13. Solution Mining and Salt Cavern Usage

Salt solution mining is just what it says, the mining of various salts by dissolving them and pumping the resulting brine to the surface where it is concentrated or processed to recover the desired chemical products. Actual dissolution and recovery methodology is predicated on the solubility of the targeted salt, A “rule of thumb” in the solution mining industry is that every 7–8 m³ of freshwater pumped into a cavity will dissolve 1 m³ of halite. Water or undersaturated brine is injected through a purpose-designed well drilled into a salt mass to etch out a void or cavern. The resulting “almost saturated” brine is then extracted for processing. The technique usually targets salts at depths greater than 400–500 m and down to 2,000 m (Fig. 13.1). The current deepest salt solution operation is in the Barradeel concession in northern Netherlands in Zechstein Z2 salts and operating at depths around 2,800 m (Geluk et al. 2007). At depths greater than 2,000 m ongoing salt creep tends to reduce cavern size. Some operating brinefield caverns are as shallow as 150 m, but this can lead to catastrophic chimneying and stoping in sediments above cavity. With deeper operations the landsurface tends to subside into a bowl of subsidence, as it does above many conventional mines. Cavern shape and the upward rise of the cavern roof is today controlled by an inert fluid blanket pumped in and maintained at the top of the zone of active brine creation. Early solution wells did not use this blanket technology. In the 1800’s and extending into first half of last century many brinefield operators perceived surface sinks, collapses and regular abandonment of caved wells as normal, during the operational life of a saltfield. Attitudes in the mining community today, across all types of exploration and production, are much changed.