Browsing by Author "Ajo-Franklin, Jonathan"

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    The EGS Collab project: Outcomes and lessons learned from hydraulic fracture stimulations in crystalline rock at 1.25 and 1.5 km depth
    (Elsevier, 2025) Kneafsey, Tim; Dobson, Pat; Blankenship, Doug; Schwering, Paul; White, Mark; Morris, Joseph P.; Huang, Lianjie; Johnson, Tim; Burghardt, Jeff; Mattson, Earl; Neupane, Ghanashyam; Strickland, Chris; Knox, Hunter; Vermuel, Vince; Ajo-Franklin, Jonathan; Fu, Pengcheng; Roggenthen, William; Doe, Tom; Schoenball, Martin; Hopp, Chet; Tribaldos, Verónica Rodríguez; Ingraham, Mathew; Guglielmi, Yves; Ulrich, Craig; Wood, Todd; Frash, Luke; Pyatina, Tatiana; Vandine, George; Smith, Megan; Horne, Roland; McClure, Mark; Singh, Ankush; Weers, Jon; Robertson, Michelle
    With the goal of better understanding stimulation in crystalline rock for improving enhanced geothermal systems (EGS), the EGS Collab Project performed a series of stimulations and flow tests at 1.25 and 1.5 km depths. The tests were performed in two well-instrumented testbeds in the Sanford Underground Research Facility in Lead, South Dakota, United States. The testbed for Experiment 1 at 1.5 km depth contained two open wells for injection and production and six instrumented monitoring wells surrounding the targeted stimulation zone. Four multi-step stimulation tests targeting hydraulic fracturing and nearly year-long ambient temperature and chilled water flow tests were performed in Experiment 1. The testbed for Experiments 2 and 3 was at 1.25 km depth and contained five open wells in an outwardly fanning five-spot pattern and two fans of well-instrumented monitoring wells surrounding the targeted stimulation zone. Experiment 2 targeted shear stimulation, and Experiment 3 targeted low-flow, high-flow, and oscillating pressure stimulation strategies. Hydraulic fracturing was successful in Experiments 1 and 3 in generating a connected system wherein injected water could be collected. However, the resulting flow was distributed dynamically, and not entirely collected at the anticipated production well. Thermal breakthrough was not observed in the production well, but that could have been masked by the Joule-Thomson effect. Shear stimulation in Experiment 2 did not occur – despite attempting to pressurize the fractures most likely to shear – because of the inability to inject water into a mostly-healed fracture, and the low shear-to-normal stress ratio. The EGS Collab experiments are described to provide a background for lessons learned on topics including induced seismicity, the correlation between seismicity and permeability, distributed and dynamic flow systems, thermoelastic and pressure effects, shear stimulation, local geology, thermal breakthrough, monitoring stimulation, grouting boreholes, modeling, and system management.
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    Monitoring Water Level of a Surficial Aquifer Using Distributed Acoustic Sensing and Ballistic Surface Waves
    (Wiley, 2024) Sobolevskaia, Valeriia; Ajo-Franklin, Jonathan; Cheng, Feng; Dou, Shan; Lindsey, Nathaniel J.; Wagner, Anna
    Groundwater resources play an increasingly crucial role in providing the water required to sustain the environment. However, our understanding of the state of surficial aquifers and their spatiotemporal dynamics remains poor. In this study, we demonstrate how Rayleigh wave velocity variation can be used as a direct indicator of changes in the water level of a surficial aquifer in a discontinuous permafrost environment. Distributed acoustic sensing data, collected on a trenched fiber-optic cable in Fairbanks, AK, was processed using the multichannel analysis of surface waves approach to obtain temporal velocity variations. A semi-permanent surface orbital vibrator was utilized to provide a repeatable source of energy for monitoring. To understand the observed velocity perturbations, we developed a rock physics model (RPM) representing the aquifer with the underlying permafrost and accounting for physical processes associated with water level change. Our analyses demonstrated a strong correlation between precipitation-driven head variation and seismic velocity changes at all recorded frequencies. The proposed model accurately predicted a recorded 3% velocity increase for each 0.5 m of head drop and indicated that the pore pressure effect accounted for approximately 75% of the observed phase velocity change. Surface wave inversion and sensitivity analysis suggested that the high velocity contrast in the permafrost table shifts the surface wave sensitivity toward the first 3 m of soil where hydrological forcing occurs. This case study demonstrates how surface wave analysis combined with an RPM can be used for quantitative interpretation of the acoustic response of surficial aquifers.
