A single-well backflow tracer test to estimate reservoir wettability has been developed. This test consists of the injection of first brine containing tracers and then oil containing tracers. The tracers used are esters, similar to those used in the measurement of residual oil saturation, and nonreactive aqueous and oleic tracers for material balance control. The esters hydrolyze during a shut-in time and then the well is produced. The tracer production data, water cuts, and bottomhole 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 In the absence of fluid drift, reservoir properties such as permeability and porosity have little influence in the test results. From the results of the test, the preferential reservoir wettability may be directly inferred. By matching the test data using a compositional reservoir simulator, these properties can be estimated and used to infer wettability- or simply used direcdy in subsequent simulations of the reservoir performance.
We illustrate this process using a chemical flooding simulator.1-5 This is a reservoir simulator that includes the usual features of a finite-difference simulator in terms of reservoir description, plus compositional and chemical features needed for this application such as kinetics, partitioning, dispersion, and capacitance. Special care has been taken to ensure numerical accuracy. The CPU time on a CRAY Y-MP required for each simulation was: 10 seconds for a ID-radial (31 blocks), 3 minutes for a 2D-areal (609 blocks), and 9 minutes for a 3D (1827 blocks) simulation, so many cases can be affordably studied.