Snap-off Events Research Articles

Fluid entrapment during forced imbibition in a multidepth microfluidic chip with complex porous geometry

Understanding and controlling fluid entrapment during forced imbibition in porous media is crucial for many natural and industrial applications. However, the microscale physics and macroscopic consequences of fluid entrapment in these geometric-confined porous media remain poorly understood. Here, we introduce a novel multidepth microfluidic chip, which can mitigate the depth confinement of traditional two-dimensional (2-D) microfluidic chips and mimic the wide pore size distribution as natural-occurring three-dimensional (3-D) porous media. Based on microfluidic experiments and direct numerical simulations, we observe the fluid-entrapment scenarios and elucidate the underlying complex interaction between geometric confinement, capillary number and wettability. Increasing depth variation can promote fluid entrapment, whereas increasing capillary number and contact angle yield the opposite effect, which seemingly contradicts conventional expectations in traditional 2-D microfluidic chips. The fluid-entrapment scenario in depth-variable microfluidic chips stems from microscopic interfacial phenomena, classified as snap-off and bypass events. We provide theoretical analyses of these pore-scale events and validate corresponding phase diagrams numerically. It is shown that increasing depth variation triggers snap-off and bypass events. Conversely, a higher capillary number suppresses snap-off events under strong imbibition, and an increased contact angle inhibits bypass events under imbibition. These macroscopic imbibition patterns in microfluidic porous media can be linked with these pore-scale events by improved dynamic pore-network models. Our findings bridge the understanding of forced imbibition between 2-D and 3-D porous media and provide design principles for newly engineered porous media with respect to their desired imbibition behaviours.

Behaviors of non-wetting phase snap-off events in two-phase flow: microscopic phenomena and macroscopic effects

Behaviors of non-wetting phase snap-off events in two-phase flow: microscopic phenomena and macroscopic effects

Experimental investigations on water sorptivity in mortars with the use of X-ray micro-CT system

The phenomenon of water sorptivity in prismatic mortar specimens was investigated in this experimental work. This phenomenon is closely linked to material durability. Sorptivity evaluated during a steady stage, combined with exposure conditions can be used for performing service life predictions. Water's initial and secondary sorptivity in unsaturated mortar specimens were measured in laboratory tests following the standard ASTM C 1585–04. The impact of initial porosity on sorptivity was investigated by varying the cement-to-sand ratio (c/s) and the water-to-cement ratio (w/c). In addition, the surface area of water intake was changed which was either vertical or horizontal in orientation. The water distribution in the test specimens was visualized using the 3D X-ray micro-CT system SkyScan 1173 which was a novelty. The mortar sorptivity was found to be highly responsive to the c/s ratio, varying by up to 100%, and to be moderately impacted by the w/c ratio, varying by up to 25% and the surface inflow direction, varying by up to 45%. The sorptivity increased with a decline in the c/s (in agreement with the existing literature data) and w/c ratio (in contrast with the existing literature data). According to micro-CT images, the water distribution in the specimens was found to be non-uniform and larger porosity contributed to the rise in water level but not to the total volume of absorbed water (in contrast to the literature data). The snap-off events were detected ahead of the waterfront.

Evaporation induced acoustic emissions in microfluidic vessels.

Fluid flow processes such as drainage and evaporation in porous media are crucial in geological and biological systems. The motion of the displacement front of a moving fluid through multi-phase interfaces is often associated with abrupt mechanical energy release, detectable as acoustic emissions (AEs). The exact origin of these pulses and their damping mechanisms are still subjects of debate. Here, we study the characteristics of such AEs during evaporation of water from artificial microfluidic vessels, inspired by the physiology of vascular water-transport in plants. From the extracted settling times of the recorded AEs, we identify three pulse types and attribute their origins to bubble formation, snap-off events and rapid pore invasion. We also show that the resonance frequencies between 10 and 70 kHz present in specific pulse types decrease with increasing vessel radius (ranging from 0.25 to 1.0 mm) and length (ranging from 2.5 to 10.0 mm). Our findings provide insight into evaporation-induced AEs from microfluidic systems, and their potential use in non-invasive inspection or vascular health monitoring.

