Characterisation of solute transport in heterogeneous porous media by multidimensional imaging and modelling

The study of solute transport in porous media continues to find applications in both traditional and emerging engineering problems, many of which occur in natural environments. Key applications include CO2 sequestration, enhanced oil recovery and soil remediation. Transport is a fundamental component in the analysis of these systems, because it provides the driving force for physical and chemical interactions between the fluid and the solid phase. However, the inherent heterogeneity of the subsurface leads to what is classically referred to as anomalous transport, which challenges classic interpretations of both field and laboratory experiments. In this context, novel laboratory protocols are needed to probe transport in heterogeneous medium by measuring the spatial structure of the concentration field in the medium, rather than relying exclusively on the analysis of breakthrough curves (BTCs). In this thesis, a combined experimental and modelling study of solute transport in a range of porous media has been presented, including sandstone and carbonate rocks, to cover a range of pore structures. At the core of the experimental work is the combination of two imaging methods, X-ray Computed Tomography and Positron Emission Tomography. While the former is used to characterise rock properties spatially, the latter allows visualising the temporal evolution of the full tracer plume within the medium in three dimensions. To this aim, a core-flooding system has been built to carry out pulse-tracer tests over a wide range of Péclet numbers (Pe=15-500) using brine- and radio-tracers. In addition to the experiments on the three rock samples (Bentheimer Sandstone, Ketton Limestone and Edwards Carbonate), control experiments on uniform beadpacks were carried out to verify the accuracy of the in-situ measurements. The experimental BTCs have been analysed in the framework of residence time distribution functions, which revealed mass transfer limitations in the microporous carbonates in the form of a characteristic flow-rate effect. Three transport models: the Advection Dispersion Equation (ADE), the Multi-Rate Mass Transfer (MRMT) and the Continuous Time Random Walk (CTRW) framework have been thoroughly evaluated with both the BTCs and the internal concentration profiles. It is shown that the ADE provides an accurate description of the results on the beadpack and the sandstone. The data on the carbonates are better described by the MRMT, which uses a fraction of stagnant, intra-granular pore space and an external fluid film resistance model to account for mass transfer between the flowing fluid and the porous particles. The CTRW theory, applied here for the first time to carbonate cores, provides a further improvement in describing the BTC, because of its ability to account for unresolved heterogeneities. In the application of the models, a distinction was made between parameters that are rocks-specific (e.g., the dispersivity) and those that depend on the flow rate, by treating the former as global fitting parameters in the optimisation routine. Accordingly, the obtained results provide a more consistent picture than what the current literature may suggest regarding the use of these models to the analysis of BTCs. The dataset obtained from the PET has been used to quantify the extent and rate of mixing in the different porous media. The 3-D images clearly reveal the presence of spreading caused by subcore-scale heterogeneities. To quantify their effects on the core-scale dispersion, various measures has been used, namely the dilution index (Π), the spreading length-scale (K) and the intensity of segregation (I). It was observed that the microporosity has a pronounced effect on mixing, thereby greatly accelerating the time scale to reach the asymptotic regime. Notably, both Π and K scale vary linearly with the square-root of time, indicating the suitability of a Fickian-based model to quantify macrodispersion. This observation suggests that the strength of heterogeneity in the rock samples investigated is moderate and that anomalous transport has evolved to normal behaviour on a length-scale O(l)∼10 cm (∼ length of the samples). In this context, to provide a more comprehensive picture of anomalous transport in laboratory rock samples, future studies should aim at increasing the spatial resolution of the measurement. Non-invasive, imaging tools such as PET are likely to go a long way in addressing this problem and provide significant opportunities to advance our understanding of miscible displacements in consolidated porous media, thus including those involving additional phenomena, such as adsorption and chemical reactions.Open Acces

Description of chemical transport in laboratory rock cores using the continuous random walk formalism

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 (DL Pe for 10 < (Pe) < 100), and the longitudinal dispersivity is found to be significantly larger than the equivalent grain diameter, De. 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 (t1 (Pe) and t2) are associated with two characteristic heterogeneity length scales (d and l, respectively). We observed that l 2.5 × De and that anomalous transport arises when ld (1). In this context, the SRMT and TPL formulations provide consistent, yet complementary, insight into the nature of anomalous transport in laboratory rock cores

Identification of a high incidence region for retroviral vector integration near exon 1 of the LMO2 locus

Therapeutic retroviral vector integration near the oncogene LMO2 is thought to be a cause of leukemia in X-SCID gene therapy trials. However, no published studies have evaluated the frequency of vector integrations near exon 1 of the LMO2 locus. We identified a high incidence region (HIR) of vector integration using PCR techniques in the upstream region close to the LMO2 transcription start site in the TPA-Mat T cell line. The integration frequency of the HIR was one per 4.46 × 10(4 )cells. This HIR was also found in Jurkat T cells but was absent from HeLa cells. Furthermore, using human cord blood-derived CD34(+ )cells we identified a HIR in a similar region as the TPA-Mat T cell line. One of the X-linked severe combined immunodeficiency (X-SCID) patients that developed leukemia after gene therapy had a vector integration site in this HIR. Therefore, the descriptions of the location and the integration frequency of the HIR presented here may help us to better understand vector-induced leukemogenesis

A general capillary equilibrium model to describe drainage experiments in heterogeneous laboratory rock cores

Macroscopic observations of two-phase flow in porous rocks are largely affected by the heterogeneity in continuum properties at length scales smaller than a typical laboratory sample. The ability to discriminate among the rock properties at the origin of the heterogeneity is key to the development of numerical models to be used for prediction. Here, we present a capillary equilibrium model that represents spatial heterogeneity in dual-porosity porous media in terms of the capillary entry pressure, 1∕, and the irreducible wetting phase saturation, ir. Both parameters are used to scale local capillary pressure curves by using three-dimensional imagery acquired during multi-rate gas/liquid drainage displacements. We verify the proposed approach by considering the case study of a dual-porosity limestone core and use the spatial variation in ir as proxy for microporosity heterogeneity. The latter places potentially next-to-leading order controls on the observed fluid saturation distribution, which is strongly correlated to the distribution of 1∕. While microporosity is by and large uniform at the observation scale on the order of 0.1 cm3, the spatial correlation of 1∕ is on the order of 1 cm and is therefore not statistically represented in the volume of typical laboratory core samples

Spatial mapping of fracture aperture changes with shear displacement using X-ray computerized tomography

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, non‐invasively. 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 (sub‐millimeter) 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 towards 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

Positron emission tomography in water resources and subsurface energy resources engineering research

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 research—and therefore imaging facilities are available at most research institutes—widespread 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

Estimating Three-Dimensional Permeability Distribution for Modeling Multirate Coreflooding Experiments

Characterizing subsurface reservoirs such as aquifers or oil and gas fields is an important aspect of various environmental engineering technologies. Coreflooding experiments, conducted routinely for characterization, are at the forefront of reservoir modeling. In this work, we present a method to estimate the three-dimensional permeability distribution and characteristic (intrinsic) relative permeability of a core sample in order to construct an accurate model of the coreflooding experiment. The new method improves previous ones by allowing to model experiments with mm-scale accuracy at various injection rates, accounting for variations in capillary–viscous effects associated with changing flow rates. We apply the method to drainage coreflooding experiments of nitrogen and water in two heterogeneous limestone core samples and estimate the subcore scale permeability and relative permeability. We show that the models are able to estimate the saturation distribution and core pressure drop with what is believed to be sufficient accuracy