Flow behaviour in the presence of wettability heterogeneities

The physical processes occurring during fluid flow and displacement within porous media having wettability heterogeneities have been investigated in specially designed heterogeneous visual models. The models were packed with glass beads, areas of which were treated with a water repellent to create wettability variations. Immiscible displacement experiments show visually the effect of wettability heterogeneities on the formation of residual oil and recovery due to capillary trapping. This work demonstrates by experiment the importance of incorporating reservoir heterogeneity into pore displacement analysis, essential for the correct interpretation of core data and for directing the route for scale-up of the processes to reservoir scale.     

Immiscible Displacement in Cross-Bedded Heterogeneous Porous Media

Direct insight into the mechanisms of flow and displacements within small-scale (cm) systems having permeability heterogeneities that are not parallel to the flow direction (cross-bedding and fault zones) have been carried out. In our experiments, we have used visual models with unconsolidated glass bead packs having carefully controlled permeability contrasts to observe the processes with coloured fluids and streamlines. The displacements were followed visually and by video recording for later analysis. The experiments show the significance that heterogeneities have on residual saturations and recovery, as well as the displacement patterns themselves. During a waterflood, high permeability regions can be by-passed due to capillary pressure differences, giving rise to high residual oil saturations in these regions. This study demonstrates the importance of incorporating reservoir heterogeneity into core displacement analysis, but of course the nature of the heterogeneity has to be known. In general, the effects created by the heterogeneities and their unknown boundaries hamper interpretation of flood experiments in heterogeneous real sandstone cores. Our experiments, therefore, offer clear visual information to provide a firmer understanding of the displacement processes during immiscible displacement, to present benchmark data for input to numerical simulators, and to validate the simulator through a comparison with our experimental results for these difficult flow problems.     

Experimental displacement patterns in a 2 × 2 quadrant block with permeability and wettability heterogeneities—problems for numerical modelling

The effect of heterogeneities on miscible and immiscible flood displacements in 2D bead packs in quadrant form, 2 × 2 block heterogeneity, with either a permeability or a wettability contrast is the subject of this paper. The physical processes occurring during miscible and immiscible flow and displacement within permeability and wettability quadrant bead pack models have been studied experimentally. This geometry occurs in a number of situations relevant to hydrocarbon production: particularly faults where adjacent rocks have large permeability contrasts with rapid changes, in the laboratory with core butting, in reservoir simulation where grid blocks have different permeability and in reservoirs having near-wellbore damage problems. The model quadrants 1–4, had 1 and 4 and 2 and 3 with identical properties, either in permeability or wettability. Reported are complete unit mobility miscible displacements, then the effects of viscosity differences (mobility modifiers) and finally immiscible displacements on displacement patterns for initial linear injection. The experiments demonstrate that nodal flow occurs for both miscible and immiscible flow, but for immiscible flow there are boundary effects due to capillary pressure differences created by water saturation changes or wettability contrasts which can leave patches of isolated fluid within a quadrant. The displacement patterns for the different models and fluids change significantly with the viscosity and wettability changes, particularly for the immiscible displacements. This is due to the changing capillary pressure between the quadrant blocks as the water saturation change. These are difficult to address in numerical modelling but should be accounted for. Other effects include coupling of all physical processes governing the flow through the node and creations of microzones of trapped residual oil. Our displacement patterns can therefore be a valuable verification benchmark tool for numerical modelling and a calibration data source for those wishing to simulate the effects of capillary pressure under differing wettability conditions and for those investigating upscaling modelling procedures. However, the possible loss of physical reality when averaging must always be considered.     

Fluid Flow Across Capillary Heterogeneities,Experiments and Simulations

A comparative study of numerical modelling and laboratory experiments of two-phase immiscible displacements in a 33 cm × 10 × 3 cm thick cross-bedded reservoir model is reported. Dynamic two-dimensional fluid saturation development was obtained from experiments by use of a nuclear tracer imaging technique and compared to numerical predictions using a full-field black oil simulator.The laboratory cross-bedded reservoir model was a sandpack consisting of two strongly waterwet sands of different grain sizes, packed in sequential layers. The inlet and outlet sand consisted of low permeable, high capillary, sand while the central crosslayer with a dip angle of 30° was a high permeable, low capillary, sand. Results on moderate contrasts in permeability and capillary heterogeneities in the cross-bedded reservoir model at different mobility ratios and capillary number floods temporarily showed a bypass of oil, resulting in a prolonged two-phase production. The final remaining oil saturations, however, were as for isolated samples. Hence, permanently trapped oil was not observed.Simulations of waterfloods, using a commercial software package, displayed correct water breakthrough at low flow rate and unity viscosity ratio, but failed in predicting local saturation development in detail, probably due to numerical diffusion.The simulator was used to test several cases of heterogeneity contrasts, and influence from different relative permeability curves. Further, by altering the capillary pressure at the outlet, the end effects were proven important.     

