Medical X-ray Computed Tomography Research Articles

In situ dynamic X-ray imaging of fluid-rock interactions inside tight sandstone during hydraulic fracturing: fluid flow process and fracture network growth

Hydraulic fracturing plays a fundamental role in unconventional hydrocarbon production and the development of enhanced geothermal systems. Thus, studying the fluid flow process and fracture network growth during hydraulic fracturing is necessary to provide guidance for creating and sustaining fracture networks in target reservoirs. The actual formation stress state and hydraulic fracturing process can be simulated in the laboratory for small-scale rock samples in hydraulic fracturing experiments. High-energy industrial X-ray computed tomography (CT) has been used to directly track the fracture propagation in millimeter-scale samples under triaxial stress loading. In this study, a large-scale fracture network with economic benefits for unconventional energy exploitation was the target of the experiment; therefore, centimeter-scale samples were used. A hydraulic fracturing simulation experiment was conducted on tight sandstone by applying triaxial compression and injection pressures. With the assistance of the PEEK core holder and third-generation medical X-ray CT scanner, in situ dynamic X-ray imaging of the fluid-rock interactions inside the sample was conducted for the first time. The fluid-rock interactions and fracture network growth can be dynamically observed, revealing the fluid-rock interactions during the hydraulic fracturing process and how macroscopic failure develops from the microfracture.

Applicability of pore networks to evaluate the inter-particle flow in heap leaching

Though many studies pertaining to heap leaching have been carried out to study the reaction kinetics in cylindrical columns, in the scaled up variants the extraction efficiency has not always complemented the laboratory expectations. Since heap leaching is a hydrometallurgical process, the reactions are contingent on the effective interactions between the two main phases (i.e. solid and the fluid). Thus, this work attempts to elicit the understanding of the liquid flow without the focus on the reactions as the prior imparts higher importance. A pseudo 2-D packed bed of 266 mm in length, 200 mm in height and 100 mm in thickness was employed in this work and subjected to X-ray computed tomography (XCT) imaging. XCT is typically used to identify the solid structures in a study specimen. The current study explores the inter-particle flow behaviour and pore networks of two packing types, namely, glass beads and ore particles. The inter-particle flow development at varied flow rates using medical XCT was attempted in this work by the addition of a clinical contrast agent. The same packed beds were subjected to laboratory experiments to determine the respective liquid drainage distributions. The flow model was proposed on a weighted random walk (RW) approach to ascertain the probabilistic behaviour of the gravity dominated flow features in the examined packings using the pore networks inferred from the XCT images. The model was able to generate comparable liquid drainage profiles for the examined packings, though the effect of flow rate was not captured. This approach could be used to specify operational parameters, such as drip emitter spacing, particle size and agglomerate size in heap leaching to enhance the overall extraction of the valuable metal species.

Inter-particle fluid flow visualisation of larger packed beds pertaining to heap leaching using X-ray computed tomography imaging

X-ray computed tomography (XCT) is a non-destructive 3-D imaging technique that permits visualisation of the internal structure within a sample, avoiding the stereological error found in conventional two-dimensional (2-D) image analysis. Its small-scale variant, in the present manuscript, referred to as X-ray micro-computed tomography (XMT) but more commonly referred to as µCT, has been widely studied for the characterisation of mineral particle beds, with a common focus in recent years being heap leaching applications. Several technical and computational factors have however limited the size of the configurations and samples studied with this methodology and thus CT flow path visualisation studies on packed beds larger than a few centimetres are uncommon in the literature. The methodology is also rarely used for fluid flow path identification, with other non-invasive alternatives being preferred. To remediate, the authors used a large medical XCT scanner to image a 266 * 200 * 100 mm3 rectangular bed configuration filled with non-porous 8 mm glass beads and porous 16–20 mm rock particles. The fluid flow was for its part imaged through the addition of Gastrografin®, a common medical contrast agent. Using Thermo Fisher Scientific's Avizo software, 3-D images of the beds, the liquid flows, as well as the pore networks were constructed and their parameters were computed. Validation experiments were also carried through ultraviolet (UV) fluorescence imaging using sodium fluorescein dye addition. By comparing the computed results of sphericity and equivalent diameter to the known values and the computed flow images to the UV fluorescence results, the authors conclude that their methodology is of adequate accuracy for further developments towards comprehensive analyses of unsaturated heap flow behaviour.

