A Tutorial on Time-Lapse Seismic Reservoir Monitoring
Authors: Johnston D.H. 
Published: October 2008

Abstract
The seismic surveillance of producing reservoirs may represent a breakthrough in our ability to efficiently manage our assets and to maximize their profitability. Changes in fluid saturation, pressure, and temperature that occur during production induce changes in the reservoir's density and compressibility that may be detected by seismic methods. As a result, seismic data can be used to help monitor and predict the interwell position and movement of reservoir fluids, locating bypassed oil, avoiding premature breakthrough, optimizing infill well locations, and evaluating pilots prior to full field implementation. During enhanced recovery, real time mapping of the EOR progress can provide the opportunity to control or modify the recovery process.

Introduction
In the later phases of a field's life, reservoir surveillance is a key to meeting goals of reduced operating costs and maximized recovery. Differences between actual and predicted performance are typically used to update the geological model of the reservoir and to revise the depletion strategy. The changes in reservoir fluid saturation, pressure, and temperature that occur during production also induce changes in the reservoir acoustic properties of rocks that may be detected by seismic methods under favorable conditions.
The key to seismic surveillance is the concept of differential imaging using time-lapse measurements. As illustrated in Figure 1, while one seismic image of a reservoir may not show any obvious production-related effects, differences in repeated surveys may be able to detect even subtle changes in reservoir properties. Acquisition of a seismic survey before production or intervention establishes the baseline conditions of the reservoir. Subsequent monitor surveys (in the common industry term 4-D seismic, the fourth dimension is calendar time) are differenced from the base survey. The result is a seismic difference volume which, when integrated with reservoir characterization and flow simulation, may be used to track the movement of fluid in a reservoir between well control.
However, the difference between two seismic surveys is not only sensitive to changes in reservoir rock properties but is also sensitive to differences in acquisition and processing, and errors in navigation. As a result, the repeatability of seismic data is one of the key issues for the successful application of time-lapse seismic monitoring. As shown in Figure I, seismic data may be differenced on a sample-by-sample basis or by differencing attributes extracted from the data. Differencing attributes is more robust in the presence of noise and data artifacts but resolution is decreased.
Many fields have multiple seismic surveys acquired over time. These legacy data are rarely acquired or processed to maximize repeatability. The applicability of legacy data to seismic monitoring depends on the magnitude of the physical property change relative to the repeatability of the seismic data. For subtle reservoir changes or noisy seismic data, seismic acquisition designed to maximize repeatability may be necessary.
The seismic difference must be interpreted in terms of fluid movement. This requires the integration of the seismic data with reservoir characterization, flow simulation, and conventional surveillance data.