Aquifer Behaviour During Brent Depressurisation and the Impact on Neighbouring Fields
Authors: Coutts S.D. 
Published: December 2007

The extent of the aquifer in communication with the Brent Field was determined by reviewing the geological and geophysical interpretation and analysing all available pressure and performance data from surrounding fields. Regional porosity and permeability data were used to determine the deterioration of aquifer properties with depth, and this formed the basis of a simulation model of the total Brent aquifer. This model incorporated voidage profiles from all the fields currently on production and was history matched against available pressure trends.

The model was used to predict the pressure response to both pressure maintenance and depressurisation of the Brent Field. In the first case, pressures will continue to decline in the upper reservoir due to unsupported production in some neighbouring fields, and additional injection would be required to maintain pressure in Brent. Under depressurisation, the timing is such that the only potentially serious impact will be on the tail end production in the Strathspey Field, but the operator is taking steps to compensate for this by maintaining plateau injection levels.

Concerns about possible communication between the Brent and Statfjord Fields as the pressure difference between them increases led to a detailed review of the major boundary fault. This combined seismic interpretation, detailed core analysis from fault zones, clay smear potential analysis, rock mechanics and a review of past pressure differences between the fields to assess the potential for fault leakage. The results were then incorporated in the simulation model which concluded that, even in worst case, the maximum cross-flow would have negligible effect.

Overall the study confirmed that the Brent Redevelopment project will have a limited effect on other fields within the Brent aquifer block. Also the possibility of communication with the Statfjord Field across the Northern Boundary fault is small and its effect would in any case be minimal.

Since the aquifer extends over several licence blocks, and data availability is limited, the study required integration of a wide range of data of varying quality and from a number of different sources in order to build up a complete aquifer model. The results highlighted various effects of pressure communication between fields which were not always apparent to engineers studying individual fields in isolation.

Introduction

Controlled depressurisation of the Brent Field (Fig. 1) to maximise hydrocarbon recovery will require back production of considerable volumes of water to gradually reduce the reservoir pressure from 5500 to 1000 psi. An understanding of the size and strength of the aquifer attached to the reservoir (Fig. 2) is a critical input to the design of this process, influencing the rate and quantity of water to be back-produced. In addition, other oil fields are thought to be in pressure communication with the Brent Field via the aquifer and the potential impact of Brent depressurisation on all these fields needed to be quantified. Thus, as part of the planning for depressurisation, an extensive integrated petroleum engineering study was undertaken to assess the range of uncertainties in the behaviour of the Brent reservoir aquifer during depressurisation and to quantify the possible impact of the redevelopment project on surrounding fields, including the effect of any possible communication between the Brent and Statfjord Fields.

This study was confirmed to the Brent reservoir as the Statfjord reservoir aquifer has already been shown to be relatively tight, with the result that depressurisation will have minimal impact on even the nearest fields. In fact, the gas reserves in the Statfjord in both the Brent South and Strathspey Fields are planned to be produced by depletion drive, allowing the reservoir pressure to drop until the wells die, without any voidage replacement. P. 73