A Method for Estimating Reinjected Water Returns Using Natural Tracers

Abstract

A lumped parameter model for the masses of fluid and natural tracers (chloride (Cl) and nitrogen gas (N2)) is developed for a reservoir system consisting of reinjection and production zones, and a shallow aquifer. Fractions of reinjected water as well as shallow aquifer water in produced fluids are estimated with this model. The method employs on-line analysis algorithm and estimates optimum values of the fractions at each moment when Cl and N2 concentrations of produced fluids are measured. Field data for more than ten years from Nigorikawa, Japan, are analyzed with the present method. Estimated fractions for the reinjected fluid indicate a relatively constant of about 0.5 during the first 1500 days followed by a steady decrease until 3200 days. This decrease is probably due to reinjecting water outside of the reservoir system. Reservoir volumes of the reinjection zone and the production zone are also estimated to be 0.6x108 and 0.7x108 m3, respectively.