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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Feasibility of using in situ deformation to monitor CO 2 storage

Deformation during CO 2 injection can lead to problems, like seismicity or fluid leaks, but small strains have the potential to be a useful signal for monitoring. The objective of this paper is to evaluate the possible evolution of the strain field during injection, and then assess existing and emerging techniques for measuring the strain field. Poroelastic analyses show that normal strains caused by injection into a reservoir are tensile in the vicinity of the well, but everywhere else at least one strain component is compressive. The vertical strain is compressive in the confining unit, and the radial strain decreases and changes sign from tensile to compressive with distance from the well. Tilting is away from the injection well at the ground surface, but it is towards the well overlying the reservoir. Methods for measuring in-situ strain include instruments that are grouted in the annulus between casing and wall rock (~ 0.1 microstrain resolution), portable strain sensors that are temporarily clamped to the borehole wall (~0.01 microstrain resolution), and strainmeters that are grouted in place (~0.001 microstrain resolution). Instruments for measuring in-situ normal strains at the magnitudes and rates expected during injection are emerging, but they have yet to be fully evaluated in applications related to CO 2 storage. Finally, in-situ strain data measured with emerging instruments promises to fill an important gap between the episodes of fast strain rates measured by seismic data, and the slow strains measured over relatively long periods of time by InSAR and GPS.

42 ENGINEERING↗
DOE OSTI2017

The Tunisian Jurassic aquifer in the North African Sahara aquifer system: information derived from two-dimensional seismic reflection and well logs

Southern Tunisia is an arid area where socio-economic activities are dependent on groundwater resources. The presented study aims to better characterize the Jurassic aquifer based on geological and geophysical data, with a view to develop a rational exploitation program. Well logs are used to precisely determine the position and composition of the known Jurassic aquifer layers and to identify others able to produce good quality water. The logs show that limestones, sandstones and dolomites of the Krachoua, Techout and Foum Tataouine formations are the main Jurassic aquifers. Sixty-eight seismic-reflection sections are integrated within this study. The interpolation between the interpreted sections leads to the construction of isochronous isopach maps and geoseismic sections, and their analysis finds that compressive and extensive tectonic deformations have influenced the Jurassic aquifer geometry. The Hercynian orogeny phase manifestation is remarkable in that there are several stratigraphic gaps in the Jurassic sequence. The E–W, NW–SE, and NNW–SSE accidents, reactivated in normal faults since the Permian to Lower Cretaceous epochs, have generated the structures found in the Jurassic series, such as subsided and raised blocks. Their syn-sedimentary activity has controlled the thickness and facies of these series. The Cretaceous, Tortonian and Post-Villafranchian compressions are responsible for the Jurassic-deposits folding in some localities. The highlighted tectonic and sedimentary events have an important impact on the Jurassic aquifer function by favoring the Jurassic aquifer interconnections and their connections with the Triassic and Cretaceous permeable series.

58 GEOSCIENCES↗
DOE OSTI2023

Hydrogen and Silicon Effects on Hexagonal Close Packed Fe Alloys at High Pressures: Implications for the Composition of Earth's Inner Core

Hexagonal close-packed (hcp) structured Fe-Ni alloy is believed to be the dominant phase in the Earth's inner core. This phase is expected to contain 4%–5% light elements, such as Si and H. While the effects of individual light element candidates on the equation of state (EoS) of the hcp Fe metal have been studied, their combined effects remain largely unexplored. Here, in this study, we report the equations of state for two hcp-structured Fe-Si-H alloys, namely Fe 0.83 Si 0.17 H 0.07 and Fe 0.83 Si 0.17 H 0.46 , using synchrotron X-ray diffraction measurements up to 125 GPa at 300 K. These alloys were synthesized by cold compression of Fe-9wt%Si in either pure H 2 or Ar-H 2 mixture medium in diamond-anvil cells. The volume increase caused by a H atom in hcp Fe-Si-H alloys is approximately eight times greater than that by a Si atom. We used the improved data set to develop a composition-dependent EoS that covers a wide range of compositions. Our calculated density and bulk sound velocity of hcp Fe-Si-H alloys suggest a large trade-off between Si and H contents in fitting the seismic properties of the inner core. Combining our new EoS with geophysical and geochemical constraints, we propose 1.6–3 wt% Si and 0.15–0.6 wt% H in the Earth's inner core.

