Engineering a robust hydraulic connection between wells is one of the most difficult aspects of enhanced geothermal systems (EGS). Designing and constructing such hydraulic connections requires an understanding of the in-situ state of stress and the heterogeneities and discontinuities that naturally exist and may control the stimulation. Even with comprehensive stress and formation characterization programs, substantial uncertainty remains in these key parameters. This is especially the case in high-temperature EGS environments where drilling conditions are often difficult and far fewer logging and testing options are available. This paper presents a new approach for explicitly quantifying the uncertainties in the state of stress using a Bayesian Markov Chain Monte Carlo method. This approach produces a probability distribution for the stress tensor, including a general 3D orientation, that reflects the uncertainties in all the observations or indicators used to constrain the stress state. This method is demonstrated using the characterization data for the EGS Collab Experiment 2 site. The output of the analysis is used to guide the design of the planned stimulations. In the case of research projects like EGS Collab, explicitly quantifying the uncertainties in the stress state allows for more rigorous hypothesis testing by allowing conclusions drawn from the experiments to be interpreted in the context of the uncertain knowledge about conditions in the test bed.
Enhanced Geothermal Systems (EGS) require use of hydraulic pressure to artificially generate fluid pathways between one or more wells. The hydraulic pressure interacts with the in-situ state of stress and rock properties to create new fractures or enhance the permeability of existing fractures. The fracturing can include contributions from both tensile (Mode I) and shearing (Mode II) displacements (e.g., Norbeck et al., 2018). One concept for creating sustainable EGS reservoirs centers on preferentially creating Mode II shearing displacements on naturally occurring fractures, which is often called shear stimulation or "hydroshearing." The potential advantage to shear-mode stimulation is that when fractures shear, they often also permanently dilate because the asperities on the fracture faces no-longer mate together perfectly after the shearing offset. This has the potential to create a fracture with significantly improved hydraulic conductivity without requiring the placement of proppant as in petroleum stimulation operations.
