Pore-scale Analysis Research Articles

Pore-scale influence of methane hydrate on permeability of porous media

The influence of methane hydrate on the permeability of hosting sediments is critical to understand natural hydrate formation and gas production. Reported experimental data from tests on laboratory synthesized hydrate-bearing specimens and hydrate-bearing cores retrieved from natural reservoirs spread in a large range, and often deviates from predictions of a few conceptual models. This is very likely due to the mismatch between the actual hydrate pore habits and distribution and the overly simplified geometric assumptions used in those models. This study simulates single phase flow through hydrate-bearing sediments with a numerical approach to explore influences of hydrate on fluid flow, based on real 3D pore structures of methane hydrate-bearing sediments obtained via micro-CT scans. Pore-scale analysis and observations show (1) hydrate particles, at low hydrate saturation, protrude into the flow channels and efficiently inhibit the flow; (2) at high saturation or in small pores, hydrate particles block some pores and pores without hydrate determine permeability. Influence of pore habit in single pores on permeability is not obvious; by contrast, distribution of hydrate in the overall pore network has a higher impact and can cause drastic permeability anisotropy. Heterogeneity in both hydrate distribution and the sediments can explain the widespread range of relative permeability as a function of hydrate saturation.

Pore-scale analysis of relations between seepage characteristics and gas hydrate growth habit in porous sediments

Seepage characteristics of gas hydrate in quartz sands play important roles in gas production from hydrate-bearing sediments. Reported seepage characteristics varied from different experimental conditions due to different growth habits of gas hydrate in pores. It is critical to capture the essence of seepage variation in the presence of gas hydrate as can be employed in the prediction of long-term hydrate exploitation. In this work, the gas hydrate with different growth habits (grain-contacting and pore-filling) is synthesized in quartz sands porous media. The pore structure characteristics of the hydrate-bearing porous media has been identified by X-ray computed tomography (X-ray CT). Experimental results show that the formation and growth of gas hydrate in pores lead to the occurrence of non-interconnected pores, further limiting the validity of pore structure analyzation and the accuracy of permeability estimation. Hence, the interconnectivity degree with definite physics meaning is proposed to clarify the variation characteristics of pore structure. According to the linear correlation of interconnectivity degree with hydrate saturation, a modified permeability reduction model based on Kozeny grain model has been proposed to estimate the permeability variation of hydrate bearing porous media with different hydrate growth habits. The modified model yields a good prediction performance as is evaluated by a number of permeability measurements from experimental studies and the normalized mean squared error (NMSE). Given that the gas production from hydrate dissociation is a more complicated process which involves multi-phase fluid flow, the modified model can be further applied in permeability estimation during gas production from hydrate-bearing sediments.

Enhancement of oil recovery by emulsion injection: A pore scale analysis from X-ray micro-tomography measurements

The efficiency of emulsion injection as an enhanced oil recovery (EOR) method is investigated in a synthetic porous medium consisting of sintered bi-disperse glass beads, which emulates the porosity and permeability of real oil-bearing rocks. Synthetic seawater and an oil-in-water emulsion are successively used to displace a mineral oil which initially saturates the porous space. Micro-CT images with a 4 μm resolution are acquired at the end of the different stages of the process; the water phase is doped with KI to optimize the contrast between the liquid phases. Thus, three-dimensional (3D) images showing the beads, doped water and residual oil present a 3-modal histogram. After denoising with a non-local means filter, alignment and segmentation, the 3D images provide the spatial distributions of the water and oil phases, and thus allow comparing the populations of residual oil ganglia prior to and after the injection of the emulsion. Visual comparison of the spatial phase distributions show that the oil droplets of the oil-water emulsion divert the water path, mobilizing previously trapped oil ganglia. The probability density functions (PDFs) of different geometrical properties of the trapped oil ganglia (104 ganglia with volumes spanning 6 orders of magnitude) after water injection show well defined power law behaviors between a size corresponding to the typical pore throat and that typical of 10 typical pore volumes, and a few very large clusters of sizes between 10 and 5×105 typical pore volumes. The largest of them alone accounts for 97% of the trapped oil. The same PDFs after emulsion injection demonstrate successful fragmentation of these few very large ganglia, which in this case is the key to efficient EOR procedure through diversion of the water flow by emulsion oil droplets to less permeable regions.