An Experimental Study on the Influence of Sub-Core Scale Heterogeneities on CO<Subscript>2</Subscript> Distribution in Reservoir Rocks

This article presents the results of CO₂/brine two-phase flow experiments in rocks at reservoir conditions. X-ray CT scanning is used to determine CO₂ saturation at a fine scale with a resolution of a few pore volumes and provide 3D porosity and saturation maps that can be use to correlate CO₂ saturations and rock properties. The study highlights the strong influence of sub-core scale heterogeneities on the spatial distribution of CO₂ at steady state and provides useful relative permeability data on a sample originated from an actual storage site (CO2CRC-Otway project, Victoria, South-West Australia). Two different samples tested, although different in nature, present strong heterogeneities, but differ in the detail of the connectivity of high porosity layers. In both samples, the results of the investigations show that sub-core scale heterogeneities control the sweep efficiency and may cause channeling through the porous medium. In one of the samples, CO₂ saturation appears uncorrelated to porosity close to the outlet end of the core. This observation is understood as a result of the position and the orientation of high porosity layers with respect to the inlet face of the core. Finally, in the operating conditions of the two experiments, the saturation maps demonstrate that gravity does not play a major role since no detectable buoyancy driven flow is observed.     

An Experimental Method for One Dimensional Permeability Characterization of Heterogeneous Porous Media at the Core Scale

Many reservoir simulator inputs are derived from laboratory experiments. Special core analysis techniques generally assume that core samples are homogeneous. This assumption does not hold for porous media with significant heterogeneities. This paper presents a new method to characterize core scale permeability heterogeneity. The method is validated by both numerical and experimental results. The leading idea consists in injecting a high viscosity miscible fluid into a core sample saturated with a low viscosity fluid. In such conditions, the fluid displacement is expected to be piston-like. We investigate the evolution of the pressure drop as a function of time. A continuous permeability profile is estimated along flow direction from the pressure drop assuming that the core sample is a stack of infinitely thin cross sections perpendicular to flow direction. Thus, we determine a permeability value for each cross section. Numerical and laboratory experiments are carried out to validate the method. Flow simulations are performed for numerical models representing core samples to estimate the pressure drop. The selected models are sequences of plugs with constant permeabilities. In addition, laboratory displacements are conducted for both low permeability and high permeability core samples. To investigate whether there is dispersion inside the porous medium, CT scan measurements are performed during fluid displacement: the location of the front is observed at successive time intervals. The results validate the methodology developed in this paper as long as heterogeneity is one dimensional.     

Simulation of three-phase displacement experiments

Three-phase displacement experiments for a water-benzyl alcohol-decane system are simulated. Literature experimental three-phase relative permeabilities for the system are used to describe the relative permeabilities in the three-phase region for different three-phase relative permeability models. Saturation trajectories and elliptical regions are mapped in the three-phase region. Simulations are performed to model displacement experiments including breakthrough and the formation of multiple shocks. The model can be used to predict the results for other displacements. In an experiment where significant gravity segregation is present, the displacement is more accurately modeled by assuming a uniform initial condition than by using the actual vertical saturation and assuming no cross flow. It is shown how different residual saturation values can be measured in the laboratory depending on the initial saturation conditions in the core. The experimental residual saturations can be significantly different than the ‘theoretical’ or model values.     