A New Fully Implicit Two-Phase Pore-Network Model by Utilizing Regularization Strategies

In this paper, we address the expensive computational cost resulting from limited time-step sizes during numerical simulations of two-phase flow in porous media using dynamic pore-network models. To overcome this issue, we propose a numerical method for dynamic pore-network models using a fully implicit approach. The proposed method introduces a regularization strategy considering the historical fluid configuration at the pore throat, which smooths the discontinuities in local conductivity caused by invasion and snap-off events. The results demonstrate the superiority of the proposed method in terms of accuracy, efficiency and consistency in comparison with other numerical schemes. With similar computational cost, determined by time-step sizes and number of Newton iterations, the developed method in this work yields more accurate results compared to similar schemes presented in the literature. Additionally, our results highlight the enhanced robustness of the our scheme, as it exhibits reduced sensitivity to variations in time-step sizes.

A two-dimensional transient study on the impact of pore space connectivity on the immiscible two-phase flow in a water-wet, water–oil system

A multi-relaxation time lattice-Boltzmann model is employed to investigate the dynamic evolution of two immiscible phases through an artificial, randomly generated, porous structure. The flow is driven by a constant pressure gradient, in the absence of gravitational effects. Constraining attention to two dimensions, the impact of the morphological properties of the porous structure, generated using constant radius circular solid grains, on a water-wet, oil–water two-phase flow is studied. Variations in the pore space connectivity and topology are quantified by the Euler characteristic. It is found that the wetting phase saturation and the degree of pore network homogeneity have a significant impact on the dynamic evolution of the non-wetting phase topology, which is governed by a series of coalescence and snap-off events. It is also observed that the phenomenal macro-scale steady-state based on the velocity field does not also imply a temporal topological invariance of the displaced phases. The impact of the pore space morphology on the transient dynamics of the two-phase flow is monitored and quantified through a series of hydrodynamic and topological parameters that signify the underlying flow transport processes.

Entrapment and mobilization dynamics during the flow of viscoelastic fluids in natural porous media: A micro-scale experimental investigation

Capillary desaturation process was investigated as a function of wetting phase rheological signatures during the injection of Newtonian and non-Newtonian fluids. Two sets of two-phase imbibition flow experiments were conducted on a water-wet sandstone core sample using brine and viscoelastic polymer solutions. During the experiments, a high-resolution micro-computed tomography scanner was employed to directly map pore-level fluid occupancies within the pore space. The results of the experiments revealed that at a given capillary number, the viscoelastic polymer was more efficient than the brine in recovering the non-wetting oil phase. At low capillary numbers, this is attributed to the improved accessibility of the viscoelastic polymer solution to the entrance of pore elements, which suppressed snap-off events and allowed more piston-like and cooperative pore-body filling events to contribute to oil displacement. For intermediate capillary numbers, the onset of elastic turbulence caused substantial desaturation, while at high capillary numbers, the superimposed effects of higher viscous and elastic forces further improved the mobilization of the trapped oil ganglia by the viscoelastic polymer. In the waterflood, however, the mobilization of oil globules was the governing recovery mechanism, and the desaturation process commenced only when the capillary number reached a threshold value. These observations were corroborated with the pore-level fluid occupancy maps produced for the brine and viscoelastic polymer solutions during the experiments. Furthermore, at the intermediate and high capillary numbers, the force balance and pore-fluid occupancies suggested different flow regimes for the non-Newtonian viscoelastic polymer. These regions are categorized in this study as elastic-capillary- and viscoelastic-dominated flow regimes, different from viscous-capillary flow conditions that are dominant during the flow of Newtonian fluids. Moreover, we have identified novel previously unreported pore-scale displacement events that take place during the flow of viscoelastic fluids in a natural heterogeneous porous medium. These events, including coalescence, fragmentation, and re-entrapment of oil ganglia, occurred before the threshold of oil mobilization was reached under the elastic-capillary-dominated flow regime. In addition, we present evidence for lubrication effects at the pore level due to the elastic properties of the polymer solution. Furthermore, a comparison of capillary desaturation curves generated for the Newtonian brine and non-Newtonian viscoelastic polymer revealed that the desaturation process was more significant for the viscoelastic polymer than for the brine. Finally, the analysis of trapped oil clusters showed that the ganglion size distribution depends on both the capillary number and the rheological properties of fluids.

