Introduction to Profile Tracing Test
In operating mines, the infiltration of water from the pit floor and/or its walls can cause stability issues, delays in ore recovery, and increased operational costs. For these reasons, hydrogeological studies are crucial to help design an effective mine dewatering system and control water infiltration.
Standard hydrogeological studies involve fieldwork and analysis using numerical models. Modeling helps optimize pit dewatering and provides a better understanding of various phenomena. Once initial data is collected, fieldwork forms the foundation of any hydrogeological study. If the quality and accuracy of the data collected is compromised, the analysis conducted with the numerical model may lead to incorrect interpretations.
As part of a standard approach, fieldwork typically includes diamond drilling for core recovery, slug testing, plug testing, flow measurement, and in certain cases, the use of cameras. These methods enable us to estimate hydraulic conductivity and assess the heterogeneity of the environment. However, none of these techniques can clearly identify fractures, faults, or other water-bearing structures. In fact, it is well-established that areas with high hydraulic conductivity are not necessarily significant aquifers capable of sustainably supplying water. This occurs when the fractured zone is localized and not connected to a fracture with preferential flow, commonly referred to as trapped water. Figure 1 illustrates an example of trapped water, which, when interpreted using traditional methods, could be mistaken for a high-flow area.
Example of a zone with high conductivity, but without natural flow
Figure 1 illustrates two fracture zones intersected by a vertical borehole. The upper fractured zone appears highly permeable based on the core description (e.g., RQD), although the lower fault could also contain water. Traditional tests (packer, flowmeter, etc.) would yield high conductivity values for both fracture zones.
In this diagram, only the lower fracture zone would facilitate preferential flow because the fault extends widely and is connected to a regional fault system. The upper fracture zone, while also seemingly capable of preferential water flow, consists of isolated fractures where water is trapped (i.e., not connected to a regional system). Due to the limited influence range of plug tests and/or flow tests (typically 5-10 meters), it is likely that the upper fracture zone will show a high conductivity value. This is one of the primary reasons why the Profile Tracing Test (PTT) was developed and adapted by our firm.
For an effective dewatering and depressurization program, targeting areas with contrasting flow characteristics is essential for optimization and cost reduction.
Methodology
A Profile Tracing Test (PTT) is a method recently adopted by our firm. The concept is straightforward: it involves mixing a tracer as uniformly as possible within a single open hole (such as a diamond drill hole used for exploration).
Once the tracer is introduced, its concentration is measured at various time intervals within the same vertical borehole. These concentration profiles are then created, and variations in concentration reveal the location of the natural active flow zone. Specifically, when the concentration decreases, it indicates the presence of flow. Figure 2 illustrates an example of this technique.
Result of a profile tracing test
In this figure, the Y-axis represents the depth of the borehole, which is consistent across all profiles. The X-axis corresponds to the tracer concentration, plotted on a predefined scale (0-15 mg/L), which is the same for all profiles.
The black lines represent the initial profile, immediately following the injection. The other two profiles show concentration measurements taken every 30 minutes after injection: the blue line at 30 minutes and the red line at 60 minutes. The initial profile is overlaid on the other profiles to illustrate the evolution of the concentrations.
The results clearly indicate a variation in concentrations in the upper part of the formation (approximately 10 meters in depth), suggesting that the highest flow is present in this zone. The results also provide evidence that no flow occurs in the lower part of the formation. Specifically, the numerical integration of the lower half of the three profiles yields nearly identical values, indicating that no tracer was flushed from the lower portion.
The change in concentration from the initial moment
Figure 3 shows the variation in concentration from the initial time. It is clear from this figure that the preferential flow zone is located at the top of the rock formation. For the analysis, certain theoretical aspects must be considered, one of which is diffusion.
The variation in concentration in the lower part (Figure 2) is clearly due to diffusion, as the trends are irregular (concentration decreases over time), which is not typical when evaluating natural flow along a borehole. Vertical flow could also influence the results, but this example does not show this type of flow.
PTTs not only help identify flow zones but also quantify them. With multiple profiles in a single borehole and a regional piezometric map, it is possible to calculate Darcy flux, apparent discharge, and hydraulic conductivity at any desired location along the profile. This technique is far more accurate than a plug test profile and more useful than a flowmeter profile, as it only accounts for natural flow (untrapped water).
Figure 4 illustrates the Darcy flux, discharge (Q), and conductivity profiles derived from the results in Figure 2. Note that only the upper part of the flow has been characterized, as no flow occurred in the lower section.
Flow rate, Darcy flow and hydraulic conductivity at the top of the hole.
The main advantage of using a PTT is that the results provide both local and regional flow information. For instance, if flow occurs in a specific location in a single borehole, it is clear that this flow zone is regional, since no constraints were applied to detect it. If this flow zone corresponds to a particular lithology, it is likely that the same flow signature will be observed in other boreholes crossing the same formation.
On the other hand, PTTs do not allow for the evaluation of hydraulic conductivity in trapped water zones. Without flow measurement, the hydraulic conductivity (K) value tends toward zero, which may differ from the local K value around the borehole, especially when this zone is not connected to the main fault system. However, this is not a significant issue, because in practice, we are typically looking for an active flow zone when planning a dewatering solution.
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