Recent studies have demonstrated that positron emission tomography (PET) is a valuable tool for in-situ characterization of fluid transport in porous and fractured geologic media at the laboratory scale. While PET imaging is routinely used for clinical cancer diagnosis and preclinical medical researchand therefore imaging facilities are available at most research instituteswidespread adoption for applications in water resources and subsurface energy resources engineering have been limited by real and perceived challenges of working with this technique. In this study we discuss and address these challenges, and provide detailed analysis highlighting how positron emission tomography can complement and improve laboratory characterization of different subsurface fluid transport problems. The physics of PET are reviewed to provide a fundamental understanding of the sources of noise, resolution limits, and safety considerations. We then layout the methodology required to perform laboratory experiments imaged with PET, including a new protocol for radioactivity dosing optimization for imaging in geologic materials. Signal-to-noise and sensitivity analysis comparisons between PET and clinical X-ray computed tomography are performed to highlight how PET data can complement more traditional characterization methods, particularly for solute transport problems. Finally, prior work is critically reviewed and discussed to provide a better understanding of the strengths and weakness of PET and how to best utilize PET-derived data for future studies. in many geologic materials can limit the application of MRI imaging in ge-35 ologic porous and fractured media (Reeves and Chudek, 2001; Werth et al., 36 2010; Nestle et al., 2003). Emission tomography includes two main imaging 37 techniques, positron emission tomography (PET) and single-photon emis-38 sion computed tomography (SPECT), that rely on photon emission for in 39 situ imaging. Positron emission tomography is the most common these emis-40 sion tomography methods and will be the focus of this study. PET relies on 41 the emission and detection of photons from positron-emitting radiotracers 42 using cylindrical arrays of scintillation crystals. Tomographic reconstruction 43 methods are used to obtain three-dimensional images of radiotracer distri-44 bution in the porous material as a function of time. Similarly to X-ray CT 45 and MRI, PET has been developed for medical purposes, but is increasingly 46 being used for applications in engineering and Earth science (Ferno et al., 47 2015b; Pini et al., 2016; Brattekas and Seright, 2017; Zahasky et al., 2018; 48 Kulenkamp↵ et al., 2018). 49 The distinct physics that underlie these imaging techniques determine 50 their strengths and weaknesses for various applications in water resources 51 and subsurface energy resources engineering. While X-ray computed tomog-52 raphy has become ubiquitous in geoscience experimental applications (e.g. 53 porosity distribution measurements shown in Figure 1), PET imaging has 54 had limited utilizat...

The analysis of dispersive flows in heterogeneous porous media is complicated by the appearance of anomalous transport. Novel laboratory protocols are needed to probe the mixing process by measuring the spatial structure of the concentration field in the medium. Here, we report on a systematic investigation of miscible displacements in a microporous limestone over the range of Péclet numbers, 20 < P e < 600. Our approach combines pulse-tracer tests with the simultaneous imaging of the flow by Positron Emission Tomography (PET). Validation of the

The shearing of fractures can be a significant source of permeability change by altering the distribution of void space within the fracture itself. Common methods to estimate the effects of shearing on properties, such as aperture, roughness, and connectivity, are incapable of providing these observations in situ. Laboratory protocols are needed that enable measurements of the spatial structure of the fracture aperture field in the medium, noninvasively. Here, we investigate changes in rough-walled Brazilian-induced tensile fracture aperture distribution with progressive shear displacement in Westerly granite and Carrara marble using a novel X-ray transparent core holder. The so-called calibration-free missing attenuation method is applied to reconstruct highly resolved (submillimeter) fracture aperture maps as a function of displacement (0 to 5.75 mm) in induced fractures. We observe that shearing increases the core-averaged fracture aperture and significantly broadens the distribution of local values, mostly toward higher apertures. These effects are particularly strong in Westerly granite and may be the result of the higher initial roughness of its fracture surfaces. Also, while the correlation length of the aperture field increases in both parallel and perpendicular directions, significant anisotropy is developed in both samples with the progression of shearing. The results on Westerly granite provide a direct indication that fracture aperture remains largely unaffected until 1 mm of displacement is achieved, which is important when estimating permeability enhancement due to natural and induced shear displacement in faults.