Micro-scale experimental investigation of the effect of pore space deformation on trapping of supercritical CO2

In this study, we investigate the impact of pore space deformation on various trapping mechanisms of supercritical CO2 (scCO2) relevant to storage of CO2 in deep saline aquifers. A series of unsteady-state two-phase brine/scCO2 drainage, imbibition, and dissolution tests were performed on a miniature sandpack sample under various stress conditions. High-resolution x-ray microtomography techniques were employed to acquire the pore and fluid occupancy maps at each step of the experiment, which were subsequently utilized to characterize the pore space deformation and changes in the frequencies of various pore-scale fluid displacement events. Our results revealed that, during the scCO2 injection stage, additional scCO2 was stored in the sandpack sample after the sandpack was compressed due to an increase in the pressure gradient even though the pore volume had decreased. On the other hand, the capillary trapping was impaired by the pore space contraction because all the imbibition displacement mechanisms at the pore scale, including the piston-like advance and snap-off, were facilitated as a consequence of an increase in their threshold capillary pressures. The increased frequency of snap-off events caused additional trapping, but more importantly, displaced scCO2 from throats by fragmenting large residual scCO2 clusters into smaller globules. During the dissolution process, the trapped scCO2 clusters and globules shrank in both pore bodies and throats, resulting in movement of the Main Terminal Meniscus (MTM) interfaces and snap-off events, respectively. The water relative permeability during the dissolution process decreased, at a fixed saturation, with the increase in the effective stress since the small pores occupied by the water phase deformed more significantly than the large pores.

Morphological and Euler characteristics of nonwetting phases in porous media

The geometric structure variation of the nonwetting phase (NWP) in porous media plays a critical role in numerous energy-related processes. This work conducted a series of gas/liquid displacements in three rock cores using high resolution x-ray microcomputed tomography (micro-CT), including two sandstones and a Berea core with different pore architectures. The morphological and topological characteristics of NWP were investigated at the pore scale in a full cycle of primary drainage (PD), main imbibition (MI), and main drainage (MD). It is found that roof and distal snap-off occurred during drainage, resulting in nonwetting (NW) singlets and ganglia. These NW bubbles occupied 10%–20% NWP volume. Both throat snap-off and pore snap-off were observed in MI, which generally occur at a single-pore scale. The effect of viscous force is significant even at a capillary dominant displacement. The trapped NWP during PD can be partially or totally displaced in MI through further snap-off events or viscous remobilization. Due to the temporal and spatial scale diversion in pore scale events, the amplitude of pressure drop fluctuations among PD, MI, and MD is different. The significant difference in NWP characteristics between PD and MD is connectivity, thus leading to hysteresis.

Using Direct Numerical Simulation of Pore-Level Events to Improve Pore-Network Models for Prediction of Residual Trapping of CO2

Direct numerical simulation and pore-network modeling are common approaches to study the physics of two-phase flow through natural rocks. For assessment of the long-term performance of geological sequestration of CO2, it is important to model the full drainage-imbibition cycle to provide an accurate estimate of the trapped CO2. While direct numerical simulation using pore geometry from micro-CT rock images accurately models two-phase flow physics, it is computationally prohibitive for large rock volumes. On the other hand, pore-network modeling on networks extracted from micro-CT rock images is computationally efficient but utilizes simplified physics in idealized geometric pore elements. This study uses the lattice-Boltzmann method for direct numerical simulation of CO2-brine flow in idealized pore elements to develop a new set of pore-level flow models for the pore-body filling and snap-off events in pore-network modeling of imbibition. Lattice-Boltzmann simulations are conducted on typical idealized pore-network configurations, and the interface evolution and local capillary pressure are evaluated to develop modified equations of local threshold capillary pressure of pore elements as a function of shape factor and other geometrical parameters. The modified equations are then incorporated into a quasi-static pore-network flow solver. The modified model is applied on extracted pore-network of sandstone samples, and saturation of residual trapped CO2 is computed for a drainage-imbibition cycle. The modified model yields different statistics of pore-level events compared with the original model; in particular, the occurrence of snap-off in pore-throats is reduced resulting in a more frontal displacement pattern along the main injection direction. Compared to the original model, the modified model is in closer agreement with the residual trapped CO2 obtained from core flow experiments and direct numerical simulation.

