon reserves of a particular region, operators must explore new reservoirs in remote areas, in both challenging offshore and onshore environments, to decide future field development. The services performed on these wells include drill-stem test (DST) completions, underbalance perforation, acid stimulation, placing downhole gauges/shut-in tools, extended flowback and PBU periods, and final zonal isolation.
In this candidate well, the operator wanted to test three zones, further performing high-rate acid fractures on the lower two zones to be able to determine commercial hydrocarbon gas rates. The first major challenge was for zonal isolation of the lowest zone, expected to be a highly fractured zone that might also be stimulated. A cement plug was designed to be deployed by means of coiled tubing (CT) for accurate placement and depth control. The added challenge in this exploratory well was to plug any wellbore channels and highly permeable fractured streaks in the lowest reservoir zone after acid stimulation and reservoir evaluation were completed while preventing excessive cement fluid loss into the zone from the cement volume that would challenge the plug effectiveness. This paper explains the innovative solution devised for this well, including the job execution and elaborating on the combination of CT intervention, carefully designed chemical blends, cement fluid handling systems, and implementation of best practices to deliver a successful zonal abandonment. The operation consisted of three runs: the first used a lead batch of an organically crosslinked polymer with loss circulation material (LCM) in the solution; the second was performed for cleanout, displacement, and permanent cement slurry placement; and, after waiting on cement (WOC), the third was used to tag the top of cement (TOC) and perform a pressure test. The organically crosslinked polymer with LCM solids served to bridge off the perforation tunnels and potential fractures in the rock so that a precise volume of cement plug slurry could be pumped safely and accurately.
