Radiotracer applications in reservoir evaluation

When primary oil production decreases in a field because of reduction in original pressure, water is usually injected to increase oil production. Injected water in special wells (injection wells) forces the oil remaining in certain layers to emerge from other wells (production wells) surrounding the injector. This technique, commonly called secondary recovery, contributes in extracting up to 50% of the original oil in place. Although this technique was first used in old reservoirs in which oil production had decreased, it is today a common practice to begin the exploitation of new wells with fluid injection as a way to optimize oil recovery. For this reason, the name "secondary recovery" is being replaced by the more general term "waterflooding." The efficiency of the waterflooding process is highly dependent on the rock and fluid characteristics. In general, it will be less efficient if heterogeneity is present in the reservoir (e.g., permeability barriers or high-permeability channels that impede a good oil displacement by the injected water) (International Atomic Energy Agency 2001). Most of the scales found in oil fields form either by direct precipitation from the water that occurs naturally in reservoir rocks or as a result of produced water becoming oversaturated with scale components when two incompatible waters meet downhole. The present study attempts to establish tracer technology as a reliable source of information in scaling experiments and reservoir evaluation such as reservoir heterogeneity. In a series of calcite-scaling experiments in sand, Ca²⁺ was used as a tracer to monitor the CaCO₃ precipitation. The results show that, for the first time in scaling experiments in porous media, the introduction of tracer technology has been successful (Stamatakis et al. 2003a).