Wells drilled into gas bearing carbonate reservoirs in offshore Sarawak are prone to severe lost circulation issues. In the first 3 exploration wells, total losses were encountered after drilling 20 m into the carbonate in 8.5-in. hole section. The well managed to drill to TD with application of pressurized mud-cap drilling. However the challenge of the total losses wellbore is the incomplete cementing of the 7-in. liner across the carbonate column. Poor zonal isolation and reduction of reservoir permeability were major concerns to the operator requiring significant investigation, laboratory testing and pre-job planning for future wells.
This paper will discuss an unique design technique for curing the challenging lost circulation issues while also providing improved cement placement along the fractured formation. Lab test results are presented using a bridging material simulator with different fracture sizes to evaluate the sealing effectiveness of the engineered systems. Return permeability studies using a Hasseler-style core holder were conducted for the fluids to verify their damage potential to the producing formations.
The engineered solution was the use of a cement spacer fluid containing membrane forming colloids to seal fracture sizes up to 2 mm which also yielded 100% return permeability to hydrocarbons. The use of the spacer fluid enabled successful cement placement along the critical carbonate section. A bond log evaluation showed that there was cement coverage along the zones of interest. Based on the positive lab tests and field results, the combination of a sealing spacer and lightweight cementing system was identified as a viable solution to combat lost circulation and improve cement placement for future wells drilled in this area.