Aquifer Behavior During Brent Depressurization and the Impact on Neighboring Fields

Summary

Planning for the depressurization of the Brent Field required an extensive study of the aquifer to determine the withdrawals necessary to depressurize the field and to predict the effect of depressurization on surrounding fields. Static and dynamic aquifer models were constructed and several techniques were applied to evaluate the sealing capacity of the major boundary fault. Since the aquifer extends over several license blocks, integration of a wide range of data of varying quality from different sources was required to build up a complete aquifer model. The results highlighted effects of pressure communication between fields which were not apparent to teams studying individual fields in isolation.

Introduction

Controlled depressurization of the Brent Field (Fig. 1) to maximize hydrocarbon recovery^1,2 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 depressurization on all these fields needed to be quantified. Thus, as part of the planning for depressurization, an extensive integrated petroleum engineering study was undertaken to assess the range of uncertainties in the behavior of the Brent reservoir aquifer during depressurization 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 confined to the Brent reservoir as the Statfjord reservoir aquifer has already been shown to be relatively tight, with the result that depressurization 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.

The investigation concentrated on three major aspects.

• An analysis of all available data to establish the extent of the aquifer in communication with the Brent Field and determine its properties.
• Prediction of the behavior of the aquifer during depressurization.
• An assessment of the risks of additional communication being established during depressurization, particularly by possible leakage across the Northern Boundary Fault from the Statfjord Field, and quantifying the impact of any such communication in the worst case.

Extent and Properties of Brent Aquifer

Since there is a general dearth of data in areas between fields, the study required integration of a wide variety of data from various sources to produce an overall aquifer description.

Aquifer Mapping.

Some base data were available from a limited series of time and reservoir property maps of the Brent and Statfjord Formations in the Greater Brent Area. These had been produced during an early review of the aquifer attached to the Statfjord Field. One initial task of the present study was thus to produce a depth map of the Brent Aquifer at top Brent reservoir level (Fig. 2). This was carried out by combining existing depth maps of known fields with a regional time map. The latter map was depth converted using available depth functions from the Brent Field itself, and tied in to all available wells within the aquifer. Over key areas, principally the Northern Boundary Fault area, all available seismic, both two dimensional (2D) and three dimensional (3D), was reevaluated to provide a consistent seismic interpretation.

A set of cross sections over the aquifer is shown in Fig. 3. To the north, west and east the Brent aquifer is seen to be bounded by major faulting. To the south, in the area of North Alwyn, the aquifer is effectively bounded by a combination of faulting and poor quality reservoir.

Historical Aquifer Pressure Data.

All available Brent reservoir pressure data from wells in the Greater Brent area were collated and corrected to the Brent Field datum level of 8700 ftss for comparison. The data consisted of repeat formation tests (or equivalent) pressure data from exploration, appraisal and early development wells (Fig. 4), together with average pressure trends from the fields on production. The early data from the 1970s suffered from inaccuracies in both absolute pressure measurements from Amerada gauges and in true-vertical depth conversion, since full deviation surveys were not run in supposedly vertical wells.

Representative average data were plotted against time for each cycle³ (Figs. 5 to 7), from which several conclusions were drawn:

• All fields in the Brent and Statfjord aquifer blocks were initially in the same pressure regime, which was some 100 psi below that in the Dunlin block to the west.
• Subsequent performance of the Brent and Statfjord Fields shows no evidence of any communication between the two blocks over producing times.
• All fields within the Brent aquifer block are in some degree of pressure communication. However, the downdip well 3/3-11, drilled in 1989, was still undepleted, indicating that faulting and permeability deterioration with depth severely limit the effective western extent of the aquifer.