58 GEOSCIENCES↗
NASA NTRS1994

Structure and stability of hydrous minerals at high pressure

The presence of even small amounts of hydrogen in the Earth's deep interior may have profound effects on mantle melting, rheology, and electrical conductivity. The recent discovery of a large class of high-pressure H-bearing silicates further underscores the potentially important role for hydrous minerals in the Earth's mantle. Hydrogen may also be a significant component of the Earth's core, as has been recently documented by studies of iron hydride at high pressure. In this study, we explore the role of H in crystal structures at high pressure through detailed Raman spectroscopic and x ray diffraction studies of hydrous minerals compressed in diamond anvil cells. Brucite, Mg(OH)2, has a simple structure and serves as an analogue for the more complex hydrous silicates. Over the past five years, this material has been studied at high pressure using shock-compression, powder x ray diffraction, infrared spectroscopy, Raman spectroscopy, and neutron diffraction. In addition, we have recently carried out single-crystal synchrotron x-ray diffraction on Mg(OH)2 and Raman spectroscopy on Mg(OD)2 at elevated pressure. From all these studies, an interesting picture of the crystal chemical behavior of this material at high pressure is beginning to emerge. Some of the primary conclusions are as follows: First, hydrogen bonding is enhanced by the application of pressure. Second, layered minerals which are elastically anisotropic at low pressure may not be so at high pressure. Furthermore, the brucite data place constraints on the effect of hydrogen on seismic velocities and density at very high pressure. Third, the stability of hydrous minerals may be enhanced at high P by subtle structural rearrangements that are difficult to detect using traditional probes and require detailed spectroscopic analyses. Finally, brucite appears to be unique in that it undergoes pressure-induced disordering that is confined solely to the H-containing layers of the structure.

Duffy, T. S.↗
DOE OSTI2026

Storage Field Development Plan: One Earth Energy

This Storage Field Development plan presents the Storage Complex characterization results, construction, monitoring, and operational plans, and costs associated with the proposed One Earth Sequestration Carbon Capture and Storage (OES-CCS) site in McLean County, Illinois, near Gibson City. The proposed storage complex, known as the Mt. Simon Storage Complex, comprises the Cambrian Mt. Simon Sandstone reservoir and the primary seal, the Cambrian Eau Claire Formation. The lowermost Underground Source of Drinking Water (USDW) identified for the site is the Ordovician St. Peter Sandstone. Geologic characterization of the Mt. Simon Storage Complex at the OES-CCS site was performed by the Illinois Storage Corridor CarbonSAFE Phase III project, which also prepared and submitted three UIC Class VI applications to construct three injection wells; the permit applications were submitted and are in the federal EPA review process. A characterization well, OEE #1, was drilled to collect site-specific data. These data were analyzed and used to develop the UIC Class VI applications. The OEE #1 well will be converted to an in-zone monitoring (IZM) well for the injection phase. The proposed buildout for the OES-CCS site includes (1) three injection wells (OES #1, OES #2, and OES #3), (2) two IZM wells, (3) two above confining zone (ACZ) monitoring wells, one of which will be used to monitor the lowermost USDW, (4) capture and compression facilities, and (5) transportation facilities, i.e., pipelines. A pre-operational testing program was proposed in the Class VI permit application and will be employed at the site pending approval. Additional pre-injection (baseline), syn-injection, and post-injection monitoring and site care procedures will be followed by OES to ensure that injection activities are protective of human health and the environment. Injection is scheduled to begin in 2025, distributed across the three injection wells in accordance with the permit operating conditions. One Earth Sequestration intends to inject up to 90 million tonnes of CO 2 over a period of approximately 20 years. Injection will begin at approximately 0.5 million tonnes of CO 2 annually and ramp up to a maximum of 4.5 million tonnes annually. Daily injection rates are expected to range from 1,400 to 1,500 tonnes per day initially and reach a maximum of approximately 4,225 tonnes per day, depending on site geology and injectivity at each injection well location, and CO 2 availability. The costs associated with the OES-CCS project include pre-operational costs (e. g. additional seismic data acquisition and well drilling), capture and transportation facility and equipment costs, predicted field operating expenditures (OpEx), and decommissioning and post-injection site care (PISC) costs. The risks associated with project activities, such as site construction, injection operations, and verification of secure storage were evaluated, and mitigation strategies proposed to alleviate those risks.

