In workover phase prior to commencing sidetrack operation, it is required to recover old existing completion string for isolating & abandoning existing reservoir section in accordance with well integrity and global well abandonment standards. Prior to utilization of the coiled tubing cementing approach, the practice was to recover all existing completion by cutting and pulling out the dual tubing or mill the permanent packer. After all the completion recovery, spot and squeeze cementing operations were conducted. However a major drawback of this process is, until recovering some part of completion string, the actual physical condition of the completion strings remains unknown and it poses high risk to get stuck in cased hole or end up in loosing accessibility inside completion string due to corrosion. Furthermore, in some of the old wells had failure to recover completion components like a dual flow assembly and a dual packer due to completion age, had led to improper zonal isolation. Even if all the old existing completion is recovered successfully, it consumes a lot of operation time and several fishing trips with overshot or junk mill BHA (Bottom Hole Assembly).
In order to minimize the risk of being stuck or loosing accessibility and ending up failing to recover existing completion and to save operational time, the coiled tubing cementing was conducted to isolate existing reservoir and leave remaining parts of completion downhole.
During the operation phase, injectivity test was performed by pumping sea water followed by bull heading kill fluid in to the reservoir. Losses rate was evaluated while observing the well, a high viscosity pill was spotted in order to treat losses and control loss rate. Coiled tubing was rigged up on Long string and run in hole to tag a landing nipple in existing completion string in order to have reference of depth corrected against ORTE (Original Rotary Table Elevation) depths while using the coiled tubing for operations. After having correct reference of depth with tagging completions nipple accessory, coiled tubing with slim OD cementing BHA was run in hole to tag PBTD (Plug Back Total Depth) and then picked up to certain depth while spotting cement slurry at controlled speed. Once the complete amount of slurry was spotted during picking up coiled tubing was pulled out to be away from cement slurry and then coiled tubing BOP (Blow Out Preventer) was closed and cement was squeezed in to the formation. After squeezing pre determined volume or archiving the lock up pressure, coiled tubing was pulled further up and circulated out to ensure all cement slurry out from coiled tubing (inside and outside). Top of cement was confirmed by tagging with the milling assembly connected to coiled tubing and the pressure test was performed after waiting on cement to confirm the integrity of the barrier.
For short string, similar abandonment plug process was followed as that of the long string. After performing tagging operations, cement was spotted while pulling out the coil tubing to certain depth and then coil tubing was picked up above the cement to squeeze cement in to the formation.
Similar coiled tubing cement operation for isolating lower perforations was performed on three other wells, and proper zonal isolation was achieved against reservoirs. This improved approach of abandoning lower reservoir prior to completions recovery proved to save 2-3 days of rig operational time in comparison to previous operations practices of recovering existing completion completely & then perform cementing operations for zonal isolation against each reservoir.
Based on the successful result in three wells, it is concluded that this coiled tubing cement operation is effective for zonal isolation and provide savings in operation days.