Stable Displacement Research Articles

Effects of fluid displacement pattern on complex electrical impedance in Berea sandstone over frequency range 104–106 Hz

ABSTRACTTo better understand the effect of fluid distribution on the electric response of rocks saturated with oil and brine, we conducted experimental studies on the complex electrical impedance in a Berea sandstone, together with in situ acquisitions of oil distribution images employing a high‐resolution medical X‐ray computed tomography. We performed two tests of brine displacement by oil under high (10 MPa) and low (5 MPa) pressures, which were accompanied by fingering and stable displacement patterns, respectively. The measured complex impedance data were fitted to the Cole model to obtain the resistance, capacitance, peak frequency of the imaginary impedance, and the exponent α of the rock–fluid system. With increasing oil saturation, the resistance showed an increasing trend, whereas the other three parameters decreased. The fingering displacement exhibited lower resistance and capacitance than the stable displacement. The analysis of the resistance changes using a simple parallel connection model indicates that there are more components of residual brine in parallel connections in the fingering pattern than in the stable displacement pattern at the same saturation. We also interpreted the normalised changes in the capacitance (or apparent dielectric constant) with respect to the oil saturation via an analysis of the shape factor of fluid distribution based on the Maxwell–Wagner–Brugermann–Hanai model. The changes in the shape factor suggest that the pinch‐off of the brine in parallel connection by the oil is a dominant mechanism reducing the capacitance. In the stable displacement, most of the connections in the brine phase are immediately pinched off by oil displacement front at a local oil saturation of 65%. Conversely, in the fingering displacement, there is a transition from the bulk or layered brine to the pinched‐off at a local oil saturation below 60%. The analyses indicate that the difference in the fluid distribution under different fluid conditions is responsible for the non‐Archie behaviour.

Hydrodynamics of fingering instability in the presence of a magnetic field

The hydrodynamics of two immiscible fluids in a rectangular Hele-Shaw cell under the influence of a magnetic field is studied, both theoretically and numerically. A linear stability analysis is conducted to determine the effect of magnetic fields on the formation of viscous fingers. As a result, an analytical solution is found to calculate the growth rate of perturbations. For numerical simulation of the two-phase flow, the interfacial tension is treated as a body force using the continuum surface force model and the interface tracking is performed by the volume of fluid method. The variations of the width and growth rate of fingers in an unstable displacement versus Hartmann number, a dimensionless number characterizing the strength of the applied magnetic field, are investigated. By varying the value of Hartmann number systematically, a suppressing effect on the formation of viscous fingers is observed. Consequently, it is detected that there exists a minimum Hartmann number preventing the formation of viscous fingers and ensuring a stable displacement. Our numerical simulations are in agreement with the results of the linear stability analysis and quantify the effect of magnetic fields in mitigating viscous fingering effects and improving the efficiency of the fluid displacement.

Uniqueness, repeatability analysis and comparative evaluation of experimentally determined MMPs

Miscible gas injection processes like CO2 flooding are recommended to be carried out at minimum miscibility pressure (MMP) for effective enhanced recovery of trapped oil. There are several methods for MMP determination, but slim tube and vanishing interfacial tension (VIT) are deployed experimental techniques in this study. Uniqueness and repeatability of slim tube determined MMP was tested by conducting 55 slim tube runs for three different Omani crude oil samples (L-721, MZE and N) using three different coil lengths of constant diameter and three different injection rates. VIT technique accuracy and reliability was evaluated by comparing VIT determined MMPs, for same three considered crude oil samples, with aforementioned different slim tube systems determined MMPs. A trend of decrease in MMP with increase in coil length was found. No unique trend was found between MMP and injection rate. Lowest MMP and highest recovery were observed with highest coil length and lowest injection rate. It may be attributed to more stable displacement process which results in earlier developed miscibility and almost complete displacement of remaining more coil length retained oil. Therefore, both slim tube design and procedure need to be standardized. PVT software predicted MMP close to lower coil length slim tube system determined MMP for sample L-721 and higher coil length measured MMP for sample MZE. VIT approach determined MMPs were found to lie in between the MMPs measured by slim tube system with medium and longer coil lengths for same three oil samples. Since VIT estimated MMPs are in close match with more liable slim tube design determined MMPs, this technique can be utilized as a reliable and cheap alternative compared to more expensive and time consuming slim tube technique for accurate MMP determination without any potential of significant error.

