Abstract

In July 2004 Statoil introduced underbalanced drilling technology to the Gullfaks Field offshore Norway. The main driver for this technology was to overcome existing pressure control problems experienced while drilling conventionally through the cap rock in order to reach the reservoir. The project represents the first application of underbalanced drilling offshore Norway. The focus of the process design was on being environmentally friendly; consequently, no hydrocarbons were released to atmosphere or flared during the operation.

This paper addresses the challenges in implementing the UBO technology in Norway. There were issues related to equipment, procedures, standards and personnel. Most of the existing Norwegian requirements and guidelines have been developed for conventional drilling. The UBO surface equipment used by the project had to go through a detailed review and extensive modifications to fulfill the Statoil internal requirements as well as the external Norwegian standards and demands. Special quantitative risk assessment methods and software were developed throughout the project to define risk associated with UBO compared to conventional methods. Procedures, emergency measures and contingencies were established to ensure a safe operational environment for personnel at any time. Interfaces with existing platform production facilities had to be made in order to process produced gas and liquid hydrocarbons thereby ensuring an environmentally friendly operation. Some of the major challenges were related to communication and teams that consisted of personnel with very different backgrounds working towards a common goal, i.e. the production philosophy with automated processes vs. the drilling philosophy with manual processes. High focus was kept on open communication throughout the project.

Background

The Gullfaks field is located in the northern part of the North Sea. It consists of three concrete gravity based platforms and is now entering its phase of tail production. The main driver for production from the Gullfaks reservoir is seawater injection. The combination of geology and focus on maximum production has created increasing drilling problems in the Shetland formation, which is the cap-rock of the Gullfaks reservoir. The problems have originated from excessive water injection over a limited time period. This has caused pressure increase in certain parts of the reservoir and related fracturing of the cap-rock. The injected water has increased the pressure in the cap-rock in certain areas to an extent where there is no longer a window between fracturing pressure and pore pressure. Some areas of the Gullfaks field are therefore no longer drillable using conventional drilling technology. The hydrocarbon reserves left in the affected areas are substantial, and Statoil therefore introduced underbalanced technology to gain access to these additional reserves. At the same time, Statoil wanted to increase the internal competency and experience for future UBO projects both locally and internationally.

The project represents the first application of underbalanced drilling offshore Norway. There were no existing rules and regulations made specifically for underbalanced drilling in Norway; therefore, the first step of the project was to approach the authorities in order to verify that the concept of underbalanced drilling was feasible. The authorities' attitude towards the project was positive, as they recognized the importance of getting UBO technology implemented in Norway, where the focus is high on tail-end production optimization. Next, Statoil's partners in the Gullfaks license had to evaluate and approve the project. Initially, they found the technology to be too expensive, but the project was approved when the need for the technology and economy of the project was proven by a detailed study. Financially, the investment cost of the whole project had to be written off on one single well, as the Gullfaks field is in the tail production phase and future wells may not generate enough income to support the introduction cost of UBO technology.

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