09 BIOMASS FUELS↗
NASA NTRS1987

Compression of oceanic lithosphere - An analysis of intraplate deformation in the Central Indian Basin

The development of intraplate structure in the Central Indian Basin is examined using models in which deformation is due to flexural buckling and the hydrodynamic growth of instabilities. Comparison of the models reveal that in a strong viscous lithosphere, deformation of the layer occurs by flexural folding at a wavelength which corresponds to the flexural buckling theory; in a lithosphere of intermediate strength, the layer deforms by folding characterized by thickening which localizes beneath topographic heights; and in a relatively weak lithosphere, the layer incurs an even greater amount of localized thickening and deforms in the symmetric or pinch-and-swell mode by inverse boudinage. It is noted that the models in which the layer folds either flexurally or with periodic thickening correspond with observed depth distribution of seismicity in the Central Indian Basin and with experimental rock rheological data.

Zuber, Maria T.↗
DOE OSTI2020

Evidence of shock-compressed stishovite above 300 GPa

SiO 2 is one of the most fundamental constituents in planetary bodies, being an essential building block of major mineral phases in the crust and mantle of terrestrial planets (1–10 M E ). Silica at depths greater than 300 km may be present in the form of the rutile-type, high pressure polymorph stishovite (P4 2 /mnm) and its thermodynamic stability is of great interest for understanding the seismic and dynamic structure of planetary interiors. Previous studies on stishovite via static and dynamic (shock) compression techniques are contradictory and the observed differences in the lattice-level response is still not clearly understood. Here, laser-induced shock compression experiments at the LCLS- and SACLA XFEL light-sources elucidate the high-pressure behavior of stishovite on the lattice-level under in situ conditions on the Hugoniot to pressures above 300 GPa. We find stishovite is still (meta-)stable at these conditions, and does not undergo any phase transitions. This contradicts static experiments showing structural transformations to the CaCl 2 , α-PbO 2 and pyrite-type structures. However, rate-limited kinetic hindrance may explain our observations. These results are important to our understanding into the validity of EOS data from nanosecond experiments for geophysical applications.

58 GEOSCIENCES↗
DOE OSTI2017

Source characteristics and geological implications of the January 2016 induced earthquake swarm near Crooked Lake, Alberta

On 2016 January 12, an intraplate earthquake with an initial reported local magnitude (M L ) of 4.8 shook the town of Fox Creek, Alberta. While there were no reported damages, this earthquake was widely felt by the local residents and suspected to be induced by the nearby hydraulic-fracturing (HF) operations. We determine the earthquake source parameters using moment tensor inversions, and then detect and locate the associated swarm using a waveform cross-correlation based method. The broad-band seismic recordings from regional arrays suggest a moment magnitude (M) 4.1 for this event, which is the largest in Alberta in the past decade. Similar to other recent M ~ 3 earthquakes near Fox Creek, the 2016 January 12 earthquake exhibits a dominant strike-slip (strike = 184°) mechanism with limited non-double-couple components (~22 per cent). This resolved focal mechanism, which is also supported by forward modelling and P-wave first motion analysis, indicates an NE–SW oriented compressional axis consistent with the maximum compressive horizontal stress orientations delineated from borehole breakouts. Further detection analysis on industry-contributed recordings unveils 1108 smaller events within 3 km radius of the epicentre of the main event, showing a close spatial-temporal relation to a nearby HF well. The majority of the detected events are located above the basement, comparable to the injection depth (3.5 km) on the Duvernay shale Formation. The spatial distribution of this earthquake cluster further suggests that (1) the source of the sequence is an N–S-striking fault system and (2) these earthquakes were induced by an HF well close to but different from the well that triggered a previous (January 2015) earthquake swarm. Reactivation of pre-existing, N-S oriented faults analogous to the Pine Creek fault zone, which was reported by earlier studies of active source seismic and aeromagnetic data, are likely responsible for the occurrence of the January 2016 earthquake swarm and other recent events in the Crooked Lake area.