Exploring the relationship between critical state and particle shape for granular materials

The relationship between critical state and particle shape corresponds to the most fundamental aspect of the mechanics of granular materials. This paper presents an investigation into this relationship through macro-scale and micro-scale laboratory experiments in conjunction with interpretation and analysis in the framework of critical state soil mechanics. Spherical glass beads and crushed angular glass beads of different percentages were mixed with a uniform quartz sand (Fujian sand) to create a sequence of mixtures with varying particle shape. On the micro-scale, particle shape was accurately measured using a laser scanning technique, and was characterized by aspect ratio, sphericity and convexity; a new shape index, taken as the average of the three shape measures and referred to as overall regularity, was proposed to provide a collective characterization of particle shape. On the macro-scale, both undrained and drained triaxial tests were carried out to provide evidence that varying particle shape can alter the overall response as well as the critical states in both stress space and volumetric compression space. The mixtures of Fujian sand and spherical glass beads were found to be markedly more susceptible to liquefaction than the mixtures of Fujian sand and crushed angular glass beads. The change in liquefaction susceptibility was shown to be consistent with the change in the position of the critical state locus (CSL) in the compression space, manifested by a decrease in the intercept and gradient of the CSL due to the presence of spherical glass beads. Quantitative relationships have been established between each of the critical state parameters and each of the shape parameters, thereby providing a way to construct macro-scale constitutive models with intrinsic micro-scale properties built in.     

Mixing of cohesive particles in a shear cell

This paper discusses two series of experiments performed in a shear cell device with six different amounts of silicone oils and using 2-mm soda lime beads as the granular materials. The first series of experiments were mixing experiments, and the developments of mixing layer thicknesses were measured. The second series of experiments had the same experimental conditions as the first series but used different combinations of colored particles. In the second series of experiments, the motions of granular materials were recorded by a high-speed camera. Using the image processing technology and particle tracking method, the positions and velocities of particles were measured. The self-diffusion coefficient could be found from the history of the particle displacements.     

Multi-scale Optical Analyses of Dynamic Gas Saturation During Air Sparging into Glass Beads

A multi-scale optical imaging technique was developed allowing for the 2D observation of two phase flow in porous media at two different scales simultaneously: Using two coupled cameras, a 2D flow cell (0.5 × 0.5m²) is recorded entirely at the bench scale and at the pore scale with a spatial resolution of 0.5 and 0.01 mm, respectively. The technique is applied to study channelized gas flow in saturated glass beads. We analyze the phase distribution at the pore scale and derive a pixel-based method for the measurement of saturation at the larger scale. This method assumes linearity between the mean reflected light intensity and the local gas saturation if averaging is performed over representative areas (REV). The REV depends on the irregularity of the local pore structure and has a lower limit at the correlation length of the porous medium (somewhat above the size of the glass beads) and an upper limit which correlates with the width of gas channels. These limits could be quantified through optical analysis. The optical approach to estimate phase saturations was validated by gravimetric analysis where a characteristic ratio between the optically observed flow cell wall and the saturation within the bulk material was identified, which corresponds to the expectation based on geometrical considerations of the glass bead packing. Considering a transient flow experiment the optical method is demonstrated to be able to quantify the temporal evolution of the residual and the convective gas phase. We conclude that the new technique provides a valuable tool to improve our quantitative understanding of multiphase phenomena across different scales.     

Application of X-ray CT investigation of CO<Subscript>2</Subscript>–brine flow in porous media

A clear understanding of two-phase flows in porous media is important for investigating CO₂ geological storage. In this study, we conducted an experiment of CO₂/brine flow process in porous media under sequestration conditions using X-ray CT technique. The flow properties of relative permeability, porosity heterogeneity, and CO₂ saturation were observed in this experiment. The porous media was packed with glass beads having a diameter of 0.2 mm. The porosity distribution along the flow direction is heterogeneous owing to the diameter and shape of glass beads along the flow direction. There is a relationship between CO₂ saturation and porosity distribution, which changes with different flow rates and fractional flows. The heterogeneity of the porous media influences the distribution of CO₂; moreover, gravity, fractional flows, and flow rates influence CO₂ distribution and saturation. The relative permeability curve was constructed using the steady-state method. The results agreed well with the relative permeability curve simulated using pore-network model.     

Grain shear flow in a rotating drum

 In the present paper we report on the experimental activities carried out on a rotating drum partially filled with grains or glass beads. The experiments give information about rheology through velocity profiles and through the velocity covariance tensor structure. We used a LDV system to measure the velocity of the grains at several points along three vertical sections. The data were also used to obtain the grain volume concentration, with encouraging results. Instantaneous velocity data were elaborated in order to obtain velocity and pseudotemperature profiles for all the experiments; for a subset of the experiments a large set of data were elaborated to obtain the velocity covariance. The velocity covariance is not collinear with the rate of deformation tensor. An attempt to justify the rotation of the tensor axes as a consequence of the kinetically induced anisotropy and of some free surface perturbations slowly moving upstream was partially successful. Received: 23 April 1999 / Accepted: 15 July 2001     

