Shale rock fragmentation behaviors and their mechanics by high pressure waterjet impinging
Abstract
High pressure waterjet technology is a potentially advanced technology to improve the drilling efficiency of shale rock formation. The success of waterjet drilling requires more insight into the rock fragmentation behaviors and their mechanics. In the present study, a series of submerged waterjet impinging trials were carried out on the outcrop shale rock collected from the Longmaxi Formation, Sichuan Basin, China. Simultaneously, the sandstone was selected as the candidate of the control group. The macro fragmentation behaviors and their micro structures were acquired by the scanning electron microscope (SEM) and computed tomography (CT) techniques. The rock mechanics and mineral composition before and after experiments were measured by the standard ISRM method and X-ray diffraction, respectively. Particularly, the grain size distributions of cuttings were obtained by a laser particle size analyzer. Results show that the efficiency of shale waterjet drilling is ultralow, which the rate of penetration and the ultimate drilling depth cannot reach the half of those sandstone. SEM imaging shows that shale is ultra-tight in which the illite clay weaves into a network and the dolomite embeds into the network. It just likes a concrete material resulting in the high compression strength. Since the static pressure of waterjet is lower than the compression strength, the common compression-shear failure does not happen, but only the tensile fracture occurs and makes the rock spallation for the initial crushing. With the penetration depth increasing, the waterjet impinging force is not able to make the rock tensile or shear failure. Only the particle erosion becomes a dominant failure mechanism for shale rock where the mineral particles are removed or crushed from rock matrix as fine cuttings. This work provides the theoretical guide for improving the shale rock drilling efficiency by water jetting.
