Introduction – Motivation
In order to meet the increasing worldwide energy and mineral resource demand in the next decades, the access to heat, oil, natural gas and minerals at significant depths, often in hard rocks, will be a key requirement. However, the construction of wells accessing ever deeper resources requires large investments, mainly due to the involved drilling operations (Tester et al., 2006). The drilling costs are shown to increase exponentially with depth and, furthermore, they occur in an early, considerably high-risk phase of the project (Tester et al., 2006; Lukawski et al., 2014). For example, the large share of drilling costs is a principal driving factor for the economic viability assessment of enhanced geothermal systems (EGSs) (Sigfusson et al., 2015; Stefansson, 1992). Indeed, drilling of deep wells into hard basement rocks represents a major challenge for conventional mechanical drilling (e.g. rotary drilling) syste
ms, featuring high rates of drill bit wear with consequent bit replacement and high non-productive time (NPT), low rates of penetration (ROP) and poor process efficiency (Fay, 1993; Diaz et al., 2017).
Hence, to improve the economic viability of utilizing deep georesources (heat, hydrocarbons or minerals), improvements on the drilling process efficiencies, particularly to drill hard, crystalline rocks, are needed. In this project, we propose to use a combined thermo-mechanical drilling (CTMD) technology to intensify the drilling process in hard rocks (Rossi et al., 2020a). This hybrid drilling technology is based on thermally-assisting conventional drilling using a flame-jet (see Figure 1), and is expected to facilitate the drilling process by increasing ROP and decreasing drill bit wear, with a consequent reduction of overall project costs. Therefore, the objectives of this project are to investigate, through laboratory and field testing, the viability and performance of the novel combined drilling technology.
- Investigate the feasibility of a thermally-assisted drilling concept
- Analyze the thermal cracking behavior of rocks
- Study the rock removal mechanisms during CTMD
- Demonstrate readiness of the technology though field testing
- Evaluate the potential improvements in drilling performance using CTMD
Combined Thermo-Mechanical Drilling (CTMD) technology
The (CTMD) concept is based on integrating a thermal assistance, for example by a flame jet, into conventional mechanical drilling. Thereby, the following drilling modes of the CTMD technology can be distinguished: (i) The flame jet can be operated as a standalone mode to induce thermal spallation when the rock exhibits the required properties (Rauenzahn & Tester, 1989; Kant et al., 2017; Rossi et al., 2020b) for thermal spallation (thermal spallation drilling, Mode I).
(ii) Alternatively, the flame jet can provide thermal assistance to conventional, rotary drilling, by thermally weakening the rock (Rossi et al., 2018), facilitating the rock removal, performed by the drilling cutters (Rossi et al., 2020c, 2020d, 2020e) (flame-assisted rotary drilling, Mode II).
Finally (iii), CTMD allows, as a third drilling mode, the use of conventional, standalone-mechanical drilling technology, in cases, where the rock formation can be drilled with adequate performance using such a conventional method, for example, in soft rocks.
Within the CTMD concept, the second drilling mode, flame-assisted rotary drilling, is key in extending the applicability of thermal-based removal approaches to any deep drilling project. A conceptual view of the flame-assisted rotary drilling mode of the CTMD technology is given in Figure 2. The flame-assisted rotary drilling mode employs flame jets to thermally weaken the rock material, prior to the mechanical rock removal, performed by conventional drilling cutters. Drill mud is used to provide cooling to the drilling components and to transport the produced cuttings, cleaning the bottom-hole region. In order to protect the flame jets in the aqueous environment, the CTMD concept includes co-axial flows of compressed air, which are used to air-shield the heat sources, thereby increasing the heat transfer impinging on the rock surface and reducing the thermal wear of the drill bit cutters (Figure 2).
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