Wrinkling of sandwich wide panels/beams based on the extended high-order sandwich panel theory: formulation, comparison with elasticity and experiments

There exist several high-order sandwich panel theories, most notably, the first to be introduced high-order sandwich panel theory (HSAPT) assumes a constant shear stress in the core. Recently, the extended high-order sandwich panel theory (EHSAPT) was introduced, its novelty being that it allows for three generalized coordinates in the core (the axial and transverse displacements at the centroid of the core, and the rotation at the centroid of the core) instead of just one (shear stress in the core) of the earlier theory. In this paper, the EHSAPT formulation for predicting the critical wrinkling load is presented for a simply supported sandwich of general asymmetric construction. The cases of (i) applying the loading just on the face sheets with a linear core assumption and (ii) applying uniform strain loading throughout the thickness of the panel and a nonlinear core assumption are examined. The results are compared with a benchmark elasticity solution. In addition, edgewise compression experiments were conducted on glass face/Nomex honeycomb core and the ensuing wrinkling point is compared with the theoretical predictions. A comparison is also made with earlier edgewise compression experiments on aluminum face/granulated-cork core reported in literature. Other wrinkling formulas that are included in the comparison are: Hoff–Mautner and the HSAPT.     

The Effect of Reservoir Wettability on the Production Characteristics of the VAPEX Process: An Experimental Study

The impact of fractional wettability on the production characteristics of a VAPEX process at the macroscale was investigated. Conventional VAPEX experiments were conducted in a 220 Darcy random packing of glass beads in a rectangular physical model and n-pentane was used to recover the Cold Lake bitumen from the oil-saturated model in the absence of connate water. The composition of oil-wet beads in the packed bed was altered from completely water-wet beads to completely oil-wet beads at different proportions of oil-wet beads mixed with water-wet beads. A substantial increase (about 40%) in the production rate of live oil was observed during the VAPEX process when the wettability of the porous packing was entirely oil-wet beads. A critical oil-wet fraction of 0.66 was found for the heterogeneous packing of water-wet and oil-wet beads of similar size distribution. Above this critical composition, the live oil production rate was not affected by further increase in the proportion of the oil-wet beads. It is believed that above this critical composition of the oil-wet beads, the crevice flow process is dominated by the continuity of higher conductivity live oil films between particles through the oil-wet regions. Below this critical composition, the live oil production rate increased linearly with the fraction of the oil-wet beads in the packing. The oil-wet regions favor the live oil drainage compared to that of the water-wet regions as they enhance the rate of imbibition of the live oil from the oil-filled pores to the vacated pores near the nominal VAPEX interface. These two factors enhance the live oil production rate during the VAPEX process. The solvent content of the live oil, the solvent-to-oil ratio (SOR), and the residual oil saturation did not correlate strongly with the proportion of the oil-wet beads in the packing. The average solvent content of the live oil and the residual oil saturation were measured to be 48% by weight and 7% by volume respectively.     

A piezoelectric biaxial loading device for interfacial fracture experiments

In this paper we describe the development of a new biaxial loading device for investigating mixed-mode fracture at bimaterial interfaces. The new device makes use of piezoelectric actuators and specially arranged flexures to provide independent displacements normal and tangential to the interface. Capacitive probes and special washer load cells were used for measuring the displacements and reactive loads, respectively. A closed-loop circuit was formed with a personal computer to control the applied displacements to within 10 nm. Preliminary experiments with quartz/epoxy/aluminum sandwich specimens with cracks growing between the quartz and the epoxy found that the intrinsic toughness of this interface was 30% lower than the value for a glass/epoxy interface. Crack opening interferometry measurements having a resolution of 30 nm revealed the presence of a cohesive zone whose size was about 0.5 μm.     

