Integration of 4D Seismic Data into the Management of Oil Reservoirs with Horizontal Wells between Fluid Contacts
Authors: Clifford Peter J. , Trythall Robert , Parr Ronnie S. , Moulds Tim P. , Cook Tim , Allan Peter M. , Sutcliffe Phil 
Published: December 2007

Abstract
This paper describes how 4D seismic data have been incorporated into the modelling and management of the Andrew and Harding reservoirs. These two fields pose similar challenges in that both use horizontal wells to produce from a 50 - 100 m oil column between gas cap and aquifer. The seismic data must therefore be used to tell us about the vertical movement of fluid contacts in different areas. The two fields also have important differences, notably in vertical communication and aquifer strength, and demonstrate a range of data applications and consequences.
Andrew 4D seismic data confirm strong aquifer influx, which has moved the oil-water contact above most wellbores, so that oil is now produced by coning from above. They also provide a detailed picture in which tens of baffling faults, and extensive shales, cause pockets of oil to be poorly swept by the producers, though without total pressure isolation. The paper describes how the 4D baffling fault picture and the coning process have been interpreted and included into history matching of the Andrew FFM. It also discusses the consequences and value of the new understanding, for well optimisation, infill drilling and reservoir management.
Harding 4D seismic data resulted in a significant change to horizontal infill well locations, and were a key match parameter in the new FFM. Prior to the 4D survey, the original history-matched FFM indicated a fairly uniform vertical rise in OWC, with local cones around producing wells. The original infill concept was to drill between these cones to drain unswept oil. However, 4D data indicated a monoclinal, tilted geometry to the OWC, suggesting a significantly different sweep pattern than predicted by the original FFM. The paper describes how 4D was used successfully to define infill locations, as well as the iterative process between 4D and the reservoir simulator that was required to develop a robust history match.

Introduction

4D Seismic Method
4D seismic (or time lapse 3D seismic) involves repeat acquisition of 3D seismic after a period of oil, gas or water production, to determine the changing fluid or pressure distribution within the reservoir. It started as a research and development tool for BP in the mid 1990's and was tested in the UKCS West of Shetland fields1 and on Forties field. The technique has been applied to a growing number of BP's North Sea assets2 and increasingly has been extended to other fields worldwide. In general it has been applied in the marine environment by re-acquiring high quality marine 3D surveys, in the same orientation as the pre-production baseline, and performing very careful cross matching processing to minimise any non-4D differences.

Andrew Field
Andrew field, in Block 16/28 of the UK sector of the North Sea (Fig. 1), is a low-relief dome structure over a salt diapir with 4-way dip closure (Fig. 2), at a depth of between 2450 to 2554 m tvdss. It comprises high net to gross Palaeocene sandstones of the Lista Formation, with porosity of 20 to 22% and permeability typically in the range 150 to 1000mD. The reservoir consists of dipping stacked sands divided by shales. The dip is typically about 4°. Certain shales, especially in the upper A sand, extend laterally over the whole reservoir, broken only by faults, while others, in the lower B sand, are less laterally extensive. Andrew is characterised by tens of defined faults in a range of orientations (Fig. 3), which are believed to influence the fluid flow, in some cases by acting as baffles to lateral fluid movement, and in others by allowing vertical fluid movement between sands, through sand-to-sand juxtaposition.
Initially there was a 58m saturated oil column with an overlying primary gas cap, and underlying large and active aquifer (Fig. 2). The oil is relatively light, with 40° API, and reservoir viscosity of 0.3 cp, leading to a favourable mobility ratio for water sweep.