Seismic surveys acquired at different stages in the life of a reservoir can provide time-lapse snapshots of the fluid distribution over production time. This technique, called four-dimensional (4D) seismic reservoir monitoring, is helping operators delineate bypassed hydrocarbons and design development programs to optimize recovery and extend theuseful life of fields. Lars Pedersen Statoil Bergen, Norway Sarah Ryan Colin Sayers Cambridge, England Lars Sonneland Helene Hafslund Veire Stavanger, Norway
For help in preparation of this article, thanks to Olav Holberg, Geco-Prakla, Oslo, Norway; Dominique Pajot and Robin Walker, Geco-Prakla, Gatwick, England; and Benoit Reymond, Geco-Prakla, Stavanger, Norway. RST (Reservoir Saturation Tool) andTRISOR are marks of Schlumberger. Some work described in this article was performed as part of the Thermie project “4D Seismic,” European Commission Contract No. OG 117/94 UK.
Reservoir management today is a science of approximation when it comes to the rate and direction of fluid-front movement. Optimal management requires up-to-date information throughout the entirereservoir volume. Access to the latest data on fluid distribution in a reservoir, and knowledge of how that distribution is changing with time, allows engineers to develop cost-effective strategies to get the most out of every field at the lowest possible risk. Today, in addition to static, or one-time measurements, time-dependent answers from various oilfield disciplines help constrain, refineand improve the accuracy of reservoir models. Time-lapse logging of fluid saturation through casing can show which zones are contributing to production and which are watering out or being bypassed.1 Permanent downhole sensors provide continual observations of pressure, temperature and other diagnostics of reservoir performance.2 These measurements supply crucial information about fluid behavior atthe well location, but fail in the vast interwell region. One measurement technique, the 3D seismic survey, has routinely been relied on to provide interwell data. In the past, seismic surveys were mainly interpreted for struc-
tural features and stratigraphic variations within the reservoir, but they can also be sensitive to contrasts in fluid type. Applied in surveys separated by periods ofproduction, time-lapse, or four-dimensional (4D)— 3D plus time—seismic images can map fluid changes in a producing reservoir (below). This article describes the technique, the rock physics and seismic modeling required for successful application, constraints in seismic acquisition, and new interpretation methodologies that allow changes in seismic response to be interpreted as changes in saturation.Application of these techniques will be discussed in an example from the North Sea Gullfaks field where water is displacing oil—the ultimate challenge in seismic monitoring.
How 4D Works
As a reservoir is exploited, pore fluid undergoes changes in temperature, pressure and composition. For example, enhanced oil recovery (EOR) processes such as steam injection increase temperature. Productionof any fluid typically lowers fluid pressure,
increasing effective pressure of overburden on the formation rock. Gas injection and waterflooding mainly change fluid composition and pressure. These fluid changes alter the formation’s seismic velocity and density, which combine to affect travel times, amplitude and many other features, or attributes, of reflected seismic waves. When these changesare great enough, a seismic survey acquired after years of production—called a monitor survey in this article—will show different attributes than one acquired earlier, perhaps even before production begins—the baseline survey. With today’s computer technology, it is possible to take the difference between two surveys, be that the change in amplitude, frequency, phase, polarity, reflection...