Solution Mining
Sump Development
The Milestone potash resource will be mined using the dual-well cavern technique, taking successive mining cuts. The dual-well technique lends itself well to potash extraction, since flow can be controlled for a more uniform cavern size.
The following solution mining methodology describes the sequence of solution mining for the Milestone potash resource.
Sump Connection
Once sumps are developed to the appropriate depth, the flow in each well system is reversed. The difference in density between a) the injected fresh water and b) the saturated NaCl brine, cause the fresh water to remain at the top of the sump—resulting in horizontal growth of the cavern roof. The oil blanket is maintained to prevent growth of the cavern into the potash-bearing member. This continues until the two sumps become connected, forming a single cavern.
Roof Development
Once both wells are connected as a single cavern, the direction of flow is again reversed. In this case, water is injected down one well, while the brine is extracted up the other well. This flow is periodically reversed to keep the cavern symmetrical. The roof of the cavern is expanded horizontally, until the desired cavern span is reached. The oil blanket is still maintained to prevent growth of the cavern into the potash-bearing member.
Primary Mining
Once the cavern roof is fully developed to the desired cavern span, primary mining of the potash member can begin. The blanket is then elevated. Hot water is injected down one well and brine is extracted through the other well. The oil blanket is carefully controlled and flow is reversed periodically to ensure symmetry. Multiple “passes” are taken in the potash member until the member is completely mined to the desired span. Insolubles and undissolved halite remain in the cavern and are collected in the sump.
Isolation of Esterhazy Member
Once primary mining is completed in the Esterhazy member, tubing and casings are raised, and cement plugs are installed to isolate the existing cavern from both wells. The purpose of this interruption in solution mining is to bypass the large volume of low grade ore in Interbed 2.
Primary Mining Complete
Once the existing Esterhazy cavern is isolated, the sump development process is initiated for the Belle Plaine member. The Sump connection, roof development, and primary mining of the Belle Plaine member, Interbed 1 and the Patience Lake member are completed, as previously described for the Esterhazy member. The Belle Plaine and Esterhazy members remain separated by Interbed 2.
Secondary Mining
Secondary mining involves the injection of NaCl-rich brine into the cavern, which is used to selectively dissolve potash from the walls of the cavern. Secondary mining also reduces operating costs due to the relatively low energy requirement.
After all primary mining activities are completed; the cement plugs are drilled out, restoring access to the lower cavern. Brine is then pumped into the cavern at a reduced rate, allowing the cavern walls to filter. The roof will also need to be controlled with the insoluble oil blanket to prevent excessive cavern height. Undissolved halite and insolubles remain in both the upper and lower portions of the cavern.
