P.L.J. Zitha

Enhanced Mass Transfer of CO2 into Water: Experiment and Modeling

Concern over global warming has increased interest in quantification of the dissolution of CO 2 in (sub-)-surface water. The mechanisms of the mass transfer of CO 2 in aquifers and of transfer to surface water have many common features. The advantage of experiments using bulk water is that the underlying assumptions to the quantify mass-transfer rate can be validated. Dissolution of CO 2 into water (or oil) increases the density of the liquid phase. This density change destabilizes the interface and enhances the transfer rate across the interface by natural convection. This paper describes a series of experiments performed in a cylindrical PVT-cell at a pressure range of p i = 10―50 bar, where a fixed volume of CO 2 gas was brought into contact with a column of distilled water. The transfer rate is inferred by following the gas pressure history. The results show that the mass-transfer rate across the interface is much faster than that predicted by Fickian diffusion and increases with increasing initial gas pressure. The theoretical interpretation of the observed effects is based on diffusion and natural convection phenomena. The CO 2 concentration at the interface is estimated from the gas pressure using Henrys solubility law, in which the coefficient varies with both pressure and temperature. Good agreement between the experiments and the theoretical results has been obtained.

Modeling of External Filter Cake Build-up in Radial Geometry

Abstract The problem of formation damage (i.e., permeability reduction due to injection of particulates), is a matter of interest in several engineering fields. In the previous attempts to model the external cake formation, cake thickness has been considered to be only dependent on time; even though in practical applications, the dependency of the cake profile on space can be important. In this article, a novel model has been developed to describe the steady state external filter cake thickness profile along the wellbore. A set of equations is derived from the force balance for a deposited particle on the cake surface and the volume conservation of the fluid in the wellbore. These equations are combined with Darcys law in radial geometry and the equation of flow in the wellbore, and solved numerically to obtain the cake thickness and fluid velocity profiles along the wellbore.

Density-driven Natural Convection in Dual Layered and Anisotropic Porous Media with Application for CO2 Injection Project

In this paper we investigate the mass transfer of CO2 injected into a layered and anisotropic (sub)-surface porous formation saturated with water. Solutions of carbon dioxide in water and oil are denser than pure water or oil. We perform our analysis to a rectangular part of the porous medium that is impermeable at the sides except at the top, which is exposed to CO2. For this configuration density-driven natural convection enhances the mass transfer rate of CO2 into the initially stagnant liquid. The analysis is done numerically using mass and momentum conservation laws and diffusion of CO2 into the liquid. This configuration leads to an unstable flow process. Numerical computations do not show natural convection effects for homogeneous initial conditions. Therefore a sinusoidal perturbation is added for the initial top boundary condition. It is found that the development of fingers is fastest for mass transfer enhancement by natural convection is largest for large anisotropy ratio’s and smaller for small ratios. It is found that the mass transfer increases and concentration front moves faster with increasing Rayleigh number if the high permeability layer is on top. Of particular interest is the case when the Rayleigh number for the high permeable layer is above the critical Rayleigh number (Racr = 40) and smaller than Racr for the low permeable layer. The results of this paper have implications in enhanced oil recovery and CO2 sequestration in aquifers.

Permeability reduction in porous materials by in situ formed silica gel

The effect of in situ formed silica gel on the permeability of a porous material was investigated experimentally. Gelling solutions of tetra-methyl-ortho-silicate (TMOS) and methanol in water were imbibed into dry sandstone plates and cured for several days. The permeability of the untreated sandstone is on the order of 1 μm2, whereas the intrinsic permeability of the silica alcogel is 5–6 orders of magnitude lower. The method of beam bending was employed to measure concurrently the permeability D and Young’s modulus Ep of cylindrical gel rods, prepared from the TMOS-based sol-gel solutions. Second, the permeabilities and moduli of the treated sandstones were measured. For both types of samples the gel structure was varied by varying the concentration of the TMOS in a solution and the pH of the water used. The parameters D and Ep follow from a detailed analysis of the measured relaxation of the load that is applied to the sample under constant deflection. In case of the gels, the relaxation was interprete...

Analysis of coupled mass transfer and sol-gel reaction in a two-phase system

The coupled mass transfer and chemical reactions of a gel-forming compound in a two-phase system were studied in detail. Tetra-methyl-ortho-silicate (TMOS) is often used as a precursor in sol-gel chemistry to produce silica gels in aqueous systems. TMOS can also be mixed with many hydrocarbons without chemical reaction, which allows for various applications in multiphase systems. In this study, TMOS was mixed with n-hexadecane and placed together with water in small cylinders. Upon contact of the mixture with the water, TMOS transfers completely to the aqueous phase where it forms a gel through a heterogeneous reaction. Nuclear magnetic resonance imaging and relaxation time measurements were employed to monitor the mass transfer of TMOS from the oleic to the aqueous phase. The longitudinal relaxation time (T1) was calibrated and used to determine the concentration of TMOS in n-hexadecane during the transfer. The mass transfer rate was obtained at various temperatures (25–45?°C) and for several initial concentrations of TMOS. In the aqueous phase a sharp decrease in the transversal relaxation time (T2) is observed which is attributed to the gel reaction, in particular the formation of methanol in the initial stage. The minimum in T2 indicates the gelation point, and was found to be strongly dependent on temperature and concentration.

