MANAGEMENT OF IN-SITU RECOVERY (ISR) MINING FLUIDS IN A CLOSED AQUIFER SYSTEM

摘要

The Beverley uranium mine, operated by Heathgate Resources (Heathgate), is located on the arid plain between the northern Flinders Ranges and Lake Frome, approximately 550 km North of Adelaide in South Australia. The deposit exists as coffinite mineralisation hosted in permeable sands within the confined, saline Beverley aquifer, some 125m below the ground surface. Uranium is mined using moderately acidic in-situ recovery (ISR) technology which entails circulating oxidizing fluid through the ore body to mobilize the uranium which is then pumped to the surface and recovered. Oxidizing fluid is then re-circulated and as the chloride levels become unmanageable the excess, high chloride, fluids are stored within the mined-out areas of the aquifer.The demonstrated ability to manage both the mining fluids and the high chloride "disposal stream" within the aquifer system was a critical criterion of permitting and regulatory approval of the mine in the late 1990s. This approval was contingent on Heathgate demonstrating a robust understanding of the hydrogeological structure of the aquifer hosting mineralisation. Studies into the structure of the aquifer, to demonstrate that fluids could be controlled comprised the interpretation of data obtained through a number of hydraulic (pumping) tests.From a hydrogeological perspective this provided an interesting and unique case. Firstly the aquifer at Beverley was found to be an almost completed bounded aquifer system. Secondly, the very detailed geological and hydraulic data that subsequently emerged during ISR mining provided an excellent validation of the initial, pre-mining, aquifer hydraulic test analysis.The paper will describe analysis of a series of hydraulic tests, in the Central Beverley ore body, using classical hydrogeological methods, supported through the adaptation of petroleum industry reservoir analysis techniques for hydraulically bounded systems, which demonstrated that the aquifer at Beverley exhibits a response to pumping stress which is consistent with an aquifer system of finite extent, which is completely bounded by lower permeability sediments.Subsequent very high density drilling, and hydraulic stress analysis during mining of the ore body, continued to support this initial interpretation.Operation of an ISR mining operation in a hydraulically bounded system presents specific challenges in that a very precise water balance must be maintained to prevent, under or over-pressurising the aquifer during mining.