Low Pressure Injection – Enhanced Recovery

HGI conducted one of the first test cases for Low pressure injection into a copper mine, starting small with four injection wells to determine feasibility of the method to drawdown trapped metal inventory. That test case was very successful <<link to first Carlota paper>> and is currently ongoing with over 220 injection wells and nearly 8 million lbs of copper removed from the leach pad. Our strategy for success was simple: monitor frequently and locally to the injection area in order to optimize production. With the data in hand, we could be nimble and change engineering parameters that were more appropriate for other parts of the leach pad. Well spacing, injection duration, well design, and reagent grade we all part of the suite of variables that could be tested and tweaked to ensure an optimal drawdown of inventory. We have repeated this success across several copper mines to reduce trapped metal inventory locked in leach pads.

Low pressure injection into a heap is best conducted within the post-operational period of the mine, prior to closure. The amount of infrastructure on the surface of the heap that includes wellheads and piping is cumbersome to move and redrilling is needlessly expensive. The photo montage below shows the myriad of solutions we have helped to deliver raffinate from headers to wellheads.

HGI has been instrumental in helping mines drawdown excessive metal inventory through Low pressure injection. Our strategy has been simple: monitor the injections frequently and be flexible with design changes.

Unlike high pressure injection <<link to high pressure injection page>>, where steel cased wells can easily accommodate 350 psi and be spaced over 150ft apart, Low pressure injection is typically conducted with multiple closely spaced wells running simultaneously. We use the term “pod” describe a 4-spot injection well pattern that operates in the 0-50 psi range, each receiving 100-200 gpm. Monitoring wells are placed in the center of the pod to keep track of PLS grade and water levels. The area can be expected to be saturated, but adequate drainage after cessation of injection is an important consideration for successful injection.

Our suite of monitoring tools include hydrological and metallurgical monitoring. Electrical resistivity tomography (ERT) using our state-of-the-art Geotection system <<link to hgiworld.com Geotection page>> can track the injected raffinate as it propagates through the ore. The data are used to calculate coverage (or sweep efficiency), directionality, drainage, and can be used to refine well designs for expanded operations. HGI’s ERT technology provides the most critical piece of data needed to optimize the delivery of reagent and extraction of metal for inventory drawdown.

Examples for ERT results from Geotection are shown below. In this set of examples, we show the differences in coverage and operation between pods containing 3, 4, and 6 injection wells. In each case, all of the injection wells (in orange) are running simultaneously. At this particular mine, the 4-well pod provided the most even distribution. The plume generated from the 3-well pod has gaps between the wells that are missed by the reagent and the 6-well pod has difficulty balancing flow and pressure to ensure raffinate is delivered to all of the wells (e.g., ST18 and ST19).

HGI’s ERT technology provides the most critical piece of data needed to optimize the delivery of reagent and extraction of metal for inventory drawdown.

Another aspect to HGI’s services for Low pressure injection is metallurgical monitoring, which has been shown to provide clues for ore that is difficult to leach, ensure sufficient free acid, pH-eH remains in an optimal leaching state, and an economic model can be created to justify injections based on production. As an example, we were asked to investigate an issue of low PLS copper grade.  The process for injection used raffinate that was derived from PLS of a nearby heap, hence the raffinate was already enriched and was used to stack grade for increased efficiency in the SX/EW.  The plot below shows two monitoring wells, one with higher grade and one with lower grade than the raffinate. We suspected that the copper was precipitating in the well with low grade, and the pH data is shown to be higher than it should be.  Our recommendation was to start delivering raffinate directly to the monitoring well to avoid adverse metallurgical conditions in this part of the heap.