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Non-monotonicity of Contact Angle from NaCl and MgCl2 Concentrations in two Petroleum Fluids on Atomistically Smooth Surfaces Seyma Aslan, Nariman Fathi-Najafabadi, and Abbas Firoozabadi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00175 • Publication Date (Web): 21 Mar 2016 Downloaded from http://pubs.acs.org on March 22, 2016
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Non-monotonicity of Contact Angle from NaCl and MgCl2 Concentrations in two Petroleum Fluids on Atomistically Smooth Surfaces Seyma Aslan a, Nariman Fathi Najafabadi b, Abbas Firoozabadia,* a
Department of Chemical & Environmental Engineering, Yale University, New Haven, Connecticut, 06520-8286
b
Chevron Energy Technology Company
* E-mail
[email protected] ABSTRACT Wetting and alteration of wetting are among the most important material properties of fluid-fluid-substrate systems in biological and industrial systems. An important industrial application of wetting and wetting alteration is related to displacement of crude oil by water injection in porous media. Water injection in oil reservoirs has been used since the early periods of oil production. Recently it has been discovered that salt concentration in the injected water may have a significant effect on the oil recovery. The process is under active research for the need of improved understanding. In this work, we investigate the
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governing elements of surface wettability with two different crude oils on two atomistically smooth surfaces (mica, quartz) and one smooth surface (calcite) as a function of salt concentration (0 M to 3 M) and type (monovalent vs. divalent). We investigate the change of wettability from NaCl and MgCl2 salts over a wide concentration for the first time. The measurements are based on long enough aging times and droplet sizes that give equilibrium and size independent contact angles. Our measurements show a non-monotonic behavior in that as NaCl concentration increases, there is a decrease (increase of water-wetting) and then an increase (decrease of waterwetting) of contact angle in all the systems we have studied. MgCl2 salt shows two trends with increasing concentration. For mica and quartz, there is first a decrease of contact angle and then an increase followed by a second sharp decrease at high MgCl2 concentrations. For calcite substrate, we observe an increase of contact angle reaching a maximum and then a decrease with increasing salt concentration. These observations have profound implications on the effect of salts on wettability alteration. The measurements have set the stage for atomistic simulations for molecular understanding of salt effect in complex fluids. 1. INTRODUCTION Wetting relates to adsorption of two different fluid molecules on a solid substrate. It is often expressed as macroscopic contact angle. Wetting and alteration of wetting are among the most important material properties of fluid-fluid-substrate systems in biological and industrial systems. The concentration and type of salts may have profound effects on wetting state. Waterflooding –injection of water into oil reservoirs– has been employed in improved oil recovery for many decades. As early as 1960’s researchers
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started to pay attention to the composition of the injected brine and its effect on waterflood oil recovery.1 Bernard studied the effect of water salinity on oil recovery in coreflooding.1 He concluded that fresh water recovers more oil from clay-rich rocks, but the high recovery is only achieved with a large pressure gradient. Pressure gradients presented by Bernard were in fact very large –in the order of 100s of psi/ft– his results were not taken seriously for field applications. It has been recognized that pressure gradient has a large effect on recovery in mixed-wet rocks and there is no such effect in strongly water-wet rocks.2 Jadhunandan and Morrow revisited the effect of brine composition on rock wettability and waterflood oil recovery and concluded that brine composition can affect core wettability.3 These effects were not as significant as other parameters affecting wettability such as the initial water saturation and aging temperature and were not fully investigated. Tang and Morrow re-introduced the idea of low salinity waterflooding to improved oil recovery and showed that decreasing brine salinity could increase oil recovery from outcrop cores saturated with dead oil.4 Their work had a few practical limitations: 1) injected brine and the aging brine were the same –in reservoir conditions one can only control the injected brine, and 2) a single core was used over and over by re-saturating it, the effects of which on core wettability and initial fluid distributions are unclear. Nevertheless, their work sparked a significant interest among researchers to investigate the effect of brine chemistry on waterflood oil recovery and reintroduced low salinity waterflooding as a potential enhanced oil recovery/increased oil recovery (EOR/IOR) technique. Subsequently, many authors have investigated various rock-fluid systems, in a wide range of pressure and temperature conditions (including high temperature and high pressure corefloods), single and multiple well field trials,5-9