Localized lattice Boltzmann equation model for simulating miscible viscous displacement in porous media

A localized lattice Boltzmann equation (LBE) model for simulating the miscible viscous displacement in porous media is proposed. The Darcy’s law for flow and the convection–diffusion equation (CDE) describing the transport of solute are solved numerically by the present model. To ensure the local implementation of the collision process in this model, the pressure and concentration gradients are computed from the moments of the nonequilibrium distribution functions, which are of second-order accuracy. Consequently, the advantages of the lattice Boltzmann method (LBM) are retained. The model is validated with a stable displacement problem, and is employed to study the viscous fingering instability that occurs in the process of the miscible viscous displacement. The results agree well with previous studies. Furthermore, although the present model is an explicit scheme, it is interesting to find that it is capable of simulating the viscous displacement over a wide range of Péclet (Pe) numbers, indicating the superior stability of this model.

Uplift tests on full-scale pipe segment in lumpy soft clay backfill

Upheaval buckling of pipelines caused by thermal- and pressure-induced loading is an important issue in pipeline design. The uplift capacity of pipelines is determined by the pipe–soil interaction during pipeline upheaval in soil. Pipelines to be installed in soft clay are usually placed into trenches and then backfilled. In this paper, a set of test devices were developed and a series of full-scale model tests were carried out on a pipe segment buried in lumpy soft clay backfill, including backfilling tests, load-controlled uplift tests, and a displacement-controlled test. Eight total pressure transducers were embedded in the wall of the pipe segment to measure soil pressures on the pipe segment, and five linear variable differential displacement transducers (LVDTs) were arranged to record the vertical displacement of the pipe segment and the surface of the soft clay ground. The stabilizing force keeping the pipe segment in place during the backfilling process was found to fit a nearly linear relationship with the dimensionless undrained shear strength of soft clay. The variation of soil pressures on the pipe segment during uplift loading was significantly affected by the buried depth of the pipe segment and the undrained shear strength of the soil. For all present load-controlled tests in lumpy soft clay backfill, the test ultimate uplift resistances were only about 19%–81% of the results calculated by the Det Norske Veritas approach. Mainly due to the voids’ compression, shearing and strain softening of lumpy soft clay backfill, the difference between initial and stable displacements in a loading step for a load-controlled test or initial and stable loads in a displacement step for a displacement-controlled test is remarkable. The limits of uplift resistances are recommended for the instant and sustaining behaviors of the pipe segment, respectively.

Gas-assisted gravity drainage process for improved oil recovery in Bao Den fractured basement reservoir

Gas injection has been widely used for Improved Oil Recovery (IOR)/ Enhanced Oil Recovery (EOR) processes in oil reservoirs. Unlike the conventional gas injection (CGI) modes of CGI and Water Alternating Gas (WAG), the Gas-Assisted Gravity Drainage (GAGD) process takes advantage of the natural segregation of reservoir fluids to provide gravity stable oil displacement. It has been proved that GAGD Process results in better sweep efficiency and higher microscopic displacement to recover the bypassed oil from un-swept regions in the reservoir. Therefore, dry gas has been considered for injection in fractured basement reservoir, Bao Den (BD) oil field located in Cuu Long basin through the GAGD process application. This field, with a 5-year production history, has nine production wells and is surrounded by a strong active edge aquifer from the North-West and the South East flanks. The depth of basement granite top is about 2,800 mTVDss with a vertical oil column of 1,500m. The pilot GAGD project has been designed to test an isolated domain in the BD fractured basement reservoir where there is favorable reservoir conditions to implement GAGD. Both reservoir simulation and Lab test have been run and confirmed the feasibility and the benefit of GAGD project in the selected area.The Dry gas will be periodically injected through existing wellwith high water cut production that located in the isolated area. As the injected gas rises to the top to form a gas zone pushing GOC (gas oil contact) downward, and may push WOC (water oil contact) to lower part of this producer (or even away from bottom of the well bore) could lower down water cut when switch this well back to production mode. The matched reservoir model with reservoir and fluid properties have been used to implement sensitivity analysis, the result indicated that there is significantly oil incremental and water cut reduction by GAGDapplication. Many different scenarios have run to find the optimal reservoir performance through GAGD process. Among these runs, the optimal scenario, which has distinct target, requires high levels of gas injection rate to attain the maximum cumulative oil production.

