Destruction of Rock Microstructure: An Experimental and Numerical Modelling Study of High-Pressure Water Jet Rock Cutting Under Subsurface Confining Pressure Conditions

ABSTRACT: High Pressure Water Jetting (HPWJ) can be applied at the drill bit-rock interface and is especially promising for deep highly confined geothermal reservoir rocks if used for peripheral groove cutting in the bottom hole. The groove can greatly reduce the mean stress making the work of the percussive hammer drill easier and faster. A numerical FDEM model of the jet loading and mineral fragment removal of a coarse-grained granite under confining pressure is introduced. The loading of a 1 mm nozzle delivering a 240 MPa jet is modelled using a spatio-temporal pressure distribution. A quartz-feldspar-mica Voronoi-based microstructure of the Sidobre granite with inter- and intra-grain properties was calibrated. A non-traversing jetting test performed in a confining pressure cell rig was used to calibrate the jet-rock interaction model which applies a defect intensity parameter. The model matched the 4-6 mm experimental groove depths for the validation study benchmark conditions of a rotating jet traversing at 157 mm/s (20 RPM) with 20 MPa back pressure. We observe a spectrum of groove or crater depths with widths normally more than twice the depth. These variations are influenced by factors such as the jet's starting point, crystalline topology, and the local mineral phases. 1. INTRODUCTION Technological innovation in drilling deeper faster is widely cited as the key requirement to unlocking greater investment in deep geothermal energy. The ORCHYD project (https://www.orchyd.eu/) introduces a hybrid technology that employs a high-pressure water jet (HPWJ) to create a peripheral groove together with the percussive action of a mud hammer. Stress analysis of the bottom hole suggests grooves of approximately 20 mm in depth will considerably lower the mean stress state of the rock being loaded by repeated blows, hence lowering the rock's resistance to brittle failure. But the positive stress-relief effect of the groove is increasingly more difficult to achieve the deeper we drill, because increased backpressure and in-situ confining stress make it more difficult to jet a sufficiently deep groove. Geothermal drilling is increasingly targeting deep ∼4-6 km rock. Experiments (Stoxreiter et al., 2019) suggest that it is possible to overcome 50 MPa of backpressure and the resistance to fracture of tough rocks like granites if: (i) a jet is from a nozzle chamber pressure with at least 220 MPa for the case of 50 MPa back pressure, (ii) there is enough hydraulic power at the rock surface, and (iii) the jet does not transverse too fast. The operational conditions of the jetting system downhole that are needed to cut rock grooves in different rock and in-situ conditions in the most effective way, remains poorly understood. In this work, we develop a predictive model to help optimise groove depth under the physical conditions of deep geothermal drilling, i.e., conditions beyond the realm of laboratory experiments.