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CHEMICAL ALTERATION OF WETTABILITY OF SANDSTONES WITH POLYSORBATE 80. EXPERIMENTAL AND MOLECULAR DYNAMICS STUDY Ivan Moncayo-Riascos, Farid B. Cortés, and Bibian Alonso Hoyos Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02263 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017
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CHEMICAL ALTERATION OF WETTABILITY OF SANDSTONES WITH POLYSORBATE 80. EXPERIMENTAL AND MOLECULAR DYNAMICS STUDY Ivan Moncayo-Riascosa*, Farid B. Cortésa and Bibian A. Hoyosa a
Departamento de Procesos y Energía, Facultad de Minas, Universidad Nacional de
Colombia Sede Medellín, Carrera 80 No. 65-223, Núcleo Robledo, 050041 Medellín, Colombia
ABSTRACT
In this work an experimental and theoretical evaluation of the wettability alteration of sandstones, using the commercial surfactant polysorbate 80 (P80) is presented. The experimental evaluation started from a virgin sandstone core (water-wet); damage was then induced to modify the wettability of the rock from water-wet to oil-wet, with the purpose of evaluating the surfactant’s ability to restore the wettability of the core to the water phase. The contact angles of water and n-decane droplets were used to evaluate the wettability alteration of the surface. The theoretical evaluation was made using molecular dynamics to determine the configuration of the surfactant adsorbed on the surface and to calculate the surfactant–liquid interaction energy. Experimental results showed that a concentration of 100 ppm of P80 in the
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impregnation solution generated the greatest contact angle for n-decane droplets and a small contact angle for water droplets using the least amount of surfactant, restoring the water-wet state of the solid and generating a lipophobic surface. The molecular dynamics results showed that adsorption of the surfactant on the surface is associated mainly with interactions between the chains of the surfactant that contain the ester group and the surface, whereas the chains containing ethylene oxides are exposed toward the liquid phase. By evaluating the interaction energy between the P80 coating and the liquid phases it was established that the chains containing the ethylene oxides are key to restoring the water-wet state of the surface since they exert an attractive interaction over water and a slightly repulsive interaction over n-decane, generating the lipophobic surface. The molecular model developed in this work allows the performance of predictive calculations of the contact angle of liquid droplets, with deviations, in most cases, of less than 5% compared to the experimental value.
KEYWORDS: wettability alteration, contact angle, non-ionic surfactant, EOR-chemical, molecular dynamics.
INTRODUCTION Waterflooding is one of the most popular methods of enhanced oil recovery (EOR) and until a few years ago it was considered the process that had contributed the most to increasing oil recovery.1 However, the performance of this method decreases in applications where the porous medium exhibits high horizontal and vertical heterogeneity. After a waterflooding process, more than 50% of the original oil can be left in situ. That is why alternative processes, such as the injection of surfactants or polymers of diverse nature, have appeared.2
The addition of surfactants has been applied successfully in mature oil fields. The main objective of this treatment is to recover the residual oil left after primary or secondary recovery processes and to change the wettability of the reservoir rock.3,4 The efficacy of the treatments by addition of surfactants is affected by their adsorption on the surface of the porous medium, 2 ACS Paragon Plus Environment
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which determines the amount of surfactant needed to obtain the additional oil, and therefore the operating costs.
It is worth mentioning that the applicability of the chemical enhanced oil recovery depends on different aspects of the reservoir (mainly the brine and oil compositions, temperature, pressure, quantity of water production, rock mineralogy and degree of rock fracture, among others).3,5,6 For instance, carbonate reservoirs are often strongly oil-wet whereas sandstones are normally neutral or even water-wet.6,7 Despite this, when the sandstone has aged or the reservoir has formation damage by asphaltenes deposition (altering the wettability of the rock), the sandstones can become oil-wet. The latter phenomenon is common in reservoirs of light and intermediate crude oil, in which surfactant flooding is more commonly used.
