Four deepwater wells in the Gulf of Mexico were identified for permanent abandonment in accordance with local regulations. The abandonment plan called for multiple cement plugs to isolate the production zones from seabed in each well. The most challenging cement plugs in each well were the ones directly above the production packer isolating the casing-tubing annulus and the production tubing. To avoid cement left in the Christmas tree at seabed and potential plugging off any valves in the manifold, cement plugs could not be placed the direct way, pumping down the production tubing.

An unconventional approach was proposed to address the challenge. It involved reverse placement of cement plugs, which is not common in deepwater even in these days. Using this technique, cement was pumped down the casing-production tubing annulus and, through perforations, back up the production tubing. Risks analysis indicated very low likelihood of plugging off any valves in the tree. However, this configuration did not allow using of any mechanical separators between fluids to prevent intermixing. Additional challenges in placing the plug were the high deviation of the section and the completion brine in the wellbore.

Simulation of reverse placement is not possible with existing software. Therefore, the jobs were designed using experimental software, which enabled the design engineer to accurately reconstruct field conditions. Additional attention was given to the job procedure to minimize contamination with the brine and optimize cement placement. Viscous spacer was pumped ahead and behind the slurry to displace the brine. The slurry was designed with low fluid loss to be squeezed through perforations in the production tubing without plugging them off.

A total of seven plugs were placed using the reverse placement technique. Specific requirements with regards to the top of cement and plug integrity had to be met before any of the cement plugs could be accepted by the operator. All plugs were tagged and pressure tested successfully in the annulus as well as inside the production tubing on the first attempt. As a result of the campaign, the four wells were abandoned as per schedule and within budget.