Norwegian operator Statoil acknowledged last month that it could have been clearer in its dialogue with Norway's Petroleum Safety Authority following the May 2010 incident where pressure build-up in well 34/10-C06A resulted in a near blowout and the shutdown of the Gullfaks C platform. Meg Chesshyre reports.
Øystein Michelsen, EVP for development & production Norway, said there had been frequent contact between Norway's Petroleum Safety Authority (PSA) and Statoil since the C6 well incident. ‘After we intensified the work of mapping the condition of wells in the Gullfaks field, we presented our findings in meetings with the PSA and the 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,' he added.
Statoil has systematically surveyed all of the wells in the main Gullfaks field since May 2010. Inspections made last autumn indicated a pressure build-up in several wells – chiefly water injection wells – in certain areas, suggesting a connection between the location of these wells and pressure build-up in the Shetland group and the Lista formation.
This led to 20 of the field's wells, mostly water injectors, being shut down. A further 30 wells were subsequently shut down, but Statoil explained that this had been done to extend reservoir life and not for safety reasons. Work is continuing to get as many as possible of these wells into production again.
According to Michelsen, well integrity and other subsurface matters have been high on the agenda of the Gullfaks organisation since the C well incident. ‘The investigation revealed several areas that need to be tackled, among them well monitoring,' he said. ‘The shut-in wells are now being scrutinised in order to evaluate their integrity. If it should emerge that there is a fault in the barriers, remedial measures will be implemented. The incident at Gullfaks C last May, however, was caused by a leakage in a casing. Nothing similar has occurred in the wells that have now been shut down.'
Michelsen emphasised that in future 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,' he noted. ‘There are natural pockmarks in the Gullfaks field resulting from the movement, or migration, of gas in the subsurface. The field is monitored continuously by gathering well data. Seismic data and seabed surveys are also analysed regularly. For the past 10 years Statoil has 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, due partly 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,' said Michelsen.
Statoil has been studying these pressure conditions since 2003 and the latest study of the Shetland and Lista formations, in November last year, showed that the high pressure had contributed to reducing the margins between the formation's pressure and strength.
Analyses of the Hordaland group and the Utsira formation overlying Shetland had concluded that they would serve as a barrier against movement of liquids and gas and prevent any leakage to the seabed should a fracture occur in the Shetland formation.
Michelsen made it clear that the safety of its staff, colleagues and suppliers on the Norwegian continental shelf remained the company's highest priority. ‘This should be clearly understood by everyone and it requires a good dialogue between management, our employees, their associations and our safety delegates. We are now focusing intensely on this dialogue and Gullfaks management is now looking specifically into how we can strengthen it,' he added.
Another shutdown
Another Gullfaks well incident reported by Statoil last month occurred on an independent exploration prospect unconnected to the main Gullfaks field reservoir. At 03.25 hours on 10 February, the semisubmersible Deepsea Atlantic (pictured above) initiated emergency shutdown procedures while completing operations on a gas condensate discovery well, 34/10-53 near the Gullfaks South field. The incident led to the crew being mustered onboard in accordance with regulations. Normalisation efforts on the rig began half an hour later and the situation was quickly clarified. Work on the well was then resumed. The incident was reported to the Norwegian Petroleum Safety Authority and investigations are ongoing.
According to Statoil, the well had been closed for seven days due to bad weather in the area but when the BOP was re-opened and mud circulated downhole, gas was detected in the mud processing unit on the rig, triggering an immediate well shutdown. ‘The gas in the well was circulated in a controlled manner and the second well barrier was thereby re-established.'
The operator added that conventional drilling technology was being used on the well at the time, and that the area did not contain the same demanding reservoir pressure conditions found on the main Gullfaks field. Also pressure conditions in the well were within the calculations made in advance of drilling the well. OE
. . . as a 2009 incident on Shell's Bardolino field raises more questions |