One of the challenges is that, after many years of water injection, it is more difficult to drill in the structures using traditional drilling technology. The reason for this is that extremely high pressure has built up in the Shetland formation directly above the reservoir, which has made it impossible to drill in certain wells using conventional methods.
Billions to gain
After extensive study of the reservoirs, as well as development and testing of equipment and training of personnel, the first underbalanced drilling operation using existing drilling equipment on Gullfaks C was carried out this past summer, with good results. The equipment used for underbalanced drilling was rented from Halliburton, while the drilling itself was carried out by Prosafe.
"We see a great potential in this new drilling technology. On the Gullfaks field alone we expect an extra gain in the form of enhanced production of NOK 1.6 billion," says Statoil's project manager for underbalanced drilling, Johan Eck Olsen.
The technology is primarily interesting in difficult zones, i.e. zones with abnormal pressure. There are a number of good candidate fields where this technology could be used, and Eck Olsen emphasizes that the experience gained on Gullfaks will also benefit other fields.
"For instance, Statoil is considering using this technology on the Kvitebjørn field, situated to the south of Gullfaks. Similar methods could also be used for drilling operations from mobile drilling rigs and on fields where we are a licensee," says the project manager, who adds that Statoil is also considering using this technology for drilling on fields operated by Statoil in Algeria, and in the development of fields in Iran and Venezuela.
"Will become standard drilling technology"
Underbalanced drilling and managed pressure drilling are forms of rotation drilling where the weight of the drilling mud column is reduced and the pressure on the top of the well is controlled by means of a rotating seal and choke valves.
In underbalanced drilling, the pressure in the well will be lower than the formation pressure and the well will thus flow. The wellstream is led through choke valves on the surface, and then through a separation unit where oil and gas are separated together with cuttings before the mud is returned to the well.
In managed pressure drilling, the weight of the drilling mud and the pressure at the top of the well will be sufficient to hold back the formation fluid, and the well will thus not flow. Both methods use the same equipment at the surface.
During the past 40 years, underbalanced drilling technology and managed pressure drilling have been used for special operations in more than 2000 wells around the world. These procedures are undergoing continuous development through new methods, technologies and operational procedures. Generally speaking, underbalanced drilling and managed pressure drilling can yield good results when drilling long horizontal wells, or in fields with hard source rocks.
"Experience shows that we can drill cheaper and safer using the technology associated with underbalanced drilling. I predict that this type of drilling operation will become standard during the next five years," says Statoil's project manager.
Close follow-up by the Petroleum Safety Authority Norway
Planning of the underbalanced drilling operations on Gullfaks started as early as in 2001, with the PSA as active observer and cooperative partner as regards development of guidelines and safety procedures.
"The authorities, represented by the Norwegian Petroleum Directorate and now the Petroleum Safety Authority Norway, have been important partners in this phase through their technical input and feedback," says Eck Olsen, referring to the fact that underbal anced drilling requires closer cooperation between drilling and production than has been customary.
The PSA has been able to develop its own expertise in this technology thanks to research funds allocated by the Ministry of Labor and Government Administration (AAD). Expertise in this type of technology is primarily found in Canada and the USA, and the AAD funds have made it possible for the PSA to seek out resource persons working for the authorities, contractors, equipment suppliers and operating companies. Shell and its introduction of underbalanced drilling on the British shelf has also made an important contribution.
"Development of expertise has been essential in order to understand the challenges entailed in this type of technology, and in order for us to be able to "speak the same language" as the operator," says the PSA's Statoil contact, Johnny Gundersen. Gundersen adds that the PSA is very satisfied with the fact that Statoil got in touch with them early in the process and paved the way for the close dialogue that has ensued.
"We were able to establish our expectations and requirements for this type of operation early on, and Statoil's follow-up has been excellent. The company's planning process has set a good example for future operations on the Norwegian shelf, particularly as regards requirements for training and implementation of training both on land and in the field.
Better planning - safer drilling
Gundersen emphasizes that there are substantial safety benefits to be gained through good planning of underbalanced drilling operations.
"There are substantial safety benefits to be gained through good planning and implementation of necessary risk analyses, drawing on the expertise of contractors and suppliers, training of personnel, involvement of employees and a gradual approach to this technology. Experience gained around the world shows that there are very few serious safety problems in wells drilled using underbalanced/managed pressure drilling," says Gundersen, who adds:
"Moreover, there is an enormous potential for increased value creation through enhanced production and less reservoir damage. Many underbalanced wells are drilled in Lithuania, and the well path is continuously guided according to the oil flow from the well. Here they have increased well production by several hundred percent since they started to use this technology."