Study identifies safe practice for Norwegian injectors

At least six offshore operators maintain injection wells for operational discharges on the Norwegian continental shelf, according to a new survey by Petroleum Safety Authority Norway (PSA).

Offshore staff

OSLO -- At least six offshore operators maintain injection wells for operational discharges on the Norwegian continental shelf, according to a new survey by Petroleum Safety Authority Norway (PSA). Of these, Statoil and ExxonMobil, have recorded leaks. 

The findings are based on a response to a circular issued in February by PSA and the Norwegian Climate and Pollution Agency (Klif). The circular also asked how the injector/discharge wells were monitored and the status of possible leaks.

Of the 16 operators that responded, 10 replied that they did not inject drill cuttings. Statoil, Talisman, BP, ConocoPhillips, ExxonMobil, and Marathon confirmed using injectors for operational discharges, and each submitted the requested information.

Responses indicate that injection is performed mainly in shale zones underlying the Utsira formation, with fracturing to the Utsira at a depth of 800-1,200 m (2,624-3,937 ft).

Some respondents said they had injected from the outset in deeper zones, while others decided to shift operations from the Utsira to depths of around 2,000-2,100 m (6,561-6,889 ft).

Early on, cuttings typically were injected in the annulus between 20-in. (51 cm) and 13 3/8-in. casing (alongside production from the well). Lately, wells have been designed purely as injectors.

Statoil currently has 20 wells in service on the Norwegian shelf for injection of drill cuttings and produced water, gas, carbon dioxide, and in some cases injection of produced water and chemicals.

The company had a further 50 wells for injection purposes, but which have since ceased to be employed for this purpose. Of these, 13 had to be shut in following detection of seabed leaks.

Others were shut in due to the need for excessive injection pressure, while others were temporarily abandoned because of suspected faults in the wells, following an assessment of downhole data in the light of a well-publicized incident at the Veslefrikk field in the North Sea.

Injection wells shut in because of established or suspected leaks are on Åsgard (seven leaks in 1997-2001), Oseberg South, Visund, Veslefrikk A, Snorre B, Oseberg C, and Njord.

Among the other respondents, ExxonMobil reported a failure in 2004 in a cuttings injector on the Ringhorne field, which led to breakthrough to the seabed. The field’s four other injectors were shut in as a result, with a new injector drilled to the Statfjord formation at 2,190 m (7,185 ft).

Talisman Energy has an injector well in the Varg field beneath and into the Utsira formation. The well has injected without problems since 1997. At Gyda, most of the injector wells have been shut in after reaching the pressure limit for injection.

ConocoPhillips currently has four injection wells, all to deeper formations at 2,000-2,100 m. No problems have been reported.

BP has two wells at Valhall injecting without problems at 1,850 and 2,500 m (6,069 and 8,202 ft), and two injector wells at Valhall flank not currently in use, but which will be at some point in connection with future planned discharges.

Marathon has two wells at Alvheim injecting produced water into the Utsira formation. Another water injector has been drilled for pressure support on Volund, but is not yet operational.

PSA plans to check whether factors related to cuttings injectors have relevance for other types of injection wells, such as water/gas injectors for production support. At present, PSA believes current regulations take sufficient account of the requirements for well integrity and the barriers needed for this type of well.

Some of the incidents occurred because of a failure to comply with the regulations. Others underline the need for achieving secure injection through the selection of suitable zones that can accept the injected medium.

06/01/2010

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