Application of Air-Mercury and Oil-Air Capillary Pressure Data In the Study of Pore Structure and Fluid Distribution

Abstract

Many physical properties of the porous media-immiscible liquid system are dependent upon the distribution of fluids within the pores; this in turn, is primarily a function of pore structure, liquid-liquid interfacial tension and liquid-solid wetting conditions. The capillary pressure hysteresis process provides a means of investigating the influence of pore structure upon fluid distribution for consistent surface conditions. Investigations indicate that residual non-wetting-phase saturations following the imbibition process (i.e., wetting phase displacing non-wetting phase) are dependent upon both pore structure and initial non-wetting phase saturation and suggest that residual fluid is distributed to discontinuous globules, one to a few pore sizes in dimension, through the entire range of pore sizes originally occupied. It appears that air-mercury capillary pressure data adequately reflect the distribution of fluids in a water-oil system when strong wetting conditions prevail. An oil-air counter-current imbibition technique has also been found to provide a rapid means of obtaining residual-initial saturation data. In a majority of cases, residual saturations determined from the oil-air or air-mercury process reasonably approximate residual oil and saturation following water drive of a strongly water-wet medium. Introduction A reliable estimate of recoverable reserves depends not only on the amount of original oil-in-place but also on pore geometry and distribution of fluids within the pores. A critical parameter determining the recovery from a reservoir under waterflood, for example, is the amount and distribution of residual oil within the various rock types present. The purpose of this paper is to investigate the mechanism of capillary trapping and assess its importance in laboratory measurements of residual oil saturation. The degree of wettability of a reservoir rock is recognized as an important factor in waterflood or imbibition experiments. In this paper, however, only the water-wet case has been considered. Considerable experimental evidence1 suggests that for water-wet rocks, capillary forces predominate in the distribution of fluids and that viscous forces in the range normally of interest in the reservoir have a minimum influence on residual oil saturation. It follows that if the ultimate recovery is controlled by pore geometry, a unique residual non-wetting phase saturation should exist for a given set of initial conditions. Two laboratory procedures found to be extremely useful in the study of pore structure and degree of fluid interconnection at various saturations are described. Although air-mercury capillary injection curves have been used2 previously to characterize the drainage case, the withdrawal or imbibition case can provide valuable supplementary data. The air-mercury process, however, has several disadvantages; it is difficult to run in a sufficiently accurate manner, mercury does not always act as a strongly non-wetting liquid and in the air-mercury process the sample is rendered unsuitable for future analyses. An alternative process is described in which air is the non-wetting phase and naptha, heptane, octane or toluene is the wetting phase. Interfacial Tension and Capillary Pressure Interfacial tension between immiscible fluids is due to the difference in attraction of like molecules as compared with their attraction to molecules of the neighboring fluid. This net attraction results in a tension at the interface. To extend the interface; thus, interfacial tension s can also be thought of as free surface energy. Interfacial tension is normally expressed as dynes/cm, and interfacial energy is measured in ergs/cm2 hence, both have dimensions mLt-2 and are numerically equal.

Characterization of Fontainebleau Sandstone: Quartz Overgrowth and its Impact on Pore-Throat Framework

“…Pickell [31] highlighted that for water-wet rocks (i.e. like Fontainebleau), capillary forces predominate in the distribution of fluids and that viscous forces in the range normally of interest in the reservoir have a minimum influence on residual oil saturation [31].…”

Characteristics and influence factors for oil displacement efficiency determination in low-permeability and complex reservoir simulation

“…During laboratory core analyses, such as those described above, a very good correlation has been documented between capillary pressure (excess pressure of the non-wetting fluid), the initial saturation of the non-wetting fluid, and the residual saturation, as shown in Figure 2 (Pickell et al, 1966;Wardlaw and Taylor, 1976;and Chatzis and Dullien, 1981). These researchers observed that as porous media is exposed to progressively higher non-wetting phase pressures the maximum (initial) saturation of the non-wetting phase also increased, as is depicted by points (1), (2), and (3) in Figure 2.…”

“…As described earlier, laboratory coreflood methods produce higher estimates for residual saturation than insitu tracer test determinations of residual saturation because the laboratory methods force large initial NAPL saturation throughout the entire pore space of the sample and displace water from progressively smaller pores (Morrow, 1987 andPickell et al, 1966). During laboratory core analyses, such as those described above, a very good correlation has been documented between capillary pressure (excess pressure of the non-wetting fluid), the initial saturation of the non-wetting fluid, and the residual saturation, as shown in Figure 2 (Pickell et al, 1966;Wardlaw and Taylor, 1976;and Chatzis and Dullien, 1981).…”

“…As the maximum saturation of the non-wetting phase increased, the residual saturation was also found to increase, points S r (1) through S r (3 ) in Figure 2. Thus, the residual saturation of a porous medium is a function of the non-wetting phase pressure history (Pickell et al, 1966 andMorrow, 1987). …”

“…The pressure of the non-wetting fluid is then slowly decreased until the pressure of the wetting and non-wetting fluids are equal, capillary pressure is zerocorresponding to saturated or water table conditions (see the spontaneous imbibition curve in Figure 1). Relative to the saturated zone, many researchers, Pickell et al (1966), Bear (1972), Dullien (1979), Fetter (1992, Freeze and McWhorter (1997) have defined residual saturation based on drainage and imbibition experiments defining residual saturation as the non-wetting phase saturation at zero capillary pressure at the terminus of the spontaneous imbibition curve. Summarizing the soil science and environmental definitions for residual NAPL saturations there tend to be two subset definitions: 1) residual NAPL saturation in the vadose zone generally defined in a fashion analogous to field capacity, the NAPL saturation which remains in the soil after gravity drainage has become negligible, and 2) residual NAPL saturation in the saturated zone.…”