Crude-Oil/Brine Interaction as a Recovery Mechanism for Low-Salinity Waterflooding of Carbonate Reservoirs

Abstract

Summary Low-salinity waterflooding in limestone formations has been less explored and hence less understood in enhanced-oil-recovery (EOR) literature. The mechanisms leading to improved recovery have been mostly attributed to wettability alteration, with less attention given to fluid/fluid-interaction mechanisms. In this work, we present a thorough investigation of the formation of water-in-oil microdispersions generated when low-salinity brine encounters crude oil and the suppressed snap-off effect caused by the presence of sulfate content in seawater-equivalent-salinity brines as recovery mechanisms in limestone rocks. We believe this is a mechanism that leads to the improved oil recovery experienced with low-salinity waterflooding and seawaterflooding in carbonate formations. This novel interpretation was studied by integrating petrographic and spectroscopic observations, dynamic interfacial-tension (IFT) measurements, thermogravimetrical analyses, and coreflooding techniques. Our data show that low-salinity brine caused a greater change in the crude-oil composition compared with seawater brine and formation-water brine. Formation-water brine created nearly no changes to the crude-oil composition, indicating its limited effect on the crude oil. These compositional changes in crude oil, caused by the low-salinity brine, were attributed to the formation of water-in-oil microdispersions within the crude-oil phase. Fourier-transform infrared (FTIR) spectroscopy data also showed that at brine-concentration levels greater than 8,200 ppm, this phenomenon was not experienced. Oil-production data for nonaged limestone cores showed an improved recovery of approximately 5 and 3% for seawater and low-salinity brines, respectively. Although the wettability-alteration effect was minimized by the use of nonaged cores, improved oil recovery was still evident. This was interpreted to represent the formation of water-in-oil microdispersions when low-salinity water (LSW) of 8,200-ppm salinity and less was used. The formation of the microdispersions is believed to increase the sweep efficiency of the waterflood by swelling and therefore blocking the pore throats, causing low-salinity-brine sweeping of the unswept pore spaces. Improved recovery by seawater brine was attributed to the changes in dynamic IFT measurement experienced using seawater brine as the continuous phase, compared with the use of LSW and formation-water-salinity (FWS) brine. This change caused a higher surface dilatational elasticity, which leads to a suppression of the snap-off effect in coreflooding experiments and hence causes improved oil recovery. Our studies conclude that the formation of microdispersions leads to improved oil recovery in low-salinity waterflooding of limestone rocks. Furthermore, the use of seawater as a displacing fluid succeeds in improving recovery because of its high surface elasticity suppressing the snap-off effect in the pore throat. We also present an easy and reliable mixing procedure representative of porous media, which could be used for screening brine and crude-oil samples for field application. Fluid/fluid interaction as well as high surface elasticity should be investigated as the causes of wettability alteration and improved recovery experienced by the use of LSW and seawater-salinity (SWS) brines interacting with limestone formations, respectively.

Novel Insights into the Pore-Scale Mechanism of Low Salinity Water Injection and the Improvements on Oil Recovery

“…The possible role of crude oil and fluid–fluid interactions during LSWI in carbonate and sandstone reservoirs had been underestimated until microdispersion formation was introduced as the main responsible mechanism of LSWI . The microdispersion theory could support the extra oil recovery during LSWI regardless of rock type, which was a remarkable discovery for the oil and gas industry. , It was shown that microdispersions were water clusters surrounded with oil surface-active materials leading to wettability alteration through the detachment of surface-active materials from the rock surface; − this work was also supported by other researchers. , Several studies have been published in the literature reporting the formation of water microdispersion as the main mechanism of LSWI irrespective of different terminologies. ,− Other mechanisms were also introduced to attribute the additional oil recovery during LSWI to the fluid–fluid interaction, such as interfacial viscoelasticity variations and osmotic effects . Alvarado et al suggested that either increasing the sulfate content or decreasing the salinity of injection water would lead to incremental oil recovery through increasing the interfacial viscoelasticity.…”

Surface Reactivity Analysis of the Crude Oil–Brine–Limestone Interface for a Comprehensive Understanding of the Low-Salinity Waterflooding Mechanism

“…EDX analysis together with XRD data for the Indiana limestone indicated a more than 99% CaCO 3 composition representing different forms of calcite. The XRD data peaks were presented in our previous publication . The absence of clay and anhydrite in the limestone sample helps study wettability alteration mechanisms solely associated to calcite mineral serving as a good proxy to limestone formations .…”

“…Calcite dissolution was observed by Mahani et al, explaining that such an observation could only be a secondary trigger to carbonate rock wettability alteration because the wettability alteration was still observed in the absence of calcite dissolution . Other proposed mechanisms for the observed IOR in carbonate rocks include formation of water-in-oil microdispersions, , suppression of the snap-off effect, − and osmosis, − which are all related to the interactions at the oil–brine interface.…”

Oil and Water Interactions during Low-Salinity Enhanced Oil Recovery in Water-Wet Porous Media

“…On the basis of these characteristics of surfactants, LSW disperses into oil that contains a considerable amount of surfactants as a microemulsion and replaces trapped oil from water-wet or mixed-wet reservoirs through micromodel experiments . Moreover, it has been reported that microdispersion of water is induced by the variation in the ion compositions of water surrounding oil based on experimental results obtained through visualization and element mapping with energy-dispersive X-ray spectrometry (EDX) and environmental scanning electron microscopy (E-SEM) . In addition, larger scale emulsions affect the flow path.…”

“…11 Moreover, it has been reported that microdispersion of water is induced by the variation in the ion compositions of water surrounding oil based on experimental results obtained through visualization and element mapping with energydispersive X-ray spectrometry (EDX) and environmental scanning electron microscopy (E-SEM). 12 In addition, larger scale emulsions affect the flow path. For example, the role of mobilized clay particles has been considered as an emulsifier 13,14 In this study, we investigated the EOR mechanism of LSWF focusing on the oil/water interaction by targeting a specific offshore oil reservoir.…”