Experimental evaluations have been widely used with the purpose of defining the optimal conditions for surfactant injection in EOR applications.3,8–11 These evaluations generally consist of selecting a promising surfactant and determining the concentration at which it produces the greatest change in the wettability state of the rock (by measuring the contact angle of oil and water droplets and imbibition tests), and determining the recovery incremental factor through coreflooding tests.12–14
The molecular characteristics of the surfactant are key in its performance in increasing the recovery factor in EOR processes.6,15–18 The selection of the surfactant to be used depends on the reservoir conditions mentioned above.3,16,17 In applications where it is required that the surfactant adsorbs onto the solid surface, the most common approach is to take advantage of the strong electrostatic interaction that can be established between the polar heads of the surfactants and the excess surface charge on the solid.16 Anionic surfactants are typically used for the treatment of carbonate formations12,19–21 and cationic surfactants for the treatment of sandstones.22–24
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Non-ionic surfactants have the advantage of being more stable than ionic surfactants under high salinity conditions, since they do not present inhibition by solvation.16,25,26 Among the non-ionic surfactants typically used in processes of alteration of wettability, fluorocarbon structures27–29 are widely known for their ability to remove condensate banks, and surfactants based on ethylene oxides (EO)6,30–33 appear as promising alternative for restoring the initial water-wet state of surfaces. In particular, the surfactant polyoxyethylene sorbitan monooleate, known as polysorbate 80 (in short P80), which has three chains containing EO in its molecular structure, is a promising alternative for recovering the water-wet state of sandstone surfaces.16,17,34
Despite the good results obtained by studies based exclusively on experimental tests, this type of evaluation does not reveal the specific mechanism by which each selected surfactant promotes the alteration of wettability, and does not show the effect of the surfactant’s molecular structure.12,13,35 Recent research has focused on developing a theoretical framework based on molecular simulation in order to complement the experimental findings, providing an atomistic description of the phenomena involved.34,36,37 These theoretical studies consider the effect of the molecular structure of the surfactant27,30,36 and allows quantification of the energetic interaction of the surfactant with the production fluids.27,38–41 In the case of EO surfactants, for example, molecular simulations have allowed the study of the adsorption of these surfactants in silicawater interfaces,36 and the study of micelle behavior of P80 in water.32 Even DFT and DPD approximations have been used to understand the effect of EO chains on the properties of a series of nonylphenol ethoxylate surfactants,33 obtaining that the solvation energy shows a linear trend with the number of EO units. This type of theoretical evaluations allows to suggest the improvement of an existing surfactant. For example, tolerance to the presence of salts containing Ca2+ could be improved by introducing EO groups in the molecular structure of anionic surfactants.42
To the best of our knowledge, in the specialized literature there is no research that involves both experimental and theoretical approaches to study the alteration of wettability of sandstones. 4 ACS Paragon Plus Environment
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Also, it is worth noting that most studies are focused on the wettability analysis in carbonate reservoirs. Instead we focused our study in sandstones that become oil-wet due to formation damage. To achieve this, we conducted a theoretical and experimental evaluation of the alteration of wettability of sandstone surfaces by P80. The experimental evaluation started from a virgin sandstone core (water-wet); damage was then induced to modify the wettability of the rock from water-wet to oil-wet, with the purpose of evaluating the surfactant’s ability to restore the water-wet state of the solid phase. The contact angles of water and n-decane droplets were used to quantify the wettability alteration of the surface. The phenomenological aspects of the process of wettability alteration were evaluated from an atomistic representation, using molecular dynamics.16,17,32 This theoretical evaluation was made with the aim of building a phenomenological model to establish which particularities of the molecular structure of the surfactant agent hold the key for the wettability alteration, to determine the configuration of surfactant adsorption on the surface, and to calculate its interaction energy with the fluids.
The theoretical-experimental approach presented in this work contributes to gaining a better physical understanding of the process of wettability alteration. In this sense, we obtained some insights that clarified the relation between the surfactant’s molecular structure, the configuration of adsorption on the solid and its impact on the wettability. Finally this work illustrates a methodology that allows the quantification and prediction of the contact angle of liquid droplets on surfaces.
EXPERIMENTAL PROCEDURES Sodium chloride (99%, PanReac, Spain) and deionized water were used to prepare brine for preparing the surfactant solutions. The surfactant P80, ethanol (99.9%) and n-decane (99%) were purchased from Sigma-Aldrich (USA). An extra-heavy crude oil of 7.2º API and 18 wt % of asphaltenes from a Colombian reservoir was used for aging and to produce the restored samples. 5 ACS Paragon Plus Environment
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Test cores were made, from an outcrop sandstone rocks, with a diameter of 3.8 cm and length of 4.83 cm. All samples were cleaned with toluene, followed by methanol in order to remove any impurities and then dried at 60 ºC for 48 h. These samples were named virgin samples, which were water-wet. For the process of restoration to the original wettability of the core samples (oil-wet state), the virgin samples were washed using steam and then the cores were aged using crude oil for at least 200 h at 65 °C, following the method proposed by McGhee et al.,43 these samples were named restored samples. For application of the surfactant treatment, the restored samples were soaked in a series of surfactant solutions to alter the wettability under static conditions. Both high concentrations (>20000 ppm) and low concentrations (
