Frieder Enzmann

Real-time 3D imaging of Haines jumps in porous media flow

Newly developed high-speed, synchrotron-based X-ray computed microtomography enabled us to directly image pore-scale displacement events in porous rock in real time. Common approaches to modeling macroscopic fluid behavior are phenomenological, have many shortcomings, and lack consistent links to elementary pore-scale displacement processes, such as Haines jumps and snap-off. Unlike the common singular pore jump paradigm based on observations of restricted artificial capillaries, we found that Haines jumps typically cascade through 10–20 geometrically defined pores per event, accounting for 64% of the energy dissipation. Real-time imaging provided a more detailed fundamental understanding of the elementary processes in porous media, such as hysteresis, snap-off, and nonwetting phase entrapment, and it opens the way for a rigorous process for upscaling based on thermodynamic models.

From connected pathway flow to ganglion dynamics

During imbibition, initially connected oil is displaced until it is trapped as immobile clusters. While initial and final states have been well described before, here we image the dynamic transient process in a sandstone rock using fast synchrotron-based X-ray computed microtomography. Wetting film swelling and subsequent snap off, at unusually high saturation, decreases nonwetting phase connectivity, which leads to nonwetting phase fragmentation into mobile ganglia, i.e., ganglion dynamics regime. We find that in addition to pressure-driven connected pathway flow, mass transfer in the oil phase also occurs by a sequence of correlated breakup and coalescence processes. For example, meniscus oscillations caused by snap-off events trigger coalescence of adjacent clusters. The ganglion dynamics occurs at the length scale of oil clusters and thus represents an intermediate flow regime between pore and Darcy scale that is so far dismissed in most upscaling attempts.

Digital Rock Physics - Effect of Fluid Viscosity on Effective Elastic Properties

Abstract This paper is concerned with the effect of pore fluid viscosity on effective elastic properties using digitized rocks. We determine a significant velocity dispersion in wave propagation simulations by the variation of the pore fluid viscosity. Several attenuation regimes are considered which may contribute to this observation. Starting point is a virtual rock physics approach. Numerical simulations of effective transport and effective mechanical properties are applied to statistically representative rock samples. The rock microstructure is imaged by 3D X-ray tomography. Permeability values were estimated through Lattice-Boltzmann flow simulations. The dry rock moduli and the tortuosity are derived by dynamic wave propagation simulations. We apply a displacement-stress rotated staggered finite-difference grid technique to solve the elastodynamic wave equation. An accurate approximation of a Newtonian fluid is implemented in this technique by using a generalized Maxwell body. We give a practical description of how to use this approach and discuss the application limits. Additionally, we show the simulated signature of a theoretically predicted slow S -wave.

Microstructural evolution of gas hydrates in sedimentary matrices observed with synchrotron X‐ray computed tomographic microscopy

The formation process of gas hydrates in sedimentary matrices is of crucial importance for the physical and transport properties of the resulting aggregates. This process has never been observed in situ at submicron resolution. Here we report on synchrotron-based microtomographic studies by which the nucleation and growth processes of gas hydrate were observed at 276 K in various sedimentary matrices such as natural quartz (with and without admixtures of montmorillonite type clay) or glass beads with different surface properties, at varying water saturation. Both juvenile water and metastably gas-enriched water obtained from gas hydrate decomposition was used. Xenon gas was employed to enhance the density contrast between gas hydrate and the fluid phases involved. The nucleation sites can be easily identified and the various growth patterns are clearly established. In sediments under-saturated with juvenile water, nucleation starts at the water-gas interface resulting in an initially several micrometer thick gas hydrate film; further growth proceeds to form isometric single crystals of 10–20 µm size. The growth of gas hydrate from gas-enriched water follows a different pattern, via the nucleation in the bulk of liquid producing polyhedral single crystals. A striking feature in both cases is the systematic appearance of a fluid phase film of up to several micron thickness between gas hydrates and the surface of the quartz grains. These microstructural findings are relevant for future efforts of quantitative rock physics modeling of gas hydrates in sedimentary matrices and explain the anomalous attenuation of seismic/sonic waves.

Microtomographic Quantification of Hydraulic Clay Mineral Displacement Effects During a CO2 Sequestration Experiment with Saline Aquifer Sandstone

We combined a noninvasive tomographic imaging technique with an invasive open-system core-flooding experiment and compared the results of the pre- and postflooded states of an experimental sandstone core sample from an ongoing field trial for carbon dioxide geosequestration. For the experiment, a rock core sample of 80 mL volume was taken from the 629 m Stuttgart Formation storage domain of a saline sandstone aquifer at the CCS research pilot plant Ketzin, Germany. Supercritical carbon dioxide and synthetical brine were injected under in situ reservoir p/T-conditions at an average flow rate of 0.1 mL/min for 256 h. X-ray computed microtomographic imaging was carried out before and after the core-flooding experiment at a spatial voxel resolution of 27 μm. No significant changes in microstructure were found at the tomographic imaging resolution including porosity and pore size distribution, except of an increase of depositional heterogeneous distribution of clay minerals in the pores. The digitized rock data were used as direct real microstructure input to the GeoDict software package, to simulate Navier-Stokes flow by a lattice Boltzmann equation solver. This procedure yielded 3D pressure and flow velocity fields, and revealed that the migration of clay particles decreased the permeability tensor probably due to clogging of pore openings.

