Published by the DTI Oil & Gas Directorate for the reservoir engineering and IOR community in the UK.
Send comments on this issue and contributions for next issue to iornewsletter@senergyltd.com by 15 August 2002.

Brent Field Depressurisation

Rob Kuyper of Shell U.K. Exploration and Production reports on the world's largest offshore oil field depressurisation project (Rob.Kuyper@expro.shell.co.uk)

Introduction
The Brent Field is located 186 km northeast of the Shetlands Islands in the U.K. North Sea and has a STOIIP of 3.8 MMMstb and a GIIP of 7.5 Tscf. The field was discovered in 1971 and was brought on production in 1976, with annual oil production peaking in 1984 at 410 Mbbl/d.

Since the mid 1980s, oil production has been declining, but because of the high solution GOR substantial gas reserves remain, dissolved in the residual and bypassed oil. In 1992 the decision was taken to depressurise the Brent Field in order to release solution gas from the bypassed (unswept) and remaining (swept) oil, and to produce the gas, once it has migrated to the crest of the structure. Brent depressurisation was planned to recover an additional 1.5 Tscf of gas and 34 MMstb of oil, extending the field's life by 5-10 years. Three of the four Brent platforms were redeveloped at a total cost of £1.3 billion to install process facilities for low-pressure operations, to reduce operating costs, to implement safety upgrades, and to refurbish facilities. The fourth platform was also upgraded but no low-pressure facilities were installed. Preparation of the platforms for long-term field development was completed at the end of 1997.

Implementation
On 1st January 1998, 450 Mbbl/d water injection was switched off. Four years after ceasing West Flank pressure maintenance, reservoir performance is, in broad terms, according to the 1992 plan which formed the basis for project sanction. West Flank reservoir pressures are declining in line with this plan and oil ultimate recovery is virtually unchanged. Gas ultimate recovery has been increased, due to a lower than expected critical gas saturation.

The novel reservoir processes and rapidly changing reservoir conditions require a high degree of flexibility in development planning, and a much reduced reaction time between observations and remedial actions compared to a conventional development. The criteria to be managed include

• gas production and availability
• oil production
• water production
• gas and water injection
• reservoir pressure and gas cap size.

Essential to the management process is an intensive reservoir surveillance programme in each reservoir unit and area of the field. The key to depressurisation reservoir management lies in systematic and regular mapping of gas-oil and oil-water contacts. Every six months, the calculation of fluid contacts and free gas cap size is updated. The observed reservoir response enables us to determine the fundamental parameters in depressurisation: the critical gas saturation and aquifer strength.

As the reservoir pressure declines, rates from gas lifted wells will reduce and a number of high rate ESPs will be installed to enhance voidage: the so-called Enhanced Voidage project. This will sustain depressurisation, to fully depressurise the reservoir.

Progress
Operationally, 1999-2001 have been the busiest years ever in the history of the Brent field: liquids production, gas throughput and well work reached "all time record highs". The reservoir off-take of up to 1 million reservoir barrels per day allowed the reservoir pressure to drop by 70 psi per month over the last three years.

The planned pressure decline has mainly been achieved by gas production. Because of a slower than predicted aquifer response, the need for "Enhanced Voidage water wells" has reduced compared to the 1992 plan. Replenishment of gas released from the swept oil zones has stabilised the gas cap size at reservoir conditions.

Conclusion
The world's largest offshore oil field depressurisation project is currently taking place in the Brent field. The depressurisation process is well on track and gas production has been higher than forecasted during the depressurisation design phase

Disclaimer: The material available on this website is designed to provide general information only. Whilst every effort has been made to ensure that the information provided is accurate, it does not constitute legal or other professional advice.
Please note: The Department of Trade and Industry cannot be held responsible for the contents of any pages referenced by an external link.