Evaluation of Meso-damage Processes in Concrete by X-Ray CT Scanning Techniques Under Real-Time Uniaxial Compression Testing

Concrete damage characteristics were investigated by X-ray computed tomography (CT) under uniaxial compression, with medical X-ray CT and a loading device used to carry out real-time uniaxial compression scanning tests, obtaining two-dimensional CT images of crack generation, propagation, coalescence, and failure. Damage deterioration of internal structures under different loading conditions was analyzed by means of a digital image-processing technique, which allowed the recognition and extraction of meso cracks. Furthermore, the crack lengths were extracted and measured from CT images. Methods of divisional regions were applied to carry out the quantitative analysis of CT numbers in specific representative regions. The fractal dimensions (FDs) of crack propagation in CT images were estimated using the box-counting dimension, and the relationship between the strain, crack propagation, and FD examined. Complicated and disordered internal cracks were clearly observed with increased load. The results indicated that such CT tests were a promising means for identifying the evolution and propagation of internal cracks and for studying the mesoscopic damage fracture mechanism under uniaxial compression.

Determination of binary diffusion coefficients between hot liquid solvents and bitumen with X-ray CT

Abstract The Steam Assisted Gravity Drainage (SAGD) process has been used for in-situ thermal bitumen recovery in Canada. Nevertheless, there are several drawbacks that could impair the SAGD performance. One of the main drawbacks of SAGD is nontrivial consumption of energy to generate steam. To curtail the consumption of steam, many SAGD variants have been proposed. For example, Vapor Extraction (VAPEX) was proposed as a solvent-based cold production method and Solvent-Assisted SAGD (SA-SAGD) was proposed as a solvent-assisted hybrid method. In those solvent-based/assisted methods, the dissolution of solvents in bitumen enhances the in-situ oil mobility. The speed of the mixing between solvents and bitumen in solvent-based/assisted recovery processes is dictated by dispersive mass flux which is proportional to the transverse dispersion coefficient. Diffusion coefficient, tortuosity and transverse dispersivity should be quantified to characterize the transverse dispersion coefficient. In this research, the diffusion coefficients of solvents in Athabasca bitumen were determined with X-ray computed tomography (CT). A medical X-ray CT scanner was used to capture the diffusion phenomenon occurring between fresh liquid solvents and bitumen. The experiments were designed to reproduce the in-situ high pressure, high temperature condition (3.5 MPa, 135C) where solvent and bitumen actually interact at the vapor chamber edge in the SA-SAGD process. Using our new °experimental system, we successfully measured molecular diffusion coefficients at the condition of the vapor chamber edge for the first time. In conclusion, the binary diffusion coefficients between solvents and bitumen were successfully measured at the condition where the mixing of solvents and bitumen actually occurs in the SA-SAGD process. The characterized diffusion coefficients will be input parameters for the reservoir simulation and semi-analytical models.

Consecutive Short-Scan CT for Geological Structure Analog Models with Large Size on In-Situ Stage.

For the analysis of interior geometry and property changes of a large-sized analog model during a loading or other medium (water or oil) injection process with a non-destructive way, a consecutive X-ray computed tomography (XCT) short-scan method is developed to realize an in-situ tomography imaging. With this method, the X-ray tube and detector rotate 270° around the center of the guide rail synchronously by switching positive and negative directions alternately on the way of translation until all the needed cross-sectional slices are obtained. Compared with traditional industrial XCTs, this method well solves the winding problems of high voltage cables and oil cooling service pipes during the course of rotation, also promotes the convenience of the installation of high voltage generator and cooling system. Furthermore, hardware costs are also significantly decreased. This kind of scanner has higher spatial resolution and penetrating ability than medical XCTs. To obtain an effective sinogram which matches rotation angles accurately, a structural similarity based method is applied to elimination of invalid projection data which do not contribute to the image reconstruction. Finally, on the basis of geometrical symmetry property of fan-beam CT scanning, a whole sinogram filling a full 360° range is produced and a standard filtered back-projection (FBP) algorithm is performed to reconstruct artifacts-free images.