A Quantitative Technique to Compare Experimental Observations and Numerical Simulations of Percolation in Thin Porous Materials

Quantitative validation of numerical simulations of percolation in porous media has always been challenging due to the stochastic nature of the material structure and morphology regardless of the numerical technique being used. In this article, we present a technique that allows for a quantitative comparison between numerical and experimental percolation. The experimental observations are of injection pressure and liquid intrusion within thin porous materials in a Hele-Shaw style test. The thin porous materials tested had a surface treatment such the contact angle was larger than \(90^{\circ }\) resulting drainage percolation. Flow rates were adjusted to encompass the range of capillary numbers for drainage flow patterns between the stable displacement and capillary fingering. In parallel to the experimental observations, a series of numerical simulations using a two-dimensional pore network model were performed mimicking the Hele-Shaw experiments. The material properties of the pore network realizations, pore size distribution, contact angle, and representative volume for the pore space were based on measurements of the porous materials tested. Boundary and initial conditions were matched between numerical simulations and experiments. To compare and validate the numerical simulation against the experiments, a new scaling of the dissipated energy during percolation in thin porous media was used. This scaling has been recently used to identify the transition between capillary fingering and stable displacement for drainage in different thin porous materials and provides a unique method for characterizing percolation in thin porous materials. Though the experiments and simulations presented in this article are for drainage, the technique described is equally applicable to imbibition. Excellent agreement is obtained between experiment and simulation with clear delineation between different types of thin porous materials.

Subdivision based mixed methods for isogeometric analysis of linear and nonlinear nearly incompressible materials

This paper addresses the use of isogeometric analysis to solve solid mechanics problems involving nearly incompressible materials. The present work is focused on extension of two-field mixed variational formulations in both small and large strains to isogeometric analysis. Inf–sup stable displacement–pressure combinations for mixed formulations are developed based on the subdivision property of NURBS. Stability and convergence properties of the proposed displacement–pressure combinations are illustrated by computing numerical inf–sup constants and error norms. The performance of the proposed formulations is assessed by studying several benchmark examples involving nearly incompressible and incompressible elastic and elasto-plastic materials in both small and large strain regime.

A visualization study on two-phase gravity drainage in porous media by using magnetic resonance imaging

Gravity drainage characteristics are important to improve our understanding of gas–liquid or liquid–liquid two-phase flow in porous media. Stable or unstable displacement fronts that controlled by the capillary force, viscous force, gravitational force, etc., are relevant features of immiscible two-phase flow. In this paper, three dimensionless parameters, namely, the gravity number, the capillary number and the Bond number, were used to describe the effect of the above mentioned forces on two-phase drainage features, including the displacement front and final displacing-phase saturation. A series of experiments on the downward displacement of a viscous fluid by a less viscous fluid in a vertical vessel that is filled with quartz beads are performed by using magnetic resonance imaging (MRI). The experimental results indicate that the wetting properties at both high and low capillary numbers exert remarkable control on the fluid displacement. When the contact angle is lower than 90°, i.e., the displaced phase is the wetting phase, the average velocity Vf of the interface of the two phases (displacement front velocity) is observably lower than when the displaced phase is the non-wetting phase (contact angle higher than 90°). The results show that a fingering phenomenon occurs when the gravity number G is less than the critical gravity number G’=Δμ/μg. Moreover, the higher Bond number results in higher final displacing-phase saturation, whereas the capillary number has an opposite effect.

