Christos D. Tsakiroglou

Characterization of the pore structure of reservoir rocks with the aid of serial sectioning analysis, mercury porosimetry and network simulation

Processes of fluid transport through underground reservoirs are closely related with microscopic properties of the pore structure. In the present work, a relatively simple method is developed for the determination of the topological and geometrical parameters of the pore space of sedimentary rocks, in terms of chamber-and-throat networks. Several parameters, such as the chamber-diameter distribution and the mean specific genus of the pore network are obtained from the serial sectioning analysis of double porecasts. This information is used in the computer-aided construction of a chamber-and-throat network which is to be used for further analysis. Mercury porosimetry curves are fitted to either 2-parameter or 5-parameter non-linear analytic functions which are identified by the median pressures, mean slopes and breakthrough pressures. A simulator of mercury intrusion/retraction, incorporating the results of serial tomography, in conjuction with the experimental mercury porosimetry curves of the porous solid are used iteratively to estimate the throat-diameter distribution, spatial correlation coefficients of pore sizes and parameters characterizing the pore-wall roughness. Estimation of the parameter values is performed by fitting the simulated mercury porosimetry curves to the experimental ones in terms of the macroscopic parameters of the analytic functions. The validity of the pore space characterization is evaluated through the correct prediction of the absolute permeability. The method is demonstrated with its application to an outcrop Grey-Vosgues sandstone.

Mercury intrusion and retraction in model porous media

Abstract Chamber-and-throat pore networks etched in glass (or plastic) are often used as model porous media to study the pore-scale mechanisms and their cooperative effects on macroscopic transport coefficients of important multiphase processes, such as hydrocarbon recovery from rock reservoirs, pollution of soils and aquifers by liquid organic wastes, etc. Such models have special built-in characteristics, notably, small pore depth (compared to the pore widths and diameters), equivalent capillary diameter that is nearly equal to the pore depth and has small variation and small chamber to throat equivalent capillary diameter ratio. Given that all these features strongly affect the various multiphase transport processes that are studied experimentally in model porous media, it is necessary to take them into account quantitatively. The present work develops a 2-D network mercury intrusion–retraction simulator that is adapted to the specific geometrical and topological characteristics of model pore networks. Based on experimental observations, the mercury meniscus motion in chambers and throats during intrusion and the mercury disconnection events during retraction are analyzed at pore-scale. Mercury intrusion or retraction in chamber-and-throat networks is simulated as a sequence of flow events occurring at progressively increasing or decreasing external pressures, respectively. It is found that the location and the degree of hysteresis of mercury intrusion-retraction curves are determined primarily by the depth of pores and the values of contact angles. The pressure of residual air may influence the high pressure region of intrusion curve and the low pressure region of retraction curve. Mercury retraction from the pore network is carried out through cluster growth, over a narrow pressure region and the residual mercury saturation is low because of the strong dependence of capillary pressures of retraction from pore chambers on the local fluid topology. The simulator predicts the experimental capillary pressure curves of two glass models satisfactorily; furthermore, the simulated patterns of mercury intrusion–retraction are in agreement with corresponding experimental ones. The experimentally validated simulator provides a reliable tool for the thorough characterization of the structure and the prediction of the capillary properties of model porous media as well as of real porous media with similar pore scale characteristics.

Pore Network Analysis of Resistivity Index for Water-Wet Porous Media

Pore network analysis is used to investigate the effects of microscopic parameters of the pore structure such as pore geometry, pore-size distribution, pore space topology and fractal roughness porosity on resistivity index curves of strongly water-wet porous media. The pore structure is represented by a three-dimensional network of lamellar capillary tubes with fractal roughness features along their pore-walls. Oil-water drainage (conventional porous plate method) is simulated with a bond percolation-and-fractal roughness model without trapping of wetting fluid. The resistivity index, saturation exponent and capillary pressure are expressed as approximate functions of the pore network parameters by adopting some simplifying assumptions and using effective medium approximation, universal scaling laws of percolation theory and fractal geometry. Some new phenomenological models of resistivity index curves of porous media are derived. Finally, the eventual changes of resistivity index caused by the permanent entrapment of wetting fluid in the pore network are also studied.Resistivity index and saturation exponent are decreasing functions of the degree of correlation between pore volume and pore size as well as the width of the pore size distribution, whereas they are independent on the mean pore size. At low water saturations, the saturation exponent decreases or increases for pore systems of low or high fractal roughness porosity respectively, and obtains finite values only when the wetting fluid is not trapped in the pore network. The dependence of saturation exponent on water saturation weakens for strong correlation between pore volume and pore size, high network connectivity, medium pore-wall roughness porosity and medium width of the pore size distribution. The resistivity index can be described succesfully by generalized 3-parameter power functions of water saturation where the parameter values are related closely with the geometrical, topological and fractal properties of the pore structure.

