David A. DiCarlo

Physics of water repellent soils

Although it is generally well known that water repellent soils have distinct preferential flow patterns, the physics of this phenomenon is not well understood. In this paper, we show that water repellency affects the soil water contact angle and this, in turn, has a distinct effect on the constitutive relationships during imbibing. Using these constitutive relationships, unstable flow theory developed for coarse grained soils can be used to predict the shape and water content distribution for water repellent soils. A practical result of this paper is that with a basic experimental setup, we can characterize the imbibing front behavior by measuring the water entry pressure and the imbibing soil characteristic curve from the same heat treated soil. q 2000 Elsevier Science B.V. All rights reserved.

Measurement of aperture distribution, capillary pressure, relative permeability, and in situ saturation in a rock fracture using computed tomography scanning

We develop an experimental technique that uses computed tomography (CT) scanning to provide high-resolution measurements of aperture distribution and in situ saturation along with capillary pressure and relative permeability for the same rough- walled fracture. We apply this technique to an induced fracture in a cylindrical basalt core undergoing water drainage. We find that the sum of the water and gas relative permeabilities is much ,1 at intermediate saturations and the water relative permeability shows a sharp change over a narrow range of average water saturation. In situ saturation maps show channeling of gas and significant retention of the wetting phase (water). The capillary pressure initially increased and then decreased with decreasing water saturation. Although this type of capillary behavior is atypical for unsaturated flows, we find that the sizes of the gas-filled apertures are consistent with the measured capillary pressure.

Experimental measurement of air-water interfacial area during gravity drainage and secondary imbibition in porous media

A new experimental method was developed to determine air-water interfacial area as a function of capillary pressure and water saturation in unsaturated porous media. The surfactant sodium dodecyl benzene sulfonate (SDBS) was used in equilibrium column adsorption experiments to estimate air-water interfacial area for water saturations (milliliter water per milliliter void) ranging from 0.05 to 1.0 and pressures ranging from 0 to 20 cm of water. A comparison was made between columns which were equilibrated under gravity drainage versus columns equilibrated under secondary imbibition. Gravity drainage experiments showed the air-water interfacial area decreased linearly with saturation, while imbibition experiments showed a more complex nonmonotonic relation to the saturation. The interfacial area data are then compared with existing network models.

Visualization by light transmission of oil and water contents in transient two-phase flow fields

Abstract The difficulty of determining transient fluid contents in a soil–oil–water system is hampering an understanding of the systems flow characteristics. In this paper, we describe a light transmission method (LTM) which can rapidly obtain oil and water contents throughout a large two-dimensional flow field of silica sand. By appropriately coloring the water with 0.005% FD&C blue #1, the hue of the transmitted light is found to be directly related to the water content within the porous media. The hue provides a high resolution measurement of the water and oil contents in transient flow fields (such as unstable flow). Evaluation of the reliability of LTM was assessed by checking the mass balance for a known water injection and its utility in visualizing a whole flow field was exemplified for unstable fingered flow by comparing fluid contents to those obtained with synchrotron X-ray radiation.

The Effect of Wettability on Three-Phase Relative Permeability

We study three-phase flow in water-wet, oil-wet, and fractionally-wet sandpacks. We use CT scanning to measure directly the oil and water relative permeabilites for three-phase gravity drainage. In an analogue experiment, we measure pressure gradients in the gas phase to determine the gas relative permeability. Thus we find all three relative permeabilities as a function of saturation. We find that the gas relative permeability is approximately half as much in a oil-wet medium than in an water-wet medium at the same gas saturation. The water relative permeability in the water-wet medium and the oil relative permeability in the oil-wet medium are similar. In the water-wet medium the oil relative permeability scales as kro ∼ So4 for So > Sor, where Sor is the waterflood residual oil saturation. With octane as the oil phase, kro ∼ So2 for So < Sor, while with decane as the oil phase, kro falls sharply for So < Sor. The water relative permeability in the oil-wet medium resembles the oil relative permeability in the water-wet medium for a non-spreading oil such as decane. These observations can be explained in terms of wetting, spreading, and the pore scale configurations of fluid.