Microstructure characteristics during hydrate formation and dissociation revealed by X-ray tomographic microscopy

AbstractDespite much progress over the past years in fundamental gas hydrate research, frontiers to the unknown are the early beginning and early decomposition of gas hydrates in their natural, submarine environment: gas bubbles meeting ocean water and forming hydrate, and gas starting to escape from the surface of a hydrate grain. In this paper we report on both of these topics, and present three-dimensional microstructure results obtained by synchrotron radiation X-ray cryo-tomographic microscopy (SRXCTM). Hydrates can precipitate when hydrate-forming molecules such as methane exceed solubility, and combine with water within the gas hydrate stability zone. Here we show hydrate formation on surfaces of bubbles from different gas mixtures and seawater, based on underwater robotic in situ experiments in the deep Monterey Canyon, offshore California. Hydrate begins to form from the surrounding water on the bubble surfaces, and subsequently grows inward into the bubble, evidenced by distinct edges. Over time, the bubbles become smaller while gas is being incorporated into newly formed hydrate. In contrast, current understanding has been that hydrate decomposition starts on the outer surface of hydrate aggregates and grains. It is shown that in an early stage of decomposition, newly found tube structures connect well-preserved gas hydrate patches to areas that are dissociating, demonstrating how dissociating areas in a hydrate grain are linked through hydrate that is still intact and will likely decompose at a later stage. FigureThe boundaries of a gas hydrate grain: excepting for the matrix (transparent, not shown), one can see tubular structures, pores from decomposition, and bubbles.

Simultaneous segmentation and beam-hardening correction in computed microtomography of rock cores

We propose a post-reconstruction correction procedure for the beam-hardening artifact that neither requires knowledge of the X-ray spectrum nor of the attenuation coefficients in multi-mineral geologic samples. The beam-hardening artifact in polychromatic X-ray computer tomography (CT) hampers segmentation of the phase assemblage in geologic samples. We show that in cylindrically shaped samples like rock cores, the X-ray attenuation value for a single phase depends mainly on the distance from the center of the cylinder. This relationship could be easily extracted from the CT data for every phase and used to infer the presence of these phases at all positions in the sample. Our new approach enables simple segmentation of the phases for further analysis. Additionally, we propose a simple mathematical exercise for parallel beam geometry to show how recent experimentally observed deviations from Beers law could be incorporated to model the beam-hardening artifact and simultaneously be used for its removal. The beam-hardening artifact (BHA) in computer tomography depends on the shape of the scanned sample.In cylindrical samples: beam-hardening artifact is a function of spatial distance from the center of the cylinder (BHA curve).The knowledge of BHA curves of different phases are the key to segregate phases.The segmentation provides the possibility to use computer tomography data in a quantitative manner.

Processing of rock core microtomography images

The abilities of machine learning algorithms to process X-ray microtomographic rock images were determined. The study focused on the use of unsupervised, supervised, and ensemble clustering techniques, to segment X-ray computer microtomography rock images and to estimate the pore spaces and pore size diameters in the rocks. The unsupervised k-means technique gave the fastest processing time and the supervised least squares support vector machine technique gave the slowest processing time. Multiphase assemblages of solid phases (minerals and finely grained minerals) and the pore phase were found on visual inspection of the images. In general, the accuracy in terms of porosity values and pore size distribution was found to be strongly affected by the feature vectors selected. Relative porosity average value of 15.92?1.77% retrieved from all the seven machine learning algorithm is in very good agreement with the experimental results of 17?2%, obtained using gas pycnometer. Of the supervised techniques, the least square support vector machine technique is superior to feed forward artificial neural network because of its ability to identify a generalized pattern. In the ensemble classification techniques boosting technique converged faster compared to bragging technique. The k-means technique outperformed the fuzzy c-means and self-organized maps techniques in terms of accuracy and speed. Testing of machine learning algorithms to process X-ray CT rock images.Unsupervised, supervised, and ensemble clustering techniques were applied.k-Means technique is the fastest in terms of CPU performance.

Simulation of Fluid Flow on Fractures and Implications for Reactive Transport Simulations

The diverse numerical simulation techniques employed to predict fluid flow properties of fractures yield differing results which limits their applicability for reactive transport simulations. Basically the fluid flow simulation techniques can be divided in two groups: (i) techniques that yield average fluid flow characteristics and (ii) techniques that produce space-resolved properties. These differences may have substantial impact on the reactive transport simulations but may also depend on the fracture characteristics. For this reason, a sensitivity analysis of the geometrical properties of fractures on the fluid flow properties is conducted and evaluated with respect to their impact on reactive transport modeling. Although employing space-resolved simulation techniques, the results of the tests show average values for permeability and fluid velocity that are comparable to previous studies that used other simulation techniques. Observed fluid flow channeling appears to be related to fracture surfaces matching and anisotropy. However, average flow velocities at potential sites for reactive transport differ up to a factor of five from the average ones for the entire fracture. Furthermore, extreme values at reactive transport sites may differ even more and the flow may be directed against the applied pressure gradient. For studies concerned with simulation of reactive transport, these deviations are crucial and should be explicitly considered in the calculations. Hence space-resolved fluid flow simulations should be employed for the simulation of reactive transport.

Ion fractionation in young sea ice from Kongsfjorden, Svalbard

Abstract The fractionation of major sea-water ions, or deviation in their relative concentrations from Standard Mean Ocean Water ratios, has been frequently observed in sea ice. It is generally thought to be associated with precipitation of solid salts at certain eutectic temperatures. the variability found in bulk sea-ice samples indicates that the fractionation of ions depends on the often unknown thermal history of sea ice, which affects the structure of pore networks and fate of solid salts within them. Here we investigate the distribution of ions in Arctic sea ice that is a few weeks old with a reconstructible thermal history. We separate the centrifugable (interconnected) and entrapped (likely disconnected) contributions to the ice salinity and determine their ion fractionation signatures. the results indicate that differential diffusion of ions, rather than eutectic precipitation of cryohydrates, has led to significant ion fractionation. the finding emphasizes the role of coupled diffusive–convective salt transport through complex pore networks in shaping the biogeochemistry of sea ice.