A Semi-Analytical Model for Two Phase Immiscible Flow in Porous Media Honouring Capillary Pressure

This article describes a semi-analytical model for two-phase immiscible flow in porous media. The model incorporates the effect of capillary pressure gradient on fluid displacement. It also includes a correction to the capillarity-free Buckley–Leverett saturation profile for the stabilized-zone around the displacement front and the end-effects near the core outlet. The model is valid for both drainage and imbibition oil–water displacements in porous media with different wettability conditions. A stepwise procedure is presented to derive relative permeabilities from coreflood displacements using the proposed semi-analytical model. The procedure can be utilized for both before and after breakthrough data and hence is capable to generate a continuous relative permeability curve unlike other analytical/semi-analytical approaches. The model predictions are compared with numerical simulations and laboratory experiments. The comparison shows that the model predictions for drainage process agree well with the numerical simulations for different capillary numbers, whereas there is mismatch between the relative permeability derived using the Johnson–Bossler–Naumann (JBN) method and the simulations. The coreflood experiments carried out on a Berea sandstone core suggest that the proposed model works better than the JBN method for a drainage process in strongly wet rocks. Both methods give similar results for imbibition processes.     

Modelling fluid flow in cross-bedded sections

Beadpack experiments and numerical simulations have been carried out to study flow displacements, effluent profiles and streamline patterns for layered systems with flow not parallel to the layers. The effects of layer thickness, permeability contrast, angle of layer to flow direction, mobility ratio and flood rate have been examined. Each of these parameters influence the displacement profiles, and disperse the flood front. Such real effects must be considered when subsuming reservoir heterogeneities in average reservoir parameters in simulation studies, or interpreting core tests.     

Immiscible Displacement in the Interacting Capillary Bundle Model Part I. Development of Interacting Capillary Bundle Model

An interacting capillary bundle model is developed for analysing immiscible displacement processes in porous media. In this model, pressure equilibration among the capillaries is stipulated and capillary forces are included. This feature makes the model entirely different from the traditional tube bundle models in which fluids in different capillaries are independent of each other. In this work, displacements of a non-wetting phase by a wetting phase at different injection rates were analysed using the interacting capillary bundle model. The predicted evolutions of saturation profiles were consistent with both numerical simulation and experimental results for porous media reported in literature which cannot be re-produced with the non-interacting tube bundle models.     

Preparation and ion exchange properties of egg-shell glass beads with different surface morphologies

A subcritical water treatment method was developed for preparing porous-surfaced glass beads with an egg-shell structure in a batch reactor. Based on the "corrosion-ion-migration-recondensation" strategy, ordinary soda-lime glass beads with a diameter of about 100 m were made first to react with subcritical water to effect controlled quantity of silicate dissolution of glass by adjusting treatment time and temperature. The dissolved silicate was then made to recondense on the glass core to form different porous shell morphologies: pores, flakes and fibers. Among these, glass beads coated with fibers with surface area of 154.5 m2/g, pore volume of 0.27 cm3/g and pore size of 7.1 nm were obtained at 573 K after 2 h of treatment. The prepared porous-surfaced glass beads were then used as adsorbent for heavy metal ions, showing various ion exchange properties. Glass beads covered with fibers displayed fast kinetics and high sorption capacity because of their egg-shell structure and high surface area. More than 90% of copper ions were adsorbed within 100 min from a solution with an initial concentration of 110 mg/L at 313 K. Ion sorption capacities were 149.33, 81.33 and 42.96 mg/g respectively for Ag+, Cu2+ and Ni2+ at 313 K. A green and low-cost method was thus developed to produce egg-shell-structured porous glass with high sorption capacity.     

Dispersion Stability and Transport of Nanohybrids through Porous Media

Single-walled carbon nanotube-silica nanohybrid particles are a very promising material that could be used for enhanced oil recovery because of their interfacial activity. To demonstrate the basic principle, aqueous nanohybrid particle dispersions were evaluated by looking at the effect of pH, surfactant, and polymer. The results showed that pH did not have significant effect on the dispersion stability of nanohybrid particles. Although surfactant could improve the dispersion stability, it reduced the interfacial activity of the nanohybrid particles, causing them to stay in the aqueous phase. The best nanohybrid particle dispersion stability was found upon polymer addition, where the dispersions were stable for more than a week even at low polymer concentration (50?ppm). One-dimensional sand-pack studies were performed to evaluate the flow of the nanohybrid particles through porous media. The results showed that most of the nanohybrid particles (>99%) could pass through a column packed with glass beads while a measurable fraction of the particles was retained in the column packed with crushed Berea. When the columns contained a residual saturation of decane, additional nanohybrid particles were retained at the oil/water interface in both glass beads and crushed Berea sand media. The sand pack studies showed that not only can the nanohybrid particles flow through porous media but also about half of the particles injected will go the O/W interface when the porous medium contains a residual saturation of hydrocarbon, where they could be used to support a catalytic conversion of components of the oil in reservoirs.