Lateral expansion of preferential flow paths in sands

The stability and persistence of preferential flow paths in sands can determine the flow paths of subsequent infiltration events. We have measured the evolution of preferential flow paths in a slab of sandy soil using an array of tensiometers and light transmission. The pressure and water content measurements show that the nonuniform moisture content exists even when the potentials are equalized horizontally and that then are the result of hysteresis in the soils pressure-saturation relationship. The equalization of potential takes place over several days, if at all, and is consistent, initially, with estimates of vapor transport out of the finger cores. Once the soil is wet enough, the remainder of water movement takes place in liquid films. Hysteresis produces another interesting situation when the pack is drained. We find that the wetter portions of the soil can be at a lower potential than the drier portions, resulting in a horizontal driving force for a flow of water from the drier to the wetter soil.

Measurement of fluid contents by light transmission in transient three-phase oil-water-air systems in sand

Most three-phase flow models lack rigorous validation because very few methods exist that can measure transient fluid contents of the order of seconds of whole flow fields. The objective of this study was to develop a method by which fluid content can be measured rapidly in three-phase systems. The method uses the hue and intensity of light transmitted through a slab chamber to measure fluid contents. The water is colored blue with CuSO4. The light transmitted by high-frequency light bulbs is recorded with a color video camera in red, green, and blue and then converted to hue, saturation, and intensity. Calibration of hue and intensity with water, oil, and air is made using cells filled with different combinations of the three fluids. The results show that hue and water content are uniquely related over a large range of fluid contents. Total liquid content is a function of both hue and light intensity. The air content is obtained by subtracting the liquid content from the porosity. The method was tested with static and transient experiments. Measurements made with the light transmission method (LTM) and synchrotron X rays of the static experiment agreed well. In the transient experiments, fingers were formed by dripping water on the surface in a two-dimensional slab chamber with partially oil-saturated sand. The LTM is able to capture the spatial resolution of the fluid contents and can provide new insights in rapidly changing, three-phase flow systems.

HIGH-SPEED MEASUREMENTS OF THREE-PHASE FLOW USING SYNCHROTRON X RAYS

Accurate monitoring of flow instabilities, which can occur when nonaqueous phase liquids (NAPLs) flow through porous media, is an important component of predicting the transport and fate of these compounds in the subsurface. In particular, flow situations in which three mobile phases (such as water, NAPL, and air) exist in the porous media are inherently complex. Unfortunately, the relatively low source intensities and the nontunable source energies make traditional dual gamma techniques unsuitable to study flow instabilities which can change within seconds. We present an alternate technique, which uses synchrotron X rays from the Cornell High Energy Synchrotron Source (CHESS) to measure three-phase fluid saturations on the time scale of seconds. Using the harmonic content resulting from X ray diffraction, we obtained a high-intensity X ray beam consisting of distinct tunable energies. Three-phase saturations were measured on 5-s timescales during fingering of light NAPL into regions of dry and water wet sandy soil. In the water wet soil the oil finger was less saturated, slower, and wider than the same finger in the dry soil. The results yield insights into the nature of three-phase preferential flow.

Biocolloid retention in partially saturated soils

Unsaturated soils are considered excellent filters for preventing the transport of pathogenic biocolloids to groundwater, but little is known about the actual mechanisms of biocolloid retention. To obtain a better understanding of these processes, a number of visualization experiments were performed and analyzed.

The effect of saturation path on three-phase relative permeability

Simulation and fluid flow prediction of many petroleum enhanced oil recovery methods as well as environmental processes such as carbon dioxide (CO2) geological storage or underground water resources remediation requires accurate modeling and determination of relative permeability under different saturation histories. Based on this critical need, several three-phase relative permeability models were developed to predict relative permeability; however, for practical purposes most of them require a variety of parameters introducing undesired complexity to the models. In this work, we attempt to find out if there is a simpler way to express this functionality. To do so, we experimentally measure three-phase, water/oil/gas, relative permeability in a 1-m long water-wet sand pack, under several saturation flow paths to cover the entire three-phase saturation space. We obtain the in-situ saturations along the sand pack using a CT scanner and then determine the relative permeabilities of liquid phases directly from the measured in-situ saturations using an unsteady-state method. The measured data shows that at a specific saturation, the oil relative permeability varies significantly (up to 2 orders of magnitude), depending on the path through saturation space. The three-phase relative permeability data are modeled using standard relative permeability models, Corey-type and Saturation Weighted Interpolation (SWI). Our measured data suggest that three-phase oil relative permeability in water-wet media is only a function of its own saturation if the residual oil saturation is treated as a function of two saturations. We determine that residual saturation is the key parameter in modeling three-phase relative permeability (effect of saturation history). This article is protected by copyright. All rights reserved.