52nd U.S. Rock Mechanics/Geomechanics Symposium,
2018. American Rock Mechanics Association
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ABSTRACT: In this paper we present a discrete element modelling based fluid injection simulation in a blocky structured rock mass. The hydro-mechanical couple model is developed on the basis of a commercial software, Particle Flow Code 2D. The fluid fl ow algorithm is modified to enable simulation of primary fluid flow through the interfaces between the rock mass blocks and secondary fluid flow into the rock block matrix. The modelling results shows that several key observations in the field of geothermal reservoir stimulation and long-term wastewater disposal are well captured, which include: post shut in seismicity, large magnitude events at the fringe of the seismicity cloud, fluid pressure induced seismicity and poro-elastic stress triggered seismicity at far distance from the injection. In addition, we examined the effect of cyclic injection where there is a relaxation time between the main stimulation cycles. The results show that cyclic injection lowers the seismicity magnitudes, and mitigates the effect of poro-elastic stress triggering. The model is being applied to a few field scale studies, e.g. Enhanced Geothermal Systems and longterm waste water disposal, where a fault zone is present near to the injection and later activated with large seismic events by the fluid injection.
Developing an Enhanced Geothermal System (EGS) in deep reservoir requires the creation of highly permeable heat exchanger. It is usually achieved by injecting fluid in high-pressure condition. Achieving enhanced permeability of the rock mass is described by two mechanisms: (1) creation of new fractures in opening mode by hydro-fracturing mechanism and (2) dilation of existing fracture under shearing model by the hydroshearing mechanism. In both mechanisms, seismic events are created where the accumulated strain energy is released from a generated fracture and at sliding fracture surfaces. These seismic events are called fluid injection-induced seismicity.
Fluid injection-induced seismicity has been a major topic for both the geoscience research community and geothermal energy development. In 2006, a seismic event with a local magnitude of 3.4 occurred at the Basel EGS site (Haring et al. 2008, Kraft et al. 2009) which caused a significant amount of economic loss and at the end suspended the whole project. The geological hazards and risks associated with subsurface fluid injection in the development of EGS have been the major obstacle of the geothermal energy market, due to the public concern of having man-made earthquakes.
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