Rapid Formation of Crystalline Plugs for Deep Borehole Seals

The DOE is evaluating deep borehole disposal of nuclear waste, where waste packages are emplaced in the lower sections of holes drilled 3 to 5 km deep in crystalline rock. A variety of plug and backfill materials are placed in the boreholes above the waste packages as structural and sealing members. This Phase I projectl will develop an approach to forming high performance plugs of molten metal and rock with exceptional seal features, requiring minimal drill rig time. The technology uses high energy thermal sources to melt the media and bond to the borehole wall material. This approach can also address immediate leakage problems caused by abandoned oil and gas wells. Poor well sealing has caused contamination in surface and subsurface water supplies, and negatively impact the performance of reservoir regions planned for CO2 sequestration and injection. Two thermal sources will be evaluated in the Phase I effort: an electric plasma arc-melter, and controlled energetic thermite mixtures that would be reacted in place. The plasma technology has been demonstrated previously for soil stabilization and in-situ vitrification of hazardous waste. The design considered for this application is a DC torch capable of 15,000K arc temperature. The torch would be positioned at the plug location, energized, and raised at a controlled rate while granular media is added to the melt. Once the desired volume of plug has been formed, the torch is withdrawn, backfilling proceeds, and the plug cools slowly by conduction. The second approach is a controlled, energetic reaction using thermite, a self-oxidizing granular metal, to melt into the borehole wall to form a plug. An engineered charge of thermite fuel and additives would be lowered to the desired plug location, ignited by a high temperature spark, and the reaction proceeds until the fuel is consumed, forming a metal and oxide solid plug. This effort will evaluate the performance requirements and emplacement environment of the deep borehole program. Numerical models will develop predictions of the thermal and structural conditions in the plug and media. The thermite technology will be evaluated in small and intermediate scale tests of mix ratios and composition to demonstrate reaction control rate and consistency of performance. The plasma torch viability assessment will address engineering aspects of the deep borehole application. The result of the Phase I effort will be an assessment of the viability of the two different approaches, and a recommendation of which technology is suitable for Phase II development and large scale field testing.