Laboratory Controlled Experiments of Rock Fracture and Induced Seismicity

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Fractures are important at all scales in nature from mineral grain micro-cracks to tectonic plate boundaries. They create earthquakes, control fluid flow and at the engineering scale reduce the integrity of rock around underground structures. The recording of earthquakes on sensors all around the earth has allowed seismologists to create detailed images of the internal structure of the earth. In a similar way, it is possible to listen to small scale induced fracturing in the laboratory under controlled conditions and use the same seismology techniques with high frequency sensors to image the growth of fractures and monitor their effect on rock deformation and fluid flow. This research will utilize a novel rock deformation apparatus that can simulate the three independent stresses that exist in the earth and deform samples of rock at elevated temperatures and fluid pressure. The samples are contained in a pressure vessel that uses high frequency listening devices to monitor the creation of fractures by locating laboratory earthquakes. The pressure vessel also facilitates repeated ultrasonic imaging as the samples are deformed to failure. The rate and viscosity of fluid injection can be controlled, and measurements of permeability are possible through out the deformation tests. Pre and post testing X-ray CT scans and petrofabric analysis are used together with ultrasonic imaging during the tests for the interpretation and validation of the seismic and deformation results. In parallel, numerical models appropriate to fracturing in rock are used to model the same tests and provide a method for extending interpretations to new scenarios. The research will focus on two areas of environmental concern where controlled laboratory experiments can enhance our understanding of the processes and what controls them. The first is fluid injection of wastewater from hydraulic fracturing which can trigger felt earthquakes or bring forward in time what may have happened by natural processes. Controlled experiments of fluid injection in the laboratory, on created fractures, will be used to study the interaction of microcracking and larger fractures that slip when triggered by fluid and deformation processes. The second area is the effect of 3D loading and unloading history, induced by the creation of excavations, on the strength and behavior of fractured rock. Here, in situ stress, deformation and seismic data from Underground Research Laboratories, built to study deep geological disposal of radioactive waste (Canada, Sweden and Finland), will be used to design the 3D loading path in the pressure vessel to simulate the deformation history of rock in excavation damage zones surrounding underground openings. Results at both scales will be used to validate numerical models of fracturing and fluid flow processes and contribute to understanding how these processes scale from the laboratory to in situ conditions.