Well-to-well Tracer Tests And Permeability Heterogeneity
Authors: Ghori S.G. , Heller J.P. 
Published: March 2010

Tracer tests have been widely used in the petroleum industry to determine qualitative information about underground reservoirs. This paper describes how it is possible to obtain quantitative information about the permeability heterogeneity of underground reservoirs from well-to-well tracer tests. 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 was produced. This catalog was computed with the help of a new numerical technique that is stable and which produces negligible numerical dispersion. The effects of varying the natural logarithm of permeability (ranging from .09 to 4.0) and the dimensionless correlation scale (ranging from .13 to .3) for numerous realizations are quantified. The general effect observed of areal heterogeneity is the appearance of multiple peaks 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 range generally decreases the number of peaks. By using a sufficient number of permeability realizations, an attempt was made to assess the variance and correlation range 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. Two different types of permeability distribution were obtained, showing that downhole sampling of the tracer would be important in achieving adequate information about the reservoir rock. A tracer test was performed in which tracer concentrations were measured at the production well. 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 confirm the numerical technique and strongly suggest that tracer tests in oil fields and aquifers can be used to quantify permeability distribution.

Introduction

One of the most important aspects of reservoir heterogeneity, that has a direct impact on enhanced oil recovery projects, is the variation in permeability. Variations of other parameters, such as porosity, are of secondary importance(1,2). It is almost impossible to measure the permeability values, for example, at the scale of the grids of a numerical reservoir simulator. However, the general features of the spatial distribution can be described by probabilistic theory. It has been observed that the spatial distribution of permeability can be sufficiently described by two geostatistical parameters(3–5): the variance and the correlation range. Unfortunately, not enough actual field data are generally available, especially in the horizontal direction, to define these two parameters of permeability for most actual reservoirs.

Operating engineers use samples of radioactive or chemical tracers taken at the surface.