SUMMARY Hydraulic fracturing plays a vital role in the development of unconventional energy resources, such as shale gas/oil and enhanced geothermal systems to increase the permeability of tight rocks. In this study, we conducted hydraulic fracturing experiments in a laboratory using carbonate-rich outcrop samples of Eagle Ford shale from the United States. We used a thermosetting acrylic resin containing a fluorescent compound as a fracturing fluid. Immediately after fracturing, the liquid resin penetrated in the fractured blocks was hardened by applying heat. Then, the crack was viewed under UV irradiation, where the fluorescent resin allowed the induced fracture to be clearly observed, indicating the formation of simple, thin bi-wing planar fractures. We observed the detailed structure of the fractures from microscopy of thin cross-sections, and found that their complexity and width varied with the distance from the wellbore. This likely reflects the change in the stress state around the tip of the growing fracture. The interaction between fractures and constituent grains/other inclusions (e.g. organic substances) seemed to increase the complexity of the fractures, which may contribute to the efficient production of shale gas/oil via hydraulic fracturing. We first detected acoustic emission (AE) signals several seconds before the peak fluid pressure was observed, and the active region gradually migrated along the microscopically observed fracture with increasing magnitude. Immediately after the peak pressure was observed, the fluid pressure dropped suddenly (breakdown) with large seismic waves that were probably radiated by dynamic propagation of the fracture; thereafter, the AE activity stopped. We applied moment tensor inversion for the obtained AE events by carefully correcting the AE sensor characteristics. Almost all of the solutions corresponded to tensile events that had a crack plane along the maximum compression axis, as would be expected based on the conventional theory of hydraulic fracturing. Such domination of tensile events has not been reported in previous studies based on laboratory/in situ experiments, where shear events were often dominant. The extreme domination of the tensile events in the present study is possibly a result of the use of rock samples without any significant pre-existing cracks. Our experiments revealed the fracturing behaviour and accompanying seismic activities of very tight rocks in detail, which will be helpful to our understanding of fracturing behaviour in shale gas/oil resource production.

Summary To investigate the influence of fluid viscosity on the fracturing process, we conducted hydraulic fracturing experiments on Kurokami-jima granite specimens with resins of various viscosities. We monitored the acoustic emission (AE) activity during fracturing and estimated the moment tensor (MT) solutions for 54 727 AE events using a deep learning technique. We observed the breakdown at 14–22 MPa of borehole pressure, which was dependent on the viscosity, as well as two preparatory phases accompanying the expansion of AE-active regions. The first expansion phase typically began at 10–30 per cent of the breakdown pressure, where AEs occurred three-dimensionally surrounding the wellbore and their active region expanded with time towards the external boundaries of the specimen. The MT solutions of these AEs corresponded to crack-opening (tensile) events in various orientations. The second expansion phase began at 90–99 per cent of the breakdown pressure. During this phase, a new planar AE distribution emerged from the borehole and expanded along the maximum compression axis, and the focal mechanisms of these AEs corresponded to the tensile events on the AE-delineating plane. We interpreted that the first phase was induced by fluid penetration into pre-existing microcracks, such as grain boundaries, and the second phase corresponded to the main fracture formation. Significant dependences on fluid viscosity were observed in the borehole pressure at the time of main fracture initiation and in the speed of the fracture propagation in the second phase. The AE activity observed in the present study was fairly complex compared to that observed in previous experiments conducted on tight shale samples. This difference indicates the importance of the interaction between the fracturing fluid and pre-existing microcracks in the fracturing process.

Group predation promotes foraging efficiency because it increases the size of prey that can be killed and improves hunting success compared to solitary predation. However, group predation may increase competition among group members during feeding. Earlier studies have focused on the advantages of group predation, but little is known about the costs and benefits of group predation for individual members of the group. Here, we show that the costs and benefits of group predation for individuals of the predatory stink bug Andrallus spinidens vary with prey size in laboratory experiments. We found that when A. spinidens fed on small prey, group predation did not significantly increase foraging efficiency but did increase competition for food among group members. In contrast, when prey was large, group predation promoted foraging efficiency, and competition over food was not detected. Our results suggest that group predation by A. spinidens nymphs is advantageous for individual members because it enables each member to hunt larger prey that could not be hunted alone. However, when group size was large or prey size was small, group predation increased competition among group members.

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