Successful Installation and Operation of Downhole Isolation Valve Combinedwith Pressurized Mud Cap Drilling to Safely Develop Sour Gas, Fractured-limestone Reservoir in Gundih Field, Indonesia

摘要

The development of a sour gas reservoir (7000 ppm H2S and 20-25% CO2) in the Kujung fractured-limestone formation in theGundih Field, Java, Indonesia faced challenges related to drilling operations that are prone to kick - loss cycles. Well controlincidents were experienced on previous wells drilled in the area and large amounts of cement and lost circulation materials wereused to mitigate severe and total circulation losses, which consequently reduced the productivity of the wells.To further add to the complexity of the situation that the sour gas development project faced, the field's location is adjacent to apopulated area, thereby increasing the requirement and need to prevent the release of sour gases during drilling operations. Thiscondition ruled out the use of underbalanced drilling (UBD) to address circulation losses as bringing sour reservoir fluids tosurface, even at a controlled condition, carried too much of a risk. In the process of finding a solution to these issues, the operatorhas identified that safety is the primary issue for the development of Gundih field, closely followed by the ability to drill thesewells and maintaining reservoir performance. The solution developed involved the use of pressurized mud cap drilling (PMCD)techniques (along with a guaranteed and sufficient supply of sacrificial fluid, mostly water) and the installation of a downholeisolation valve (DIV), to isolate and prevent sour gas from the formation from reaching the surface, should severe losses andsubsequent kicks be encountered.The PMCD+DIV system has allowed recent drilling operations to properly address the situation where severe, total and immediatecirculation losses are encountered, thereby concurrently minimizing reservoir damage and the amount of sour reservoir fluidreleased at surface while penetrating deeper into the fracture limestone reservoir. In one instance, the DIV was able to keep sourreservoir fluids from reaching surface during PMCD operations, undergoing as much as 47 open/close cycles during drilling,tripping, fishing and well test operations, all of which were performed while experiencing total loss of circulation.The details of the PMCD+DIV system utilized will be presented in this paper, together with an assessment of how the systemenhanced and improved the handling of health, safety and environment (HSE) issues during the drilling process, thereby assistinggreatly in the accelerated development of the sour fractured-limestone gas reservoir in Gundih Field.