Abstract
In order to study the resonance characteristics of rock under harmonic excitation, two vibration models have been presented to estimate the natural frequency of rock encountered during the drilling. The first one is a developed single-DOF model which considers the properties and dimensions of the rock. The second one is a multi-DOF model based on the principle of least action. Subsequently, the modal characteristics, as well as the influence of excitation frequency, the mechanical properties, and dimensions of the rock on its resonance frequency, are analyzed by using FEM. Finally, the ultrasonic test on artificial sandstones and materials of drill tools are carried out indoor, and the FFT transform method is adopted to obtain their resonance frequencies. Based on the analysis undertaken, it can be concluded that the natural frequency of the rock increases with the change of vibration mode. For the same kind of rock, the resonance frequency is inversely proportional to mass, while for the different kinds of rocks, the mechanical parameters, such as density, elastic modulus, and Poisson’s ratio, determine the resonance frequency of the rock together. Besides, the shape of the rock is also one of the main factors affecting its resonance frequency. At last, the theoretical research results are further verified by ultrasonic tests.
Highlights
It is well known that the resonance is a very common physical phenomenon in nature, and it can be observed in almost every field of engineering technology
A new drilling technology which takes advantage of the damage characteristic of resonance has been proposed in the petroleum engineering, namely, Resonance Enhanced Drilling (RED). e main idea of this technology is that the bit, while rotating, applies a dynamic impacting force with an adjustable high frequency to the rock, so that makes the rock resonate to achieve rock fragmentation [1]. e RED technology can solve the common problems, such as slow rate of penetration and serious wear of drill bit, when drilling the deep and hard formations [2,3,4]
Elastic modulus [5,6,7], compressibility, density [8], shear modulus, and damping ratio [9] of the rock can be obtained by resonance ultrasonic spectrum and the pore characteristics, saturation [10], and sediment characteristics [11] of the rock can be extracted by nuclear magnetic resonance technology
Summary
It is well known that the resonance is a very common physical phenomenon in nature, and it can be observed in almost every field of engineering technology. A new drilling technology which takes advantage of the damage characteristic of resonance has been proposed in the petroleum engineering, namely, Resonance Enhanced Drilling (RED). Since the key to realizing of the RED technology is to determine the magnitude or the range of the resonance frequency of the rock so that the corresponding harmonic vibration impact can be exerted on the rock to achieve resonance fragmentation. In this paper, aiming at the small rocks, a single-DOF model of natural frequency considering the properties and dimensions of the rock is developed. En, based on the principle of least action, a multi-DOF model of natural frequency is proposed. En the ultrasonic test is conducted on the artificial sandstones and the materials of drill tool in Section 4 to verify the theoretical research results and explain the feasibility of RED technology. Equation (4) is the single–DOF model of natural frequency of the rock
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