58 GEOSCIENCES↗
DOE OSTI2019

Subtask 1.4 - Techno-Economic Assessment of Regional Carbon Utilization Scenarios and Attendant Monitoring Technology

The U.S. Department of Energy (DOE) Carbon Storage Program seeks to develop technologies to significantly improve the effectiveness of carbon capture and storage (CCS), reduce the cost of implementation, and support the readiness of widespread commercial deployment by 2035. To accomplish this goal, technical and economic barriers must be addressed and the results used to inform regulators and industry on the safety, permanence, and viability of carbon storage. To help overcome these barriers, the Energy & Environmental Research Center (EERC) conducted three activities: 1) investigate the regional techno-economic life cycle of CO2 enhanced oil recovery (EOR) in North Dakota, 2) assist DOE in determining the prospective CO2 storage resource of residual oil zones (ROZs), and 3) develop a seismic technique for monitoring geologic storage of CO2. These three activities address two strategic goals enumerated in the DOE–EERC Cooperative Agreement, namely, 1) to advance foundational science and 2) to inform data-driven policies that enhance U.S. economic growth and job creation, energy security, and environmental quality. Activity 1 In North Dakota, there is synergistic incentive to develop and implement CO2 EOR technologies. Regional industry partners are actively seeking options that can cost-effectively improve the efficiency of power production, while also coproducing CO2 for use in EOR applications. A full-value-chain techno-economic assessment that links lignite resources, power generation, and EOR is needed to determine the implications for a state or region to continue to use these resources or look elsewhere for other energy sources and revenue streams. The integration of North Dakota’s lignite power industry with the state’s oil industry has the potential to revitalize legacy oil fields and produce up to 1 billion barrels of additional oil using CO2 EOR while providing a market for a CO2 commodity captured at coal-fired power plants. Revitalization of North Dakota’s legacy oil fields will bring new industry investment to the state, resulting in the growth of oil-related jobs and revenue through oil sales, severance tax, and associated oilfield development activities (drilling, pipelines, etc.). CO2 demand for EOR and incremental oil production were calculated for 201 conventional oil fields in North Dakota. Infrastructure development in the fields was estimated using 1) a traditional vertical well arrangement of CO2 injectors and oil producers and 2) an advanced case where horizontal wells were used exclusively. Pipeline, compression, and recycling infrastructure requirements were also estimated. When viewed from the CO2 capture and CO2 EOR sectors, the advanced case showed a positive investment scenario when including potential benefits of the 45Q tax credit policy. An economic evaluation of a full implementation of CO2 EOR showed an average $60 billion potential state economic impact estimated over the 37-year construction and production time frame could be realized with an assumed oil price of $65/bbl. Oil price ranges of $35–$90/bbl cause an estimated variation of ±$5 billion over the potential effort. The total net state government revenues were estimated over the 37-year construction and production time frame. With an assumed oil price of $65/bbl, total potential fiscal impact could average $7.4 billion. This result ranges about ±$2.5 billion for a potential EOR implementation effort for oil price ranges of $35–$90/bbl. This broad-brush assessment was focused on “magnitude-of-effect” perspectives