A Mechanistic Pore-Scale Analysis of the Low-Salinity Effect in Heterogeneously Wetted Porous Media

Low-salinity (LS) waterflooding has been a topic of substantial recent interest in the petroleum industry. Studies have shown that LS brine injection can increase oil production relative to high-salinity (HS) brine injection, but contradictory results have also been reported and a mechanistic explanation of these findings remains elusive. We have recently developed a pore-scale model of LS brine injection in uniformly wetted networks (Watson et al. in Transp Porous Med 118:201–223, 2017), and we extend this approach here to investigate the low-salinity effect (LSE) in heterogeneously wetted media. We couple a steady-state fluid displacement model to an innovative tracer algorithm and track the evolving salinity front as oil and HS brine are displaced from the network. The wettability of the pore structure is locally modified where water salinity falls below a critical threshold, and simulations show that this can have significant consequences for oil recovery. Our results demonstrate that, for heterogeneously wetted networks, the oil-wet (OW) pores are the only viable source of incremental oil by LS brine injection. Moreover, we show that a LS-induced increase in the displaced OW pore fraction is a necessary, but not sufficient, condition to guarantee additional oil production. Simulations further suggest that the initial OW pore fraction, the average network connectivity and the initial HS brine saturation are factors that can determine the extent of incremental oil recovery following LS brine injection. This study clearly highlights that the mechanisms of the LSE can be markedly different in uniformly wetted and in non-uniformly wetted porous media.

Pore-Scale Analysis of Condensate Blockage Mitigation by Wettability Alteration

Liquid banking in the near wellbore region can lessen significantly the production from gas reservoirs. As reservoir rocks commonly consist of liquid-wet porous media, they are prone to liquid trapping following well liquid invasion and/or condensate dropout in gas-condensate systems. For this reason, wettability alteration from liquid to gas-wet has been investigated in the past two decades as a permanent gas flow enhancement solution. Numerous experiments suggest flow improvement for immiscible gas-liquid flow in wettability altered cores. However, due to experimental limitations, few studies evaluate the method’s performance for condensing flows, typical of gas-condensate reservoirs. In this context, we present a compositional pore-network model for gas-condensate flow under variable wetting conditions. Different condensate modes and flow patterns based on experimental observations were implemented in the model so that the effects of wettability on condensing flow were represented. Flow analyses under several thermodynamic conditions and flow rates in a sandstone based network were conducted to determine the parameters affecting condensate blockage mitigation by wettability alteration. Relative permeability curves and impacts factors were calculated for gas flowing velocities between 7.5 and 150 m/day, contact angles between 45° and 135°, and condensate saturations up to 35%. Significantly different relative permeability curves were obtained for contrasting wettability media and impact factors below one were found at low flowing velocities in preferentially gas-wet cases. Results exhibited similar trends observed in coreflooding experiments and windows of optimal flow enhancement through wettability alteration were identified.

Advances in the modeling of the soil–water characteristic curve using pore-scale analysis

A theoretical model for the prediction of the soil–water characteristic curve (SWCC) is presented using pore-scale analysis and three-dimensional approximations of pore geometry employing the concept of unit cells. The model considers the effect of particle size and packing porosity on funicular and pendular retention. The unit cell was upscaled to non-uniform soils using the grain-size distribution curve. SWCC experiments on uniform and graded glass beads were carried out to provide verification data. Predictions for the glass beads showed reasonable results for drying SWCCs. The nonionic surfactant Triton X-100 and sodium chloride were utilized to demonstrate the role of the contact angle and surface tension. The modeling of wetting curves using advancing contact angles indicated the need to consider additional hysteresis mechanisms. The hypothesis of independent grain-size fractions often underestimated matric suctions, leading to the proposal of a correction function using the coefficient of uniformity. The proposed approaches were compared to four previously published models, using 58 glass beads and sandy materials. The new models provided superior results for most of the materials studied. The proposed framework offers a general approach that may be modified in the future by including other retention mechanisms and unit cell geometries.