The transport of chemically reactive fluids through fractured clay‐rich rocks is fundamental to many subsurface engineering technologies. Here, we present results of direct‐shear laboratory experiments with simultaneous imaging by X‐ray Computed Tomography in Opalinus claystone with subsequent fluid injection to unravel the interplay between mechanical fracture deformation, fluid sorption, and flow. Under constant radial stress (σc = 1.5 MPa), the average mechanical aperture trued¯CT increases with shear displacement. Upon brine injection, trued¯CT is reduced by 40% relative to initial conditions (trued¯CT0=140−250 μm) and fluid‐sorption induces a divergent displacement of the two sample halves (Δh = ±50 − 170 μm) quantified by digital image correlation. None of these changes are observed in a control experiment with decane, indicating that creep is subordinate to swelling in sealing the fracture. Swelling‐induced changes in permeability within the fracture are heterogeneous and largely affect the fracture flow field, as computed using numerical simulations.

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We investigate chemical transport in laboratory rock cores using unidirectional pulse tracer experiments. Breakthrough curves (BTCs) measured at various flow rates in one sandstone and two carbonate samples are interpreted using the one-dimensional Continuous Time Random Walk (CTRW) formulation with a truncated power law (TPL) model. Within the same framework, we evaluate additional memory functions to consider the Advection-Dispersion Equation (ADE) and its extension to describe mass exchange between mobile and immobile solute phases (Single-Rate Mass Transfer model, SRMT). To provide physical constraints to the models, parameters are identified that do not depend on the flow rate. While the ADE fails systematically at describing the effluent profiles for the carbonates, the SRMT and TPL formulations provide excellent fits to the measurements. They both yield a linear correlation between the dispersion coefficient and the Péclet number (D L ∝ Pe for 10 < (Pe) < 100), and the longitudinal dispersivity is found to be significantly larger than the equivalent grain diameter, D e. The BTCs of the carbonate rocks show clear signs of nonequilibrium effects. While the SRMT model explicitly accounts for the presence of microporous regions (up to 30% of the total pore space), in the TPL formulation the time scales of both advective and diffusive processes (t 1 (Pe) and t 2) are associated with two characteristic heterogeneity length scales (d and l, respectively). We observed that l ≈ 2.5 × D e and that anomalous transport arises when l∕d ≤ (1). In this context, the SRMT and TPL formulations provide consistent, yet complementary, insight into the nature of anomalous transport in laboratory rock cores.

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Quantification of heterogeneous multiscale permeability in geologic porous media is key for understanding and predicting flow and transport processes in the subsurface. Recent utilization of in situ imaging, specifically positron emission tomography (PET), enables the measurement of three‐dimensional (3‐D) time‐lapse radiotracer solute transport in geologic media. However, accurate and computationally efficient characterization of the permeability distribution that controls the solute transport process remains challenging. Leveraging the relationship between local permeability variation and solute advection behavior, an encoder‐decoder based convolutional neural network (CNN) is implemented as a permeability inversion scheme using a single PET scan of a radiotracer pulse injection experiment as input. The CNN can accurately capture the 3‐D spatial correlation between the permeability and the radiotracer solute arrival time difference maps in geologic cores. We first test the inversion accuracy using synthetic test datasets and then test the accuracy on a suite of experimental PET imaging datasets acquired on four different geologic cores. The network‐predicted permeability maps from the geologic cores are used to parameterize forward numerical models that are directly compared with the experimental PET imaging data. The results indicate that a single trained network can generate robust 3‐D permeability inversion maps in seconds. Numerical models parameterized with these permeability maps closely capture the experimentally observed solute arrival time behavior. This work provides an unprecedented approach for efficiently characterizing multiscale permeability heterogeneity in complex geologic samples.