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    Source mechanism of kHz microseismic events recorded in multiple boreholes at the first EGS Collab testbed
    (Elsevier, 2024) Qin, Yan; Li, Jiaxuan; Huang, Lianjie; Schoenball, Martin; Ajo-Franklin, Jonathan; Blankenship, Douglas; Kneafsey, Timothy J.; EGS Collab Team
    Continuous microseismic monitoring using three-component (3C) accelerometers deployed in multiple boreholes allows for tracking the detailed evaluation of mesoscale (∼10 m scale) fracture growth during the fracture stimulation experiments at the first Enhanced Geothermal Systems (EGS) Collab testbed. Building on a well-constrained microseismic event catalog, we invert for moment tensor of the events to better understand the fracture geometry and stress orientations. However, it is challenging because of the unknown orientation of 3C accelerometers and low signal-to-noise-ratio nature of high-frequency (several kHz) monitoring. To address these challenges, we first perform the hodogram analysis on the continuous active-source seismic monitoring (CASSM) data to determine the orientations of the 18 3C accelerometers. We then apply the principal component analysis (PCA) to the observed microseismic waveforms to improve the signal-to-noise ratios. We perform a grid search for the full moment tensor by fitting the PCA-denoised waveforms at a frequency range of 5 to 8 kHz. The moment tensor results show both the creation of hydraulic fractures and the reactivation of natural fractures during the hydraulic stimulations. Our stress inversion based on the inverted moment tensors reveals the alteration of stress regime caused by hydraulic fracture stimulations.
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    The role of stress and fluid saturation on the acoustic response of fractured rock
    (Frontiers Media S.A., 2023) Lisabeth, Harrison P.; Ajo-Franklin, Jonathan
    Standard rock physics models are formulated to describe the behavior of porous sedimentary reservoirs, with clean sandstones being the archetypal system; however, many situations demand geophysical monitoring of rocks with significantly different structures, such as low porosity, fractured reservoirs. Conventional models also suggest that these “stiff” reservoirs can be challenging to monitor seismically due to small fluid substitution effects, but the presence of fractures leads to stress dependence which may be leveraged for remote monitoring purposes. Using samples from the Duperow Formation (dolostone) obtained from the Danielson test well in Kevin Dome, MT, we conducted ultrasonic and multi-scale structural (profilometry, synchrotron micro-tomography, pressure sensitive film) measurements on naturally fractured core in order to characterize the effects of fluid substitution and effective stress on the acoustic response of fractured reservoir rock with a focus in particular on the textural and seismic characteristics of natural fractures. We find that changes in effective stress can yield changes in velocity of up to 20% and changes in attenuation up to 200%. Measured fluid substitution effects are resolvable, but stress effects dominate. These measurements provide insight into the physical processes controlling acoustic response of fractured rocks in general and can also be used to inform monitoring efforts in fractured reservoirs.
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    Understanding Subsurface Fracture Evolution Dynamics Using Time-Lapse Full Waveform Inversion of Continuous Active-Source Seismic Monitoring Data
    (Wiley, 2023) Liu, Xuejian; Zhu, Tieyuan; Ajo-Franklin, Jonathan
    Predicting the behavior, geometry, and flow properties of subsurface fractures remains a challenging problem. Seismic models that can characterize fractures usually suffer from low spatiotemporal resolution. Here, we develop a correlative double-difference time-lapse full waveform inversion of continuous active source seismic monitoring data for determining high-spatiotemporal-resolution time-lapse Vp models of in-situ fracture evolution at a shallow contamination site in Wyoming, USA. Assisted by rock physics modeling, we find that (a) rapidly increasing pore pressure initializes and grows the fracture, increasing the porosity slightly (from ∼13.7% to ∼14.6%) in the tight clay formation, thus decreasing Vp (∼50 m/s); (b) the fluid injection continues decreasing Vp, likely through the introduction of gas bubbles in the injectate; and (c) final Vp reductions reach over ∼150 m/s due to a posited ∼4.5% gas saturation. Our results demonstrate that high-resolution Vp changes are indicative of mechanical and fluid changes within the fracture zone during hydrofracturing.