A Pore-Scale Investigation of the Transient Response of Forced Convection in Porous Media to Inlet Ramp Inputs

Abstract This paper investigates the transient response of forced convection of heat in a reticulated porous medium through taking a pore-scale approach. The thermal system is subject to a ramp disturbance superimposed on the entrance flow temperature/velocity. The developed model consisted of ten cylindrical obstacles aligned in a staggered arrangement with set isothermal boundary conditions. A few types of fluids, along with different values of porosity and Reynolds number, are considered. Assuming a laminar flow, the unsteady Navier Stokes and energy equations are solved numerically. The temporally developing flow and temperature fields as well as the surface-averaged Nusselt numbers are used to explore the transient response of the system. Also, a response lag ratio (RLR) is defined to further characterize the transient response of the system. The results reveal that an increase in amplitude increases the RLR. Nonetheless, an increase in ramp duration decreases the RLR, particularly for high-density fluids. Interestingly, it is found that the Reynolds number has almost negligible effects upon RLR. This study clearly reflects the importance of conducting pore-scale analyses for understanding the transient response of heat convection in porous media.

Digital core construction of fractured carbonate rocks and pore-scale analysis of acoustic properties

Fractures in carbonate rocks make the reservoir and its elastic properties complex, which creates challenges in the geophysical exploration of oil and gas resources. To study the elastic properties of fractured carbonate rocks, pore-scale numerical simulations and microscopic analyses are necessary for theoretically comprehensive analysis. First, a digital core of carbonate rock was established from X-ray CT scan using medium filtering and threshold segmentation method technologies. Then, some carbonate rock digital cores with different types of fractures were constructed by inserting fractures into scanning slices. Based on the theory of elastic mechanics, the elastic properties of carbonate rock cores with fractures were calculated using the finite-element method, and the effects of the microscopic factors on the rock elastic properties were studied in detail. The calculated stiffness-matrix elements of the digital core are consistent with the Gassmann fluid-substitution theory in isotropic rock. For the digital cores with coin-shaped and penetrating fractures, the influence of fracture density, fracture opening, and fracture length on the stiffness-matrix elements and the wave velocity were studied in detail through pore-scale simulations. This work provides a new perspective for carbonate rock physics and lays a foundation for multi-scale analysis of acoustic properties in carbonate rock reservoirs.

Pore-Scale Investigation on the Plugging Behavior of Submicron-Sized Microspheres for Heterogeneous Porous Media with Higher Permeability

Reservoir heterogeneity is regarded as one of the main reasons leading to low oil recovery for both conventional and unconventional reservoirs. High-permeability layers or fractures could result in ineffective water or gas injection and generate nonuniform profile. Polymer microspheres have been widely applied for the conformance control to overcome the bypass of injected fluids and improve the sweep efficiency. For the purpose of examining the plugging performance of submicron-sized microspheres in high-permeability porous media, systematic investigations were implemented incorporating macroscale blocking rate tests using core samples and pore-scale water migration analysis via nuclear magnetic resonance (NMR). Experimental results indicate that microsphere particle size dominates the plugging performance among three studied factors and core permeability has the least influence on the plugging performance. Subsequently, microsphere flooding was conducted to investigate its oil recovery capability. Different oil recovery behaviors were observed for cores with different permeability. For cores with lower permeability, oil recovery increased stepwise with microsphere injection whereas for higher permeability cores oil recovery rapidly increased and reached a plateau. This experimental work provides a better understanding on the plugging behavior of microspheres and could be employed as a reference for screening and optimizing the microsphere flooding process for profile control in heterogeneous reservoirs.

A comprehensive pore scale and volume average study on solid/liquid phase change in a porous medium

The pore scale and volume average analysis of the heat and fluid flow for a solid/liquid phase change in a 3D cubic Lattice Metal Frame (LMF) is performed numerically. The computational domain consists of 125 cells and the solid phase is consolidated. The governing equations which are continuity, momentum and energy equations for fluid phase and heat conduction equation for solid phase are solved. The volume average transport parameters are obtained from the pore scale results, then the volume average governing equations are also solved. A good agreement between the pore scale and volume average results is observed for the different Rayleigh numbers. A reverse heat transfer (heat transfer from Phase Change Material (PCM) to the solid frame) is observed in some cells (such as the center and upper cells) when the convection heat transfer is strong (i.e. Ra=106). In the reverse heat transfer region, the heat transfer rate between the solid frame and PCM takes negative value and the PCM temperature becomes greater than solid frame temperature. The change of sign of heat transfer rate and temperature difference between the solid frame and PCM does not occur simultaneously and this makes two discontinuities in the instant Nusselt number. Furthermore, the volume average governing equations are also solved by employing the interfacial heat transfer correlations reported in literature and the obtained results are compared with the results of present study. A good agreement between the results from pore scale analysis and the results based on some interfacial heat transfer correlations reported in literature is observed for wide range of Rayleigh number.

