Abstract
This paper presents the mathematical basis for a new and cost effective method to estimate reservoir pressure and effective water permeability in low permeability reservoirs. This method, called Baseline/Calibration, has been successfully tested in the Wamsutter field. This approach, which is an alternative to time-consuming DFIT tests and conventional pressure buildup tests, requires injection of water in multiple short stages. Sandface pressure and flow rate are analyzed to estimate reservoir pressure and permeability.
We derived analytical expressions which provide a mathematical basis for this method. The analytical formulation assumes piston-like displacement of reservoir fluids with injected water in a water-invaded region and conventional transient flow of gas outside this region.
We validated the analytical model and the proposed test-interpretation technique with a numerical simulation model. In addition, we analyzed flow rate and pressure data from a field trial performed in the Almond formation of the Wamsutter field. The results of numerical simulation and the field trial of this method verified that our method accurately determines reservoir pressure with short injection tests.
Measurements of reservoir pressure and in-situ reservoir permeability are important for variety of reasons including estimation of ultimate recovery, production forecasting, and optimization of depletion planning. However, conventional well tests methods, another potential source of these properties, are often impractical in unconventional reservoirs because of the long shut-in times required.
