Tracer tests have been widely used in the petroleum industry to determine qualitative information about underground reservoirs, such as the existence of flow barriers, directional flow paths, etc. This paper describes how well-to-well tracer tests can be used to obtain quantitative information about the geostatistical parameters (variance and correlation scale) of permeability. Both theoretical and experimental results are presented.

In the theoretical work, a catalog of tracer production curves for different sets of geostatistical parameters for five-spot flooding patterns is produced. This catalog is computed with the help of a new numerical technique that is stable and devoid of any numerical dispersion. The effects of variance of the natural logarithm of permeability (ranging from .14 to 2.25), dimensionless correlation scale (ranging from .2 to .5), and of numerous realizations are quantified. The general effect of areal heterogeneity is that multiple peaks appear in the output tracer concentration curve, even when the flow is confined to a single layer. An increase in variance generally increases the number of prominent peaks in the output concentration profile, decreases the time between the emergence of peaks, and increases the time span of the occurrence of the tracer. An increase in correlation scale generally decreases the number of peaks. By using a sufficient number of permeability realizations, an attempt is made to assess variance and correlation length of the permeability field from the tracer and output concentration curves.

In the experimental work, a slab of rock (Arizona flagstone) representing a quarter of a five-spot pattern was selected. A simple minipermeameter was used to measure areal permeability on grids of points on both the top and bottom surfaces of the rock. A tracer test was performed and the tracer output concentrations were measured at six intervals along the depth of the rock at the production well. Six different types of concentration curves were obtained, showing that downhole sampling of the tracer would be important in achieving adequate information about the reservoir rock. Permeabilities measured with the minipermeameter were used as input into the simulator and the numerical production curves were compared with the experimental profiles. The occurrences of similar patterns of multiple peaks confirms the numerical technique and strongly suggests that well-to-well tracer tests in oil fields and aquifers could be used to estimate the geostatistical parameters of the permeability field.

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