Safe and secure operations at Gullfaks
The Gullfaks C platform in the North Sea. (Photo: Marit Hommedal)
Safety takes top priority
“Our people must be safe at work, that is our top priority,” says Øystein Michelsen, Statoil’s executive vice president for Development and Production Norway.
Statoil takes any expression of concern from its personnel seriously.
“When our people say that they don’t feel safe at work, then we take action,” says Michelsen. We are also working with our union reps and safety delegates to make sure that people feel safe.”
Several meetings for the whole Gullfaks workforce and union reps and safety delegates have been held. Experience gained at these meetings has also been imparted to employees on other platforms.
Øystein Michelsen, Statoil’s executive vice president for Development and Production Norway.
“We work continuously to improve our safety efforts,” says Michelsen. “Our most important common task is to secure a good development of our work processes and compliance with our governing documentation. This will mean that the safety level offshore will be strengthened further.
“We recognise that we could have been clearer in our dialogue with the Petroleum Safety Authority Norway (PSA). After the well C6 incident in 2010 there has been frequent contact between the PSA and Statoil. After we intensified the work of mapping the condition of wells on the Gullfaks field, we presented our findings in meetings with the PSA and the Norwegian Petroleum Directorate (NPD) on 16 and 21 December. We acknowledge that the level of detail in the information given at these meetings has been inadequate and we will clarify with the PSA the need for further information in this work.”
Since May 2010, Statoil has worked systematically to survey all of the wells in the whole of the main field. Inspections made during the autumn have indicated a pressure build-up in several wells in certain areas, chiefly in wells which are used for injecting water. There would appear to be a connection between pressure build-up in the Shetland group and the Lista formation, and the position of wells with pressure build-up.
This led to the shut-down of 20 wells on the field. Most of these wells are water injectors, wells which are made to avoid pressure decrease in the reservoir. To safeguard the reserves and to facilitate production at a later date, we have shut down a further 30 wells. This will also ensure a longer lifetime for the reservoir and it has not been done for safety reasons.
The incident at Gullfaks C last May was caused by a leakage in a casing. Nothing similar has occurred in the wells we have now shut down.
Subsurface conditions at Gullfaks
“The Gullfaks situation is demanding but manageable, with regard to both personnel and the environment.” says Michelsen.
He emphasises that in the time ahead Statoil will maintain its dialogue with the supervisory authorities, partners, the NPD and the safety delegate service, making sure that they are kept constantly updated about the status and development of the Gullfaks field.
No leakage has been established from the reservoir, either to the seabed or to the shallow layers as a result of Statoil’s operations at Gullfaks. There are natural pockmarks in the Gullfaks field resulting from the movement, or migration, of gas in the subsurface.
These are natural leakages which have occurred since gas and oil were formed in the formations tens of millions of years ago. This is also why the Gullfaks field today consists mainly of oil, and only a little gas. We have not detected any increase in the number of pockmarks after the field was mapped in 1979 and we are monitoring this continuously.
The natural movement, or migration, of gas in the formations is one of the factors which have made Gullfaks a complex field to drill wells in. The combination of somewhat unpredictable pressure conditions in the rocks above the reservoir, and how much pressure the formation can tolerate is an important challenge. This sometimes causes demanding conditions that we have been familiar with since we began operations for oil recovery from Gullfaks in 1986.
“We invest considerable resources to develop technology and solutions which will ensure that all of our drilling operations and all of our production are safe and secure,” says Michelsen.
The recovery strategy at Gullfaks has been based on injecting water into the main reservoir to replace the volumes we extract in oil. The result of this technology has made it possible to realise a recovery factor of more than 60%, while the normal factor for this type of reservoir is between 30% and 50%.
The field is monitored continuously by gathering well data. Seismic data and seabed surveys are also analysed regularly. For the past 10 years we have observed and mapped a high-pressure zone which lies partly at the top of the Shetland rock formation and at the bottom of the Lista rock formation which lies above Shetland.
This high-pressure zone has spread to a certain extent. This is partly due to pressure build-up which occurs naturally and partly to changes in pressure resulting from water injection which has unintentionally taken different paths than the reservoir.
Statoil has studied these pressure conditions since 2003 and the result of the latest study was in November 2010. The study of the Shetland and Lista formations showed that the high pressure has contributed to reducing the margins between the formation’s pressure and strength.
The Hordaland group and the Utsira formation lie above Shetland. Analyses have been carried out of these. The conclusion is that they will be able to contain any leakage should a fracture occur in the Shetland formation. They will thereby serve as a barrier against movement of liquids and gas and prevent any leakage to the seabed.
Ola Anders Skauby, press spokesperson Development and Production Norway
Phone: +47 90 59 85 19