Seismic Reservoir Monitoring: Application of Leading Edge Technologies in Reservoir Management

Abstract

Absbact Seismic reservoir monitoring (4D seismic) involves time lapse or periodic acquisition of 3D seismics designed to provide a detailed image of the reservoir. The interpreted changes in reservoir parameters are then associated directly with the production recovery process. To be successful, a seismic reservoir monitoring program requires more than repeatability of 3D seismic acquisition and processing. To be able to understand the changes observed in repeat seismic images, these changes must have a clear and understandable relationship to the production- recovery process. Development of these relationships requires a detailed understanding of the impact of the production and recovery processes on the rock-fluid system within the reservoir. Once an understanding of the changes occurring in the reservoir is developed, the effect that each of these has on the seismic response (velocity/density) must be understood quantitatively. The changes in reservoir conditions (saturation, pressure, temperature, and pore-volume) due to production and recovery mechanisms can then be visualized by coupling 3D flow simulation with 3D seismic simulation (forward modeling) and can be evaluated to plan repeat 3D seismic surveys. oth reservoir characterization and reservoir monitoring are part of the reserves optimization process. Effective implementation of this process requires careful attention to both elements. Detailed reservoir descriptions and the resulting flow models provide a quantitative understanding of reservoir heterogeneity and processes related to production of hydrocarbons. Characterizing changes in properties due to the production/recovery process that can be monitored by seismic data then determines the structure of the seismic monitoring system. Phillips' 8 billion BOE Ekofisk field in the Norwegian North Sea provides a valuable opportunity to focus new technologies in reservoir characterization and reservoir monitoring on improving reservoir management and recovery of a significant portion of the reserves. A major characterization effort initiated in 1994 resulted in a 25 million cell reservoir description and coupled fluid flow model. Insights gained in the characterization have led to a broad evaluation of the seismic reservoir monitoring potential on the field. These efforts include substantial rock physics, log analysis, and seismic forward modeling programs. Results from these studies show a clear seismic response to the ongoing waterflood. This work forms the basis of a seismic reservoir monitoring program to be implemented field wide in 1998. Introduction Phillips has been actively engaged in evaluation of 4D technologies since 1995. This has taken place, in part, in the context of the evaluation of opportunities for seismic monitoring on one of Phillips' largest assets, the Ekofisk field. This paper focuses on identifying opportunities for seismic reservoir monitoring and defines a detailed methodology for moving beyond the single-point-in-time reservoir characterization program to a dynamic reservoir monitoring program that substantially improves the reserves optimization process. Figure 1 is a diagram describing our view of this reservoir characterization/reservoir monitoring methodology. A complete reservoir characterization program results in a detailed reservoir description in terms of geology and fluid flow. This detailed model is a platform for understanding the effect of production and recovery processes on the rock-fluid system, and the measurements required for observing changes in reservoir properties associated with production.