Dutler, Nathan

2020, Dutler, Nathan

Hydraulic fracturing is a common technique used in a variety of fields like civil and mining engineering, oil & gas and geothermal industry. It can be used to enhance the permeability of low permeable rocks, to increase the connectivity of natural fractures, to modify the rock mass strength, or to measure the Earth’s stress field. In the context of deep geothermal energy exploitation, a heat exchanger needs to be created at depth with characteristics favorable for heat extraction i.e. sufficient permeability and heat exchanger area. The creation of the heat exchanger for geothermal heat extraction remains a critical element with high associated risks including poor reservoir performance and induced seismicity. Hence the need for a better understanding of the coupled seismic-hydromechanical processes during stimulation operations. The execution of experiments on the intermediate-scale has the advantage of a better control on the processes associated with induced seismicity and reservoir performance compared to full-scale and allow to use comprehensive real time monitoring of pore pressure, rock mass deformation and seismicity. This scale is closer to the full-scale stimulation than laboratory scale, where seismo-hydromechanical interactions are generally focused on single fractures. The decameter-scale In-situ Stimulation and Circulation (ISC) project took place between 2015 and 2018 at the Grimsel Test Site (GTS), Switzerland. The GTS is located in the Central Swiss Alps, beneath the mountains of the Grimsel Pass. Overall, the moderately crystalline fractured rock mass shows a pervasive foliation and was intersected by six major sub-vertical shear zones. For each of the two assumed stimulation endmembers, hydraulic shearing and hydraulic fracturing, six experiments were conducted. Prior to the experiments, the test volume was characterized in great detail with respect to geology, geophysics, hydrogeology and in-situ stress field. This doctoral thesis aims at better understanding tensile fracture growth. It includes study of fracture toughness and fracture process zone on laboratory scale and the investigation of the seismo-hydromechanical coupled processes during in-situ hydraulic fracturing experiments. The tested intact Grimsel Granodiorite samples indicate that the resistance against material failure is significantly higher across the foliation plane than along it. The results from Digital Image Correlation (DIC) confirm the development of a semi-elliptical fracture process zone (FPZ) with an average length to width ratio of about two for both principal directions. This agrees well with the available results in the literature. The experimental results of the length of the FPZ give supporting evidence to the fact that a nonlinear cohesion stress distribution provides an accurate cohesive model that agrees well with the experimental results. Additionally, the conformity of the ratio of the FPZ length in two principal directions with the theoretical predictions gives supporting evidence to the proportionality of the FPZ length with respect to the square of fracture toughness to tensile strength. At the decametric scale during the in-situ experiment, the hydromechanical coupled responses of the rock mass and its fractures were captured by a comprehensive monitoring system installed along the tunnels and within dedicated boreholes. At the borehole scale, these processes involved newly created tensile fractures intersecting the injection interval while at the cross-hole scale, the natural network of fractures dominated the propagation process. The six HF experiments can be divided into two groups based on their injection location (i.e., south or north to a brittle ductile shear zone), their similarity of injection pressures and their response to deformation and pressure propagation. The experiments executed north of the shear zone, show smaller injection pressures and larger backflow during bleedoff phases. In addition, we observe re-orientation of the seismic cloud as the fracture propagated away from the wellbore. The re-orientation during propagation is interpreted to be related to a strong stress heterogeneity and the intersection of natural fractures striking different from the propagating hydraulic fracture. This leads in the details to complex geometry departing from theoretical mode I fracture geometries. The seismic activity was limited to about 10 m radial distance from the injection point. In contrast, strain and pressure signals reach further into the rock mass indicating that the process zone around the injection point is larger than the zone illuminated by seismic signals. Furthermore, strain signals indicate not just single fracture openings but also the propagation of multiple fractures. Various methods to estimate the fracture opening and fracture contact pressure were applied and compared from single injection borehole observations with the strain gauge in distance from the injection point. The results show, that the fracture opening pressure was also observed at the strain gauge, associated with a strong increase of fracture transmissivity. The combination of injection pressure and strain observation allows to define an aperture-stress relationship with a general trend toward decreasing normal fracture stiffness during fracture opening. The fracture contact pressure can be estimated, but hydromechanical superposition of pump shut-in and corresponding pressure loss and interaction of the connected surrounding fractures make this task very challenging and error-prone. The pore pressure data set differentiate two distinct responses based on lag time and amplitudes. This allow to distinguish a near- and far-field response. The near-field response is due to pressure diffusion and the far-field response is due to stress perturbation. The far-field pore pressure response is consistent for all experiments, indicating the dominant failure mechanism. This change in the far-field are very sensitive and can be used as a complementary method to seismic monitoring during hydraulic stimulations. The exceptional hydromechanical dataset allow to test numerical stimulations and can help to improve injection strategies, the monitoring design and the numerical modelling.