A new visualization technique for the study of solute dispersion in model porous media

A new technique of high resolution is developed to perform visualization experiments of the hydrodynamic dispersion of pollutants in transparent glass-etched pore networks, which are regarded as representative models of natural porous media and single fractures. The technique is based on the continuous detection of the sharp colour changes caused on an aqueous solution, as the solute concentration varies, because of the strong sensitivity of a system of indicators to pH. Image analysis is used for the transformation of the spatial distribution of colour intensity to solute concentration profiles. Unidirectional miscible displacement and single source-solute transport experiments are used to identify and quantify the transient and steady-state solute dispersion regimes in a pore network, and estimate the longitudinal and transverse dispersion coefficients as functions of Peclet number. The dispersion coefficients are estimated by fitting the spatial/temporal distribution of solute concentration over various regions of the network to analytic solutions of the convection–dispersion equation, obtained by using a flux-type boundary condition at solute sources. The experimental technique and the method of analysis of its results may be proved very useful for model validation, sensitivity analysis of dispersion coefficients with respect to pore space parameters, and identification of liquid pollutant dispersion regimes in underground aquifers.

Experimental study of the immiscible displacement of shear-thinning fluids in pore networks

The pore scale mechanisms and network scale transient pattern of the immiscible displacement of a shear-thinning nonwetting oil phase (NWP) by a Newtonian wetting aqueous phase (WP) are investigated. Visualization imbibition experiments are performed on transparent glass-etched pore networks at a constant unfavorable viscosity ratio and varying values of the capillary number (Ca), and equilibrium contact angle (theta(e)). Dispersions of ozokerite in paraffin oil are used as the shear-thinning NWP, and aqueous solutions of PEG colored with methylene blue are used as the Newtonian WP. At high Ca values, the tip splitting and lateral spreading of WP viscous fingers are suppressed; at intermediate Ca values, the primary viscous fingers expand laterally with the growth of smaller capillary fingers; at low Ca values, network spanning clusters of capillary fingers separated by hydraulically conductive noninvaded zones of NWP arise. The spatial distribution of the mobility of shear-thinning NWP over the pore network is very broad. Pore network regions of low NWP mobility are invaded through a precursor advancement/swelling mechanism even at relatively high Ca and theta(e) values; this mechanism leads to irregular interfacial configurations and retention of a substantial amount of NWP along pore walls; it becomes the dominant mechanism in displacements performed at low Ca and theta(e) values. The residual NWP saturation increases and the end WP relative permeability decreases as Ca increases and both become more sensitive to this parameter as the shear-thinning behavior strengthens. The shear-thinning NWP is primarily entrapped in individual pores of the network rather than in clusters of pores bypassed by the WP. At relatively high flow rates, the amplitude of the variations of pressure drop, caused by fluid redistribution in the pore network, increase with shear-thinning strengthening, whereas at low flow rates, the motion of stable and unstable menisci in pores is reflected in strong pressure drop fluctuations.

Gas sensing and structural properties of variously pretreated nanopowder tin (IV) oxide samples

Abstract The correlation between the gas sensing properties and the pore structure of pure nanopowder SnO 2 samples subjected to varying degrees of thermal pretreatment has been investigated. It is shown that both the intrinsic air resistance and the sensitivity to reducing gases are generally more dependent upon operating temperature than the degree of pretreatment. Nevertheless, there is evidence for a correlation between the porosity and the observed gas sensitivities of samples which have not undergone significant grain growth. It is concluded that under fixed operating conditions, the pore structure of the oxide strongly influences the observed gas sensitivity in such samples.

A multi-flowpath model for the interpretation of immiscible displacement experiments in heterogeneous soil columns

This work focuses on the phenomenon of the immiscible two-phase flow of water and oil in saturated heterogeneous soil columns. The goal is to develop a fast and reliable method for quantifying soil heterogeneities for incorporation into the relevant capillary pressure and relative permeability functions. Such data are commonly used as input data in simulators of contaminant transport in the subsurface. Rate-controlled drainage experiments are performed on undisturbed soil columns and the transient response of the axial distribution of water saturation is determined from electrical measurements. The transient responses of the axial distribution of water saturation and total pressure drop are fitted with the multi-flowpath model (MFPM) where the pore space is regarded as a system of parallel paths of different permeability. The MFPM enables us to quantify soil heterogeneity at two scales: the micro-scale parameters describe on average the effects of pore network heterogeneities on the two-phase flow pattern; the macro-scale parameters indicate the variability of permeability at the scale of interconnected pore networks. The capillary pressure curve is consistent with that measured with mercury intrusion porosimetry over the low pressure range. The oil relative permeability increases sharply at a very low oil saturation (<10(-3)) and tends to a high end value. The water relative permeability decreases abruptly at a low oil saturation (~0.1), whereas the irreducible wetting phase saturation is quite high. The foregoing characteristics of the two-phase flow properties are associated with critical (preferential) flowpaths that comprise a very small percentage of the total pore volume, control the overall hydraulic conductivity, and are consistent with the very broad range of pore-length scales usually probed in soil porous matrix.

