Extended application of radon as a natural tracer in oil reservoirs

In the 80's it was a common practice in the study of contamination by NAPL to incorporate a tracer to the medium to be studied. At that time the first applications focused on the use of 222 Rn, a naturally occurring radioactive isotope as a natural tracer, appropriate for thermodynamics studies, geology and transport properties in thermal reservoirs. In 1993 the deficit of radon was used to spot and quantify the contamination by DNAPL under the surface. For the first time these studies showed that radon could be used as a partitioning tracer. A methodology that provides alternatives to quantify the oil volume stored in the porous space of oil reservoirs is under development at CDTN. The methodology here applied, widens up and adapts the knowledge acquired from the use of radon as a tracer to the studies aimed at assessing SOR. It is a postulation of this work that once the radon partition coefficient between oil and water is known, SOR will be determined considering the increased amount of radon in the water phase as compared to the amount initially existent as the reservoir is flooded with water. This paper will present a description of the apparatus used and some preliminary results of the experiments. The determination of residual oil saturation, SOR, has been used for a long time in order to model the performance of the reservoirs and predict its fall. In the seventies, the knowledge of SOR started to be used to identify remaining resources during the process of flooding. This knowledge was also important for recovering projects. The quantity and the distribution of SOR are known to be critical parameters for maximizing production from an oil field. The estimation of such parameters is helpful to decide which method is more appropriate to the recovery and is supposed to be applied in order to obtain an economically viable production. SOR varies widely according to the type of formation, with the properties of the oil and also with water properties. A generally accepted method to quantify the SOR is not known yet. Different measuring techniques are available in literature (1-3), but the results are not necessarily the same when applied to the same well. Such variations can be caused by the heterogeneity of the reservoir rock or even by the alterations caused in the reservoirs by the applied process of recovery. In the method proposed here, it is postulated that if the radon partition coefficient between the oil and the water is known, as the reservoir is flooded with water, it would be possible to determine the Residual Saturation by the increased quantity of radon in the aqueous phase in relation to the quantity initially present. In this work, the methodologies that allow quantifying SOR in oil reservoirs are similar to the ones that use radon to monitor remediation of DNAPL in the contaminated areas (4-10). These ideas were adapted and modified in order to try to develop a new method which can be able to estimate SOR in oil reservoirs by using natural radon present in the reservoir. This methodology, which is being developed, is based on five main activities and will be tested in the field soon: 1. Knowledge of the area to be studied by sampling water,