Elsevier

Fuel

Volume 285, 1 February 2021, 119144
Fuel

Functional compounds of crude oil during low salinity water injection

https://doi.org/10.1016/j.fuel.2020.119144Get rights and content

Highlights

  • Acidic Materials are the most functional compounds during low salinity water injection (LSWI).

  • Asphaltenes are the second most functional compounds during LSWI.

  • Spontaneous formation of water-in-oil microdispersion is the underlying mechanism of LSWI.

Abstract

Due to the intrinsic complexity of crude oil, the advanced chemical compositional analysis would be required to detect the crucial interactions that may take place between water and crude oil during low salinity water injection (LSWI). In this study, a series of analytical techniques were combined to discover the functional compounds of crude oil contributing to low salinity water effect (LSE) during LSWI. The Fourier Transform Infrared (FT-IR) analysis and Fourier Transform Ion Cyclotron Resonance Mass Spectroscopy (FT-ICR MS) were deployed to characterise the oil/water interface of a chosen crude oil with a high potency toward formation of water microdispersion. Using the negative and positive electrospray ionisation modes (−ESI and +ESI, respectively), acidic compounds with aliphatic nature and asphaltene molecules were determined to be the most functional compounds at the interface promoting the spontaneous formation of water-in-oil microdispersion. These species are key to designing any waterflood operation in oil reservoirs.

Introduction

Enhanced Oil Recovery (EOR) by injection of Low Salinity Water (LSW) into the deep subsurface sandstone and carbonate geological layers containing crude oil has attracted a great attention in the past decade. Despite the list of mechanisms being proposed in the literature, to date, the microscopic interactions between rock, crude oil, and brine leading to further oil recovery remain poorly known. Low Salinity Water Injection (LSWI) was first reported to be effective in sandstones cores [1] which was subsequently confirmed by more researchers [2], [3], [4], [5], [6], [7]. Most of the early works emphasized on the predominant role of clay minerals during LSWI in sandstone rocks [8], [9], a factor that was absent in carbonate rocks. Surprisingly in 2011, LSWI were reported with positive results in carbonate rocks [10]. The reported extra oil recovery from the tertiary LSWI in carbonate cores was substantial (7–8.5% IOIP for two times diluted seawater and 9–10% IOIP for 10 times diluted seawater). While the previously proposed clay-related mechanisms could not predict and explain the reported extra oil recovery in carbonate rocks, other mechanisms were proposed for LSWI in carbonate rocks.

Despite the discrepancy on the role of clay and rock type in low salinity water effect (LSE), the consensus on the mechanism of LSWI centres around the wettability alteration toward more water-wet or less oil-wet state (mixed-wet state) [2], [6], [8], [11], [12], [13], [14]. Mixed wettability conditions mean that the larger pores within a rock are preferably oil-wet and water phase paths through smaller pores [15]. This is in turn allows the water drainage (waterflooding) to continue until the residual oil saturation reaches a small value (very low residual oil saturation is achievable in mixed-wet rocks). However, wettability alteration is a macroscopic definition that arises from the more-fundamental interactions that need to be defined themselves. Several mechanisms have been introduced for explaining the wettability alteration during LSWI [1], [8], [14], [16], [17], [18] which are all concentrated around the possible role of rock/fluid (i.e., rock/brine) interactions [19], [20], [21], [22], [23], [24] and the possible role of fluid/fluid (i.e., oil/brine) interactions were thoroughly overlooked. However, there are some refutations in the literature challenging the proposed mechanisms [25], [26], [27].

In 2013, a new mechanism [28] was introduced which revealed the paramount importance of fluid/fluid (i.e., crude oil/brine) interactions during LSWI. It was articulated through visual evidence that formation of water microdispersion within the crude oil is the main mechanism of LSWI, something that could explain the reported additional oil recovery from both sandstone and carbonate rocks. It was after the introduction of this mechanism that even the inability of LSWI in producing additional oil in some oil reservoirs was clarified [29], [30]. In recent years, the water microdispersion mechanism has been progressively reported as the main mechanism of LSWI [30], [31], [32], [33], [34], [35], [36], [37], [38], [39] for different systems by several researchers encouraging the oil industry to put more time and efforts into determining the key components within the crude oil that are important for LSWI to optimize this EOR tool.

Asphaltenes are the polar aromatic species in crude oil constituting a high sulphur, oxygen, and nitrogen contents with a trace of heavy metals such as Nickel and Vanadium [40], [41]. Also, they are defined to be a fraction insoluble in paraffinic solvents (i.e., n-hexane and n-heptane) and soluble in aromatic solvents (i.e., benzene and toluene) [42], [43]. Asphaltenes have been reported to be the most surface-active material with the ability to diffuse into the oil/water interface and decrease the oil–water interfacial tension [44], [45], [46]. They have been extensively analysed through mass spectroscopy by Fourier Transform Ions Cyclotron Resonance (FT-ICR MS) in both positive and negative electrospray ionization modes (+ESI and −ESI) [47], [48], [49], [50], [51], [52], [53] generating precise and valuable information.

