Rock Fracturing under Pulsed Discharge Homenergic Water Shock Waves with Variable Characteristics and Combination Forms

Abstract

High voltage pulsed discharge in water (HVPD) is used throughout industry for fracturing both natural and man-made materials. Using HVPD, we modeled crack propagation of rocks under homenergic water shock waves (HWSW) with different characteristics and combination forms using a combination of experimental analysis and numerical simulation. The experimental results show that, under the same discharge energy (2 kJ), water shock waves (WSW) with different characteristics fractured the rock mass distinctly different. With a higher the peak pressure (PP) of WSW, more long cracks and microcracks were formed, creating a larger damage area. The numerical simulation results show that a single HWSWs impact with different characteristics will still only cause three long cracks to be well developed and almost no microcracks, when PP of HWSW was 3 MPa. With the increase of PP, the number of both long cracks and microcracks increased. This is consistent with the experimental results. When the peak pressure became greater than 15 MPa, crack propagation gradually became concentrated and the surrounding borehole wall became more severely broken. The rock model had optimal fracturing under the impact of the HWSW with a PP of 10 MPa. Also, the simulations showed that, under repeated-impacts of HWSWs with consistent characteristics, the fracturing characteristics were basically identical to those by a single-impact. While under the repeated-impact of HWSWs with variable characteristics, there was almost no relationship between the fracturing effect and the sequence of repeated-impacts. Finally, under a single-impact of HWSW with low PP and hydrostatic pressure (PH) acting within an initial crack (similar to hydraulic fracturing in a hydrocarbon well), the initial crack had excellent propagation with an increase in hydrostatic pressure. However, when PP of HWSW was too high, increasing PH had no effect on initial crack propagation.