Descripción y Evaluación de los Potenciales Impactos Identificados

tube bundle model with triangular tube cross-sections in which both flu- ids can coexist at the same tube cross-section. They also investigated trapping of oil during water flooding by assuming that the size and dis- tribution of the triangular tube cross-sections varied with positions along the tube bundle length.

2.3

Wettability

Wettability is one of the essential properties that control multiphase flow in porous media as it affects the distribution and transport behavior of flu- ids during displacement. Several methods have been applied to quantify the wettability of a porous rock, such as contact angle measurements and wetting indexes computed from capillary pressure curves (Cuiec, 1991). However, wettability classification of reservoir as water-wet or oil-wet is a gross oversimplification (Morrow, 1990; Cuiec, 1991). Recent advanced imaging techniques allow us to investigate the wettability distribution down to pore scale and thus provide physical fundamental knowledge to establish pore scale wettability models, which could increase the insight of multiphase flow in porous media (Kumar et al., 2009, 2012).

2.3.1 Wettability alteration

Hydrocarbon reservoirs are initially saturated with water and generally considered as uniformly water-wet (Tiab & Donaldson, 2004; Dandekar, 2006). This uniformly wetting state may change to either a homogeneous oil-wet state, or to a mixed-wet state, in which some parts of the rock surface become oil-wet while other parts remain water-wet (Donaldson & Alam, 2008). Hydrocarbon components (e.g., asphaltenes) that get adsorbed on the pore surface, when the hydrocarbon phase invades from the source rock, is the necessary condition of wettability alteration. In order to have an effective hydrocarbon-components adsorption, the pres- sure difference between the oil and water phases needs to be sufficiently

high, first to overcome the capillary entry pressure and second to over- come the critical pressure when thin water films collapse (Kovscek et al., 1993).

The first necessary condition of wettability alteration during primary drainage is that the pressure difference exceeds the capillary entry pres- sure. This is because only the part of the pore space exposed to crude oil could result in wettability changes on both core and pore scales (Masalmeh, 2003). The capillary entry pressure depends on several parameters, such as pore shape, pore size, interfacial tension and pore surface wettability. General speaking, the capillary entry pressure can be quantified by the so called MS-P equation (Mayer & Stowe, 1965; Princen, 1969a,b, 1970; Ma et al., 1996); please refer to Section 2.5 for a review of the capillary entry pressure for piston-like invasion.

After primary drainage, most of the original water has been displaced from the pores by oil, leaving water only in the pore corners/necks and as thin water films along the solid surfaces. The stability of the water films depend on the disjoining pressure and the geometry of the solid surface, and the critical pressure when the thin films start to collapse can be examined by the augmented Young-Laplace equation (Hirasaki, 1991b,a; Kovscek et al., 1993; Frette et al., 2009). The local curvature of a pore wall has a significant effect on the critical pressure.

Kaminsky & Radke (1997) discussed whether wettability alteration can occur in pores with intact the films due to asphaltene diffusion across the water film, which however is a much more time consuming process. The authors concluded that this process is not sufficient to initiate signif- icant wettability alteration. While more recent studies performed using advanced imaging techniques generally support this conclusion, Kumar et al. (2009); Kumar & Fogden (2010); Kumar et al. (2012) also observed that the deposition of crude oil components slightly intruded the wa- ter/solid area in pore geometries saturated by both oil and water. Hence, after primary drainage and through the geological time, the reservoir rock might therefore end up with three distinctively different simultane-

2.3. Wettability

ous wetting populations, namely strongly water-wet, where oil has not invaded, oil-wet, where the water films have collapsed, and weakly to strongly water-wet, where the films remain.

2.3.2 Mixed-wettability model

The pore scale model proposed by Kovscek et al. (1993) is currently the most acceptable theoretical model for mixed-wet conditions. In this model, the pore space is represented by capillary tubes with four-cornered curved star-shape cross-sections that are formed by four touching spheres; and thus the critical capillary pressure required for a thin water film collapse decreases with decreasing pore size. In their model, three types of wetting states could be formed after drainage. The smaller pores do not get invaded by oil and remain water-wet as the maximal drainage capillary pressure is smaller than the capillary entry pressure. The medium-sized pores become mixed-wet as the water filled corners remain water-wet and the middle portions of the pore walls, that are contacted by oil, become oil-wet. The larger pores invaded by oil remain water-wet as the critical capillary pressure has not been exceeded at the end of the drainage process and the protective water films are still stable. As explained by Kovscek et al. (1993), the wetting state after primary drainage depends strongly on maximal drainage capillary pressure and the pore wall curvature. For example, for curved convex pore shapes (e.g., eye-shaped), the larger pores become mixed-wet and the smaller pores remain water-wet. Contact angle hysteresis could be incorporated in a mixed-wet model by introducing contact lines that get pinned while the corresponding arc meniscus hinges at an angle which changes according to capillary pres- sure (Ma et al., 1996). This conceptual hysteresis model has been used broadly in pore-network modeling to investigate the hysteresis effects on multiphase flow in mixed-wet porous media (Øren & Bakke, 2003; Piri & Blunt, 2004, 2005; Helland & Skjæveland, 2006b; Ryazanov et al., 2009; Helland & Frette, 2010; Kim & Lindquist, 2012).

Recently, advanced imaging techniques based on micro-CT have been utilized to investigate the microscopic wetting state directly in porous materials (Kumar et al., 2009; Kumar & Fogden, 2010; Kumar et al., 2012). Generally, it was demonstrated that deposition of crude oil com- ponents occurred in regions exterior to the bulk water phase and at lo- cations where thin water films ruptured. These results support the pore scale mixed-wet model proposed by Kovscek et al. (1993) and are also in agreement with the study of Kaminsky & Radke (1997). However, Kumar et al. (2009); Kumar & Fogden (2010); Kumar et al. (2012) argue that the mixed-wettability model (Kovscek et al., 1993) could be poten- tially improved by including a transition zone near the contact line, as deposition of crude oil components is observed to occur non-uniformly at a finite distance outside of the remaining bulk water phase.