Estimating fractions of reinjected water return to produced fluids using natural tracers

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Lumped parameter reservoir models were developed for estimating fractions of reinjected water return to produced fluids. The model basically consists of production zone and reinjection zone, and employs continuity equations to chemical constituents and mass of fluid. These zones can represent a combination of production and reinejection zones as well as production and reinejection wells. The chemical constituents such as chloride (Cl), stable isotope of water were found to be appropriate natural tracers through literature survey. In particular, Cl can be an excellent tracer because of its nonreactivity and conservativity in reservoirs. Fractions of reinjected water return to produced fluids were estimated with this model.Estimating methods employ on-line analysis algorithm and estimate optimum values of fractions at each moment when measurements were made. Field data from three different geothermal areas in Japan were analyzed for the fractions. Respective field showed different characteristics in terms of chemical constituents that were considered in developing the model. Estimated fractions vary with time which implies changes in reinjection program such as relocation of reinjection wells. Changes in fractions with time also suggests magnitude of reinjection return varies not directly by reinjection program but due to changes in reservoirs and/or production conditions.Further improvement of the models includes1) estimating reservoir volume2) simultaneous estimation of fractions in multiple well