Simulating stress-dependent fluid flow in a fractured core sample using real-time X-ray CT data

Abstract. Various geoscientific applications require a fast prediction of fracture permeability for an optimal workflow. Hence, the objective of the current study is to introduce and validate a practical method to characterize and approximate single flow in fractures under different stress conditions by using a core-flooding apparatus, in situ X-ray computed tomography (CT) scans and a finite-volume method solving the Navier–Stokes–Brinkman equations. The permeability of the fractured sandstone sample was measured stepwise during a loading–unloading cycle (0.7 to 22.1 MPa and back) to validate the numerical results. Simultaneously, the pressurized core sample was imaged with a medical X-ray CT scanner with a voxel dimension of 0.5 × 0.5 × 1.0 mm3. Fracture geometries were obtained by CT images based on a modification of the simplified missing attenuation (MSMA) approach. Simulation results revealed both qualitative plausibility and a quantitative approximation of the experimentally derived permeabilities. The qualitative results indicate flow channeling along several preferential flow paths with less pronounced tortuosity. Significant changes in permeability can be assigned to temporal and permanent changes within the fracture due to applied stresses. The deviations of the quantitative results appear to be mainly caused by both local underestimation of hydraulic properties due to compositional matrix heterogeneities and the low CT resolution affecting the accurate capturing of sub-grid-scale features. Both affect the proper reproduction of the actual connectivity and therefore also the depiction of the expected permeability hysteresis. Furthermore, the threshold value CTmat (1862.6 HU) depicting the matrix material represents the most sensitive input parameter of the simulations. Small variations of CTmat can cause enormous changes in simulated permeability by up to a factor of 2.6 ± 0.1 and, thus, have to be defined with caution. Nevertheless, comparison with further CT-based flow simulations indicates that the proposed method represents a valuable method to approximate actual permeabilities, particularly for smooth fractures (< 35 µm). However, further systematic investigations concerning the applicability of the method are essential for future studies. Thus, some recommendations are compiled by also including suggestions of comparable studies.

Seismically induced soft-sediment deformation structures revealed by X-ray computed tomography of boring cores

X-ray computed tomography (CT) allows us to visualize three-dimensional structures hidden in boring cores nondestructively. We applied medical X-ray CT to cores containing seismically induced soft-sediment deformation structures (SSDSs) obtained from the Kanto region of Japan, where the 2011 off the Pacific coast of Tohoku Earthquake occurred. The CT images obtained clearly revealed various types of the seismically induced SSDSs embedded in the cores: a propagating sand dyke bent complexly by the preexisting geological structure, deformed laminations of fluidized sandy layers, and two types of downward mass movement (ductile downward folding and brittle normal faulting) as compensation for upward sand transport through sand dykes. Two advanced image analysis techniques were applied to the sand dyke CT images for the first time. The GrowCut algorithm, a specific digital image segmentation technique that uses cellular automata, was used successfully to extract the three-dimensional complex sand dyke structures embedded in the sandy sediments, which would have been difficult to achieve using a conventional image processing technique. Local autocorrelation image analysis was performed to detect the flow pattern aligned along the sand dykes objectively. The results demonstrate that X-ray CT coupled with advanced digital image analysis techniques is a promising approach to studying the seismically induced SSDSs in boring cores.

Investigating Aperture-Based Permeability and Capillary Pressure in Rock Fractures

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 170819, “Investigating Aperture-Based Stress-Dependent Permeability and Capillary Pressure in Rock Fractures,” by Da Huo, SPE, Boxiao Li, SPE, and Sally M. Benson, SPE, Stanford University, prepared for the 2014 SPE Annual Technical Conference and Exhibition, Amsterdam, 27–29 October. The paper has not been peer reviewed. Laboratory measurements of fracture-aperture distributions have been made by use of computed-tomography (CT) scanning under various conditions of effective stress. The stress-dependent aperture-distribution data demonstrate that increasing stress results in two effects: The mean aperture will decrease and the variance of aperture distribution will increase. By analyzing the stress-dependent mean and variance of aperture distributions, this paper provides a more straightforward method for estimating stress-dependent permeability and capillary pressure in rock fractures. Experimental Setup A Berea sandstone is cut to create a fracture in the middle of the core. The sample is moderately homogeneous, with little porosity variation. The fracture surface is very rough and has large aperture variations. The length of the sample is 8 cm, and the radius is 5 cm. The porosity of the sample is 0.22. The rock sample is wrapped in a sleeve and then placed in an aluminum core holder. Water is injected around the sleeve to create the confining stress. Two high-accuracy pressure transducers are used to measure pressure at the inlet and outlet of the core, and electric heaters maintain the temperature of the core. Pore pressure is kept constant at atmospheric pressure, and temperature stays constant at 50°C. For permeability measurements, a dual-pump system is used to inject water into the core. A pressure-regulating pump is connected to the outlet of the core to maintain 2.07 MPa at the downstream end of the core throughout the experiment. The experimental setup is shown in Fig. 1. Permeability is measured with the steady-state method. Fluid is injected at three different rates while the pressure drop across the core is measured. On the basis of core size, fluid viscosity, slope of pressure drop, and fluid-injection rate, Darcy’s law is used to calculate permeability. The intact-core permeability is measured at different stress levels and is subtracted from the fractured-core permeability. Permeability is calculated using a mean aperture of 0.1 mm. The core holder is placed in a medical X-ray CT scanner. Five repeated scans are taken and averaged to reduce the uncertainty of the CT scanning.

