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

Abstract

The development of a sour gas reservoir (7000 ppm H2S and 20–25% CO2) in the Kujung fractured-limestone formation in the Gundih Field, Java, Indonesia faced challenges related to drilling operations that are prone to kick - loss cycles. Well control incidents were experienced on previous wells drilled in the area and large amounts of cement and lost circulation materials were used 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 a populated area, thereby increasing the requirement and need to prevent the release of sour gases during drilling operations. This condition ruled out the use of underbalanced drilling (UBD) to address circulation losses as bringing sour reservoir fluids to surface, even at a controlled condition, carried too much of a risk. In the process of finding a solution to these issues, the operator has identified that safety is the primary issue for the development of Gundih field, closely followed by the ability to drill these wells 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 downhole isolation valve (DIV), to isolate and prevent sour gas from the formation from reaching the surface, should severe losses and subsequent kicks be encountered. The PMCD+DIV system has allowed recent drilling operations to properly address the situation where severe, total and immediate circulation losses are encountered, thereby concurrently minimizing reservoir damage and the amount of sour reservoir fluid released at surface while penetrating deeper into the fracture limestone reservoir. In one instance, the DIV was able to keep sour reservoir 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 system enhanced and improved the handling of health, safety and environment (HSE) issues during the drilling process, thereby assisting greatly in the accelerated development of the sour fractured-limestone gas reservoir in Gundih Field.