Evolution of Reservoir Models Incorporating Different Recovery Mechanisms and 4-D Seismic Ð Implications for CO2 Sequestration Assurances
by Alan P. Byrnes, Richard D. Miller, and Abdelmoneam E. Raef
CO2 sequestration in aquifers and oil reservoirs requires reservoir modeling to predict the nature of gas distribution, its transport paths, and its long-term fate. The process of reservoir modeling is constrained by the nature and volume of the information available at the time of model construction. Reservoir models almost invariably increase in complexity with each recovery process. Simple reservoir models that accurately represent primary recovery usually must increase in complexity to accurately model reservoir response to waterflood. With CO2 tertiary recovery, and associated three-phase flow and mobility ratio differences, further increases in reservoir model complexity are usually required. Understanding this evolution permits the necessary advancement in the complexity of reservoir models prior to or contemporaneously with injection providing improvement of injection and recovery processes.
The DOE-sponsored CO2 enhanced recovery project in central Kansas illustrates this evolution of reservoir modeling and the role 4-D seismic can play in accelerating model development. One- and two-layer models adequately predicted primary production. Subsequent waterflooding required the introduction of more layers and lateral heterogeneity. Injection of CO2 for enhanced recovery, begun in December 2003, exhibited delayed breakthrough response and different well response than predicted by models. 3-D seismic revealed lateral heterogeneities that were also indicated by well interference testing but not fully quantified. 4-D seismic revealed that injected CO2 was constrained in its motion both in response to the observed heterogeneities and the interaction of pressures generated by water containment injectors. Information provided by 4-D seismic can play a key role in developing accurate reservoir models.