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.     

Multi-Scale Characterization of Fractured Rocks Used as a Means for the Realistic Simulation of Pollutant Migration Pathways in Contaminated Sites: A Case Study

An integrated methodology is developed to quantify the geostatistical and transport properties of fractured media at multiple scales. Such information is helpful in developing numerical models and estimating the up-scaled transport coefficients of fractured formations. An oil-contaminated fractured site, overlying granite rock and situated in northern Spain, is investigated, and a macroscopic geological model that quantifies the regional distribution of faults and fractures over the entire area is established. The methodology is based on the measurement of fractured outcrops in the field (scale ～1-100 m), the collection of representative fractured samples and measurement of the fracture aperture (scale ～0.01-1 mm), and the analysis of macroscopic characteristics (scale ～1-5 km) of fracture/faults. The multi-scale fracture properties are utilized to construct a discrete fracture/fault network model which provides input data to a macroscopic simulator of contaminant transport in fractured porous media. The transient NAPL migration pathways are predicted for one scenario of pollution. Such information is helpful in the risk assessment of fractured contaminated sites.     

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     

Soil column experiments used as a means to assess transport,sorption, and biodegradation of pesticides in groundwater

Soil column experiments are used to investigate the fate of three pesticides of high, intermediate, and low solubility in groundwater: N- phosphonomethyl glycine (glyphosate); O,O-diethyl-S-[(ethylthio)methyl]phosphorodithioate (phorate); (2,4-dichlorophenoxy)acetic acid (2,4-D). Feed solutions are prepared by adding each pesticide (100 mg/L glyphosate, 50 μ g/L phorate, 50 mg/L 2,4-D) along with conservative tracer, KBr, in synthetic groundwater. The concentration of the pesticides in effluents is detected by ion chromatography (glyphosate, 2,4-D) and GC-FID (phorate). The Br^? breakthrough curves are employed to estimate the dispersion coefficient and mean pore velocity in each column. Solute transport and reactive models accounting for equilibrium/non-equilibrium sorption and biodegradation are coupled with inverse modeling numerical codes to estimate the kinetic parameters for all pesticides.     

Soil column experiments used as a means to assess transport, sorption, and biodegradation of pesticides in groundwater

Soil column experiments are used to investigate the fate of three pesticides of high, intermediate, and low solubility in groundwater: N- phosphonomethyl glycine (glyphosate); O,O-diethyl-S-[(ethylthio)methyl]phosphorodithioate (phorate); (2,4-dichlorophenoxy)acetic acid (2,4-D). Feed solutions are prepared by adding each pesticide (100 mg/L glyphosate, 50 micro g/L phorate, 50 mg/L 2,4-D) along with conservative tracer, KBr, in synthetic groundwater. The concentration of the pesticides in effluents is detected by ion chromatography (glyphosate, 2,4-D) and GC-FID (phorate). The Br(-) breakthrough curves are employed to estimate the dispersion coefficient and mean pore velocity in each column. Solute transport and reactive models accounting for equilibrium/non-equilibrium sorption and biodegradation are coupled with inverse modeling numerical codes to estimate the kinetic parameters for all pesticides.     

Large-scale effects on resistivity index of porous media

The estimation of humidity in the unsaturated zone of soils and NAPL saturation in contaminated aquifers may be based on the interpretation of electrical resistivity index logs. In the present work, concepts of the theory of the two-phase flow in pore networks are employed to interpret the form of the equilibrium and dynamic resistivity index curves of large porous samples. A resistivity cell is constructed to measure the capillary and electrical properties of large samples of unconsolidated porous media. The drainage capillary pressure and resistivity index curves of a sand column are measured by using the micropore membrane (porous plate) method, where a 0.5% wt/vol NaCl aqueous solution is displaced by n-dodecane. The dynamic resistivity index curves are measured by using the continuous injection technique for various orientations of the sand column. Based on concepts of the two-phase flow theory, concerning the dominant displacement growth pattern in a pore network and arising from the cooperative effects of capillary, buoyancy, and viscous forces, approximate relationships are developed for the resistivity index and saturation exponent as functions of the water saturation. The saturation exponent decreases as the displacement advances and the fluid distribution across the sand column tends to be homogenized after oil breakthrough. Both the resistivity index and saturation exponent increase as the displacement pattern tends to become compact and stable. In the destabilized flow pattern, as the Bond number decreases, the resistivity index may increase respectably within a narrow range of values of the Bond number. This happens when the thickness of the unstable capillary finger exceeds the lateral dimension of the porous sample and becomes a fractal percolation cluster. The saturation exponent becomes almost constant and independent of water saturation only over the destabilized displacement pattern at high values of the Bond number.     

Combining steam injection with hydraulic fracturing for the in situ remediation of the unsaturated zone of a fractured soil polluted by jet fuel

A steam injection pilot-scale experiment was performed on the unsaturated zone of a strongly heterogeneous fractured soil contaminated by jet fuel. Before the treatment, the soil was stimulated by creating sub-horizontal sand-filled hydraulic fractures at three depths. The steam was injected through one hydraulic fracture and gas/water/non-aqueous phase liquid (NAPL) was extracted from the remaining fractures by applying a vacuum to extraction wells. The injection strategy was designed to maximize the heat delivery over the entire cell (10 m × 10 m × 5 m). The soil temperature profile, the recovered NAPL, the extracted water, and the concentrations of volatile organic compounds (VOCs) in the gas phase were monitored during the field test. GC-MS chemical analyses of pre- and post-treatment soil samples allowed for the quantitative assessment of the remediation efficiency. The growth of the heat front followed the configuration of hydraulic fractures. The average concentration of total hydrocarbons (g/kg of soil) was reduced by ～ 43% in the upper target zone (depth = 1.5-3.9 m) and by ～ 72% over the entire zone (depth = 1.5-5.5 m). The total NAPL mass removal based on gas and liquid stream measurements and the free-NAPL product were almost 30% and 2%, respectively, of those estimated from chemical analyses of pre- and post-treatment soil samples. The dominant mechanisms of soil remediation was the vaporization of jet fuel compounds at temperatures lower than their normal boiling points (steam distillation) enhanced by the ventilation of porous matrix due to the forced convective flow of air. In addition, the significant reduction of the NAPL mass in the less-heated deeper zone may be attributed to the counter-current imbibition of condensed water from natural fractures into the porous matrix and the gravity drainage associated with seasonal fluctuations of the water table.