On the unsteady forced convection in porous media subject to inlet flow disturbances-A pore-scale analysis

Heat convection response of a porous medium to the harmonic disturbances in the inlet flow is investigated in a configuration consisting of several obstacles. Navier Stokes and energy equations are solved computationally and the average Nusselt number around the obstacles is favourably compared against the existing empirical data. The Nusselt number fluctuations are then examined, revealing that the dynamical relations between the inlet flow fluctuations as the input and those of Nusselt number as the output, can be nonlinear. The extent of encountered nonlinearity is determined quantitatively through introduction of a measure of nonlinearity. It is shown that increases in the pore-scale Reynolds number can strengthen the nonlinearity. However, this is not a global trend and further increases in Reynolds number may push the system dynamics back to linear. Application of the concept of transfer function to the identified linear cases reveals that the frequency response of the Nusselt number closely resembles a classical low-pass filter. Further, through a statistical analysis, it is shown that thermal response of the porous medium is strongly dominated by those of the first few obstacles. This highlights the importance of taking pore-scale approach in the dynamical problems that involve heat convection in porous media.

Significance of shape factor on permeability anisotropy of sand: representative elementary volume study for pore-scale analysis

This paper is a study of determination of representative elementary volume (REV) size suitable for pore-scale flow simulation (PFS) and evaluation of permeability anisotropy for sand based on Kozeny–Carman (KC) equation. We extracted small digitized samples for PFS from the 3D structures of Toyoura sand and glass beads obtained by X-ray computed tomography. Using the digitized samples with various sizes, we performed a series of flow simulations. We characterized the sample size using the ratio of the porous length L to the median grain diameter $$D_{50}$$ . The $$L/D_{50}$$ of REV was determined on the basis of statistical evaluation on coefficients of variations of the simulated values of void ratio, specific surface, hydraulic conductivity and hydraulic tortuosity. We confirmed that the hydraulic properties obtained from the PFS using the REV size determined agreed well with the past experimental studies. We also conducted PFSs in the horizontal direction to evaluate the permeability anisotropy. We clarified that the shape factor in the KC equation has a strong influence on permeability anisotropy even though previous studies treated hydraulic tortuosity as the only factor that affects the permeability anisotropy.

Pore-scale analysis of axial and radial dispersion coefficients of gas flow in macroporous foam monoliths using NMR-based displacement measurements

A micro-scale analysis of mass transport in ceramic foams that are used as catalyst supports in gas phase reactions is of high interest. Although the effects of flow rate and foam parameters on the radial and axial dispersion are known for liquid flows, no pore-scale experimental analysis has been yet reported to correlate the mechanical and diffusional dispersion of gas flows to the geometry of open-cell foams. Here, a spatially resolved Pulsed Field Gradient NMR method is applied to determine dispersion coefficients of thermally polarized gas along axial and transversal directions of open-cell foams. The comparative study of three commercial foam samples with different morphologies shows the effect of open porosity, window size, and flow rate on gas dispersion. Additionally, the influence of mechanical and diffusional dispersion at each flow rate is investigated for individual samples. By observing the transition from diffusional dispersion to mechanically driven dispersion of gas, it is found that diffusional dispersion plays an important role, even at higher flow rates after a transition from Darcy to Darcy-Forchheimer regime occurs. The measured values for dispersion coefficients of methane can be directly used in pseudo-heterogeneous models for the methanation reaction.

