Our goal is to design a robust stimulation and flow-through experiment to enable model validation. However, we cannot know everything about the rock mass and uncertainty in stress orientation and magnitude along with the unknown presence of preexisting discontinuities that can dominate system response. Consequently, our first objective was to select a specific location at SURF consistent with a borehole geometry that would best facilitate direct observation of stimulation via hydraulic fracturing and subsequent flow-through tests.
4850 level testbed characterization
4850 level natural fracture system
4850 level testbed geophysical monitoring system
Multiple methods will be employed to monitor the hydrofracture and flow-through experiments. We anticipate utilizing continuous active-source seismic monitoring (CASSM), passive microseismic (MEQ) monitoring, acoustic emissions (AE), electrical resistivity tomography (ERT), borehole pressure monitoring, in situ borehole deformation monitoring using the SIMFIP tool, and continuous distributed monitoring of temperature, seismicity, and strain using fiber optic cables. A number of these methods were successfully utilized in prior field experiments (e.g., Oldenburg et al., 2016; 2017; Knox et al., 2016). Each of these methods are described briefly below.
- Continuous Active-Source Seismic Monitoring (CASSM):. CASSM instrumentation will be deployed within the monitoring wells of Experiment 1. The instruments operated both during the active fracturing, and in a background before and after fracturing events. The results are expected to be a measurement of stress sensitivity and a continuous monitoring of stress changes and fracture growth between the boreholes associated with the induced hydrofractures.
- Passive Microseismic Monitoring:Microseismic (or micro-earthquake, MEQ) monitoring is a passive observation of small-scale seismic events. In EGS Experiment 1, MEQ monitoring was conducted to measure induced seismicity from hydraulic fracturing activities (e.g., Pettitt et al., 2012). These include its triggers, magnitude and propagation in crystalline rocks. MEQ data can help to understand patterns of fracture developments, connectivity and the impacts from in-situ stress, rock fabric and existing fractures or discontinuities. MEQ monitoring at the EGS Collab site was carried out using high sensitivity three component (3C) accelerometers that were installed in the monitoring boreholes
- Acoustic Emissions: Acoustic emissions (AE): monitoring is an approach targeting very high frequency seismic energy generated during tensile and shear failure of brittle materials. While similar in principle to MEQ, the higher frequencies of AE recording require (a) faster recording rates, often including triggering, (b) special purpose transducers tuned to the AE band, and (c) special attention to transducer installation to preserve higher frequency coupling. AE records, when processed, can provide information on periods of active fracturing through examination of event frequency. When integrated with an appropriate velocity model, AE events can be located to provide a time-resolved image of fracture advancement. Lastly, AE events can potentially be analyzed using moment tensor methods to evaluate failure modes.
- Electrical Resistivity Tomography (ERT): ERT data are sensitive to changes in temperature, fluid saturation and fluid conductivity caused by fracture stimulation and flow. The objective of the ERT monitoring is to provide unique information for model validation by 1) imaging the conductivity distribution of the host rock prior to fracturing, as part of the initial characterization phase, 2) imaging the fracture extent by imaging the change in conductivity caused by the presence of fracture fluid and geophysical tracers within the fracture zone, 3) monitoring fracture fluid transport during fracture flow experiments using time-lapse imaging, and 4) monitoring the thermal zone
- Borehole Deformation Monitoring: Deformation monitoring was conducted in the injection well with the Step-rate Injection Method for Fracture In-situ Properties tool. This tool allows a zone of a borehole to be hydraulically isolated by packers to allow standard fluid injection tests, while precisely monitoring deformations along and transverse to the borehole, and downhole injection pressures and flow rates from the fluid injection (Guglielmi et al., 2015).
- Distributed temperature, seismicity, and strain monitoring: Distributed temperature (DTS), strain (DSS), and acoustic (DAS) sensing offer opportunities for geothermal monitoring due to their low deployment cost and excellent performance at high temperatures when appropriate packaging is chosen. We deployed several multi‐fiber cables to measure these properties, focusing on grouted cables in the monitoring boreholes. The DSS datasets, particularly data acquired in the fracture orthogonal wells, will provide constraints on fracture intersection in any of the 6 monitoring boreholes. The DTS recording will provide additional constraints on the evolving temperature field during the thermal tests.
4850 level testbed flow and monitoring system
Each flow test will be conducted within the fracture zone between the stimulation well and a production well. Both the stimulation and production wells will be instrumented with a packer system and sensors that monitor fracture aperture and shear, as well as temperature and pressure above, below, and within the isolated interval. These tests will be preceded by numerical modeling simulations to help design the flow, thermal and tracer testing.
Major phases of experiment 1
4850 level hydraulic stimulations
2018 circulation test
2019 to 2020 circulation test
Decommission of the 4850 level testbed
