51st U.S. Rock Mechanics/Geomechanics Symposium,
San Francisco, California, USA
2017. American Rock Mechanics Association
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ABSTRACT: The applicability of a combined thermo-mechanical drilling technique is investigated. The working principle of this method is based on the implementation of a heat source as a mean to either provoke thermal spallation on the surface or to weaken the rock material, when spallation is not possible. Thermal spallation drilling has already been proven to work in hard crystalline rocks, however, several difficulties hamper its application for deep resource exploitation. In order to prove the effectiveness of a combined thermo-mechanical drilling method, the forces required to export the treated sandstone material with a polycrystalline diamond compact (PDC) cutter are analyzed. The main differences between oven and flame treatments are studied by comparing the resulting strength after heat-treating the samples up to temperatures of 650°C and for heating rates ranging from 0.17 °C/s to 20 °C/s. For moderate temperatures (300-450°C) the unconfined compressive strength after flame treatments monotonously decreased, opposed to the hardening behavior observed after oven treatments. Thermally induced intra-granular cracking and oxidation patterns served as an estimation of the treated depth due to the flame heat treatment. Therefore, conclusions on preferred operating conditions of the drilling system are drawn based on the experimental results.
The exploitation of hydrocarbons and deep geothermal resources relies on finding cost-effective solutions to increase the drilling performance in hard crystalline rocks (Gill, 1983, Bejerano et al., 2006). Conventional rotary drilling techniques, based on mechanical rock abrasion and exportation, result in high rates of drilling tool wearing, causing significant costs (Fay, 1993, Pessier et al., 1992). Additionally, rotary drilling is characterized by low drilling speeds in hard crystalline basement rocks targeted for enhanced geothermal energy utilization. Further, even lower overall drilling rates result when considering tripping times required to exchange worn drill tools. Therefore, alternative drilling methods, such as hammering (Han et al., 2005, Teodoriu, 2011), thermal spallation (Rudolf von Rohr, 2015), electro-pulse (Anders et al., 2015), plasma drilling (Timoshkin et al., 2003, Kocis et al., 2015), and hydraulic jetting processes (Pekarek et al., 1963) are investigated worldwide in order to enhance the drilling capabilities for exploration of deep geological resources.
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