Wettability Alteration during Low-Salinity Waterflooding in Carbonate Reservoir Cores

Abstract

Production enhancement by low-salinity waterflood in carbonate formations is a subject of intense speculation. Several mechanisms are attributed to enhanced oil recovery by low-salinity waterflooding in carbonate formations. Review of experimental data in the literature indicates that the main mechanism involves interaction of Na+, Cl−, Ca2+, Mg2+, SO42− and crude oil carboxylate ions (R-COO−) with the rock in the electrical double layer (EDL) near the surface of carbonate pores, leading to wettability alteration. In this study, we performed four seawater floods in heterogeneous low-permeability carbonate cores followed by low-salinity floods. The core permeability is between 0.5 to 1.5 md, and porosity in the range of 18 to 25%. Cores were aged for eight weeks at reservoir pressure and temperature. We also conducted pendant drop oil-brine IFT measurement, and captive oil-droplet contact angle at different brine salinity, with and without the presence of surfactant. The carbonate core flood results show that removing NaCl from seawater or diluting the seawater twice and four times yielded about 8% incremental oil. In one experiment, the change in the effluent ionic concentrations was measured, and it was observed a decrease in Ca2+, Mg2+, Cl−, and SO42−. Using pendant drop IFT measurements, oil-brine IFT increased with decreasing salinity both in presence and in absence of 1,000-ppm surfactant. From captive oil-droplet contact-angle measurements, it was observed that cleaned un-aged carbonate core slabs were water-wet, and became more water-wet as salinity decreased (both in presence and in absence of 1000-ppm surfactant). The wettability of crude-aged carbonate core slabs altered from oil-wet to intermediate-wet as salinity decreased. And, the wettability changed from intermediate-wet to water-wet with decreasing salinity in presence of 1,000-ppm surfactant. Moreover, addition of small amount of surfactant alters the wettability of crude-aged or cleaned un-aged carbonate core slabs towards water-wet. The degree of water-wetness achieved by surfactant solution depends on salinity level.

Response of Non-Polar Oil Component on Low Salinity Effect in Carbonate Reservoirs: Adhesion Force Measurement Using Atomic Force Microscopy

“…Our results are in agreement with Rezaei et al [67], who reported that increasing the pH of n-decane (non-polar oil)-brine-calcite system from 5 to 10 reduces the contact angle from 148° to 51°, suggesting that the wettability of n-decane-brine-calcite system is dependent on pH. Alameri et al [13] also reported a similar pH effect on the contact angle of the oil-brine-carbonate system. It is also worth noting that pH likely significantly affects the interaction of the polar-oil-brinecarbonate system, and thus contact angle and wettability.…”

“…Extensive research has been done to show that lowering injected brine salinity can improve oil recovery from carbonate reservoir at secondary and tertiary mode from 5 to 30% of the original oil in place (OOIP) [5][6][7][8][9][10][11][12]. Wettability alteration towards a more water-wet state is believed to be one of the main physiochemical processes behind the low salinity effect, which shifts oil relative permeability towards a lower residual oil saturation [9,[13][14][15], thus enhancing oil recovery. To understand the factors controlling the wettability alteration process in carbonate reservoirs, several explanations have been developed and proposed to predict and quantify the wettability alteration, such as electrostatic bridging [7,8,[16][17][18], electrical double layer [13,[19][20][21], and surface complexation modelling [22,23].…”

“…To be more specific, the existing literature shows that the adsorption of the sulphate onto the pore surface would likely shift the surface charge from positive to negative, thus leading to a repulsion force between the oil (base and acidic functional groups) and carbonate pore surfaces [7]. Additionally, previous studies show that electrical double layer force also plays an important role in wettability alteration and release of the oil component from the carbonate surface [13,18,20,21,23]. For example, Mahani et al [18] measured the zeta potential of the oil-brine and brine-carbonate (limestone) interfaces.…”

 Brine-Dependent Recovery Processes (Smart-Water/Low-Salinity-Water) in Carbonate and Sandstone Petroleum Reservoirs: Review of Laboratory-Field Studies, Interfacial Mechanisms and Modeling Attempts

“…These different scales of investigation have been well tested, though some were more tested compared to others, many of which will be discussed below. However, studies [26,[34][35][36][37] that reported interfacial tension measurements were not highlighted in this review because changes in the brine-oil interfacial tension observed is considerably less to associate the recovery process to IFT reduction compared to those associated with gas-dependent recovery processes and alkaline waterflooding. Based on these previous studies, various aspects of the experimental method have been investigated, including reservoir and the injected brine parameters, to identify the optimum condition for brine-dependent process performance.…”

Effect of Initial Wettability on Performance of Smart Water Flooding in Carbonate Reservoirs—An Experimental Investigation with IOR Implications

“…As discussed above, sandstone reservoirs were the focus of several studies on the effect of initial wettability on oil recovery. However, very few studies have been reported on carbonates [21][22][23], and the investigation process has yet to give rise to a definitive conclusion regarding carbonate reservoirs. The potential of LSW in carbonate reservoirs has been proven and it was observed that wettability alteration is the main mechanism of LSW/smart water injection.…”