Benchmarking the Viability of 3D Printed Micromodels for Single Phase Flow Using Particle Image Velocimetry and Direct Numerical Simulations

Holistic understanding of multiphase reactive flow mechanisms such as CO2 dissolution, multiphase displacement, and snap-off events is vital for optimisation of large-scale industrial operations like CO2 sequestration, enhanced oil recovery, and geothermal energy. Recent advances in three-dimensional (3D) printing allow for cheap and fast manufacturing of complex porosity models, which enable investigation of specific flow processes in a repeatable manner as well as sensitivity analysis for small geometry alterations. However, there are concerns regarding dimensional fidelity, shape conformity and surface quality, and therefore, the printing quality and printer limitations must be benchmarked. We present an experimental investigation into the ability of 3D printing to generate custom-designed micromodels accurately and repeatably down to a minimum pore-throat size of 140 μm, which is representative of the average pore-throat size in coarse sandstones. Homogeneous and heterogeneous micromodel geometries are designed, then the 3D printing process is optimised to achieve repeatable experiments with single-phase fluid flow. Finally, Particle Image Velocimetry is used to compare the velocity map obtained from flow experiments in 3D printed micromodels with the map generated with direct numerical simulation (OpenFOAM software) and an accurate match is obtained. This work indicates that 3D printed micromodels can be used to accurately investigate pore-scale processes present in CO2 sequestration, enhanced oil recovery and geothermal energy applications more cheaply than traditional micromodel methods.

Dynamics of water injection in an oil-wet reservoir rock at subsurface conditions: Invasion patterns and pore-filling events.

We use fast synchrotron x-ray microtomography to investigate the pore-scale dynamics of water injection in an oil-wet carbonate reservoir rock at subsurface conditions. We measure, in situ, the geometric contact angles to confirm the oil-wet nature of the rock and define the displacement contact angles using an energy-balance-based approach. We observe that the displacement of oil by water is a drainagelike process, where water advances as a connected front displacing oil in the center of the pores, confining the oil to wetting layers. The displacement is an invasion percolation process, where throats, the restrictions between pores, fill in order of size, with the largest available throats filled first. In our heterogeneous carbonate rock, the displacement is predominantly size controlled; wettability has a smaller effect, due to the wide range of pore and throat sizes, as well as largely oil-wet surfaces. Wettability only has an impact early in the displacement, where the less oil-wet pores fill by water first. We observe drainage associated pore-filling dynamics including Haines jumps and snap-off events. Haines jumps occur on single- and/or multiple-pore levels accompanied by the rearrangement of water in the pore space to allow the rapid filling. Snap-off events are observed both locally and distally and the capillary pressure of the trapped water ganglia is shown to reach a new capillary equilibrium state. We measure the curvature of the oil-water interface. We find that the total curvature, the sum of the curvatures in orthogonal directions, is negative, giving a negative capillary pressure, consistent with oil-wet conditions, where displacement occurs as the water pressure exceeds that of the oil. However, the product of the principal curvatures, the Gaussian curvature, is generally negative, meaning that water bulges into oil in one direction, while oil bulges into water in the other. A negative Gaussian curvature provides a topological quantification of the good connectivity of the phases throughout the displacement.

A pore-scale experimental investigation of process-dependent capillary desaturation

