Casing Design and Barriers Related to Shallow Gas
MetadataShow full item record
The top hole section in an offshore well is conventionally drilled riserless utilizing seawater with high viscosity sweeps for cuttings removal. In recent years, the possibility of eliminating the pilot hole and setting the surface casing below shallow gas or shallow water flow zones has been investigated. This would allow for setting the surface casing below potential problem zones, so that they do not cause problems deeper in the well. Furthermore this could provide sufficient formation strength at the casing shoe, which can open for reducing the number of subsequent casing strings. In order to safely penetrate a shallow gas zone without drilling a pilot hole with dynamic kill possibilities, weighted mud must be utilized when using a conventional top hole drilling system. Owing to the fact that this is done riserless, which results in huge mud losses to seabed, alternative methods should be considered to avoid the "pump and dump" strategy. Drilling the top hole section whilst circulating the drilling fluid back to the rig can be achieved by utilizing the riserless mud recovery (RMR) system or the low riser return system (LRRS). These new "dual-gradient" systems have a significant benefit in terms of avoiding "pump and dump" when drilling the top hole section. Well barrier designs have been proposed which incorporate techniques and solutions to handling shallow gas with regards to barrier requirements, including procedures for handling the problem of gas trapped between the intermediate casing and surface casing (trapped annuli). The LRRS system is primarily intended for dual-gradient drilling with a BOP, but can be modified for use in top hole drilling. However, seeing that this system involves utilizing a riser, this system will not be approved for top hole drilling on the Norwegian Continental Shelf (NCS). Based on my studies of different techniques of tackling shallow gas in the top hole section, it is found that the RMR system is favorable for eliminating the pilot hole and setting the surface casing below a shallow gas or shallow water flow zone in a safe and time-efficient manner. The riserless return system enables return of fluid and cuttings in a closed system. Since weighted mud is used the risk of shallow gas influx is reduced compared to conventional riserless drilling. Owing to the fact that the return is routed to the platform, shallow gas is a cause of concern. The existing regulations for drilling on the NCS do not allow for elimination of the pilot hole when shallow gas is expected. However, simulations have shown that the system can detect an influx prior to gas reaching the return line, and also handle the influx in a safe manner. This verifies the feasibility of the RMR system related to safety and well control.