- Copyright © 2000 Society of Exploration Geophysicists
Time‐lapse seismic, commonly known as 4-D, can dramatically improve reservoir understanding and ultimate recovery. However, 4-D seismic monitoring is not applicable to all reservoirs and all development strategies. To be a strong candidate for 4-D seismic, a field must have favorable geophysical characteristics and a development plan that produces observable changes in repeat 3-D surveys.
A traditional flow simulator that includes a petrophysical model has the ability to predict “dynamic” reservoir changes and resultant changes in seismic response. This interdisciplinary approach to reservoir modeling blurs the distinction between geophysics and reservoir engineering. This “integrated flow model” is a powerful tool that can be used to determine the feasibility of a 4-D seismic monitoring project before initiation. In addition, it can be used throughout the life of the monitoring project to ensure correct and thorough understanding of the reservoir as it matures.
The underlying principle of 4-D seismic monitoring is to observe changes in 3-D surveys as a function of the fourth dimension, time. The seismic signature of a reservoir depends on two primary elements, static reservoir rock properties (e.g., porosity and lithology) and “dynamic” time-varying properties (e.g., fluid saturation and pore pressure). The comparison of two or more 3-D surveys over the same area in effect cancels the static contribution. Therefore, any observable change is due to “dynamic” changes of the reservoir and effects of fluid flow.
Three key parameters—the effective compressibility modulus, the shear modulus, and the bulk density—affect the elastic response of a reservoir. The effective compressibility modulus depends on both rock and fluid compressibility. This means that each reservoir will have a unique effective modulus based on its porosity, structure, lithology, and fluid content. In addition, the effective compressibility modulus will vary spatially as a consequence of reservoir heterogeneity and variable fluid saturation. The shear modulus is …