Abstract
Experimental evidence shows that injecting low-salinity water during the oil recovery process can lead to an increase in the amount of oil recovered. Numerous mechanisms have been proposed to explain this effect, and, in recent years, two which have gained notable support are multicomponent ionic exchange (MIE) and pH increase. Both mechanisms involve ion exchange reactions within the thin film of water separating the oil in a reservoir from the clay minerals on the surface of the reservoir rock. Since the reactions occur on the molecular scale, an upscaled model is required in order to accurately determine the dominant mechanism using centimetre-scale experiments. In this paper, we develop the first stages of this upscaling process by modelling the pore-scale motion of an oil slug through a clay pore throat. We use a law-of-mass-action approach to model the exchange reactions occurring on the oil–water and clay–water interfaces in order to derive expressions for the surface charges as functions of the salinity. By balancing the disjoining pressure in the water film with the capillary pressure across the oil–water interface, we derive an expression for the salinity-dependent film thickness. We compare the two mechanisms by modifying an existing model for the velocity of an oil slug through a pore throat. Numerical results show that the velocity increases as the salinity decreases. The percentage increase is larger for the MIE mechanism, suggesting that MIE may be the dominant causal mechanism; however, this will vary depending on the particular clay and oil being studied.
Highlights
Due to the ever-increasing demand for energy, the oil and gas industry is continually developing enhanced oil recovery methods to increase the amount of oil that can be extracted from oil reservoirs
It has been observed in numerous experiments (Jadhunandan 1990; Ligthelm et al 2009; RezaeiDoust et al 2011; Tang and Morrow 1999; Yildiz and Morrow 1996) that using low-salinity waterflooding (LSW) as either a secondary process, in which the low-salinity water is injected into a reservoir saturated with oil, or as a tertiary process, in which low-salinity water is injected after a high-salinity waterflood has been performed, can lead to an increase in the amount of oil recovered; this is known as the low-salinity effect (LSE)
We observe that the multicomponent ionic exchange (MIE) and local pH increase mechanisms are unaffected by the change in the surface area, but that the increase in the speed of the oil slug as the salinity decreases is greater for the global pH increase mechanism
Summary
Due to the ever-increasing demand for energy, the oil and gas industry is continually developing enhanced oil recovery methods to increase the amount of oil that can be extracted from oil reservoirs. Lager et al (2008) perform core flood experiments in which they observe that an increase in pH, or an increase in the number of fines produced, is not consistent across experiments, and determine that these are secondary effects that occur during LSW, rather than the causal mechanism They conclude that cation exchange on the clay surfaces is the primary cause of the LSE in a sandstone reservoir. 3, we determine the thickness of the film of water separating a static oil from a clay surface as a function of the concentration of the surrounding brine This is done by considering a balance between the intermolecular forces
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