Reservoir Characterization Using Single-Well Tracer Tests

A single-well backflow tracer test to estimate reservoir wettability and to determine in-situ relative permeability parameters and residual phase saturations has been developed. Wettability controls the distribution and characteristics of the flow of fluids through a reservoir rock, affects the efficiency of the displacement of oil by water, and affects the estimated reserves of a petroleum reservoir. An in-situ evaluation of reservoir wettability would be an attractive alternative, because it is unaffected by extraneous fluids and samples a much more representative volume of the reservoir. This test consists of the sequential injection of brine containing tracers and then oil containing tracers. The tracers are reactive and partitioning tracers, similar to those used in the measurement of residual oil saturation, and nonreactive aqueous and oleic tracers used for material balance control. The tracers react during a shut-in time and then the well is produced. The sequential injection of brine and oil has the purpose of exposing the reservoir rock to the whole spectrum of saturations from residual oil to residual water saturations, so that the whole relative permeability curves are scanned during the test. It is shown that the tracer production data, producing water cuts, and bottornhole pressures are all sensitive to wettability-dependent properties. The most important by far is relative permeability, but capillary pressure and mixing behavior (dispersion and capacitance) are also important. From the results of the test, the reservoir wettability may be directly inferred. By matching the test data using a compositional reservoir simulator, the in-situ relative permeability parameters and the residual phase saturations can be estimated and used to infer wettability or simply used directly in subsequent simulations of the reservoir performance. The history matching procedure has been automated in a chemical flooding simulator by using optimization techniques. Sensitivity studies to various reservoir properties and test parameters are shown. Besides the estimation of the preferential reservoir wettability, the simultaneous determination of the in-situ relative permeability parameters, residual water saturation, and residual oil saturation is new and the most important contribution of this study.