A series of micro-scale core-flooding experiments were conducted in a miniature water-wet Berea sandstone sample to examine how changes in the desaturation (water-displacing-oil) protocol impact the non-wetting phase recovery and to provide, to the best of our knowledge, the first pore-scale evidences of the associated displacement subtleties. We studied (i) the continuous displacement mode (CDM), which consisted of a series of imbibition tests with increasing capillary numbers and similar initially connected oil, and (ii) the discontinuous displacement mode (DDM), which included a drainage followed by a multi-rate imbibition test. During all experiments, a cutting-edge imaging platform was employed to visualize the fluid occupancies in the pore space.The in-situ saturation measurements showed that, at a given capillary number, the non-wetting phase recovery was more efficient under the continuous mode during which the oil phase was connected prior to each imbibition. This is attributed to the changes in the order by which the pore-scale displacement events take place before entrapment. For the DDM, however, the entrainment of oil clusters is the governing recovery mechanism, and the desaturation process was observed only when a threshold capillary number was reached. These findings are supported by pore-scale fluid occupancy maps demonstrating piston-like and snap-off events during the capillary-driven flow processes as well as trapped cluster entrainment during the viscous-dominated flow tests. Moreover, this investigation presents previously unidentified pore-scale displacement events in natural porous media, such as fragmentation and coalescence of trapped non-wetting phase globules prior to entrainment.These mechanisms took place at intermediate capillary numbers and mostly under the discontinuous displacement mode before the threshold of mobilization was reached. The capillary desaturation process under CDM was found to be gradual and more pronounced compared to that under the DDM mode. It is also demonstrated that even though the pore-scale displacement mechanisms responsible for oil recovery are different for CDM and DDM, the viscous-dominated imbibition tests produce similar distributions of trapped oil clusters under both cases.

Sphere and cylinder contact mechanics during slip

Using molecular dynamics (MD) we study the dependency of the contact mechanics on the sliding speed when an elastically soft slab (block) is sliding on a rigid substrate with a sin(q0x) surface height profile. The atoms on the block interact with the substrate atoms by Lennard-Jones potentials. We consider contacts with and without adhesion. The contact area and the friction force are nearly velocity independent for small velocities (v < 0.25 m/s) in spite of the fact that the shear stress in the contact area is rather non-uniform. For the case of no adhesion the friction coefficient is very small. For the case of adhesion the friction coefficient is higher, and is mainly due to energy dissipation at the opening crack tip, where rapid atomic snap-off events occur during sliding. We comment on the role of elastic non-linearity on the change in the contact area with increasing tangential force.

Local instabilities during capillary-dominated immiscible displacement in porous media

Fully understanding the mechanism of pore-scale immiscible displacement dominated by capillary forces, especially local instabilities and their influence on flow patterns, is essential for various industrial and environmental applications such as enhanced oil recovery, CO 2 geo-sequestration and remediation of contaminated aquifers. It is well known that such immiscible displacement is extremely sensitive to the fluid properties and pore structure, especially the wetting properties of the porous medium which affect not only local interfacial instabilities at the micro-scale, but also displacement patterns at the macro-scale. In this review, local interfacial instabilities under three typical wetting conditions, namely Haines jump events during weakly-wetting drainage, snap-off events during strongly-wetting imbibition, and the co-existence of concave and convex interfaces under intermediate-wet condition, are reviewed to help understand the microscale physics and macroscopic consequences resulting in natural porous media. Cited as : Liu, Y., Iglauer, S., Cai, J., Amooie, M.A., Qin, C. Local instabilities during capillary-dominated immiscible displacement in porous media. Capillarity, 2019, 2(1): 1-7, doi: 10.26804/capi.2019.01.01

Microfluidic investigations of crude oil-brine interface elasticity modifications via brine chemistry to enhance oil recovery

Waterflooding has been widely adopted as an improved oil recovery technique since water is, in general, inexpensive and readily available. More recently, low salinity waterflooding (LSW) has been applied to oil production. LSW has successfully demonstrated higher recovery efficiencies than conventional water flooding, yet the underlying mechanism is still quite unclear. Extensive research has focused upon rock-fluid interactions under low salinity conditions, however very little work has considered interactions between oil and brine and how fluid-fluid interfacial properties are influenced by brine salinity and composition. To fill this gap, we have developed a microfluidic device for multiphase interfacial testing with length scales comparable to the pore scale of oil reservoir rocks. Unlike conventional core flooding tests, fluid-fluid phenomena can be easily visualized within microfluidic devices via optical microscopy. Oil snap-off, a phenomenon that represents a major dispersion mechanism, occurs when crude oil flows through porous media in the reservoir and induces trapping hysteresis, can be investigated within a well-controlled microfluidic flow-focusing geometry. The size of formed droplets is examined as a measure of the resistance to snap-off. It has been previously demonstrated that snap-off events are influenced by brine chemistry, oil composition, and the interaction time between crude oil and brine. Here, we show that asphaltenes in the oil phase contribute directly to the development of the elasticitic film. We also propose the reduction of interfacial elasticity by other polar components, such as naphthenic acids, which counteract asphaltene behavior over different time scales. The contribution of these effects on interfacial elasticity is shown to be mediated by both the composition and concentration of brine and oil.

