Defining the Wettability State of Mixed Wet Reservoirs: Measurement of Critical Capillary Pressure for Crude Oils
Authors: Basu Subhayu , Sharma Mukul M. 
Published: December 2007

The wetting properties of crude oils are measured using a new technique that measures the capillary pressure required to rupture thin films of brine on mineral substrates. An Atomic Force Microscope is used to record the force required to rupture thin brine films on quartz surfaces. Low values of critical capillary pressure imply that large sections of the reservoir may be rendered mixed-wet because the brine films are unstable. High critical capillary pressures imply water-wet reservoirs. The technique also provides a direct indication of the height above the water table, i.e., the capillary pressure at which mixed-wet conditions may be expected in the reservoir. Data presented for a pure hydrocarbon (octadecane) and a Prudhoe Bay crude oil indicate that the critical capillary pressure varies with pH, salinity oil composition and mineralogy. The pH and salinity dependence is explored in some detail in this paper.

Introduction

The distribution and flow of fluids in porous media is significantly impacted by the wetting properties of the pore walls. Nonwetting fluids occupying larger pores will have larger relative permeabilities. Most minerals in their natural state are water-wet in an oil-water or gas-water system. However, it has been consistently reported in the literature that variations from water wet behavior are observed in a wide variety of reservoirs.

It is generally agreed that during the process of hydrocarbon accumulation in reservoirs the oil and gas migrate into a reservoir rock that is water-wet. However, as the fluid phases are separated by the action of gravity and capillary pressure, the wetting properties of the core surfaces can be altered. It has been conjectured that mixed-wet conditions can arise as a consequence of destabilization of thin brine films coating the rock grains. The adsorption of polar components such as resins and asphaltenes from the crude oil has also been implicated in wettability reversal. In a recent study on the wettability of Prudhoe Bay reservoirs it was observed that the wettability of the rock changed systematically with height above the water table. More mixed-wet conditions were obtained for core samples taken higher above the water table. This data together with that provided by earlier authors suggest that the destabilization of the brine film caused by the application of a large enough capillary pressure is responsible for the establishment of mixed wetting conditions. A pore level description of this process is provided in Reference 10.

While the mechanism for the establishment of mixed-wet conditions through the rupture of thin wetting films is now fairly well accepted, it has not been possible to date to quantify this effect when working with crude oil-brine-rock systems. Contact angle measurements do not adequately address issues related to wettability alteration. Tests on core samples such as the USBM index or the Amott index do not address pore scale wettability alteration issues either. Adhesion tests proposed by Buckley, et. al. provide a useful way of measuring wettability alteration. However, since the applied capillary pressure cannot be carefully controlled in such experiments the measurements will depend on the force applied to the nonwetting phase.

In this study we propose a new method for characterizing the wettability state of mixed-wet reservoirs through Atomic Force Microscope measurements. AFM measurements provide us with direct way of measuring the critical capillary pressure for crude oils, i.e., the capillary pressure required to rupture brine films on mineral surfaces.

The magnitude of Pc, crit. depends in a complex way on factors such as water chemistry, solid surface characteristics that include both physical and chemical heterogeneity, and oil composition. Investigation of these effects on dewetting is often frustrating, since the behavior one observes involves the combined effects of many factors, including surface structure, fluid composition, and solute adsortion effects. P. 29