Hybrid Filter Based on Neural Networks for Removing Quantum Noise in Low-Dose Medical X-ray CT Images

The main source of noise in computed tomography (CT) images is a quantum noise, which results from statistical fluctuations of X-ray quanta reaching the detector. This paper proposes a neural network (NN) based hybrid filter for removing quantum noise. The proposed filter consists of bilateral filters (BFs), a single or multiple neural edge enhancer(s) (NEE), and a neural filter (NF) to combine them. The BFs take into account the difference in value from the neighbors, to preserve edges while smoothing. The NEE is used to clearly enhance the desired edges from noisy images. The NF acts like a fusion operator, and attempts to construct an enhanced output image. Several measurements are used to evaluate the image quality, like the root mean square error (RMSE), the improvement in signal to noise ratio (ISNR), the standard deviation ratio (MSR), and the contrast to noise ratio (CNR). Also, the modulation transfer function (MTF) is used as a means of determining how well the edge structure is preserved. In terms of all those measurements and means, the proposed filter shows better performance than the guided filter, and the nonlocal means (NLM) filter. In addition, there is no severe restriction to select the number of inputs for the fusion operator differently from the neuro-fuzzy system. Therefore, without concerning too much about the filter selection for fusion, one could apply the proposed hybrid filter to various images with different modalities, once the corresponding noise characteristics are explored.

Compaction effects on soil macropore geometry and related parameters for an arable field

Soil compaction decreases soil pores are important for root growth as well as infiltration of water and nutrients. A study was conducted to evaluate the effects of soil compaction on macropore parameters measured using X-ray computed tomography (CT). Macropore parameters included number of pores, number of macropores (> 1000 μm diam.), number of coarse mesopores (200 to 1000 μm diam.), porosity, macroporosity, coarse mesoporosity, area of largest pore, pore circularity, and fractal dimension of macroporosity. A field experiment was conducted on Mexico silt loam (fine, smectitic, mesic Vertic Epiaqualfs) with field treatments including four replicates of uniformly Compacted (C) and Non-Compacted (NC) plots arranged in a randomized complete block design. Soil cores (76.2 mm diam. by 76.2 mm long) were removed from three selected depths (0 to 10 cm, 10 to 20 cm, and 20 to 30 cm). Cores were scanned using a medical X-ray CT scanner with four scans taken in each sample at 15 mm spacing starting at 25 mm from the core surface. Images were analyzed using Image-J software. The C treatment was found to increase bulk density by 8% (1.34 to 1.45 g cm −3) and decrease saturated hydraulic conductivity by 69% (47.1 to 14.6 cm hr − 1 ). CT-measured number of pores decreased by 71%, number of macropores by 69%, and coarse mesopores by 75% with the C treatment used in the study. Compaction was also found to significantly decrease CT-measured porosity and macroporosity by 64%. Differences between treatments for the parameters were most pronounced in the upper 10 cm; differences between treatments were not significant below 20 cm. A regression equation with CT-measured macroporosity, area of largest pore and porosity explained most of the variability in saturated hydraulic conductivity ( R 2 = 0.79). Efforts should be made to minimize soil compaction due to its harmful effects on soil pores and subsequent challenges for plant root growth and enhanced runoff of water and nutrients.

