Chapter 4 - Seismic Reservoir Monitoring

Abstract

Abstract Seismic reservoir monitoring is normally done by comparing the results of repeated seismic reflection surveys over the same area. This technique is known as time lapse or four-dimensional (4D) seismic, where the fourth dimension is time. 4D reservoir monitoring involves acquisition, processing, and interpretation of repeated seismic data over a field to monitor the changes occurring in the reservoir either due to production of hydrocarbons or enhanced oil recovery/improved oil recovery (EOR/IOR)-related changes, such as injection of water/gas, steam, polymer, etc., into the reservoir. EOR has been defined in Chapter 6 , citing real field examples. The basic principle of time-lapse seismic reservoir monitoring is to subtract out the first-order static geology part to produce clear images of the time-variant fluid flow changes. The reference survey in 4D is termed as baseline and repeated surveys are termed as monitor surveys. This technique helps to identify unswept oil and gas zones in the reservoirs. The idea behind its application is to generate a time-lapse difference dataset, which should be close to zero, except in the reservoir where changes have occurred. The reservoir properties may change due to change in saturation of rock fluids under the effect of production or EOR/IOR. For example, if oil is being produced under water flooding conditions, then saturation of both oil and water changes in the reservoir. This will lead to a change in the seismic response of the reservoir. Apart from saturation, the reservoir may undergo pressure changes either due to production or an EOR/IOR process. Change in pressure will lead to changes in seismic velocities. In general, the expected time-lapse change in the reservoir is small; hence, good quality control is required for 4D monitoring.