Experimental investigation on perforation of shale with ultra-high pressure abrasive water jet: Shape, mechanism and sensitivity

Abstract

Perforation is critical to create a flow path between a shale reservoir and the production tubing to ultimately achieve supercritical carbon dioxide (SC–CO2) hydro-jet fracturing and natural gas production. However, the mechanisms of ultra-high pressure abrasive water jet (UHP-AWJ) perforation in shale remain unclear. To address this concern, experiments are designed using the Taguchi method to simulate UHP-AWJ perforating in practice. The UHP-AWJ perforation mechanisms are investigated by measuring the reflection angle, the length of perforation, testing strain, and perforation pressure. The results indicate that there are two typical perforation tunnel shapes, depending on jetting flow, back flow, and abrasive action. During UHP-AWJ perforating, the reflection angle decreased from 180° to 40° (by 77.4%), while the length of perforation increased following an exponential trend, and the perforation pressure rose with fluctuations owing to dynamical jetting. In addition, the effects of influential factors, including AWJ pressure, AWJ distance, AWJ time, and casing thickness on the evaluation indexes (such as length of perforation, the maximum diameter of perforation, mass loss of shale, perforating speed, and effective diameter of perforation) are discussed thoroughly. Moreover, the axial pressure is more than twice the radial pressure due to the strong impingement of the water jet. The experimental error of the perforation pressure is only 1.09%, indicating the reliability of our results. Three main stages of UHP-AWJ perforation based on the above results are outlined and summarized.