Non-destructive Analysis of Oil-Contaminated Soil Core Samples by X-ray Computed Tomography and Low-Field Nuclear Magnetic Resonance Relaxometry: a Case Study

Non-destructive measurements of contaminated soil core samples are desirable prior to destructive measurements because they allow obtaining gross information from the core samples without touching harmful chemical species. Medical X-ray computed tomography (CT) and time-domain low-field nuclear magnetic resonance (NMR) relaxometry were applied to non-destructive measurements of sandy soil core samples from a real site contaminated with heavy oil. The medical CT visualized the spatial distribution of the bulk density averaged over the voxel of 0.31 × 0.31 × 2 mm3. The obtained CT images clearly showed an increase in the bulk density with increasing depth. Coupled analysis with in situ time-domain reflectometry logging suggests that this increase is derived from an increase in the water volume fraction of soils with depth (i.e., unsaturated to saturated transition). This was confirmed by supplementary analysis using high-resolution micro-focus X-ray CT at a resolution of ∼10 μm, which directly imaged the increase in pore water with depth. NMR transverse relaxation waveforms of protons were acquired non-destructively at 2.7 MHz by the Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence. The nature of viscous petroleum molecules having short transverse relaxation times (T2) compared to water molecules enabled us to distinguish the water-saturated portion from the oil-contaminated portion in the core sample using an M 0–T2 plot, where M 0 is the initial amplitude of the CPMG signal. The present study demonstrates that non-destructive core measurements by medical X-ray CT and low-field NMR provide information on the groundwater saturation level and oil-contaminated intervals, which is useful for constructing an adequate plan for subsequent destructive laboratory measurements of cores.

X-ray computed tomography to quantify tree rooting spatial distributions

Poor root development due to constraining soil conditions could be an important factor influencing health of urban trees. Therefore, there is a need for efficient techniques to analyze the spatial distribution of tree roots. An analytical procedure for describing tree rooting patterns from X-ray computed tomography (CT) data is described and illustrated. Large irregularly shaped specimens of undisturbed sandy soil were sampled from various positions around the base of trees using field impregnation with epoxy resin, to stabilize the cohesionless soil. Cores approximately 200 mm in diameter by 500 mm in height were extracted from these specimens. These large core samples were scanned with a medical X-ray CT device, and contiguous images of soil slices (2 mm thick) were thus produced. X-ray CT images are regarded as regularly-spaced sections through the soil although they are not actual 2D sections but matrices of voxels ∼0.5 mm×0.5 mm×2 mm. The images were used to generate the equivalent of horizontal root contact maps from which three-dimensional objects, assumed to be roots, were reconstructed. The resulting connected objects were used to derive indices of the spatial organization of roots, namely: root length distribution, root length density, root growth angle distribution, root spatial distribution, and branching intensity. The successive steps of the method, from sampling to generation of indices of tree root organization, are illustrated through a case study examining rooting patterns of valuable urban trees.

Algorithms for density and composition-discrimination imaging forfourth-generation CT systems

This paper shows that if the off-beam idle detectors in the detection ring of a fourth-generation x-ray computed tomography (CT) system are used to measure the scattered radiation, it is numerically feasible to reconstruct electron-density images to supplement the conventional attenuation-coefficient images of transmitted radiation. It is also shown that by combining these two images, composition changes can be detected with the aid of an effective-atomic-number indicator. The required image-reconstruction algorithms are developed and tested against Monte Carlo simulated measurements, for a variety of phantom configurations. In spite of the relatively poor statistical quality of scattering measurements, it is demonstrated that electron-density images of reasonable quality can be obtained. In addition, it is shown that composition discrimination is possible for materials of effective atomic number greater than five, in the photon energy range of a typical medical x-ray CT system operating at 102 kVp. The obtained supplementary electron-density and composition images can be useful in radiotherapy planning and for studying tumour histology, as well as in industrial and security applications where identification of materials based on density and composition is important.

How computed X-ray tomography can be used to study crosslink density in non-filled peroxide cured polyisoprene rubber

As part of an ongoing research programme to investigate how medical X-ray computed tomography (CT) can be used in non-destructive testing of polymeric materials, a series of peroxide cured, non-filled polyisoprene rubbers with different crosslink densities were investigated. It was found that a normal medical X-ray CT scanner, thanks to its excellent contrast resolution, is capable of detecting density variations, caused by crosslinking, which exceed 2–3 mg cm −3.