of what CO2 capture and use for EOR could mean to the state. Whether CO2 EOR ever becomes widely adopted is not the issue at this stage of evaluation. The question being answered is, How valuable could CO2 capture and CO2 EOR be for the state and local jurisdictions in western North Dakota? The results of the current economic assessment remove any ambiguity to that question. Full implementation of CO2 EOR as modeled in this study provides substantial economic benefits to the state, even at low oil prices (i.e., $35/bbl). Job creation and government revenues could be substantial, depending on the scale of implementation. With a better understanding of what CO2 EOR might mean for the state’s economy, the focus can now shift to the policies and financial mechanisms needed to make private investment in CO2 capture and use as EOR competitive with other energy-related pursuits. Activity 2 ROZs contain remnants of oil not swept away by natural waterflooding. Although nearly all of the identified ROZs are found in West Texas, there is evidence that this category of oil reservoir can be found in other major oil-producing basins of the United States. Because the formation of these oil reservoirs is distinct from other conventional reservoirs, the method to estimate the prospective CO2 storage resource in ROZs may be equally distinct from the method used for conventional fields. As such, the EERC collaborated with DOE’s National Energy Technology Laboratory (NETL) to investigate the feasibility of CO2 storage in ROZs and determine what specific modification (if any) needs to be made to the current DOE volumetric method to estimate prospective CO2 storage resource in oil reservoirs. This activity built on a previous EERC investigation to identify ROZs in the Williston and Powder River Basins and upon collaborations between the EERC and NETL to develop storage resource estimation methods for different types of geologic CO2 storage. Review of the volumetric approach raised questions about the applicability of such an approach to hydrocarbon-bearing zones without the inclusion of potential hydrocarbon production. A production-based approach was presented by the EERC to inform the working group of additional methods of increasing the CO2 storage resource potential. Because the production-based method focuses more heavily on oil recovery than CO2 storage, it was not included in the working group manuscript. Activity 3 The EERC has used sparse and irregular seismic arrays to detect CO2 injected into a reservoir. The variability of seismic noise recorded by the sensors directly impacts the seismic interpretation. However, designing noise attenuation workflows effectively can be a more challenging task using sparse and irregular arrays for data acquisition. Based on the hypothesis that noncoherent noise is stationary during weekly data acquisition, a new machine-learning algorithm was developed to automatically estimate noise conditions using passive records, noise from active records, and ancillary information. The new noise attenuation processing workflow can improve the time-lapse detection of CO2 from sparse and irregular array acquisitions. Results show spatial and temporal noise patterns can be related to the location of the sensors and the weather conditions during the data acquisition and that our attenuation processing workflow can improve the time-lapse detection of CO2 from sparse and irregular array acquisitions. This effort was funded in part through the EERC–DOE Joint Program on Research and Development for Fossil Energy-Related Resources Cooperative Agreement No. DE-FE0024233. Nonfederal funding was provided by the North Dakota Department of Commerce.

