In Saskatchewan, the target beds for potash are the potash-bearing members of the Prairie Evaporite sequence contained within the Elk Point Group; a Devonian Aged sedimentary sequence in Western Canada, Montana and North Dakota. There is a total of three (3) significant potash-bearing members that make up the Prairie Evaporite Formation (in descending order): the Patience Lake member (PLM), the Belle Plaine member (BPM) and the Esterhazy member (EZM), which are situated at the top of a halite-dominated sequence, overlying the dolomitic Winnipegosis Formation. The halite-dominated evaporite sequence is itself overlain by the Dawson Bay Formation, another dolomitic-dominated formation, with the Second Red Beds, red dolomitic shales, as the boundary between itself and the underlying evaporites. The potash-bearing members are relatively flat lying, with a very slight regional southward dip. “Mounds” may occur in the Winnipegosis, corresponding to ancient reefs in the carbonates, and are sometimes associated with thinning or “leaching” anomalies in the overlying potash members.

The potash mineralisation within the Prairie Evaporite Formation is relatively uncomplicated. The dominating constituents are halite (NaCl), sylvite (KCl) and carnallite (KMgCl3.6(H2O)); sylvite being the main ore mineral. The salt (i.e., non-clay seam) is generally confined to halite, sylvite and minor anhydrite, as well as carnallite (mostly in the Esterhazy member). The anhydrite content in the salt ranges from absent to 2.7 % (Patience Lake) and 3% (Belle Plaine).

Selective solution mining (or “selective dissolution”) of potash consists of using a brine solution mostly consisting of salt (NaCl), and a minor amount of sylvite or potash (KCl), to selectively dissolve the KCl from a potash bed within a solution mining cavern.

Based on geologic data from the six exploration boreholes and 3D seismic survey within KL 245, the Tugaske Project is targeting, as a base case, a continuous potash zone (“sub-member”) at the base of the Patience Lake member, which is being referred to as the “PLM 1”. The PLM 1 averages 3.9 metres thick, with an average potash grade of 43% KCl, across the mining area. Analysis of the relative economics of a range of approaches for the selective dissolution at the Project indicated that the most capital-efficient approach is a multi-lateral horizontal cavern arrangement consisting of a single (1) horizontal injection well that connects to seven (7) horizontal lateral extraction wells (thus forming one cavern system) that are drilled along the base of the target potash bed in a “fan-style” configuration. This method is similar to a current solution mining method that has been utilized by Intrepid Potash at their Cane Creek Mine in Moab, Utah since 2003 (Ryan, 2012).

To attain the design production rate of 250,000 tonnes of KCl product per year (t/y) of saleable MOP, six (6) solution mining caverns are required. On this basis, each cavern will produce about 45,000 t/y of saleable MOP and has a design cavern life of approximately 12 years within the PLM 1 sub-member only.

Cavern Dimensions & Layout
The multi-lateral horizontal cavern selective mining arrangement is designed to fit within a typical Saskatchewan land section (i.e., 1.61 km by 1.61 km, or 1 mile by 1 mile). The layout provides a 100-m-wide pillar along two edges of the section, to protect the solution mining wells and to minimize surface subsidence above the pillar.

The solution mining cavern consists of a single (1) 600-m-long horizontal well that intersects seven (7) horizontal lateral wells in a fan-style pattern with lengths ranging from 1,390m to 1,483 m. For area planning purposes, a solution mining cavern can be represented by an 800-m by 1,600-m right triangle.

Using an elliptical cavern with an average height of 3.9 m and maximum width of 80 m per lateral and an average potash grade of 43% KCl within each horizontal cavern with an average cavern brine recovery factor of about 60%, it is estimated that each solution mining cavern will produce an average of about 499,000 tonnes. The planned cavern production is 45,000 t/y per cavern so that each cavern has an estimated operating life of approximately 12.1 years.

Injection & Production Well Drilling
Using directional drilling techniques, multiple wells can be drilled from a single drilling pad. Each well will contain a 406.4-millimetre (mm) (16 inches) conductor casing that will be installed before drilling is initiated. The drill pad is temporary; hence drill mats will be used for the central portion of the drill pad and will be surrounded by gravel. Each well pad will be finished upon drilling completion, for long-term access and operations of the wells.

Each solution mining cavern will be composed of a directionally drilled injection well and a directionally drilled multi-lateral production well. Solution mining will be initiated at the intersection of the injection well and each lateral of the production well. With a single drilling rig, drilling for the initial six (6) caverns will require approximately 12 months to complete. Upon completion of the initial drilling phase, drilling activities will be suspended until extraction from these caverns nears completion in approximately 12 years.

Cavern Development
Upon completion of the drilling operation, the caverns could be required to remain inactive for up to six (6) months before the plant operations can be initiated. To minimize solution mining start-up difficulties, preliminary plans include 30 days circulating unheated, fresh water in each solution mining cavern to flush the drilling muds and slightly enlarge the individual lateral borehole diameters. It is planned to inject unheated fresh water down each lateral for a period of 3 to 4 days. The resulting production brine will then be injected into the potash disposal well. Upon completion of each cavern development, it is planned to pressurize each solution mining cavern to a surface pressure of about 69 bar (6.9 MPa or 1,000 psi).

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The process plant (or, “the plant”) is nominally designed to produce 250,000 tonnes per year of saleable Muriate of Potash (“MOP”), pink or clear, granular grade potash product. Return brine from processing will be heated in a series of heat exchangers to 100 °C and pumped to the wellfield for re-injection into the mine caverns, for dissolution and recovery of potassium chloride (KCl), from the underground sylvinite ore deposit containing both KCl and sodium chloride (NaCl) minerals. Heating the return brine will increase the dissolving capacity for KCl. The wellfield production brine of approximately 60 °C will be pre-cooled in a heat exchanger, with the mine return brine, prior to feeding the crystallization circuit. There, incoming production brine will be progressively cooled down in a 6-stage crystallization circuit, consisting of 4 vacuum crystallizers, followed by 2 surface cooled crystallizers; ultimately cooling mother liquor to 10°C, with chilled glycol/water mixture, resulti ........