Simulator models for single-well tracer test parameter estimation and model selection in heterogeneous formations

The single-well chemical tracer method has been used in over 140 tests since 1968 to determine residual oil saturation after waterflooding. Interpretation of the test involves matching tracer data from the field using computer simulation programs. As the complexity of the simulator model increases, the number of trial runs that are needed to satisfactorily fit field data increases rapidly. The conventional fitting procedure can thus become very cumbersome and can involve prohibitive computer costs. A new method has been devised to model tests in heterogeneous formations. These rigorous simulators have been replaced by simple response functions capable of being used in a standard parameter estimation routine. Matching the tracer data now involves function evaluations rather than full simulator runs, thus resulting in a large reduction in computing time. An algorithm has been developed to find best fit values for all unknown test variables, such as the flow distribution into and out of the layers, the dispersive mixing lengths and the residual oil saturations in the different layers. The method bas been verified by using it to explain both simulated and actual field data from tracer tests conducted in multi-layer reservoirs.