Experimental study of stable imbibition displacements in a model open fracture. I. Local avalanche dynamics

We report the results of an experimental investigation of the spatiotemporal dynamics of stable imbibition fronts in a disordered medium, in the regime of capillary disorder, for a wide range of experimental conditions. We have used silicone oils of various viscosities μ and nearly identical oil-air surface tension and forced them to slowly invade a model open fracture at different constant flow rates v. In this first part of the study we have focused on the local dynamics at a scale below the size of the quenched disorder. Changing μ and v independently, we have found that the dynamics is not simply controlled by the capillary number Ca∼μv. Specifically, we have found that the wide statistical distributions of local front velocities, and their large spatial correlations along the front, are indeed controlled by the capillary number Ca. However, local velocities exhibit also very large temporal correlations, and these correlations depend more strongly on the mean imposed velocity v than on the viscosity μ of the invading fluid. Correlations between local velocities lead to a burstlike dynamics. Avalanches, defined as clusters of large local velocities, follow power-law distributions-both in size and duration-with exponential cutoffs that diverge as Ca→0, the pinning-depinning transition of stable imbibition displacements. Large data sets have led to reliable statistics, from which we have derived accurate values of critical exponents of the relevant power-law distributions. We have investigated also the dependence of their cutoffs on μ and v and related them to the autocorrelations of local velocities in space and time.

Theory and Implementation of a Two-Step Unconditionally Stable Explicit Integration Algorithm for Vibration Analysis of Structures

Time history analysis is becoming the routine process to quantify the response of the structure under dynamic loads. In this paper, a novel two-step unconditionally stable explicit integration algorithm, named Unconditional Stable Two-Step Explicit Displacement Method (USTEDM), is proposed for vibration analysis of structure. USTEDM is unconditionally stable, requires low memory, produces no overshoot, and is third order accurate. The accuracy and efficiency of USTEDM are presented and compared with other commonly used integration algorithms. The result shows that the proposed algorithm has superior performance and can be used efficiently in solving vibration response of civil engineering structure.

Low-voltage, High-tuning Range MEMS Variable Capacitor Using Closed-loop Control

A low-voltage, high-tuning range variable MEMS capacitor using parallel-plates electrostatic microactuators operated in close-loop is introduced in this paper. The structures were fabricated using an in-house 2-masks process on SOI wafers and the use of a closed-loop controller increases the stable displacements of the structure beyond the pull-in limitation along the full available gap (the limitation are the mechanical stoppers). Additionally, the structure has two pairs of actuators, in opposing directions, enabling bi-directional displacement leading to extended tuning range. The fabricated capacitors have a designed tuning range close to 17. A lower actuation voltage is also a main benefit for future integration on low-power devices (measured pull-in voltage of approximately 3.25V). The devices were characterized and a capacitance variation of 2.3pF was experimentally verified.

Interface evolution during radial miscible viscous fingering.

We study experimentally the miscible radial displacement of a more viscous fluid by a less viscous one in a horizontal Hele-Shaw cell. For the range of tested injection rates and viscosity ratios we observe two regimes for the evolution of the fluid-fluid interface. At early times the interface length increases linearly with time, which is typical of the Saffman-Taylor instability for this radial configuration. However, as time increases, the interface growth slows down and scales as ∼t(1/2), as one expects in a stable displacement, indicating that the overall flow instability has shut down. Surprisingly, the crossover time between these two regimes decreases with increasing injection rate. We propose a theoretical model that is consistent with our experimental results, explains the origin of this second regime, and predicts the scaling of the crossover time with injection rate and the mobility ratio. The key determinant of the observed scalings is the competition between advection and diffusion time scales at the displacement front, suggesting that our analysis can be applied to other interfacial-evolution problems such as the Rayleigh-Bénard-Darcy instability.

Fundamental investigation of foam flow in a liquid-filled Hele-Shaw cell

The relative immobility of foam in porous media suppresses the formation of fingers during oil displacement leading to a more stable displacement which is desired in various processes such as Enhanced Oil Recovery (EOR) or soil remediation practices. Various parameters may influence the efficiency of foam-assisted oil displacement such as properties of oil, the permeability and heterogeneity of the porous medium and physical and chemical characteristics of foam. In the present work, we have conducted a comprehensive series of experiments using customised Hele-Shaw cells filled with either water or oil to describe the effects of foam quality, permeability of the cell as well as the injection rate on the apparent viscosity of foam which is required to investigate foam displacement. Our results reveal the significant impact of foam texture and bubble size on the foam apparent viscosity. Foams with smaller bubble sizes have a higher apparent viscosity. This statement only applies (strictly speaking) when the foam quality is constant. However, wet foams with smaller bubbles may have lower apparent viscosity compared to dry foams with larger bubbles. Furthermore, our results show the occurrence of more stable foam-water fronts as foam quality decreases. Besides, the complexity of oil displacement by foam as well as its destabilizing effects on foam displacement has been discussed. Our results extend the physical understanding of foam-assisted liquid displacement in Hele-Shaw cell which is a step towards understanding the foam flow behaviour in more complex systems such as porous media.

