4D Seismic Aspects of Reservoir Management
Authors: Kelamis P.G. , Uden R.C. , Dunderdale I. 
Published: October 2008

Abstract
Seismic reservoir monitoring is gaining popularity within the Exploration and Production industry. Analysis of repetitive seismic surveys provides direct measurements of the fluid movements and saturation changes within the hydrocarbon accumulation. Tracking saturation fronts is essential to design and improve production strategies in order to properly manage, as well as predict, the performance of the field. Additional value can be realised by identifying areas of bypassed oil from 4D seismic and so improve ultimate recovery.
In this paper, we present the analysis and results of a seismic reservoir monitoring project over an offshore clastic field in the Arabian Gulf. Particular emphasis was given to matching the repetitive seismic datasets in order to minimise the residual noise in areas where changes due to production are expected to be minimal. After rotation and repositioning of the two seismic datasets, a difference 3D volume was computed, validated by geological data and calibrated to a reservoir simulation model that has been history matched. The seismic difference between the two 3D surveys indicates gas encroachment within the upper part of the reservoir and is in overall agreement with production logging measurements atthe wells.

Introduction
Changes within a reservoir due to production and injection can result in measurable changes in the seismic response.' Using repetitive seismic surveys it is possible to monitor the interwell movement of fluid fronts throughout the producing life of the reservoir. 4D seismic technology has been proven successful both onshore and offshore as an aid to reservoir management. However, each application must be designed and adapted to the specific reservoir properties and recovery mechanisms that are applicable to the reservoir under evaluation.
This paper describes a successful pilot study of the use of 4D seismic recently completed in a clastic oil reservoir with a large gas cap, located in the Arabian Gulf. The objective of the study was to determine whether 4D seismic is a valid technique to measure saturation changes in an offshore producing field. The main producing sandstone reservoir lies within the Khafji member of the Lower Cretaceous Wasia Formation. At initial conditions, the reservoir contained oil together with a large gas cap. During a production period of more than twenty years, productionlogging measurements have indicated significant gas encroachment within the Khafji Main Sand interval.

Methodology
The underlying principle of 4D seismic is that production from a hydrocarbon reservoir causes changes to the reservoir fluid acoustic properties and fluid saturations as a function of time, which may be detected by repeat seismic surveys. Current technology uses p-wave reflection surveys, which is the case in this project, to monitor the change in seismic response.
The principal benefit of 4D seismic to the reservoir management process is that it can provide an independent measurement of the position of the gas and/or oil saturation fronts, and this can be used as an additional history matching parameter.