The Effects of Geometry, Wettability, Viscosity And Interfacial Tension On Trapping In Single Pore-throat Pairs
Authors: Wardlaw N.C. 
Published: March 2010

Multiple displacement experiments in the same pore-throat pair show that the amount of trapped, residual oil is a function of pore-throat geometry and wettability and is not affected much by differences of viscosity or interfacial tension within the limits studied.

Displacement efficiency was least where the displacing phase was strongly wetting (contact angles &lt;30 degrees) and greatest for conditions of intermediate wettability (contact angles – 90 degrees) in systems with large pore-to-throat size ratios.

In extending these conclusions to multi-pore media with branching networks of pores and throats, it must be understood that trapping by capillary instability (snap-off) is only one of several mechanisms of trapping which may occur. However, in strongly water-wet rocks with large aspect ratios, it may be the most important mechanism of trapping.

Semi-rigid films formed by a contaminant at the interface of an aqueous solution with decane inhibited snap-off and could favour higher displacement efficiency by maintaining continuity of decane during imbibition.

Introduction

Trapping of oil and gas on a microscopic scale in a petroleum reservoir rock is affected by the geometric and topologic properties of the pores, by the properties of the fluids and by properties related to fluid-rock interaction such as wettability. Trapping also is affected by gravity, capillary and viscous forces which prevail during the displacement of one fluid by another.

In a system with two fluid phases, one fluid can be referred to as the displacing phase and the other as the displaced phase. The amount of residual displaced phase is very variable in different fluid-rock systems.

Many artificial and natural porous media consist of branching networks of elements which, individually, have irregular, converging and diverging portions. The restrictions in such elements are called throats and the bulges pores. Several distinct mechanisms of trapping may occur during the displacement of one fluid by another in porous media (Stegemeier, 1977; Mohanty et al., 1980). This paper deals only with trapping in individual pores caused by associated restricting throats. However, in strongly water-wet rocks with large aspect ratios, this may be the most important mechanism of trapping.

The objective is to define the effects of geometry, wettability, interfacial tension and viscosity on trapping in individual pore-throat pairs. The results show that pore-throat geometry and wettability are the most important factors affecting this type of trapping.

Numerous authors have cited wettability as an important variable affecting recovery during a water flood (Moore and Slobod, 1956; Coley et al., 1956; Donaldson et al., 1969; McCaffery and Bennion, 1974).

Pore-Throat Ratio, Wettability and Trapping

Figure 1 illustrates a glass micromodel with elements of differing pore-throat size ratio. At the bottom of the figure there is a simple tube of elliptical cross section with principal radii of 250 and 125 µm.

(Figure in full paper)

Similar tubes above act as throats connecting pores of various sizes. The pores have circular outlines of 600 µm, 700 µm. 1560 µm and 1940 µm radii and are confined between parallel plates with 250-µm separation.

The model was prepared by etching images into two glass surfaces which were then fused together.