Water Injection Fall-Off Tests in Deepwater Reservoir: What Do We Actually See Into Formation?
Authors: Daungkaew Saifon , Goh Kim Fah Gordon , Carnegie Andrew 
Published: March 2015

Water injection is an important recovery method in deepwater reservoirs. A certain water injection volume is required to maintain target oil production. A short injection / fall-off test is therefore often conducted to evaluate well injectivity performance prior to oil production. Following that, permanent downhole gauges are set to monitor full field development scale. Pressure transient data from the permanent gauges are then 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, such as What do we actually see into formation, i.e. injected water or reservoir oil?Do the permeability and skin values change with time?How can we capture and understand reservoir changes versus time?

Determination of in-situ reservoir permeability from pressure transient tests has always been of primary target in well test analysis. In a single-phase system, an answer is straight forward. However, in the two-phase injection system, the single phase assumptions cannot be applied any longer. 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[1]. However, the reservoir performance and production forecasts need to use the effective permeability to oil. This is usually inferred from longterm fall-off tests that investigate beyond the injection water invasion zone. Therefore, an appropriate methodology should be devised and applied in test design and interpretation to deliver accurate commercially valuable well test outputs, specifically for short-lived falloffs.

A three-dimensional dynamic simulation has been selected as a means to show the importance of the above factors, understand fall-off peculiarities, and discuss the way the outputs can be achieved when testing injection wells during continuous pressure maintenance schedule. The paper demonstrates the application of the simulation to fall-off tests performed on the two-phase reservoir model. Responses of pressure fall-off recorded at various stages of long-term injection sequence on the log-log plot are produced and compared with actual tests from deepwater reservoirs. Reflection of fall-off test duration on phase mobility dynamics is presented. Importance of rock wettability and relative permeability-saturation relation is discussed. Contribution of phase-by-phase saturation related peak flow rates radially propagated in time from the wellbore away into a reservoir during a falloff and their association with the pressure derivative[2] curve to determine effective phase permeability values are discussed in this paper. Awareness of changing of the effective phase permeability values on pressure derivative that reflects either effective permeability of a phase while recorded at various moments of injection schedule allows one to provide a comprehensive well test analysis. To understand this effect, the correct value should be reported instead of an overall absolute reservoir permeability value that could mislead model calibration issues in FDP study or other reservoir engineering campaigns. Such an approach can be a useful tool in analyzing both simulated and field fall-off data to benefit dynamic simulation in the testing business.