Introduction
Horizontal Line Drive (HLD) selective solution mining is a new technology for selectively solution mining potash by utilizing proven oilfield technologies from Steam Assisted Gravity Drainage (SAGD) and tight oil waterflooding. These proven oilfield techniques, when combined with the successful elements of Saskatchewan solution mining pilots from the 1960s and 1970s, offer a revolutionary new method of economically extracting potash from subsurface deposits.
The patent pending HLD technology does not require the construction of caverns in order to start solution mining because it creates large horizontal mining planes either within or along the base of high-grade potash seams, thus allowing warm brine to be pushed from the center horizontal injection well to the two adjacent horizontal production wells. The creation of the mining plane within or at the base of the potash seam is one of the key aspects that separates this technology from others. The technology is described in the following patent applications to the United States and Canada Patent Offices (PO):
USPO: https://patentscope.wipo.int/search/en/detail.jsf?docId=US278279342&docAn=16370589
Canada PO: https://patentscope.wipo.int/search/en/detail.jsf?docId=CA278257118&docAn=3038390
HLD solution mining is a way of extracting only the highest-grade potash mineralization over thicknesses of 1.5 to 3 metres, much like a conventional underground “tunnel” mine. This selective mining reduces the capital and operating expenditures related to managing and processing brine and water. By dissolving only, the highest grade of potash beds, the process will create a high potash concentration solvent which will minimize processing costs. If the solvent brine is warmer than the formation temperature, the Operator will be able to selectively control dissolution of the potash and in this way leave waste salt behind in the mining seam.
Mining Method
To produce a brine that is warmer than the formation temperature, a vertical well will be drilled and cased to basement. Then the basal Deadwood Formation will be perforated and a submersible pump will be used to produce formation brine, which will be of a hotter temperature than the shallower potash beds. The brine source well will be drilled at a location at the Project site where the depth to basement is greatest, thus ensuring the largest difference in formation temperatures is achieved.
Initial deployment of the system will focus on identifying locations with a historical potash test well that demonstrates the presence of high-grade deposits/seams to ensure decades of mining potential. Once the mining site has been selected a 3D seismic program would be run to cover at least two to three square kilometres around the historical wellbore. After establishing a sufficient mining resource, a large diameter vertical well will be drilled 700 to 1,000+ metres away from the historical hole, using the 3D seismic to help properly position the well for mining and brine quantity. This well will core the Prairie Evaporite section to demonstrate the presence of economic mineralization and to optimize the placement of the horizontal legs. The well will be plugged back before the horizontal leg is drilled to the bottom of the richest potash mineralization as identified from core. The horizontal leg will extend 500 – 800+ metres in the formation and will likely be drilled perpendicular to the natural fracture stress regime.
The horizontal well will have approximately 15 – 25 different stages that will be pressured up and separated simultaneously using packers and a sleeve system for packer isolation. This will create a rectangular mining plane, from the toe to the heel of the leg, extending out perpendicular to the horizontal well leg. The horizontal well will be equipped with a limited-entry zonal isolation injection string (Injection Control Device (ICD)) that will help distribute the water evenly from each of the 15 – 25 stages along the horizontal well. This will help create an injection front that will sweep outward and perpendicular to the wellbore (much like a waterflood for tight oil reservoirs). A large pump will be used to push down the injection well a suitable mining solvent brine to create a parting or separation along a pre-determined seam, thus creating the required mining plane.
Once the mining plane has been created, an open-hole horizontal production well is drilled on each side of the injection well so that the injection and production legs are parallel and in the same high-grade ore and some 100 metres out from the central injection well, i.e. the production wells are some 200 metres apart in the mining plane, with the injection well in the middle. This creates a mining plane with a surface area of over 100,000 metres2 or 25 acres. The mining plane is a closed system so that water injected into the
formation at approximately the separating pressure will result in the same amount of water being produced from the adjacent production wells.
Advantages of HLD Solution Mining
Ten advantages of the HLD system are:
1. Since only thin, high-grade seams are being removed, there will be minimal subsidence, so the extraction ratio can be higher than for conventional vertical cavern-type solution mining.
2. Selectively dissolving only the potash beds will mean salt waste will be left behind within the mining plane. This not only eliminates the need to store salt waste on surface, but the remnant salt will aid the mining process by creating a permeable void-filling “crystal mush”.
3. Using sodium chloride brines from deeper reservoirs as mining solvent will lower the operating cost per tonne, as well as eliminating the need for using fresh surficial waters for this purpose. Not needing caverns means that fresh water is also not needed to start mining.
4. The required technology is currently available from any active oilfield center or supplier.
5. Using a saturated salt solvent brine will confine vertical growth of the mining plane to the high-grade potash bed, also eliminating the need for a roof protection fluid like oil or diesel.
6. Salt does not naturally fracture, so the possibility that the mining plane separating vertically out of the mining seam is unlikely.
7. Injection and production wells can be drilled multi-directionally from pads, thus lessening the surface disturbance caused by extensive well installations.
8. Operation of the HLD system will be very similar to existing oil and gas operations, so there will be no need for extensive and specialized training of operating staff.
9. Since HLD mining is ideal for mining smaller annual tonnages, it can be deployed in areas where the deposit size is limited due to structural or mineralogical constraints.
10. Multiple stacks of mining planes can be created to allow for thorough removal of all high-grade seams at any specific location.
Special Thanks To:
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• Tim Campbell et al for creating Figure 1.
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• Peter Lucas Project Management for turning our design sketches into digital drawings (Figure 2 – 4) for our patent application.