PORE-SCALE VISUALIZATION ON POLYMER FLOODING: APPLICATION OF SINGULAR VALUE DECOMPOSITION-BASED IMAGE ANALYSIS METHOD

Through years of working with visualization studies on micromodels and Hele-Shaw cells, we have always been confronted with difficulties such as numerous images of long flooding processes to be analyzed, irregularities of laser-etched micromodels that complicate the analysis of pore images, poor lighting, and so on. Finally, we aimed to customize a simple method addressing our difficulties. The method is based on singular value decomposition coupled with contour tracing and is developed using the MATLAB programming language. It is built with an approach that allows easy application of the experience of an expert when it is needed. Singular value decomposition (SVD) has been utilized because of its attractive properties that are useful to our objectives, including image compression and image denoising. The method is experimented on pore-scale visualization of polymer flooding in a micromodel, and it showed a reliable performance. In this paper, the pore-scale analysis is chosen rather than calculating the overall oil recovery factor because it has more details to investigate and helps better to evaluate the performance of the proposed method. We calculated the residual oil and connate water saturation as a case study. The method's unique procedure and features are explained step by step through the case study. The method facilitates the analysis by reducing the calculation time and the required storage space. Also, it offers an interesting component to decrease redundancies due to lighting problems.

Permeability Prediction using multivariant structural regression

A novel method for permeability prediction is presented using multivariant structural regression. A machine learning based model is trained using a large number (2,190, extrapolated to 219,000) of synthetic datasets constructed using a variety of object-based techniques. Permeability, calculated on each of these networks using traditional digital rock approaches, was used as a target function for a multivariant description of the pore network structure, created from the statistics of a discrete description of grains, pores and throats, generated through image analysis. A regression model was created using an Extra-Trees method with an error of <4% on the target set. This model was then validated using a composite series of data created both from proprietary datasets of carbonate and sandstone samples and open source data available from the Digital Rocks Portal (www.digitalrocksporta.org) with a Root Mean Square Fractional Error of <25%. Such an approach has wide applicability to problems of heterogeneity and scale in pore scale analysis of porous media, particularly as it has the potential of being applicable on 2D as well as 3D data.

Nuclear magnetic resonance and x-ray microtomography pore-scale analysis of oil recovery in mixed-porosity carbonates

With the recognition that carbonate reservoir rocks commonly contain a significant microporosity component (pore sizes ∼1 μm), there has been an ongoing effort to understand how this affects hydrocarbon recovery. This work reports x-ray microtomography (XMT) and nuclear magnetic resonance (NMR) data on two characteristic rock types. One is micropore dominated, and the other exhibits mixed-pore sizes. Several new techniques are introduced to accommodate investigation of full-sized core-plug samples having preserved wettability and to perform such studies with native fluids. First, the uses of high-field NMR techniques are demonstrated to determine porosity, overall oil and water content, and how these fluids are distributed between micro- and macropores. It is found that microporosity values agree with those obtained by optical (total pore system) analysis. Second, the use of xenon gas to tag the native oil phase for XMT imaging is demonstrated, showing how this helps determine contributions from pores that are smaller than the instrument resolution. Finally, a new method that uses NMR measurements of oil and water saturations to constrain the image analysis is demonstrated. The analysis shows the extent to which oil recovery occurs from the different pore systems. In both rock types, oil in the macroporosity is preferentially swept. Nevertheless, in agreement with a recent theory, oil recovery from the microporosity is significant. For the 70% microporosity sample, approximately one-third of the total oil recovery comes from the micropores, a factor that agrees well with theory.

Pore-scale analysis of coal cleat network evolution through liquid nitrogen treatment: A Micro-Computed Tomography investigation

Coalbed methane extraction suffers from low permeability, and liquid nitrogen treatment has been suggested as one of the methods to address this issue. This study thus investigates cryogenic liquid N2 fracturing of a bituminous coal at pore scale through 3D X-ray micro-computed tomography. The μ-CT results clearly demonstrate that freezing the coal with liquid nitrogen increases the porosity by over 11% and creates fracture planes with large apertures originating from the pre-existing cleats in the rock. The images also suggest connection establishment of the cleat network with originally isolated pores and micro-cleats following the freezing, thereby increasing pore network connectivity. Furthermore, SEM images of the frozen sample highlights the appearance of continuous wide conductive fractures with the maximum aperture size of 9 μm. The analysis of mechanical properties through Nano-indentation technique shows a decrease of up to 25% in the indentation modulus due to increase in the compressibility of the cracked rock. Moreover, as the main purpose of this fracturing treatment, the permeability evolution of the coal is examined computationally and experimentally. Lattice Boltzmann simulations on the μ-CT images highlight two-fold enhancement in permeability measure of the treated rock. Additionally, the outcome of core flooding tests shows 2.5 times increase in the permeability measure after liquid nitrogen exposure. This study thus provides a visual understanding of fracturing mechanism associated with liquid nitrogen treatment of a coal, and quantifies the pore structure and permeability evolution of the rock.

