Successful Recovery of Behind Pipe Reserves in Highly Deviated Well Using the Balanced Cement Plug Concept for Cement Packer Installation through Coiled Tubing: Offshore Niger Delta
Authors: Onuh Haruna , Ogubuike Kenneth , Osaronwaji Emmanuel , Ibrahim Charles , Chemmarikattil Rohan , Nwaigwe Leonard 
Published: August 2020

In challenging horizontal wellbores with subhydrostatic well conditions and the inherent uncertainties associated with mechanical bridge plug installation, the balanced cement plug concept becomes the appropriate method for cement packer installation. The balanced cement plug method requires creating a balanced U-tube with the hydrostatic pressure consisting of a column of brine, spacers, and cement slurry in the annulus of a tubing/drillstring equating the hydrostatic head in the drillstring/tubing and annulus. Fluid volumes are calculated, accounting for fluids both inside and outside the pipe at the given gradient/head, resulting in a hydrostatically "balanced system."

Recently, this technology has been successfully deployed for cement packer design and installation using coiled tubing (CT) to place 813 ft of cement packer in the 3 1/2-in. tubing × 9 5/8-in.casing annulus from bottom of tubing punch at 5,913 to 5,100 ft MD (685 ft above shallowest perforations). The location of the production packer at a depth of 629 ft with 9.6-lbm/gal calcium chloride brine (cement accelerator) existing below the tubing punch interval in the tubing-casing annulus presents challenges with respect to the cement slumping through the brine. Detailed engineering design and execution is necessary to overcoming such inherent challenges. The zonal isolation of the existing subhydrostatic mature oil zone within the horizontal wellbore (793 ft blank liner and 1,698 ft predrilled liner) was achieved through placement of 575 ft of cement plug in the 3 1/2-in. tubing from 6,511 to 5936 ft MD.

A rigless through-tubing balanced cement plug concept for cement packer installation and reperforations was deployed to access behind pipe oil reserves in a high-angle (69 to 72º) deviated well, Offshore Niger Delta. Design considerations were reviewed and precautions for placement through CT were discussed. The operation was successfully executed and the temperature log confirmed the TOC as proposed during the design phase. The HUD in the tubing was also tagged as expected with no cement U-tubing from the annulus. Post-job shut-in tubing and casing pressures, quantity of cement pumped, and flow tests have proven the success of the design and procedure implemented in challenging wellbores. The stable production post-hookup to the production line presents 212% incremental production without sand exclusion against the proposed well intervention objectives.

This presents the advancement in cement packer installation in complex well trajectory with high potential of U-tubing attributed to failed retainer packer for accessing bypassed oil reserves. The balanced cement plug engineering design presents an interplay between 8.5-lbm/gal inhibited sea water, 8.33-lbm/gal spacer (drill water+surfactant), 8.5- and 9.7-lbm/gal brine, 15.8-lbm/gal neat cement slurry above the 9.6-lbm/gal existing completion calcium chloride brine, and pressures in the casing/tubing annulus and tubing to balance the fluid without a retainer packer.