Naphthenic acids are another type of surface-active materials within the crude oil which are defined as the carboxylic acids with at least one naphthenic ring [54]. The term can be used to refer to all organic acids in crude oil [55], [56]. Also, the carboxylic acids can be identified by the general chemical formula of R-COOH [57]. They can diffuse into the oil/water interface and partition into the aqueous phase through formation of organic-soaps with cations such as sodium, calcium, and magnesium [56], [58]. In addition, they have been reported to be strongly encouraging in stabilizing the water in oil (W/O) emulsions [59], [60], [61], [62] and decrease the oil–water interfacial tension [63], [64]. The surface activity of naphthenic acids has been estimated to be strongly pH-dependent as they are strongly active at the oil/water interface at neutral pH values [56], [65]. The heavier acids or asphaltene-like acids (i.e., heavy aromatic acids inside the crude oil) can contribute to the oil/water interface and enhance the stability of W/O emulsions under all conditions [66]. Stearic acid is believed to be the most reactive linear carboxylic acid within the crude oil and has been reported to have a synergic effect with asphaltenes at the oil/water interface [64]. Using electrospray ionisation mode (ESI) is believed to be the most suitable method for characterizing the naphthenic acids in crude oil [67], [68]. However, this technique has been reported to be selective [69] as in −ESI mode, the deprotonated species (with negative charges) such as carboxylic acids, carbazoles, and phenols can be detected [70], [71]. In contrast, the protonated species (positively charged compounds) such as basic materials, and positively charged asphaltene molecules can be studied through +ESI mode [72], [73]. Also, the highest offered mass resolution, mass accuracy, and mass resolving power are achievable by using this technique, and the analysis the complicated petroleum mixtures in molecular level is possible [74], [75], [76], [77], [78] as well. Taking the benefit of ionization technique (i.e., +ESI and −ESI modes) with the highest mass resolution, the discrimination of different complex compounds within the crude oil is possible appreciating the different ionization efficiencies [79], [80], [81].

In the current study, different crude oil samples were contacted with HSW and LSW. The samples were taken from the oil/water interface, then they were analysed by a Karl Fischer Titrator to quantify the amount of water microdispersion within the crude oil. The most potent crude oil toward formation of water microdispersion was chose for further analysis by FT-ICR MS. The collected crude oils from the oil/water interface of corresponding crude oil were analysed through +ESI and −ESI modes of FT-ICR MS in order to get an insight into the structure of oil/water interface during waterflooding. Another purpose of this study is to characterize the surface-active materials contributing to LSE and the interactions at the interface. Detecting the most responsible compounds of crude oil for additional oil recovery during LSWI will resolve the inconsistencies around the underlying mechanism of this EOR method. Likewise, this can lead to achieve the most reliable screening method for recognising the crude oils of suitable composition for LSWI based on the physicochemical properties of the resident oil. The interfacial crude oil samples were also investigated with Fourier Transform Infrared (FT-IR) spectroscopy to detect the compositional changes, in particular, any possible change in the intensity of functional groups.

Section snippets

Fluids preparation

The ionic aqueous solutions of LSW and high salinity water (HSW) were made by adding pure grade salts [BioXtra grade salts, 99.5 (AT), supplied by Sigma-Aldrich chemicals Co. Inc] to deionized water [molecular biology reagent grade, with a conductivity of approximately 0.05 μS/cm, supplied by Sigma-Aldrich chemicals Co. Inc]. The ionic composition of the brines used in the study are demonstrated in Table 1. The synthetic HSW brine was made based on the formation water composition of an oil

Water content measurements

Eight crude oil samples with different physicochemical properties were analysed after contact with brines. The crude oil samples were taken from the oil/water interface and investigated by KFT method to measure the amount of spontaneously formed water-in-oil microdispersion. The water content of interfacial crude oil samples was measured and converted into the water microdispersion ratio. As illustrated in Fig. 1, the ratio is higher at the oil/LSW interface indicating the stronger water-in-oil

Conclusion

The results of this study systematically demonstrate that the chemical composition of crude oil contacted by water can be thoroughly different depending on the salinity of injection water. These compositional changes provide us with important clues about the mechanism of LSWI through which further oil recovery may be achieved. It was revealed through FT-IR analysis that conjugated acidic materials or acidic asphaltenes promote the spontaneous formation of water-in-oil microdispersion at the

CRediT authorship contribution statement

Mohammad Fattahi Mehraban: Conceptualization, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization. Seyed Amir Farzaneh: Resources, Supervision. Mehran Sohrabi: Supervision, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was performed as a part of the Low Salinity Water Injection joint industry project (JIP) in the Centre for Enhanced Oil Recovery (EOR) and CO2 Solutions at Heriot-Watt University, Edinburgh, Scotland, UK. The project is equally funded by ADNOC, BP, the UK Oil and Gas Authority, Total E&P, Wintershall Dea GmbH, Woodside Energy, and ConocoPhillips which is gratefully acknowledged.

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