Snap-Off of Oil Droplets in Water-Wet Pores

Abstract

Abstract When oil emerges from a water-wet constriction into a water-filled pore, the interfacial forces are such that a leading portion of the oil may separate into a droplet (snap off). Theory indicates that for a given shape of constriction, there is a minimum size to the protruding portion of the oil that permits snap-off. If the oil/solid contact angle is zero and if the constriction has the shape of the throat of a tore (a doughnut hole), the oil must protrude for a distance of at least seven times the throat radius before snap-off can occur. Experiments were performed in approximately toric constrictions constructed of glass. Liquids used were a dyed mineral oil and a water-ethanol mixture adjusted to the same density as the mineral oil to avoid gravitational effects. Within the limitations imposed by visual distortions through the glass and the liquids, the observations appear to be in accord with the theoretical predictions. Introduction Production of oil from a reservoir involves flow of oil through a porous medium that also contains water. Because of the small size of the pores in the reservoir rock or sand capillary forces at the oil-water interface are of considerable importance in determining the nature of the flow through the pores. This report deals with one aspect of such pores. This report deals with one aspect of such How, determining the conditions that must be met in order that the oil emerging from a water-wet constriction will separate (snap off or pinch off) into a droplet in the larger parts of the channel. By applying the fundamental equation of capillarity to oil flowing through certain geometrically describable water-wet constrictions we can calculate the minimum size of the protruding body of oil that will permit snap-off of the oil to give a separate droplet. The theoretical treatment can be checked, at least semiquantitatively, by observations on dyed mineral oil flowing through glass pores previously filled with a water-alcohol mixture. This mixture should be adjusted to give be same density as that of the mineral oil in order to eliminate gravitational effects, which can be large relative to capillary effects in constrictions having an internal diameter of about 2 mm. THEORETICAL In a report on pore structure and fluid distribution, Pickell, Swanson and Hickman included a discussion Pickell, Swanson and Hickman included a discussion on some aspects of snap-off of oil into droplets as that nonwetting phase moves through constrictions in pores. The discussion may be extended to indicate some pores. The discussion may be extended to indicate some of the physical conditions that must be met if snap-off is to occur. Pickell et al considered a single pore to be generally quite angular in cross-section, as shown in Fig. 1a (after Fig. 3 of their report). However, let us first consider an isolated constriction that is circular in cross-section and then comment on a less ideal pore. DOUGHNUT-HOLE PORE Let the constriction have the shape of the hole within a tore, i.e., the hole through a doughnut,* as in Fig. 1b. This pore, initially water-filled, now has oil of the same density being forced into it. As the nose of the oil and the wall of the pore, so that thin layer of water is imagined as being retained between the oil and the wall of the pore, so that the contact angle between rock and oil remains zero. The oil is moving so slowly that the pressure (or, more properly, the head) is essentially equal at all points within the oil phase. A longitudinal section of a toric pore is indicated in Fig. 1c. Pertinent lengths are shown in Fig. 1d, where Pertinent lengths are shown in Fig. 1d, where these lengths are given relative to the radius of the throat (this word being used to indicate the narrowest part of the constriction). SPEJ P. 85