53rd U.S. Rock Mechanics/Geomechanics Symposium,
New York City, New York
2019. American Rock Mechanics Association
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52 since 2007
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ABSTRACT: Radial water jet drilling (RJD) is a method of enhancing heat recovery by accessing and connecting to high permeable zones within geothermal reservoirs. The wall rock geometry behind an advancing water jet borehole under in-situ conditions is largely unknown. Water jet drilling tests were performed on 300 mm cubical blocks of weak porous sandstone under true-triaxial boundary stress conditions at the Delft Technical University (DTU) rock mechanics laboratory. Some of these tests showed distinct breakout features depending on the applied stress field. Geometries of resulting boreholes are recovered using X-Ray CT scans, and are analysed using segmentation software (Avizo). The code Solidity, using a combined finite-discrete element method with a cohesive zone fracture model, simulates stress take-up and wall shearing giving breakouts comparable to the experiments. The results lead to the suggestion that criteria based on Kirsch solutions would be suitable to provide general guidance on in-situ stress and rock strength conditions free of breakouts. FEMDEM models appear well-suited to examine geometries and dimensions that can be sustained by given strengths under deeper in-situ conditions. Wall-rock failure and a process of jet-hole enlargement together with the potential benefits of greater heat recovery arising from larger holes is also briefly discussed.
Radial Water-jet Drilling (RJD) uses the power of a focused jet of fluids, applied to a rock through a coil inserted in an existing well. The aim of developing this technology is to provide a cost effective alternative to hydraulic fracture stimulation, as a means to increase permeability and therefore heat flow for exploitation in geothermal energy production. Considerable international effort is therefore turning to the prediction of a rock type's ‘jettability’ as a function of (a) potentially controllable parameters e.g. jet nozzle design and head pressure for operating conditions, and (b) the uncontrollable rock properties and in-situ stress conditions in a reservoir. Laboratory testing and numerical methods are being applied to investigate factors contributing to rock jetting performance and rates of penetration as part of a wider scoped European H2020 project ‘Novel Productivity Enhancement Concept for a Sustainable Utilization of a Geothermal Resource’ (SURE) (see companion papers, this conference).
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