Pore-Scale Analysis of Calcium Carbonate Precipitation and Dissolution Kinetics in a Microfluidic Device.

In this work, we have characterized the calcium carbonate (CaCO3) precipitates over time caused by reaction-driven precipitation and dissolution in a micromodel. Reactive solutions were continuously injected through two separate inlets, resulting in transverse-mixing induced precipitation during the precipitation phase. Subsequently, a dissolution phase was conducted by injecting clean water (pH = 4). The evolution of precipitates was imaged in two and three dimensions (2-, 3-D) at selected times using optical and confocal microscopy. With estimated reactive surface area, effective precipitation and dissolution rates can be quantitatively compared to results in the previous works. Our comparison indicates that we can evaluate the spatial and temporal variations of effective reactive areas more mechanistically in the microfluidic system only with the knowledge of local hydrodynamics, polymorphs, and comprehensive image analysis. Our analysis clearly highlights the feedback mechanisms between reactions and hydrodynamics. Pore-scale modeling results during the dissolution phase were used to account for experimental observations of dissolved CaCO3 plumes with dissolution of the unstable phase of CaCO3. Mineral precipitation and dissolution induce complex dynamic pore structures, thereby impacting pore-scale fluid dynamics. Pore-scale analysis of the evolution of precipitates can reveal the significance of chemical and pore structural controls on reaction and fluid migration.

In situ pore-scale analysis of oil recovery during three-phase near-miscible CO2 injection in a water-wet carbonate rock

We study in situ three-phase near-miscible CO2 injection in a water-wet carbonate rock at elevated temperature and pressure using X-ray microtomography. We examine the recovery mechanisms, presence or absence of oil layers, pore occupancy and interfacial areas during a secondary gas injection process. In contrast to an equivalent immiscible system, we did not observe layers of oil sandwiched between gas in the centre of the pore space and water in the corners. At near-miscible conditions, the measured contact angle between oil and gas was approximately 73°, indicating only weak oil wettability in the presence of gas. Oil flows in the centres of large pores, rather than in layers for immiscible injection, when displaced by gas. This allows for a rapid production of oil since it is no longer confined to movement in thin layers. A significant recovery factor of 80% was obtained and the residual oil saturation existed as disconnected blobs in the corners of the pore space. At equilibrium, gas occupied the biggest pores, while oil and water occupied pores of varying sizes (small, medium and large). Again, this was different from an immiscible system, where water occupied only the smallest pores. We suggest that a double displacement mechanism, where gas displaces water that displaces oil is responsible for shuffling water into larger pores than that seen after initial oil injection. This is only possible since, in the absence of oil layers, gas can contact water directly. The gas-oil and oil-water interfacial areas are lower than in the immiscible case, since there are no oil layers and even water layers in the macro-pore space become disconnected; in contrast, there is a larger direct contact of oil to the solid. These results could serve as benchmarks for developing near-miscible pore-scale modelling tools.

Direct thermal visualization of micro-scale hydrogen evolution reactions in proton exchange membrane electrolyzer cells

The heat generated from electrochemical reactions has been considered one of the most significant issues in terms of the reliability of energy conversion devices. So far, no systematic study on the relation between heat generation and electrochemical reaction exists, especially in the form of experiments. In this study, changes of the temperature distribution and hydrogen evolution reaction (HER) on the catalyst coated membrane (CCM) in proton exchange membrane electrolyzer cells (PEMECs) are in-situ visualized with the help of a novel PEMEC design, thermal spectroscopy and high-speed visualization system. At the channel-scale, the temperature increases rapidly for most of the active areas, and finally reaches the equilibrium state at 27 °C. The temperature distribution is non-uniform throughout the process. In addition, a series of pore-scale analyses are provided to clarify the relation between the temperature distribution and electrochemical reaction area. More interestingly, the rapid heat generation areas are found to be in a good agreement with the electrochemical reaction areas, which confirms that the heat is released during the reaction processes. Finally, the temperature evolution phenomena on the LGDL surface have also been recorded. These findings could help better understand the correlation between the cathode side electrochemical reaction and heat generation.