Summary

Polar crude oil components can adsorb on mineral surfaces and alter their wetting properties. Extrapolating from laboratory observations of wetting alteration to reservoir conditions is difficult because of the chemical complexity of the crude oils, the variability of mineral surfaces, and the presence of brine. It is essential to recognize that the polar components of crude oils can adsorb by several distinct mechanisms, depending on factors that include brine composition and the solvent quality of the oil for its asphaltenes. In this paper, several important mechanisms are identified. These have been demonstrated by experimental observations of contact angles between pure fluids on flat surfaces, after exposure to crude oil. The consequences of these mechanisms for wetting alteration by different crude oils in porous media are examined.

There are limitations inherent in this approach. The pore surfaces in reservoir rocks are neither flat nor of a single mineralogy. Nevertheless, surface studies have been useful in showing how the wetting of high energy surfaces exposed to crude oil can evolve over time. The influence of solvent quality of the crude oil for its asphaltene fraction on wetting alteration has been demonstrated. Correlation between surface wetting alteration studies and changes in the wetting of artificial cores contacted with brine and crude oil has also been shown.

In the absence of suitable model compounds to represent the interfacially active material in crude oils, the usual practice has been to separate the oils into various fractions on the basis of solubility, polarity, or other chemical and physical characteristics. An alternative strategy for understanding the effects of specific oil components on the interactions between oils and solids is to compare crude oils with varying compositions.

The mechanisms identified in this work are representative of physisorption on amorphous silica surfaces. These observations may well apply, with due consideration to differences in surface charge and surface chemistry, to a variety of minerals, especially other oxides. It should be noted, however, that there are circumstances in which other mechanisms will dominate. One example is chemisorption of carboxylic acids on hematite surfaces. Bearing in mind that exceptions will be encountered, it is still important to understand the more general mechanisms by which crude oils can alter the wetting of high energy oxide surfaces.

Introduction

Wettability research has received considerable attention over the past five decades, but definitive conclusions about the wettability of oil reservoirs remain elusive. In the absence of well characterized model systems that reproduce the essential features of wetting alteration by crude oils, studies have necessarily concentrated on very complex natural materials. Generalizing from the results of such studies can be misleading.

To bring order to conflicting observations, some understanding of the mechanisms that underlie the observed wetting changes is necessary. It is particularly difficult to unravel these mechanisms from core tests because of the coupled effects of wetting and pore morphology. Instead, we concentrate on observing interactions of crude oils and their components with smooth solid surfaces in the presence or absence of brine.

Background

Crude Oil Composition. Composition of the crude oil is crucial to wetting alteration in two distinct ways:

  1. Polar components present in the crude oil, especially in the heavy asphaltene and resin fractions, are those that exhibit surface activity and influence wetting.

  2. The oil is itself the solvent environment that influences partitioning of the surface active components between bulk oil and oil/water or oil/water/solid interfaces.

The first point has long been recognized, but the importance of the second is often neglected. Oil density (or gravity) is one measure of the solvency of an oil. In recent work, the refractive indices of oils and their mixtures with solvents and precipitants have been used to quantify the solvency of each oil with respect to its asphaltenes.

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