Offshore Kalimantan presents some unique challenges in zonal isolation. Two of the problems include low fracture pressure and a shallow gas zone. To overcome these challenges, the operator had resorted to pumping three different slurry systems, each designed to overcome one specific problem. The three slurries were:
Low-density, gas tight slurry.
Low-density filler slurry.
The three slurry systems caused logistic difficulties in transportation, storage on the rig, job execution, and job design. In designing and executing the job, a good understanding of the actual open-hole size was important. If the hole was actually smaller than predicted, the filler cement would be placed over the gas zone.
During the execution of the job, the same problem could occur if too much filler cement was pumped. If this happened, gas migration was not controlled and a costly remedial job was pumped.
An extensive technical study resulted in the development of a single engineered-particle-size slurry system that simplified the job execution procedures, mitigated gas migration, and improved the quality of the overall cement job. Changes made to the physics of the cementing operations could also be pre-evaluated using an advanced u-tube simulator. Input of well and fluid parameters allowed analysis of the impact and optimization of slurry design prior to actual execution of the first new system trials. Impact of system re-engineering effects on pump rates, wash/spacer volumes, surface/downhole pressures, and total job times could be predicted and design improvements made. Case histories are given on the first two successes using this new lightweight technology.
The field under study is located offshore Kalimantan on the outer limit of the Delta Mahakam River (Fig. 1). In the delta environment, the water depths are fairly shallow, averaging only 6 m. The temperature gradient in the field is 2.5°C/100 m, giving an average BHST of 57°C. The lithology is described as primarily sand with intermittent clay layers. The wells are deviated up to 40° with a kick-off point between 200 m and 40 0m. Over 190 wells have been drilled in the field since development started in 1987.
During this time the conductor and surface casing configuration has evolved until it has been standardized to the following. The conductor casing is driven to ±80m then cleaned with a 26-in. bit. Once this has been accomplished, a 17½-in. open hole is drilled to ±1,200 m/1,400 m and cemented to surface, covering the shallow gas zones found between 300 m and 500 m. The shallow gas zones pose one of the two biggest problems in getting good zonal isolation.
The second problem after the shallow gas zones is the low fracture pressure. The average formation fracture gradient is 1.65 sg-MWE with a pore pressure of 1.05 sg-MWE. With this small pressure window the actual placement of the slurry becomes quite critical. To have good mud removal using turbulent flow displacement, a certain flow rate has to be achieved all around the annulus. If this critical rate is not achieved then a channel of mud is left in the annulus allowing gas flow and requiring remediation. To achieve a complete zonal isolation within the limitations set by the formation, three cement systems have been used historically.