Profile tracer test (PTT)

INTRODUCTION

In mining operations, water infiltration from the pit floor and/or from the pit wall causes problems of stability, adds delays in ore recovery and increases the cost of operations. For these reasons, hydrogeological studies are essential in helping to develop an efficient dewatering system and to control water infiltration.

Standard hydrogeological studies require field investigation works and analysis with the use of numerical models. Modeling allows for pit dewatering optimisation and allows to better understand several phenomenon. After the initial information has been gathered, field works are the starting point for any hydrogeological study. If the quality and accuracy of the information is not sufficient, the analysis conducted by means of the numerical model could lead to erroneous interpretation.

In the framework of a standard approach, field works usually include diamond drilling for core recovery, slug test, packer test, borehole flowmeter measurements and in some particular situations, a televiewer survey. All these standard approaches allow us to estimate hydraulic conductivity and assess the heterogeneity of the medium. However, none of these methods allows us to clearly identify water-bearing fractures and/or faults. In fact, it is well known that an area of high hydraulic conductivity is not necessarily a high water-bearing zone. This occurs when the fractured area is local and not connected with a water-bearing fracture, normally called trapped water. Picture 1 shows an example of trapped water; using traditional interpretation methods, this could be interpreted as being a high flow zone.

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    Figure 1: Example of High K zones, but with no natural flow

Figure 1 illustrates two zones of fractures crossed by a vertical drill hole. The upper fractured zone appears very permeable, based on the description of the rock, and the lower fault could be water-bearing as well. Traditional tests (packer, flow meter, etc.) would give high K values for these two fracture areas. In this diagram, only the lower fracture would be water-bearing, because the extent of the fault is wide and connected with another regional fault system. The upper fracture area appears very water-bearing, but the fractures are isolated and the water is trapped. Because of the limited area of influence of packer testing and/or borehole flowmeter test (5-10m), it is probable that this upper fracture area is producing a high K value. This is one of the main reasons why Profile Tracer Testing has been developed and adapted by our firm.

For an efficient dewatering/depressurization program, targeting the contrasting flow zones is the key for optimisation and cost reduction.

METHODOLOGY

A Profile TT 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 purposes). Once the tracer is mixed in the hole, the concentration is measured at different periods of time in the same vertical borehole. Profiles of concentration are created and variations of concentrations indicate the location of the natural active flow zone. In fact, where concentrations decrease, it means that flow is present. Figure 2 shows an example of this technique.

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    Figure 2: Profile tracer test results.

In this figure, the Y axis corresponds to the depth in the hole and is the same for all the profiles. The X axis corresponds to tracer concentrations and the scale (0-15 mg/l) is the same for all the profiles. The black lines correspond to the initial profile, immediately after the injection. Two other profiles correspond to measurements of concentration every 30 minutes after the injection (blue line – 30 min and red line 60 min). The initial profile is projected over the other profile, to show the evolution of the concentrations. Results clearly show a variation of concentrations in the upper part of the formation (depth of about 10m), indicating the highest flow in this area. Results also show evidence of no flow in the lower part of the formation. In fact, numerical integration of the lower half of the 3 profiles shows almost the same value, indicating that no tracer has been flushed from the lower half portion.

PTT3Figure 3 shows the variation of concentration from the initial time. In this figure, flow location is very obvious in the upper part of the rock formation. For the analysis, some theoretical aspects must be taken into account. One of these is the diffusion; the variation of concentration in the lower part (figure 2) is clearly caused by diffusion, because trends are not regular (decrease of concentration with time), which is impossible when assessing natural flow along a borehole. Vertical flow could also affect the results, but this example does not show any vertical exchange.

Profile TTs not only allow for flow identification, but also for flow quantification. With several profiles in a single hole, and a regional piezometric map, it is possible to calculate Darcy’s flux, apparent flow and hydraulic conductivity at every single location required along the profile. This technique is much more accurate than a packer test profile and more useful than a flowmeter profile, because only natural flow is characterises (not trapped water). Figure 4 illustrates Darcy flux, Flow (Q) and K profiles from results derived on figure 2. Note that only the upper part of the flow has been characterised, because no flow has occurred in the bottom area.

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    Figure 4: Flow, Darcy’s flux and K values in the upper part of the tested hole.

The main advantage in using a Profile TT is that 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 flow zone is regional, because no stress has been applied in order to discover it. If this flow zone corresponds to a specific lithology, it is probable that the same flow signature would be measured in different drill holes crossing the same formation.

On the other hand, PTTs do not allow for the Hydraulic conductivity assessment of trapped water zones. In fact, when no flow is measured, this means that the K value is near zero, which could be different than the local K value around the hole, when this area is not connected with the main fault system. This is not a real problem, because in fact, we normally search for an active flow zone when it’s time to plan a dewatering solution.