Shunji Sasaki

Characteristics of microseismic events induced during hydraulic fracturing experiments at the Hijiori hot dry rock geothermal energy site, Yamagata, Japan

Microseismicity accompanying hydraulic injection experiments at the Hijiori hot dry rock site was monitored by a network of ten borehole seismic stations deployed at an average distance of 2 km from the injection well. While expanding hydraulic fractures are almost aseismic, they can induce microseismic events. These events are probably caused by shear failures induced by high pore fluid pressures occurring on planes of weakness in the rock surrounding the main hydraulic fracture. Thus we can use these induced events to locate the hydraulic fracture and follow its growth. Microseismic events induced during the 1988 hydraulic fracturing experiment with a high injection pressure were located near the injection point in the early stage of the experiment and clearly migrated towards the east and distributed along a vertical plane. The strike of seismicity is nearly parallel to the direction of the maximum principal stress. The vertical orientation and east–west strike of the seismic events are essentially coplanar with the caldera ring-fault structure in the southern portion of the Hijiori Caldera. This indicates that a preexisting fracture zone was being re-opened and developed in the direction of the maximum principal stress, although microseismic events were caused by shear failures. The space–time distribution of the microseismic events shows that the events migrated at a rate proportional to time to the power 2/3. Assuming that migration of events is attributed to fracture propagation, the propagation of the hydraulically stimulated fracture can be explained by one of two petroleum industry models tested. Seismicity accompanying the 1989 circulation test with a low injection pressure was diffuse and showed a seismic cloud. The permeability was estimated from the hypocenter migration as 10−16 m2, which is intermediate between the permeability of core samples of granodiorite taken from the production well and the permeability of fractured rocks obtained by an injection test between the injection well and the production well. It is therefore concluded that the seismic cloud accompanying the circulation test is due to the permeation of water into joints which slip when the effective stress is reduced by the increased pore fluid pressure accompanying the hydraulic injection. Microseismic events occur and migrate along those joints.

EFFECT OF GRAIN SIZE IN GRANITIC ROCKS ON HYDRAULIC FRACTURING MECHANISM

Hydraulic fracturing experiments ere conducted by employing four 20-cm-cubic granitic rock specimens of different grain size. Water was injected into a vertical hole in the specimen, bored normal to a rift plane along which mirco cracks preferentially oriented. In the two horizontal directions, 12Npa and 6Mpa confining pressures were applied to the specimen. Due to water injection, a horizontal crack along a rift plane was created in the specimens having larger grains, while a vertical crack along the direction of the maximum confining pressure was created in those having smaller grains. In addition, fault plane solutions of acoustic emission indicated that dominant micro fracturing mechanism was shear in the specimens having larger grains while it was tensile in those having smaller grains. These results demonstrate that grain size significantly affects the orientation of crack extension and the micro fracturing mechanism in hydraulic fracturing of granitic rocks.