Numerical study on impact energy transfer and rock damage mechanism in percussive drilling based on high temperature hard rocks
Introduction
Geothermal resource is one of the most competitive renewable and clean energies (Yu et al., 2018). However, High temperature hard formations and rocks, for example granite are often encountered when drilling geothermal wells, which brings huge challenges and high costs to drilling operation(He et al., 2019; Hu et al., 2018; Wu et al., 2019). Percussive drilling has proven to be a feasible drilling technology capable to significantly increase the penetration rate when drilling hard rocks. In general, percussive drilling is the use of downhole impact devices to apply additional impact energy to the drill bit, forcing the bit to impact on rocks. For its detailed working principle, see (Song et al., 2019). Impact energy transfer efficiency is the ratio of the actual impact energy used for rock fragmentation to the input energy generated by the impact devices. The study of energy transfer efficiency and rock damage characteristics is important to improve impact energy utilization as well as rock breaking efficiency in percussive drilling.
Predecessors have done a lot of studies on the energy transfer efficiency and rock fragmentation mechanism in percussive drilling. Li et al. (2001) studied the transmission process of the dynamic stress waveform and reflected energy in percussive drilling based on experiments. A bit-rock interaction model was built to study energy transfer efficiency of the impact system in percussive drilling (Franca, 2011). The energy transfer efficiency of percussive drilling by analyzing shape of the incident wave was optimized(Lundberg and Collet, 2010, Lundberg and Collet, 2015) . Yang et al. (2019) analyzed the effects of four different incident wave shapes (exponent, rectangle, triangle, and sine) on the energy transfer efficiency through theory modelling and found that the energy transfer efficiency under the condition of rectangular wave is the largest. Song et al. (2019) used the finite element method to establish a three-dimensional percussive system model to study the energy transfer efficiency of percussive drilling considering the strong nonlinearity of the percussive process. Mardoukhi et al. (2018) conducted experimental study of the dynamic indentation damage in thermally shocked granite and analyzed damage characteristics of the heated rock after impacted on. Song et al. (2020a) built an axial-torsional coupled percussive system model to study impact energy transfer in axial-torsional coupled percussive drilling. Saksala et al. (2014) conducted numerical and experimental study of percussive drilling with a triple-button bit, and analyzed the fragmentation characteristics of the rock during percussion. Song et al. (2020b) considered the strain rate effect of the material and investigated effect of the button numbers and multiple Impacts on the energy transfer and rock crushing mechanism of percussive drilling. Saksala et al. (2020) developed a two-dimensional single tooth model to study thermal shock assisted percussive drilling, and analyzed the effects of thermal shock on rock damage.
Totally, most previous studies focused on the energy transfer efficiency and rock damage in percussive drilling based on normal temperature rocks. There are few studies concerning effects of the rock temperature and the heat exchange on impact stress wave propagations, energy transfer efficiencies and rock fragmentation characteristics in percussive drilling. The mechanical properties of high temperature rocks are very different from that of normal temperature rocks. As the temperature increases, the compressive and tensile strength of the rock decreases (Yin et al., 2015). Moreover, in geothermal drilling, there is heat exchange between high temperature rocks and low temperature drilling fluid. The heat exchange process will generate thermal stress inside the rock, which may have large implications for percussive drilling. Therefore, it is essential to investigate energy transfer efficiency and rock damage characteristics in percussive drilling based on high temperature hard rocks. This study was carried out on the basis of our previous studies (Song et al., 2019; Song et al., 2020a). The main purpose of this paper is to simulate the mechanical percussion-heat transfer coupled process, and analyze effects of the rock temperature and heat exchange on the energy transfer efficiency and rock damage in percussive drilling. The outline of this study is as follows. At first, the physical model for mechanical percussion-heat transfer coupled process was built. And then heat transfer model, fully coupled thermal stress calculation method, temperature-dependent rock damage model, and impact energy transfer model were introduced. Finally, the mechanical percussion-heat transfer couplings were simulated. For heat transfer process simulation, our main object is the heat exchange process between high temperature rock and low temperature fluid, which means the high temperature rocks are cooled. Then as the control group, we also simulated the process of high temperature rocks being heated.
Section snippets
Physical model
Our main studies focus on the mechanical percussion-heat transfer coupled process in percussive drilling. As shown in Fig. 1, the percussive system contains percussive hammer with initial velocity, anvil, multiple-tooth bit, and rocks. When the percussive hammer strikes the upper end face the anvil, the upward-propagating stress waves and the downward-propagating stress waves will appear simultaneously. The downward-propagating stress waves are the carriers of the input energy in the percussive
Geometrical parameters of the percussive system
Fig. 2 presents the geometrical model of the percussive system. Rocks were meshed with the structured grid. The mechanical percussion-heat transfer coupled simulation zone of rocks was meshed by a finer grid. The size of the coupled simulation zone is 100 mm×100 mm, and the grid size is 1.25 mm. The total elements of the rock model are 317269. The hemispherical teeth and the multiple-tooth bit were selected. The size of the tooth is Φ12mm. The percussive hammer size is Φ37mm×300mm. The anvil
Simulation results and discussion
The propagation patterns of the impact stress wave were first analyzed during the percussion. Then the effects of rock temperature on percussive process were studied. Subsequently, we analyzed effects of heat exchange on the percussive process, impact energy transfer efficiency. Then the temperature, thermal stress, and rock damage distributions of the rocks under the mechanical percussion-heat transfer couplings were studied. Finally, we also studied the effects of different tooth shapes.
Conclusions
This article adopted the numerical method to study mechanical percussion-heat transfer coupled process, impact stress wave propagation, energy transfer efficiency, and rock damage in percussive drilling based on high temperature hard rocks. The effects of impact velocity, rock temperature, heat transfer, and tooth shape were analyzed. The following conclusions are drawn. As the rock temperature increases, the bit-rock interaction force decreases, and both duration of the interaction forces and
Author statement
Hengyu Song, Huaizhong Shi, and Gensheng Li conceived the idea for this manuscript. Hengyu Song built the mechanical percussion-heat transfer coupled model and conducted the simulations. Zhenliang Chen, Ran Ji, and Han Chen performed data analysis. All authors discussed the simulation results and contributed to the final manuscript.
Declaration of Competing Interest
No conflict of interest.
Acknowledgments
The authors give their thanks to the financial support of National Key R&D Program of China (Grant No. 2019YFC0604904) and the Joint Funds of the National Natural Science Foundation of China (Grant No. U19B6003-05). The authors also gratefully acknowledge the comments of the reviewers and the editors.
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