Effects of Oil on Foam Generation and Propagation in Sandstone Porous Media

Foaming of nitrogen stabilized by C14-16 alpha olefin sulfonate in natural sandstone porous media, previously subject to water flooding, was studied experimentally. Foam was generated in-situ by co-injecting gas and surfactant solution at fixed foam quality. Effect of surfactant concentration on the foam strength and foam propagation was examined. X-ray CT scans were obtained to visualize the foam displacement process and to determine fluid saturation at different times. The experiments revealed that stable foam could be obtained in the presence of water-flood residual oil. CT scan images, fluid saturation profiles and mobility reduction factors demonstrated that foam exhibited a good mobility control in the presence of water-flood residual oil. This was further confirmed by a delay in the gas breakthrough. The experiments also proved that immiscible foam displaced additional oil from water-flooded sandstone cores, supporting the idea that foam is potentially an effective EOR method. Foam flooding provided an incremental oil recovery ranging from 13±0.5% of the oil initially in place for 0.1 wt% foam to 29±2% for 1.0 wt% foam. Incremental oil due to foam flow was obtained first by a formation of an oil bank and then by a long tail production due to transport of dispersed oil within the flowing foam. The oil bank size increased with surfactant concentration, but the dispersed oil regime was less sensitive to the surfactant concentration.

A CT Scan Study of Foam Flooding in Porous Media

Foam is widely used in oil and gas recovery operations as a mobility control and profile correction agent. A brief list of foam applications includes acid diversion during matrix stimulation, gas blocking and enhanced oil recovery. This paper aims to study the dynamics of foam flooding assisted liquid displacement in a porous media. We report core-flood experiments performed using Bentheimer sandstone and N2 foam with the aid of X-ray computed tomography. A detailed description of CT images and quantification of local fluid saturation revealed that foam is formed in-situ and giving a mobility control. Furthermore, oil can be produced by a liquid slug induced by this strong immiscible foam front.

Modeling of Foam Flow Using Stochastic Bubble Population Model and Experimental Validation

The transient foam flow, forward movement of foam front until breakthrough in a one dimensional flow, in an oil-free porous medium was studied using the stochastic bubble population (SBP) model. The premise of this model is that foam flow in porous media is a complex fluid and bubble generation is a stochastic process. The SBP foam model describes the net bubble generation using three parameters: maximum bubble density and bubble generation and destruction coefficients. The corresponding governing equations, a system of non-linear partial differential equations in the saturation, pressure and bubble density, were solved using the IMPES method. The sensitivity to the main physical parameters was also analyzed. It was found that increase of the maximum bubble density leads to generation of stronger foam, characterized by a slower foam propagation rate and a larger foam mobility reduction. The bubble generation coefficient Kg mainly controlled the foam generation rate such that a higher Kg led to a more rapidly increasing bubble density. We also provided a comparison between the numerically obtained saturation and pressure data with those obtained from the experiments at which foam was generated by co-injecting nitrogen and C14-16 alpha olefin sulfonate surfactant in Bentheimer sandstone. X-ray CT scans were also obtained to visualize the foam displacement process and to determine fluid saturation at different times. A good match was obtained between the numerical and the experimental data which confirms that the SBP foam model is robust and reproduces the main features of the transient behavior of foam flow in a homogeneous porous media.

Surfactant Induced Solubilization and Transfer Resistance in Gas-Water and Gas-Oil Systems

Typically, conventional reservoir simulators underestimate the recovery factor of heavy oil reservoirs under solution gas drive. We hypothesize that natural surfactants in oil (e.g. asphaltenes) cause this phenomenon in two ways: 1) by hindering the mass transfer rate of gas molecules through the gas-oil interface and 2) by enhancing the solubility of gas in the heavy oil. We investigate effect of surfactants on mass transfer rate of gas through gas-water interface and on the solubility of gas in oil. In bulk experiments, we observe that the addition of sodium dodecyl sulfate (SDS) does not influence the gas transfer rate while in the presence of a porous medium the growth of gas bubbles becomes increasingly difficult with increasing SDS concentration, which indicates that the interaction of the grain with fluids is an essential element in bubble growth in porous media. The effect a non-ionic surfactant on the solubility of methane in n-dodecane is also examined. The bubble point pressures of the gas oil surfactant system are determined experimentally.It is found that the bubble point pressures of the system decrease with increasing surfactant concentration, i.e., the surfactant enhances the solubility of methane in the oil.

Gas Permeability of Foam Films Stabilized by an Alpha Olefin Sulfonate (AOS) Surfactant

In the present study we examine the basic properties of single foam films prepared from alpha (C14‐C16) olefin sulfonate (AOS). The film thickness was measured as a function of the electrolyte (NaCl) concentration. Special attention was focused on the gas permeability of the films defined by permeability coefficient kf(cm/s). The influence of the film thickness and surfactant adsorption on kf was followed. Supporting surface tension experiments at different surfactant concentrations were performed to obtain the adsorption of AOS at air/aqueous solution interface at different surfactant and salt concentrations.