Technical Paper: Water Injection Fall-Off Tests in Deepwater Reservoir: What Do We Actually See Into Formation?

Society: IPTC
Paper Number: 13440
Presentation Date: 2009


A certain water-injection volume is required to maintain target oil production, so a short injection or fall-off test is often conducted to evaluate injectivity performance prior to oil production. Following that, permanent downhole gauges are set to monitor full field development. Pressure transient data are also analyzed as long-term injection and fall-off tests. Reservoir parameters, such as permeability and skin factors, can be obtained from these short- and long-term tests. However, several questions must be asked about the interpretations.

What do we actually see in the formation, injected water or reservoir oil? Do the permeability and skin values change with time? How can we capture and understand reservoir changes versus time? In a single-phase system, an answer is straightforward. However, in the two-phase injection system, the single-phase assumptions can no longer be applied. When interpreting water-injection fall-off tests, it is important to consider the volume and consequent injection radius occupied by water pumped into formation. These factors are significant in log-log plot interpretation because the first, early radial flow stabilization plateau reflects the relative permeability to the water injected. However, the reservoir performance and production forecasts need to use the effective permeability to oil. This is usually inferred from long-term fall-off tests that investigate beyond the injection water invasion zone. Therefore, an appropriate methodology should be applied in test design and interpretation to deliver accurate commercially valuable well test outputs, specifically for short-lived falloffs.

The paper demonstrates the application of a 3D dynamic simulation to fall-off tests performed on the two-phase reservoir model. It shows the importance of the mentioned factors, helps in understanding fall-off peculiarities, and discusses the way the outputs can be achieved when testing injection wells during continuous pressure maintenance schedule.

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