Rock fracture initiation and propagation by mechanical and hydraulic impact

Abstract

Rock fracture due to high-pressure fluid jetting and insert indentation was experimentally investigated. In the frame of this article, indentation designates the process of pressing a tungsten carbide insert (TCI) for roller cone bits displacement-controlled into the rock surface. Under atmospheric conditions, several crystalline and one sedimentary rock type were tested. Depending on the size, type and bonding of the mineral grains, distinct differences in fracture behaviour were observed. The influence of elevated ambient pressure regimes on the evolving cracks and fractures in the tested granite revealed that microcracking ceases with increasing ambient pressure, while the specific energy is also significantly affected. Under similar conditions, indentation experiments were performed on undamaged samples and on samples containing jetted kerfs. A distinct variation in the measured maximum force and removed volume indicated different underlying rock fracture mechanisms. By applying methods for crack visualization and three-dimensional rendering, it turned out that the dimensions of the kerf significantly affected the mechanism of fracture initiation and propagation as well as the extent of the associated fracture pattern. Furthermore, the common practice of characterizing the rock excavation process only via measurable quantities such as the specific energy, without considering the created fracture pattern, yields misleading results.

Ergebnisse aus dem EU‐H2020‐Projekt ThermoDrill – Ein Beitrag der Felsmechanik zur effizienteren Gewinnung geothermischer Energie

“…To the knowledge of the authors, apart from the notch that was created, the formation of cracks in the surrounding rock was only investigated in detail in [9]. The cracking is however of major significance for the interaction of mechanical and hydraulic rock excavation processes, which is why attention must be carefully paid to investigating the extent of the crack formation.…”

Energy Requirement for Rock Breakage in Laboratory Experiments and Engineering Operations: A Review

“…To figure out how the energy transmitted to the rock is distributed, the fracture and damage pattern caused in the rock beneath an indenter or a cutter has been widely investigated (e.g. Lundberg 1974;Kumano and Goldsmith 1982a, b;Lindqvist and Lai 1983;Lindqvist et al 1984Lindqvist et al , 1994Xu and Yu 1984;Rogers et al 1986;Howarth and Bridge 1988;Pang et al 1989;Zhang 1994Zhang , 2016Kou 1995;Liu et al 2008;Franca 2011;Entacher et al 2015;Nariseti et al 2015;Stoxreiter et al 2019a). Based on the previous studies, the fracture and damage pattern under indentation can be simplified as shown in Fig.…”

“…Reported measurements on specific energy in percussive drilling or similar indentation tests are few. Stoxreiter et al (2019a) performed indentation tests with a bullet-shaped tungsten carbide indenter on Neuhauser granite specimens with a diameter of 220 mm and a height of 90 mm. The specimens were mounted in a steel ring and the annular space was filled with mortar.…”

Experimentelle Untersuchungen zu kombinierten hydraulisch-mechanischen Gesteinslöseprozessen

“…Mit fluoreszierendem Harz gefüllte Risse und Kerbe entlang der Schneidspur eines Hochdruckwasserstrahls mit 4000 bar Staudruck, 0,25 mm Düsendurchmesser und rund 13 kW hydraulischer Leistung unter atmosphärischen Bedingungen[8] …”

“…Das Mit Farbeindringverfahren sichtbar gemachter vertikaler Zugriss unterhalb eines Kraters aus einem Eindrückversuch ohne benachbarte freie Oberfläche. b Ausgeprägte subhorizontale Risse vom Krater des Eindrückversuchs (rechts) in Richtung der Kerbe (links)[8] Ausmaß dieses Rissnetzwerks hängt stark von der hydraulischen Leistung des Strahls und vom Umgebungsdruck ab.…”