Soil water repellency: its causes, characteristics and hydro-geomorphological significance

In this paper, theoretical and experimental approaches to flow, hydrodynamic dispersion, and miscible and immiscible displacement processes in reservoir rocks are reviewed and discussed. Both macroscopically homogeneous and heterogeneous rocks are considered. The latter are characterized by large-scale spatial variations and correlations in their effective properties and include rocks that may be characterized by several distinct degrees of porosity, a well-known example of which is a fractured rock with two degrees of porosity---those of the pores and of the fractures. First, the diagenetic processes that give rise to the present reservoir rocks are discussed and a few geometrical models of such processes are described. Then, measurement and characterization of important properties, such as pore-size distribution, pore-space topology, and pore surface roughness, and morphological properties of fracture networks are discussed. It is shown that fractal and percolation concepts play important roles in the characterization of rocks, from the smallest length scale at the pore level to the largest length scales at the fracture and fault scales. Next, various structural models of homogeneous and heterogeneous rock are discussed, and theoretical and computer simulation approaches to flow, dispersion, and displacement in such systems are reviewed. Two different modeling approaches to these phenomena are compared. The first approach is based on the classical equations of transport supplemented with constitutive equations describing the transport and other important coefficients and parameters. These are called the continuum models. The second approach is based on network models of pore space and fractured rocks; it models the phenomena at the smallest scale, a pore or fracture, and then employs large-scale simulation and modern concepts of the statistical physics of disordered systems, such as scaling and universality, to obtain the macroscopic properties of the system. The fundamental roles of the interconnectivity of the rock and its wetting properties in dispersion and two-phase flows, and those of microscopic and macroscopic heterogeneities in miscible displacements are emphasized. Two important conceptual advances for modeling fractured rocks and studying flow phenomena in porous media are also discussed. The first, based on cellular automata, can in principle be used for computing macroscopic properties of flow phenomena in any porous medium, regardless of the complexity of its structure. The second, simulated annealing, borrowed from optimization processes and the statistical mechanics of spin glasses, is used for finding the optimum structure of a fractured reservoir that honors a limited amount of experimental data.

Flow phenomena in rocks : from continuum models to fractals, percolation, cellular automata, and simulated annealing

wide acceptance of the conclusion that most reservoirs are at wettability conditions other than VSWW. This conclusion has led to a resurgence of interest in satisfactory proce­ dures for measuring reservoir wettability and determining its effect on oil recovery, especially with respect to waterflooding. De­ termination of reservoir wettability and its effect on oil recovery by methods that in­ volve core samples will be referred to as ad­ vanced core analysis for wettability (ACAW). Reservoir wettability is not a simply de­ fined property. Classification of reservoirs as water-wet or oil-wet is a gross oversim­ plification. Various procedures for meas­ uring wettability have been proposed. Two methods of quantifying wettability based on rocklbrine/oil displacement behavior, the modified Amott test 2 and the USBM test, 3 are in common use. Each method depends on water saturation measurements and re­ lated capillary pressures or flow conditions to define a wettability scale. The tests show that reservoir wettability can cover a broad spectrum of wetting conditions that range from VSWW to very strongly oil-wet. With­ in this range, complex mixed-wettability conditions given by combinations of preferentially water-wet and oil-wet surfaces have been identified. In this paper, the 'adopted scales of reservoir wettability and their relationships to interface boundary con­ ditions are considered together with the dra­ matic effects that wettability can have on oil recovery.

Wettability and Its Effect on Oil Recovery

A review of immiscible fluids in the subsurface: properties, models, characterization and remediation

Important features of multiphase flow in porous media that distinguish it from single-phase flow are the presence of interfaces between the fluid phases and of common lines where three phases come in contact. Despite this fact, mathematical descriptions of these flows have been lacking in rigor, consisting primarily of heuristic extensions of Darcy's law that include a hysteretic relation between capillary pressure and saturation and a relative permeability coefficient. As a result, the standard capillary pressure concept appears to have physically unrealistic properties. The present paper employs microscopic mass and momentum balance equations for phases and interfaces to develop an understanding of capillary pressure at the microscale. Next, the standard theories and approaches that define capillary pressure at the macroscale are described and their shortcomings are discussed. Finally, an approach is presented whereby capillary pressure is shown to be an intrinsic property of the system under study. In particular, the presence of interfaces and their distribution within a multiphase system are shown to be essential to describing the state of the system. A thermodynamic approach to the definition of capillary pressure provides a theoretically sound alternative to the definition of capillary pressure as a simple hysteretic function of saturation.

