PNNL Borehole Muon Detector Borehole Muon detector for 4D density tomography of subsurface reservoirs
 PI: Alain Bonneville, PhD, Laboratory Fellow, PNNL co-PI: Richard Kouzes, PhD, Laboratory Fellow, PNNL Requested Funding: $ 250K for the first year Within the New Subsurface Signals element of the Subsurface Challenge, we propose to use density-dependent attenuation of cosmic ray muon flux data to develop a time-lapse 3D (i.e., 4D) density tomographic image of the subsurface. Muons are generated in the upper atmosphere from cosmic ray interactions, and due to density variations in the Earth’s subsurface, they pass through stratigraphic units at multiple angles. By measuring muon flux at different depths in vertical, inclined, or horizontal wells, the attenuation of signal due to the different stratigraphic units, or the fluids contained within these units, can be determined. These measurements can be performed continuously to identify and interpret variations in density and fluid content as a function of time, and the data can be validated using gravity, GPS and InSAR observations. The determination of the density distribution of materials in the Earth’s subsurface, and the evolution of density as a function of time, has the potential to provide a sensitive, cost effective, and more precise monitoring technique to determine field-scale displacement of reservoir fluid induced by injection of liquid or gas. To date, the only way to collect direct and quantitative density distribution information is to measure changes in the Earth’s gravity field, also known as time-lapse gravity measurements. Time-lapse gravity has been used for more than 50 years. Substantial developments over the last decade in gravimeter technology, as well as the advent of precise GPS systems, have led to improvements in differential microgravity measurements; however, this technique is limited in that it only provides discrete values of the gravity field anomaly that represent the integral of the density distribution, and is by nature, an underdetermined problem. Cosmic-ray muon tomography promises to provide, for the first time, a complete and precise image of the desity distribution of the subsurface. Further, this novel approach has the potential to become a direct, real time, and low cost method for monitoring fluid displacement in subsurface reservoirs. Although muon flux rapidly decreases with depth, preliminary analyses (see calculations below) indicate that the muon technique will have sufficient sensitivity to effectively map density variations caused by fluid displacement at large depths up to 4500 ft. In the past five years, muon tomography has been used to successfully image the displacement of magma in active volcanoes (Tanaka et al., 2007; Lesparre et al.; 2010) with unprecedented detail using large detectors deployed at the surface. The primary technical challenge preventing deployment of this technology in the subsurface is the lack of miniaturized muon tracking detectors capable of fitting in standard boreholes. New applications developed at PNNL have substantially reduced the size of muon detectors, providing confidence that borehole-deployable systems are technically and economically feasible (Aguyo et al., 2013; Aguyo and Bonneville, 2012). The concept for a muon detector to be used in deep wells to monitor muon flux and angle of deflection are based on scintillating fibers with solid state readout and integrated threshold and coincidence electronics. The market for the development of such a tool is promising. The first challenge is to develop sensors with enough sensitivity to get a full 3D density image of the subsurface, yet small enough to fit in a borehole. The second challenge is to develop a rapid and efficient inversion method that will take into account not only the different muon paths, but also the data generated by other techniques, such as gravity, GPS and InSAR. PNNL is well positioned to resolve both of these challenges due to our advanced computing resources and expertise in complex data inversion (Johnson and Wellman, 2013; Johnson et al., 2012), as well as our demonstrated expertise in sensor development for national security purposes (Aguayo et al., 2011; Kouzes et al., 2010; Kouzes et al., 1

PNNL Borehole Muon Detector 2008, Milbrath et al., 2007). Comparatively, other organizations have primarily focused on preliminary studies, which are mostly academic and theoretical, with only one, led by a team of researchers at the University of Sheffield in the United Kingdom (Kudryavtsev et al., 2012), focused on industrial applications. This project will yield important progress on sensor development, as well as optimization of deployment strategies. We envision a 3-year project (see tentative schedule below), with the first year devoted to completing a borehole prototype design and feasibility study, including development and assessment of deployment configurations and signal inversion methods to assure adequate method sensitivity. After this initial feasibility phase, a comprehensive series of tests in deep underground laboratories (e.g., Soudan mine [~2300 feet below ground surface] in Minnesota at 1.95 kmwe) will be performed, followed by a complete borehole test in a CO2 storage or EOR site. Independent methods, including gravity and surface deformation observations, will be used for validation. Advanced contacts have been established with Schlumberger and the NASA Jet Propulsion, both of which have expressed interest in a potential partnership. DOE funding is crucial for this project and will allow transition of the technology from laboratory concept to field-scale testing. If successful, this technique could be used in multiple subsurface applications, from deep subsurface reservoirs for oil and gas exploration or natural gas and CO2 storage, to shallow depth applications, such as mapping soil contamination or imaging subsurface hydrogeology. Method sensitivity and detector concept: We use the example of CO2 injection into a deep reservoir to estimate detector sensitivity and deployment requirements. During injection, CO2 replaces reservoir fluid present in the pores, resulting in an overall decrease of bulk density within the formation. Conservatively, the muon technique should be capable of detecting as little as a 1% change in formation density over a year, corresponding to a significant change in scCO2 saturation within the formation: From Mei and Himes (2007), muon intensity: I = I1 exp(-h/1) + I2 exp(-h/2) Where, h=slant depth, I1 = 8.6x10-6 s-1cm-2sr-1, I2 = 0.44x10-6 s-1cm-2sr-1, 1 = 0.45 kmwe,  2 = 0.87 kmwe, Total flux at Soudan (1.95 kmwe) is 2 x 10-7 cm-2s-1, or ~6.2 cm-2y-1, or ~6.2 x 104 m-2y-1. A one-year measurement with a one square meter detector would have a one-sigma uncertainty of 0.4% with zero background, and 0.6% for a 1:1 signal to background ratio. Various detectors have been developed at PNNL that can be used for muon detection (gas, plastic, NaI), including compact, portable, rugged systems. The detector needs to measure very low flux rates, so coincidence counting with low signal to background resolution is required. The detector will need to measure less than 20.3 cm (8”) in diameter, should be approximately 40.6 cm (16”) long, and will need to be capable of measuring out to 30 degrees from vertical. A preliminary concept would be a series of 20 cm diameter by 40 cm long detectors on a tether. Depending on orientation, vertical offset vs. horizontal, the area of each detector would be between 314-800 cm2; therefore, it would take 13 to 32 of these detectors to generate the required 1 m2 detection area. References; Aguayo, E., J. E. Fast, R. T. Kouzes, J. Orrell, 2013, The μ-Witness detector: A ruggedized, portable, flux meter for cosmogenic activation monitoring, IEEE Transactions on Nuclear Science, Vol 60, No. 2, pp. 689-692. Aguayo, E. and A. Bonneville, A low-cost, portable, ruggedized cosmic muon detector prototype for geological applications, AGU Fall meeting, 2012.