Experimental study on nonmonotonicity of Capillary Desaturation Curves in a 2‐D pore network

AbstractImmiscible displacement in porous media is important in many applications such as soil remediation and enhanced oil recovery. When gravitational forces are negligible, two‐phase immiscible displacement at the pore level is controlled by capillary and viscous forces whose relative importance is quantified through the dimensionless capillary number Ca and the viscosity ratio M between liquid phases. Depending on the values of Ca and M, capillary fingering, viscous fingering, or stable displacement may be observed resulting in a variety of patterns affecting the phase entrapment. The Capillary Desaturation Curve (CDC), which represents the relationship between the residual oil saturation and Ca, is an important relation to describe the phase entrapment at a given Ca. In the present study, we investigated the CDC as influenced by the viscosity ratio. To do so, we have conducted a comprehensive series of experiments using a high‐resolution microscope and state‐of‐art micromodels to investigate the dynamics and patterns of phase entrapment at different Ca and M. By postprocessing of the experimental high‐resolution images, we calculated the CDC and quantified the effects of the Ca and M on the phase entrapment and number of blobs trapped in the micromodel and their size distributions during immiscible two‐phase flow. Our results show that CDCs are not necessarily monotonic for all M, and the physical mechanisms causing this nonmonotonic behavior are discussed.

Mobility Investigation of Nanoparticle-Stabilized Carbon Dioxide Foam for Enhanced Oil Recovery (EOR)

Enhanced oil recovery (EOR) can extend the life of an oil field by providing additional drive mechanism to the crude oil. The use of carbon dioxide (CO2) in EOR application has shown a good potential, but it has some weaknesses such as viscous fingering. Viscous fingering problem can be solved by reducing the CO2 gas mobility, which can be achieved by transforming the CO2 gas into surfactant-stabilized foam. However, surfactant-stabilized foam is not very stable under harsh reservoir condition, which could be handled by introducing nanoparticle-stabilized CO2 foam. Thus, this paper aims to investigate the mobility of nanoparticle-stabilized CO2 foam at varying brine salinity (1 - 4 wt%), concentration of AOS surfactant (0.01 - 1 wt%) and concentration of nanoparticle (0.05 - 1 wt%). The volumetric phase ratio was fixed at 8 CO2/aqueous. The sand pack foam flooding test was conducted to measure the effectiveness of the formulated foam to displace the oil inside the porous medium through mobility and oil recovery measurement. It was found that foam mobility is inversely proportional to oil recovery. Mobility decreased when increase of brine salinity, surfactant and nanoparticle concentration, which has increased the oil recovery. Thus, it is important to reduce the foam mobility for efficient displacement process, which could minimize viscous fingering and enhance the oil recovery. This could be achieved by increasing the viscosity of displacing fluid (foam) for more stable displacement in EOR application.

Fabrication, characteristics and electrical model of an ionic polymer metal-carbon nanotube composite

We develop an ionic polymer metal-carbon nanotube composite (IPMCC) actuator composed of a multiwalled carbon nanotube (MWCNT)/Nafion membrane sandwiched between two hybrid electrodes, composed of palladium, platinum and MWCNTs. The surface morphology and cross-sectional structure of the metal-carbon nanotube hybrid electrode were observed using scanning electron microscopy (SEM). SEM investigation indicated that the MWCNT layer can adhere very well with the platinum–palladium metal electrode, fill the cracks in the metal surface, and prevent the oxidation of nanoscale platinum particles. These observations show that the surface resistance of the total electrode is retained and the stability of electrode property is maintained. The displacement, blocking force and nonlinear current versus voltage (V–I) characteristics were measured. Compared with an ionic polymer metal composite (IPMC), the IPMCC shows a more stable displacement and blocking force under 1, 1.2 and 2 V at 0.1 Hz, and 2.34–3.29 times higher effective air-operating time under 3 V at 0.1 Hz. It can be observed from the V–I characteristics that the change in shape becomes significant at amplitudes higher than 1.2 V. An equivalent circuit is used to model the nonlinear behavior of the IPMCC, in which the leakage current was taken into account and analyzed. The values of the components in the circuit are estimated and electrical behavior is simulated by using the Pspice software. Compared with the model with no consideration of the leakage current, the simulations obtained by the model considering leakage current showed better agreement with the experimental results. The impressive leakage current (20 mA), which is successfully simulated by the proposed model with the nonlinear circuit, is shown to play an important role in the total current.

