Coupled Process Modeling at EGS Collab for Experiment Design and Model Validation

The EGS Collab project is using the Sanford Underground Research Facility in Lead, SD, USA, to conduct a series of intermediate (~10 m) scale fracture stimulation and flow experiments to better understand key processes related to the creation and operation of Enhanced Geothermal Systems (EGS). A critical objective of this project is to develop improved coupled process models that can be employed for designing and simulating field-scale EGS reservoirs. The first suite of experiments was conducted in a test bed consisting of an injection borehole, a production borehole, and six monitoring boreholes drilled from a drift located ~1480 m below ground surface. The initial borehole configuration was informed through THM modeling studies that predicted that the stimulated fractures were likely to preferentially grow in the direction of the drift due primarily to thermal stress gradients, and that drainage through the boreholes might result in limiting further growth of hydraulic fractures. The results of hydraulic fracture initiation models motivated the use of a fracture notching tool within the stimulation borehole to mitigate near-wellbore tortuosity. Detailed geologic and hydrologic characterization of the test bed environment and extensive geophysical monitoring of the stimulation and flow experiments using a wide range of sensors led to the development of a discrete fracture network model - this served as the framework for interpretation of the stimulation and flow experiments. THC simulations were conducted to model potential water-rock interaction between the injected dilute industrial water, the sulfate-bicarbonate formation waters, and the host phyllite - such reactions could lead to permeability reduction. Coupled process modeling simulations also informed the design and subsequent interpretation of a series of tracer tests as well as a long-term chilled water flow test. One of the key observations of Experiment 1 was that the presence of natural fractures had a major impact on hydraulic fractures, and that the injection of fluids likely induced mixed-mode stimulation. The modeling results and field observations from this first set of experiments are being used to help with the design of a new set of experiments that will be conducted in new test bed being developed in a drift at a depth of ~1250 m.