Example of basic analysis: First level or direct interpretation

The concentration versus time curves (experimental response curves) are analysed to measure the main characteristics of flow in the group of wells under study (Fig. 30).

For this, 370 GBq of HTO was injected into well CnE-241. The experimental response curves give transit times and allow quantification of the injected water produced in different directions. Basic interpretation or first level interpretation allows to acquisition of qualitative and semi-quantitative information that in many cases is the only type required by the end users. Tracer sampling and measurement showed that tracer came up at three out of five production wells around the injection well. The arrows represent the fraction of the injected tracer recovered in each well.

Figure 31 presents the experimental response and cumulative curves obtained at production well CnE-324.

The experimental response curve obtained in production well CnE-324 indicates:


FIG. 30. Pattern under analysis.


• Absence of channelling.

• Breakthrough time is 117 d.

• Time for the peak is 159 d.

• Mean residence time is 194 d.

• Tracer recovery is 6.5%.




FIG. 32. Extrapolated response curve for well CnE-324.

It is easy to observe that tracer sampling was interrupted before the concentration reaches background level; consequently, for more accurate results, extrapolation of the curve is required.

For this purpose, it is recommended that an exponential function be used and parameters obtained from a least squares fit using the last points in the tail of the experimental data as reference. However, extrapolation has to be used carefully and based upon the general knowledge of each particular situation in order to avoid speculation and unexpected results. Figure 32 shows the extrapolated curve generated by the Anduril software.

The new values for the parameters are:

• Breakthrough time is 117 d.

• Time for the peak is 159 d.

• Mean residence time is 217 d.

• Tracer recovery is 7.7%.

A similar treatment has been applied to other wells in the pattern shown in Fig. 30 with the following experimental results.