A METHOD OF ANALYZING TRACER DATA TO CALCULATE SWEPT PORE VOLUME AND THERMAL BREAKTHROUGH IN FRACTURED GEOTHERMAL RESERVOIRS UNDER TWO-PHASE FLOW CONDITIONS

摘要

One of the goals of using tracers in geothermal reservoirs is to predict the thermal breakthrough as early as possible. Shook (2001) has shown how this can be accomplished for the case of single-phase flow. In this paper, we show how the thermal breakthrough can be calculated from tracer data for the case of two-phase flow of steam and liquid water in a fractured reservoir without matrix permeability. In particular, we present a method to estimate the swept pore volume of the fracture in a hot, dry geothermal reservoir using partitioning tracers of high volatility. When a tracer with a high partition coefficient (concentration in the vapor divided by the concentration in the liquid) is injected in water, it will partition into the vapor phase and subsequently transport in the vapor phase (steam) toward the production wells. The method uses the first temporal moment of the tracer concentration distribution recorded at the producer to calculate the pore volume contacted by the injected tracer (the swept pore volume). This method has a rigorous theoretical basis and has been widely used in both groundwater and oil field applications. It can be used in the absence of detailed reservoir characterization data or flow and transport models since only a very simple, fast and easy integration of the production data is needed to yield the mean residence time of the tracer. Once the pore volume of the fracture is known, then the thermal breakthrough can be calculated using a retardation factor that takes into account the latent heat of the water.