Peck, Wesly↗
DOE OSTI2018

Mechanical Testing Results on Core from Borehole U-15n, NNSS, in support of SPE

The Nevada National Security Site (NNSS) will serve as the geologic setting for a Source Physics Experiment (SPE) program. The SPE will provide ground truth data to create and improve strong ground motion and seismic S-wave generation and propagation models. The NNSS was chosen as the test bed because it provides a variety of geologic settings ranging from relatively simple to very complex. Each series of SPE testing will comprise the setting and firing of explosive charges (source) placed in a central bore hole at varying depths and recording ground motions in instrumented bore holes located in two rings around the source positioned at different radii. Modeling using advanced simulation codes will be performed both a priori and after each test to predict ground response and to improve models based on acquired field data, respectively. A key component in the predictive capability and ultimate validation of the models is the full understanding of the intervening geology between the source and the instrumented bore holes including the geomechanical behavior of the site rock/structural features. This report presents a limited scope of work for an initial phase of primarily unconfined compression testing. Samples tested came from the U-15n core hole, which was drilled in granitic rock (quartz monzonite). The core hole was drilled at the location of the central SPE borehole, and thus represents material in which the explosive charges will be detonated. The U-15n location is the site of the first SPE, in Area 15 of the NNSS.

58 GEOSCIENCES↗
DOE OSTI2018

Seismic Analysis of Spatio-Temporal Fracture Generation During EGS Resource Development

This DOE GTO funded project was a collaborative effort between Array Information Technology and the University of California at Berkeley. The project goal is to improve technology to assess in-situ stress magnitude and orientation, kinematic fracture parameters, rupture size, rupture orientation as well as temporal changes and volumetric distribution of the injected fluid during Enhanced Geothermal System (EGS) resource development. The project leveraged high-frequency seismic data recorded by the LBNL 34-station permanent geophone network and seismic broadband data recorded by a temporary 33-station seismometer network that operated in The Geysers during the injection phase of the DOE GTO funded EGS demonstration project at the well Prati-32. The operation of the broadband network with high station density and good azimuthal coverage to monitor the injection phase produced an unprecedented dataset that is typically unavailable for EGS operations. The two datasets offer the advantages of studying the parameters under investigation over a broader frequency band and the higher sensitivity of the broadband seismometers resulted in a richer dataset with a lower magnitude of completeness. Moment tensor (MT) inversion have been employed to develop a catalog of earthquake source solutions including moment tensor and finite source rupture area. This resulted in a 168-event waveform-based seismic moment tensor catalog for events 0.7≤Mw≤3.9 based on a semi-automated moment tensor analysis technique. The MT solutions were used for in-situ stress estimation during the EGS injection phases. We found an approximate 15-degree counterclockwise rotation of the least compressive stress sigma-3, and a rotation of the maximum compressive stress sigma-1 toward the vertical as the injected volume of water increased. We developed a rupture-area magnitude scaling relationship of The Geysers earthquakes obtained from finite-source inversion for fault slip that reveals a very high correlation to published results. Based on the rupture area-magnitude relationship we mapped Mw from the LBNL earthquake catalog to estimate the activated fracture area, its location and orientation within the injection volume of Prati-32. The double-difference Wadati (DDW) technique was applied to high-accuracy P- and S-wave differential travel times derived from waveform cross correlation to image fluid saturation in the subsurface based on Vp/Vs-ratio. The recently developed technique has been applied to estimate temporal changes of fluid distribution in fault zones and in volcanic regions but not yet in engineered geo-reservoirs. The improvement of the technology includes high-resolution waveform cross correlation applied to the high-quality datasets with lower magnitude of completeness resulting in a higher number of recorded events. The improvement allowed to estimate both, the spatial and temporal distribution of injected fluids and the state of the injected fluid during reservoir stimulation. Results of the DDW analysis revealed the development of an injection-derived steam plume during the early injection phase, particularly in the northern part of the Prati-32 study area.

15 GEOTHERMAL ENERGY↗
DOE OSTI2021

Stratigraphically Controlled Stress Variations at the Hydraulic Fracture Test Site-1 in the Midland Basin, TX

We investigated the relationship between stratigraphy, stress, and microseismicity at the Hydraulic Fracture Test Site-1. The site comprises two sets of horizontal wells in the Wolfcamp shale and a deviated well drilled after hydraulic fracturing. Regional stresses indicate normal/strike-slip faulting with E-W compression. Stress measurements in vertical and horizontal wells show that the minimum principal stress varies with depth. Strata with high clay and organic content show high values of the least compressive stress, consistent with the theory of viscous stress relaxation. By integrating data from core, logs, and the hydraulic fracturing stages, we constructed a stress profile for the Wolfcamp sequence, which predicts how much pressure is required for hydraulic fracture growth. We applied the results to fracture orientation data from image logs to determine the population of pre-existing faults that are expected to slip during stimulation. We also determined microseismic focal plane mechanisms and found slip on steeply dipping planes striking NW, consistent with the orientations of potentially active faults predicted by the stress model. This case study represents a general approach for integrating stress measurements and rock properties to predict hydraulic fracture growth and the characteristics of injection-induced microseismicity.