Heat and mass transfer in melting porous media: Stable miscible displacements

Changes in the porosity and permeability of a porous medium due to melting are modeled. A frozen phase, which initially fills a part of the porous medium, melts and gets dissolved in the injected hot solvent. The amount of melted material, the rate of melting as well as the profiles of temperature, porosity, and concentration are analyzed to understand the nature of this naturally and industrially important phenomenon. Four reference scenarios corresponding to instantaneous thermal equilibrium, no heat transfer to the frozen phase, no-melting, and instantaneous melting conditions are solved analytically and the effects of different parameters are discussed. Numerical simulation results show that the profiles of the fluid temperature, porosity of the medium, and solvent concentration, form three fronts moving at different rates. It is found that for heat transfer coefficients above a certain value, the rate of melting is independent of this parameter and the system can be considered to have reached instantaneous thermal equilibrium. Moreover, slow heat transfer in the medium is shown to increase the rate of melting at long time periods by involving larger areas in the melting process. Heterogeneous scenarios are also analyzed by introducing frozen blocks of different geometries. The ability of the flow to bypass the frozen region involves a new heat transfer mechanism identified as outer-boundary convection. The effects of the geometry of the block along with the other parameters on the melting process are examined. Furthermore a generalizing scheme is proposed to predict the melt production for different block geometries and values of the saturation of the frozen phase.

Lattice Boltzmann simulation of immiscible fluid displacement in porous media: Homogeneous versus heterogeneous pore network

Injection of anthropogenic carbon dioxide (CO2) into geological formations is a promising approach to reduce greenhouse gas emissions into the atmosphere. Predicting the amount of CO2 that can be captured and its long-term storage stability in subsurface requires a fundamental understanding of multiphase displacement phenomena at the pore scale. In this paper, the lattice Boltzmann method is employed to simulate the immiscible displacement of a wetting fluid by a non-wetting one in two microfluidic flow cells, one with a homogeneous pore network and the other with a randomly heterogeneous pore network. We have identified three different displacement patterns, namely, stable displacement, capillary fingering, and viscous fingering, all of which are strongly dependent upon the capillary number (Ca), viscosity ratio (M), and the media heterogeneity. The non-wetting fluid saturation (Snw) is found to increase nearly linearly with logCa for each constant M. Increasing M (viscosity ratio of non-wetting fluid to wetting fluid) or decreasing the media heterogeneity can enhance the stability of the displacement process, resulting in an increase in Snw. In either pore networks, the specific interfacial length is linearly proportional to Snw during drainage with equal proportionality constant for all cases excluding those revealing considerable viscous fingering. Our numerical results confirm the previous experimental finding that the steady state specific interfacial length exhibits a linear dependence on Snw for either favorable (M ≥ 1) or unfavorable (M < 1) displacement, and the slope is slightly higher for the unfavorable displacement.

Позитивные отклонения в праве

The article aims to identify the positive potential of the deviations in law. The authors explore issues related to the definition and essence of positive deviations in law, identification of causes, factors and conditions of their occurrence, elaboration on mechanisms for the development of positive deviations in law; identification of the consequences of such deviations. Special attention is given to self-regulation and civil initiatives as the favourable environment for the emergence of positive deviations in law and legal technologies of registering such deviations. The authors emphasize the interrelationship between the legal models and legal deviations. Their coexistence is inevitable, they are inextricably linked, and only their combined study is able to explain the processes of interest. Positive deviations in law are not per se, but only from the point of view of the given legal models. The recognition of deviations as positive depends on the assessment of the relevant legal model. The authors offer their own definition of positive deviations in law as the stable displacement of means of legal regulation from regulatory purposes, which resulted in unforeseen socially useful results. The authors conclude that positive deviations in law facilitate the selection of variants of lawful behavior and legal discretion, and are a source of lawmaking, as well as they promote correction of archaic legal models or creation of new ones.