Conventional mine dewatering practices previously considered robust are increasing being exposed as sub-optimal, especially as mining increasingly occurs below the water table in permeable deposits. Furthermore, an industry-wide push to increase mining efficiency and productivity is often hindered by physical conflict with conventional (vertical in & ex-pit) dewatering wells. Construction of the first horizontal dewatering well (a/k/a Dewatering Well Placement [DWP]) for a hardrock mine in Australia is presented. DWP offers a step-change improvement in mine dewatering with the wellhead outside the pit and the production section directionally drilled to purposely target hydraulic zones beneath the pit floor. Fortescue Metals Group commissioned the construction of the first horizontal dewatering well in a Channel Iron Deposit (CID) at their Solomon Mine in the Pilbara Region of Western Australia.
The design followed an established workflow that considered ore deposit geology, mine hydrogeology and geomechanics; target pump rate and electronic submersible pump (ESP) selection; directional tooling, downhole surveying and drilling fluids; and well architecture, completion technologies, development and drilling equipment requirements. Drilling and completion combined technology from the oil and gas industry (e.g., directional drilling, steering, ESP and packers [slipover & riserless pump]) along with traditional well construction practices and civil construction. The well was constructed as a blind (single entry) completion, with an entry angle of 25 degrees from horizontal, to 930 m Measured Depth (MD). An optical gyroscopic steering tool was used to avoid magnetic interference of the iron ore body and accurately steer the drillhead to the target zone. A slotted liner was anchored in place using an inflatable slipover packer, which also provided an annular seal. A four stage ESP with an operating range up to 300 L/s, along with a riserless pump packer to reduce friction losses, was housed inside a horizontal section of the surface casing at a Total Vertical Depth of 92 m. Further optimisation of DWP is currently underway with the construction of a second well, which will be presented. DWP requires technologies and practices from different industries that when combined result in a step-change improvement and the next generation mine dewatering solution.