Introduction to Profile Tracing Test
In operating mines, the infiltration of water from the pit floor and/or its walls causes stability problems, adds delays in ore recovery and increases the cost of operations. For these reasons, hydrogeological studies are essential to help develop an effective mine dewatering system and to control water infiltration.
Standard hydrogeological studies require field work and analysis with the use of numerical models. The modeling allows the optimization of the dewatering of the pits, but also to better understand several phenomena. After the first information has been collected, field work is the starting point for any hydrogeological study. If the quality and precision of the information collected is not precise, the analysis carried out using the numerical model could lead to an erroneous interpretation.
As part of a standard approach, field work usually includes diamond drilling for core recovery, slug testing, plug testing, flow measurement and in special situations the use of a camera. All these standard approaches allow us to estimate the hydraulic conductivity and to evaluate the heterogeneity of the medium. However, none of these methods allows us to clearly identify fractures/faults and/or any other water-bearing structures. In fact, it is well known that an area of high hydraulic conductivity is not necessarily an important aquifer area capable of supplying water on a sustainable basis. This occurs when the fractured zone is local and not related to a fracture where there is preferential flow, normally called trapped water. Figure 1 shows an example of trapped water; using traditional interpretation methods, this could be interpreted as a high flow area.
Example of a zone with high conductivity, but without natural flow
Figure 1 illustrates two fracture zones traversed by a vertical borehole. The upper fractured zone appears very permeable, based on the core description (eg RQD) although the lower fault could also contain water. Traditional tests (packer, flowmeter, etc.) would give high values of conductivity for these two fracture zones. In this diagram, only the lower fracture would carry preferential flow, because the extent of the fault is wide and connected to another regional fault system. The upper fracture zone also appears to carry preferential water flow, but the fractures are isolated and the water is trapped (therefore not connected to a regional system). Due to the limited zone of influence of plug tests and/or flow tests (5-10 m), it is likely that this upper fracture zone will produce a high conductivity value. This is one of the main reasons why the Profile Tracing Test (PTT) was developed and adapted by our firm.
For an effective dewatering/depressurization program, targeting contrasting flow areas is the key to optimization and cost reduction.
Methodology
A profile tracing test is an application recently adapted by our firm. The concept is very simple; it consists of mixing a tracer as uniformly as possible in a single open hole (for example, a diamond drill hole for exploration). Once the tracer is mixed in the hole, the concentration is measured at different time periods in the same vertical borehole. Concentration profiles are created and the concentration variations indicate the location of the natural active flow zone. In fact, when the concentrations decrease, it means that a flow is present. Figure 2 shows an example of this technique.
Result of a profile tracing test
In this figure, the Y axis corresponds to the depth of the hole and is the same for all profiles. The X axis corresponds to the tracer concentration at a predefined scale (0-15 mg/L) which is the same for all profiles. The black lines correspond to the initial profile, immediately after the injection. The other two profiles correspond to concentration measurements every 30 minutes after the injection (blue line – 30 min and red line 60 min). The initial profile is projected onto the other profile, in order to show the evolution of the concentrations. The results clearly show a variation of concentrations in the upper part of the formation (depth of approximately 10 m), indicating that the highest flow is in this zone. The results also show evidence that there is no flow in the lower part of the formation. Indeed, the numerical integration of the lower half of the three profiles represents approximately the same value, indicating that no tracer was flushed from the lower half.
The change in concentration from the initial moment
Figure 3 shows the change in concentration from the initial time. In this figure, it is evident that the location of the preferential flow is at the top of the rock formation. For the analysis, certain theoretical aspects must be taken into account. One of them is diffusion;
The variation of the coPTT3ncentration in the lower part (Figure 2) is clearly caused by the diffusion, because the trends are not regular (decreasing of the concentration with time), which is impossible when evaluating the natural flow along a borehole. Vertical flow could also affect the results, but this example does not show this type of flow.
PTTs not only allow to identify flow zones, but also to quantify them. With multiple profiles in a single hole, and a regional piezometric map, it is possible to calculate Darcy flow, apparent discharge and hydraulic conductivity at any desired location along the profile. This technique is much more accurate than a plug test profile and more useful than a flowmeter profile, because only natural flow is considered (untrapped water). Figure 4 illustrates the Darcy flow, flow (Q) and conductivity profiles from the results derived from Figure 2. Note that only the upper part of the flow has been characterized, as no flow has had place in the lower area.
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 local and regional flow information. For example, if a flow occurs in a specific location in a single hole, it is clear that this area of flow is regional, because no constraint has been applied in order to discover it. If this flow zone corresponds to a particular lithology, it is likely that the same flow signature will be measured in the different holes that cross the same formation.
On the other hand, the PTTs do not allow the evaluation of the hydraulic conductivity of the trapped water zones. Indeed, in the absence of flow measurement, this means that the value of K is approaching zero, which may be different from the local value of K around the hole, when this area is not connected to the system. main fault. This isn't a real problem, because in fact we normally look for an active flow zone when it's time to plan a dewatering solution.
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