microseismicity↗
DOE OSTI2019

High pressure thermoelasticity and sound velocities of Fe-Ni-Si alloys

The Earth's iron-dominant core is known to contain nickel from cosmochemical analysis and some amount of light elements from geophysical constraints on density and seismic wave velocities. Although there have been several studies to constrain thermoelastic properties of iron-alloys, there has been no systematic study on the effects of nickel and light elements on properties of iron using the same experimental methods and data analysis approach. We conducted nuclear resonant inelastic X-ray scattering and X-ray diffraction experiments on body-centered cubic and hexagonal close-packed (hcp) Fe 0.91 Ni 0.09 and Fe 0.8 Ni 0.1 Si 0.1 up to 104 GPa and 86 GPa, respectively, and compare to similar measurements conducted on hcp-Fe up to 171 GPa. Specifically, we determine the Debye sound velocity from the low-energy transfer region of the (partial) phonon density of states (DOS) using the equation of state determined for each material and a new approach which utilizes information criteria and probability distributions. Nickel decreases the shear velocity of iron, while 10 at% Si has little to no effect on the shear velocity of Fe 0.91 Ni 0.09 . We observe that the shape of the phonon DOS of these alloys remains similar with increasing pressure. In the measured compression range, we therefore apply a generalized scaling law to describe the volume dependence of the phonon DOS and find that the vibrational Grüneisen parameters of hcp-Fe 0.91 Ni 0.09 are nearly indistinguishable from those hcp-Fe and those for Fe 0.8 Ni 0.1 Si 0.1 trend lower. From the vibrational free energy, we constrain the harmonic vibrational component of thermal pressure, which shows a significant positive deviation from theoretical calculations of hcp-Fe at pressures and temperatures of Earth's core. Collectively, our results demonstrate that the effects of nickel should be considered when modeling iron-rich planetary cores.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Phase I Project: Fiber Optic Distributed Acoustic Sensing for Periodic Hydraulic Tests

The extraction of heat from hot rock requires circulation of fluid through fracture networks. Because the geometry and connectivity of these fractures determines the efficiency of fluid circulation, many tools are used to characterize fractures before and after development of the reservoir. Under this project, a new tool was developed that allows hydraulic connectivity between geothermal boreholes to be identified. Nanostrain in rock fractures is measured using fiber optic distributed acoustic sensing (DAS). This strain is measured in one borehole in response to periodic pressure pulses induced in another borehole. The strain in the fractures represents hydraulic connectivity between wells. DAS is typically used at frequencies of Hz to kHz, but strain at mHz frequencies were measured for this project. The tool was demonstrated in the laboratory and in the field. In the laboratory, strain in fiber optic cables was measured in response to compression due to oscillating fluid pressure. DAS recorded strains as small as 10 picometer/m in response to 1 cm of water level change. At a fractured crystalline rock field site, strain was measured in boreholes. Fiber-optic cable was mechanically coupled borehole walls using pressured flexible liners. In one borehole 30 m from the oscillating pumping source, pressure and strain were measured simultaneously. The DAS system measured fracture displacement at frequencies of less than 1 mHz (18 min periods) and amplitudes of less than 1 nm, in response to fluid pressure changes of less 20 Pa (2 mm of water). The attenuation and phase shift of the monitored strain signal is indicative of the permeability and storage (compliance) of the fracture network that connects the two wells. The strain response as a function of oscillation frequency is characteristic of the hydraulic structure of the formation. This is the first application of DAS to the measurement of low frequency strain in boreholes. It has enormous potential for monitoring geothermal reservoirs for purposes of understanding reservoir compliance and for assuring security of injection fluids. Periodic pressure pulses can be induced by oscillating injection or pumping during operation so the system could provide real time reservoir data. DAS cable may already be installed at a site as it is becoming increasingly used for seismic observation. Simulations conducted for this project indicate that strain should be propagated through borehole cements, so observations can be made outside of well casing. In uncased holes, the cable would need to be mechanically coupled to the borehole wall to provide measurements. One option would be to install fiber into cemented and abandoned boreholes to extend their utility.

15 GEOTHERMAL ENERGY↗