Investigation of the Contamination of Fractured Formations by Non-Newtonian Oil Pollutants

In many practical applications, non-aqueous phase liquid (NAPL) pollutants exhibiting a clearly non-Newtonian rheological behavior (e.g. crude oil, suspensions of engine oils, asphalt, creosote, etc.) may migrate through fractured formations and contaminate aquifers. The present work is the first step toward the development of non-Darcian models concerning the non-linear NAPL flow in single fractures, and detemiination of the coupled effects of non-Newtonian NAPL rheology and flow rate on the transient immiscible displacement of an aqueous phase by a NAPL. Initially, a protocol is developed for the preparation and rheological characterization of synthetic non-Newtonian NAPLs, which are based on waxy oils. Then, an artificial transparent glass-etched single fracture of controlled morphology is fabricated and used for the measurement of the non-linear pressure gradient--superficial velocity relationship for the flow of NAPL of varying rheology. Pore network simulations and effective medium approximation (EMA) are used for the interpretation of the experimental results and derivation of an analytic non-Darcian one-phase flow model. Visualization experiments of the immiscible displacement of an aqueous phase by Newtonian and non-Newtonian NAPLs are performed on the artificial fracture under controlled values of the viscosity ratio and capillary number (ratio of viscous to capillary forces). Comparative study of the Newtonian and non-Newtonian NAPL flow patterns allows us to evaluate the interactive effects of NAPL rheology, flow rates and fracture morphology on the spatial and temporal distribution of such liquid pollutants within single fractures

Non-aqueous phase liquid-contaminated soil remediation by ex situ dielectric barrier discharge plasma

Abstract Non-thermal dielectric barrier discharge plasma is examined as a method for the ex situ remediation of non-aqueous phase liquid (NAPL)-contaminated soils. A mixture of equal mass concentrations (w/w) of n-decane, n-dodecane and n-hexadecane was used as model NAPL. Two soil types differing with respect to the degree of micro-heterogeneity were artificially polluted by NAPL: a homogeneous silicate sand and a moderately heterogeneous loamy sand. The effect of soil heterogeneity, NAPL concentration and energy density on soil remediation efficiency was investigated by treating NAPL-polluted samples for various treatment times and three NAPL concentrations. The concentration and composition of the residual NAPL in soil were determined with NAPL extraction in dichloromethane and GC-FID analysis, while new oxidized products were identified with attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The experimental results indicated that the overall NAPL removal efficiency increases rapidly in early times reaching a plateau at late times, where NAPL is removed almost completely. The overall NAPL removal efficiency decreases with its concentration increasing and soil heterogeneity strengthening. The removal efficiency of each NAPL compound is inversely proportional to the number of carbon atoms and consistent with alkane volatility. A potential NAPL degradation mechanism is suggested by accounting for intermediates and final products as quantified by GC-FID and identified by ATR-FTIR.

Spatial distribution of jet fuel in the vadoze zone of a heterogeneous and fractured soil

The goal of the present work is to screen and evaluate all available data before selecting and testing remediation technologies on heterogeneous soils polluted by jet fuel. The migration pathways of non-aqueous phase liquids (NAPLs) in the subsurface relate closely with soil properties. A case study is performed on the vadoze zone of a military airport of north-west Poland contaminated by jet fuel. Soil samples are collected from various depths of two cells, and on-site and off-site chemical analyses of hydrocarbons are conducted by using Pollut Eval apparatus and GC-MS, respectively. The geological conceptual model of the site along with microscopic and hydraulic properties of the porous matrix and fractures enable us to interpret the non-uniform spatial distribution of jet fuel constituents. The total concentration of the jet fuel and its main hydrocarbon families (n-paraffins, major aromatics) over the two cells is governed by the slow preferential flow of NAPL through the porous matrix, the rapid NAPL convective flow through vertical desiccation and sub-horizontal glaciotectonic fractures, and n-paraffin biodegradation in upper layers where the rates of oxygen transfer is not limited by complexities of the pore structure. The information collected is valuable for the selection, implementation and evaluation of two in situ remediation methods.