Rock breaking characteristics of cyclic electrohydraulic shockwaves

Abstract

In recent years, to further increase the rate of penetration (ROP), cyclic electrohydraulic shockwaves drilling (EHSD) has been proposed. It is of great significance to study the rock breaking characteristics of this technology. Firstly, a series of experiments were carried out using three different types of sandstone rock samples. The results showed that all the rock samples were fractured after 15 to 40 impacts. Before fracture, the damage of rock samples can be divided into internal damage and external damage, including surface pits, orthogonal central tensile cracks developed from the bottom face, and other tensile cracks. Secondly, based on stress wave theory, a theoretical model for calculating the P-wave peak stress in the rock sample was established, and the calculation was carried out by MATLAB. The simulation results for S2-1 showed that the maximum transmitted compressive stress near the top face reaches 163.5 MPa. The P-wave reflected from the lateral faces formed a superposition area of tensile stress, and the maximum tensile stress reaches 5.44 MPa. With the increase of impact times, the maximum tensile stress gradually decreases and approaches the top face, which explains the formation of the orthogonal central cracks. In addition, it was found that rock samples are subject to the alternating changes of compressive and tensile stress under a single impact. Finally, based on the analysis and discussion of the above results, the rock breaking mechanism of EHSD was analyzed. The results showed that the EHSD mainly damages the rock through the water wedge effect, the tensile stress near the crack boundary, and alternating stress. The research can improve our understanding of how EHSD breaks rocks, which can promote the application of this technology in field drilling.