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PNNL Borehole Muon Detector

Aguayo Navarrete E, RT Kouzes, and JL Orrell. 2011. Development of a Portable Muon Witness System. PNNL-20194, Pacific Northwest National Laboratory, Richland, WA. Kudryavtsev, Vitaly A., Spooner, Neil J. C., Gluyas, Jon, Fung, Cora & Coleman, Max. 2012. Monitoring subsurface CO2 emplacement and security of storage using muon tomography. International Journal of Greenhouse Gas Control 11: 21-24. Johnson TC, and DM Wellman. 2013. Re-Inversion of Surface Electrical Resistivity Tomography Data from the Hanford Site B-Complex. PNNL-22520; RPT-DVZ-AFRI-014, Pacific Northwest National Laboratory, Richland, WA. Johnson TC, L Slater, D Ntarlagiannis, FD Day-Lewis, and M Elwaseif. 2012. Monitoring groundwater-surface water interaction using time-series and time-frequency analysis of transient three-dimensional electrical resistivity changes. Water Resources Research 48:Article No. W07506. doi:10.1029/2012WR011893 Kouzes RT, JH Ely, LE Erikson, WJ Kernan, DC Stromswold, and ML Woodring. 2010. Full Scale Coated Fiber Neutron Detector Measurements . PNNL-19264, Pacific Northwest National Laboratory, Richland, WA. Kouzes RT, ER Siciliano, JH Ely, PE Keller, and RJ McConn, Jr. 2008. Passive Neutron Detection for Interdiction of Nuclear Material at Borders. Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 584(2-3):383-400. Lesparre, N., Gibert, D., Marteau, J., Déclais, Y., Carbone, D. & Galichet, E. 2010. Geophysical muon imaging: feasibility and limits. Geophysical Journal International 183: 1348-1361. Mei D. and A, Hime, 2006, Muon-induced background study for underground laboratories, Phys Rev D 73, 053004. Milbrath BD, BJ Choate, JE Fast, WK Hensley, RT Kouzes, and JE Schweppe. 2007. Comparison of LaBr3:Ce and NaI(Tl) Scintillators for Radioisotope Identification Devices. Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 572(2):774-784. Tanaka, Hiroyuki K. M., Nakano, Toshiyuki, Takahashi, Satoru, Yoshida, Jyunya, Takeo, Minoru, Oikawa, Jun, Ohminato, Takao, Aoki, Yosuke, Koyama, Etsuro, Tsuji, Hiroshi & Niwa, Kimio. 2007. High resolution imaging in the inhomogeneous crust with cosmic-ray muon radiography: The density structure below the volcanic crater floor of Mt. Asama, Japan. Earth and Planetary Science Letters 263: 104-113.

Tentative schedule Year 1  Complete feasibility study: amplitude of density anomaly linked to various scenarios of fluid injection in several realistic cases, exposure time, optimized geometry of the detectors and boreholes (one or two horizontal boreholes, different depths of observation).  Development of a borehole prototype. Year 2  3-month test of the tool in an underground observatory (Soudan);  Choice of a pilot site where fluid injection is conducted. Initial contacts have been made with operators of EOR fields and US CO2 pilot storage sites.  Acquire background level space-borne and airborne INSAR, gravity and GPS data on the selected site. Year 3:  Deployment onsite with other geophysical techniques (gravity sensors, GPS).  Testing and validation of muon detector system. Year 4:  Cross validation of muon tomography against deformation and gravity models.  Joint inversion of muon density tomography with deformation and gravity data.  Conclusions on muon detector efficiency and operations. .

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