Uranium symposium covers state of the art (Part 2)

Another presentation in the U2009 uranium symposium in Keystone, Colo., was given by Ted Way of In-Situ Inc. The paper dealt with well-field mechanics for in-situ uranium mining, and was based on a journal article in the November-December issue of Southwest Hydrology.

“Basic to an in-situ mining operation is a thorough understanding of the site’s hydrogeology, particularly the degree to which fluid movement can be predicted and controlled,” Way says.

The questions that the hydrologist must address include: Is uranium deposited in the saturated zone with sufficient available drawdown? Do the upper and lower confining units of the aquifer provide enough vertical confinement for the leaching solution? Is the formation’s hydraulic conductivity high enough for wells to achieve reasonable productivity and injectivity?

Another important parameter is the storage coefficient – the ratio of the water pumped to the volume of the cone of depression.

Once a site’s hydrogeology is considered feasible, engineers can use three aspects to enhance the operation’s economy and minimize environmental effects: recovery process design; well-field design; and monitoring programs.

The first aspect, recovery process design, is based on laboratory tests which indicate the distance that the leaching solution can travel underground before losing its leaching ability. The results are used to determine the distance between injection wells and production wells.

The second aspect, well-field design, can be quantified by areal sweep efficiency – the percentage of the field leached by the solution. Efficiency can be raised by using more wells and by arranging wells in certain optimum geometric configurations.

Ideally the solution from different injection holes should reach production holes simultaneously (in other words, they should have identical breakthrough times), but in reality, this can only be approximated.

To ensure containment, the pumping rate is typically 1-3% higher than the injection rate. A groundwater model is usually developed when the field is designed.

Apart from areal sweep efficiency, another two parameters that are important to well-field design are productivity and injectivity. Productivity depends on well-field uranium reserves, while injectivity is limited by the highest pressure that the rock can withstand before fracturing.

One final technique used to boost recovery is flow reversal. When uranium levels in production wells drop, more uranium can be recovered by using production wells as injection wells and vice versa.

The third and final aspect is a groundwater monitoring program, which is essential for protecting areas surrounding the well-field. The program is implemented by using monitoring wells which surround the production field at a distance of 120-150 metres. Another layer of protection can be added by inner monitoring wells which give an early warning in case of seepage outside the production field.

Additional monitoring wells overlaying and underlying the aquifer are needed to monitor vertical leakage.

Parameters monitored include pH, conductivity, groundwater level and uranium concentration.

Way concludes that a field’s uranium reserves coupled with hydrologic characteristics are the most important parameters in determining the economy of the operation.