http://www.soilmech.polito.it/content/download/169/887/version/1/file/Hass_Gray_WRR93_Basis+of+Pc.pdf

Thermodynamic basis of capillary pressure in porous media

Wettability is a major factor controlling the location, flow and distribution of fluids in a reservoir. The wettability of a core will affect almost all types of core analyses, including capillary pressure, relative permeability, waterflood behavior, electrical properties, and simulated tertiary recovery. The most accurate results are obtained when native- or restored-state cores are run with native crude oil and brine at reservoir temperature and pressure. Such conditions provide cores that have the same wettability as the reservoir. The wettability of originally water-wet reservoir rock can be altered by the adsorption of polar compounds and/or the deposition of organic material that was originally in the crude oil. The degree of alteration is determined by the interaction of the oil constituents, the mineral surface, and the brine chemistry. The procedures for obtaining native-state, cleaned, and restored-state cores are discussed, as well as the effects of coring, preservation, and experimental conditional conditions on wettability. Also reviewed are methods for artificially controlling the wettability during laboratory experiments.

Wettability Literature Survey- Part 1: Rock/Oil/Brine Interactions and the Effects of Core Handling on Wettability

Article de synthese sur les limites et les avantages des methodes de mesure de la mouillabilite. Ces methodes sont quantitatives: methode Amott, methode du Bureau des Mines des Etats-Unis (USBM), methode de l'angle de contact ou qualitatives: imbibition, examen microscopique, flottation, ecoulement vitreux, courbes de permeabilite relative, courbes de pression capillaire, capillarimetrie, pression capillaire de deplacement, relations permeabilite/saturation et diagraphie de reservoir. Les methodes de RMN et d'adsorption de colorant destinees a mesurer la mouillabilite fractionnelle sont egalement discutees. Enfin l'auteur propose une methode qui permet de determiner si une carotte a une mouillabilite mixte

Wettability Literature Survey- Part 2: Wettability Measurement

The wettability of a core will strongly affect its waterflood behavior and relative permeability. Wettability affects relative permeability because it is a major factor in the control of the location, flow, and distribution of fluids in a porous medium. In uniformly or fractionally wetted porous media, the water relative permeability increases and the oil relative permeability decreases as the system becomes more oil-wet. In a mixed-wettability system, the continuous oil-wet paths in the larger pores alter the relative permeability curves and allow the system to be waterflooded to a very low residual oil saturation (ROS) after the injection of many PV's of water. The most accurate relative permeability measurements are made on native-state core, where the reservoir wettability is perserved. Serious errors can result when measurements are made on cores with altered wettability, such as cleaned core or core contaminated with drilling-mud surfactants.

Wettability literature survey - Part 5: The effects of wettability on relative permeability

The capillary-pressure/saturation relationship depends on the interaction of wettability, pore structure, initial saturation, and saturation history. No simple relationship exists that relates the capillary pressures determined at two different wettabilities. Therefore, the most accurate measurements are made with cores that have native reservoir wettability. The article describes pore geometry and the pore medium and concludes that because of wettability, pore structure, initial saturation, and inhibition behavior interactions, there is a large range of contact angles where neither oil nor water will imbibe freely into a uniformly wetted reservoir core. In contrast it is sometimes possible for both fluids to imbibe freely into a core with fractional or mixed wettability.

Wettability literature survey - Part 4: Effects of wettability on capillary pressure

Cet article est le sixieme d'une serie consacree a l'influence de la mouillabilite sur l'analyse des carottes. L'auteur etudie l'effet de la mouillabilite sur l'injection d'eau et notamment le coefficient de viscosite de l'huile, sa percee, sa saturation residuelle dans les systemes uniformement ou non mouilles

William G. Anderson

Papers

Wettability Literature Survey- Part 1: Rock/Oil/Brine Interactions and the Effects of Core Handling on Wettability

Wettability Literature Survey- Part 2: Wettability Measurement

Wettability literature survey - Part 5: The effects of wettability on relative permeability

Wettability literature survey - Part 4: Effects of wettability on capillary pressure

Wettability Literature Survey-Part 6: The Effects of Wettability on Waterflooding