A Study of Microscale Gas Trapping Using Etched Silicon Micromodels

Immobilization and trapping of carbon dioxide (CO2) enhances the security of geological storage. Trapping mechanisms have been characterized in four groups: structural, residual, dissolution, and mineralization. While structural trapping acts immediately when injection starts and is well investigated, the contribution of residual and dissolution trapping increases over storage time and these contributions need to be better understood for better predictions. This paper focuses on an experimental pore-scale investigation of residual and capillary trapping. CO2–water imbibition experiments were conducted in micromodels whose homogenous pore space is geometrically and topologically similar to Berea sandstone. Microvisual data, photographs and video footage, describes the trapping mechanism and, especially, the disconnection and shrinkage of the CO2 phase. Results show that depending on the flow rate of the imbibing water different trapping mechanisms are observed. Lower flow rates, comparable to the trailing edge of a CO2 plume, lead to more snap-off events and greater trapped residual saturation, whereas rates comparable to the near wellbore area during enhanced sequestration showed displacement of gas bubbles and greater dissolution that ultimately leads to very low or zero gas saturations. Furthermore, complete dissolution events showed that homogenous as well as heterogeneous dissolution occurs. Whereas the latter is subdivided into microbubble formation and dissolution on crevices or pore roughness, the former occurs without the influence of pore walls. Based on the observations we suggest that the type of rock and its roughness as well as the fines present at the CO2 brine interface are important factors determining the dissolution mechanism.

A mercury porosimeter for investigating capillary phenomena and microdisplacement mechanisms in capillary networks

Mercury porosimetry was studied in glass-etched micromodels with the aid of an experimental apparatus that enables the accurate measurement of capillary pressures and mercury saturations, as well as the observation of microdisplacement mechanisms at the pore level. The effect of fluid topology, pore size, and pore body to pore throat aspect ratio during quasi-static imbibition for the air—mercury system is demonstrated in terms of experimentally obtained capillary pressure curves. Imbibition is shown to be determined by the interplay of bond-withdrawal (snap-off in throats) and site-withdrawal (withdrawal from pores) processes. Under conditions of small variability in pore body and pre throat size and for relatively small pore body to pore throat aspect ratio, imbibition phenomena are controlled by the fluid topology in a deterministic manner. That is, withdrawal occurs first from pore throats by the snap-off mechanism and proceeds in pore bodies in a manner that preserves the continuity of the nonwetting phase (nwp). Critical capillary pressures were measured for the withdrawal of mercury from pores and throats under various configurations of capillary interfaces. Theoretical calculations were in qualitative agreement with experimental values. For fully saturated capillary networks, snap-off events in pore throats initiate the withdrawal of mercury. The conjecture that the frontal advance mechanism dominates mercury withdrawal over cluster growth at high initial mercury saturation is not valid for conditions whereby all faces of the pore network are exposed to a mercury sink.

Experimental determination of gas-bubble breakup in a constricted cylindrical capillary

To gain insight into the mechanisms of gas-foam generation in porous media, this work quantifies how smaller bubbles snap off from a single, larger bubble as it moves through a smooth, cylindrical constriction. Time to snap off, generated bubble size, and bubble velocity are obtained from viewing 16-mm movies taken of snap-off events. The bubble capillary number, Ca/sub GAMMA, is varied from 10/sup -5/ to 5 x 10/sup -2/. The authors study two narrowly constricted capillaries of different tube radii (neck radius/tube radius of 0.17 and 0.20) and one widely constricted capillary (neck radius/tube radius = 0.40). In the widely constricted capillary, time to breakup is proportional to Ca/sub GAMMA/sup -2/, and the generated bubble length is proportional to Ca/sub GAMMA/sup -1/. In the narrow constrictions, the same results apply above Ca/sub GAMMA = 5 x 10/sup -4/; below this critical Ca/sub GAMMA, time to breakup is independent of Ca/sub GAMMA but depends on the Ohnesorge number. Snap-off behavior with surfactant solutions is found to be similar to that in surfactant-free solutions.