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and on a wide range of mineralogies including clastics and carbonates.9, 10 Because of its significance, experimental investigation of the effect of salt concentration has accelerated in the last couple of years. Some authors show success in low salinity EOR in field studies;11-13 and some report no increase in oil recovery.14, 15 The understanding of the underlying mechanisms for low salinity water injection is the key.4, 16-18 Myint and Firoozabadi have recently provided a review on the proposed mechanisms for improved oil recovery from low salinity waterflooding.19 The review underlines that most authors single out the wettability alteration as the leading mechanism for low salinity waterflooding.19 Nasralla et al. have employed a contact angle goniometer and zeta potential analyzer to examine the wettability alteration of crude oil by low salinity water.20 They suggest the wettability alteration to be the primary mechanism of increased oil recovery with low salinity waterflooding (LSW). They have performed contact angle measurements on mica substrate (muscovite and biotite) after 1 hour of aging time and used Berea sandstone cores for waterflooding. They have studied saline water from 0 to 174 g TDS/L and reported contact angles to decrease in low-salinity water. A decrease in contact angle (by ca. 15° for muscovite, and ca. 40° for biotite surfaces) signifies the surface to become more water-wet at low salt concentration. They performed zeta potential measurements to investigate the effect of water salinity on electrokinetic charge of rock/brine and oil/brine interfaces. The data showed high salinity water to have ca. zero charge at the rock/brine and oil/brine interfaces; whereas, low-salinity water made the colloids to be highly negative at both interfaces and the negative charge renders the surface to be more waterwet. Based on the measurements, they concluded that improved oil recovery is only due
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to wettability change. These authors have not considered substrates other than mica and have not assessed the variation in brine composition as one of the key players of wettability alteration. Yousef et al. were among the first to report on the EOR from low-salinity brine in carbonates.21 They employed coreflooding and contact angle measurements in carbonate surfaces and reported improved oil recovery by varying brine composition from 100times diluted seawater (570 ppm) to 213,000 ppm connate water. They showed the highest oil recovery in twice-diluted seawater with ca. 9 to 10% improved oil recovery. At the highest salt concentration (213,000 ppm), the contact angle on a carbonate-rock sample is measured as 90° and it monotonically decreases to ca. 60° at 100-times diluted seawater rendering the surface more water-wet. They measured the contact angle after 2 days without providing the motivation for aging time. Aging may have a significant effect on wettability, as we will discuss later. Schultz et al. investigated the surface energy forces and wettability of water on muscovite mica submerged in n-alkanes.22 They present contact angle measurements during the first 30 minutes of the experiments and conclude that aging may not affect the results in nalkane solutions. Mugele et al. have recently published results on wettability of water in decane on solid surfaces with 2 µL droplets for contact angle measurements.23 In simple systems such as n-alkenes, the size may not have an effect. Aging may have also a substantial effect in crude oil systems as we will discuss in this work. Lashkarbolooki et al. have studied the effect of salinity, asphaltenes, and resins on wettability of an acidic crude oil in carbonate rocks.24 They have measured interfacial tension and contact angle of oil-brine-substrate interface at various NaCl, MgCl2, and
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CaCl2 concentrations (0 - 45,000 ppm) with 1 h and 24 h aging time – they did not observe significant change with time and only reported the final reading. They report no change in contact angle for NaCl and slight change for divalent salts (by ca. 6° decrease for CaCl2 and ca. 15° decrease for MgCl2) making the surfaces more water-wet when the salt concentration is decreased. Their conclusion is not in line with the work of other authors –they report no change in contact angle with a change in NaCl concentration while all other studies cited above show change. The acidity of the crude oil or the surface properties of the carbonate rocks may have contributed to their results. Mahani et al. have investigated the effect of low salinity on wettability alteration in carbonates.25 They have reported zeta potential measurements and contact angle analysis on two different carbonate rocks – limestone and dolomite– from 1800 mg TDS/L to 180,000 mg TDS/L brine concentration. They report up to 40° reduction in contact angle (more water-wet) when salt concentration is decreased. They have investigated only carbonate rocks and have not assessed the effect of different salt types. Alameri et al. have investigated the mechanism of wettability change with low salt by performing contact angle analysis on crude-aged carbonate discs.26 They have reported ca. 20° reduction in contact angle when salinity decreased from 50,000 ppm to 1,000 ppm. They have used
