The Effect of Brine Concentration on Asphaltene Stability

Abstract

The stability of asphaltenes is a critical parameter which may affect the flow in porous media significantly. This study investigates the change in the stability of asphaltenes after the interaction of asphaltenes originated from five different reservoirs with reservoir components? namely sand, clay, water, and brine. Asphaltenes are polar like water and brine. Hence, the initial lab-scale sensitivity studies were carried out first with asphaltene-water and asphaltene-brine systems. Then, the impact of porous medium has been investigated on asphaltene stability? asphaltenes are mixed with either sand or sand+clay mixture and then exposed to water or brine. Since, both sand and clay show water-wet behavior, the effect of water-wet rock surface on asphaltene stability has been aimed to understand. Moreover, different brine types and concentrations on asphaltene stability have been tested. A divalent and a monovalent salts were used to prepare brine solutions at 2%, 4%, 6%, and 8% concentrations. All laboratory tests were achieved under optical microscope and the interaction has been examined overtime. Our experimental results suggest that asphaltenes from all five crude oil samples are dispersed in water and they aligned themselves within water due to polar nature of asphaltenes and water. In asphaltene-brine systems, the monovalent salt (NaCl) interacts more with asphaltenes than divalent salt (CaCl2). This is due to higher water uptake capacity of CaCl2 than NaCl. The microscopic images revealed that a water layer is formed around CaCl2 inhibits the direct interaction of asphaltene-CaCl2. This behavior is enhanced within the porous media; salt crystals form bridges between the sand grains, clay and asphaltenes contribute to the formation of those bridges. The impact of brine has been observed more significantly at high concentration of the brine solutions. The asphaltenes-brine interaction also shows variations among different asphaltenes. Thus, all asphaltene samples have been analyzed for their elemental compositions to check if cation exchange is possible between the salts and asphaltenes. These experiments revealed that for the asphaltenes rich in calcium and sodium content, their interaction with brine is greater than for the asphaltenes poor in calcium and sodium content. Moreover, this interaction contributes to the formation of bigger asphaltene clusters which decreases the asphaltene stability and promotes more asphaltene precipitation. This study provides some general trends observed in asphaltene-water and asphaltene-brine systems and explains the reasons why some outliers do not fit the trends by examining the differences in the chemical composition of asphaltenes. Hence, this study enriches our knowledge towards asphaltene behavior in porous media.

Critical Review of Underlying Mechanisms of Interactions of Asphaltenes with Oil and Aqueous Phases in the Presence of Ions

“…The cations neutralize the polar sites and prevent asphaltenes from aggregation compared to those only in contact with water. Asphaltenes construct an ionic bonding with pure water, resulting in larger aggregates . This phenomenon helps the asphaltenes remain at the brine–oil phase interface than in the oil phase.…”

“…Demir and Bilgesu studied the stability of asphaltenes in water and brine systems. They extracted two types of asphaltenes ( n -pentane and n -heptane).…”

“…The dispersion of asphaltenes in brine solution was less than pure water and continued to decrease with increasing salt concentration. Demir and Bilgesu prepared four different brines, 2000, 4000, 6000, and 8000 ppm, using NaCl and CaCl 2 individually. The captured images showed that the sizes of asphaltene aggregates increased with salt concentration, especially after 2% salinity.…”

“…We have learned that it is essential to thoroughly study the interactions between the injected phase (brine) and oil at different salinity conditions. The emulsion stability depends on pressure, , temperature, , salinity, type of salt, ,, pH, , resin to asphaltene ratio, , and the presence of solids particles. , All the factors mentioned above are different from atmospheric conditions. Asphaltene’s behavior and interactions may significantly differ in reservoir conditions.…”

Intermolecular Interaction between Heavy Crude Oils and Surfactants during Surfactant-Steam Flooding Process

“… 24 , 30 , 31 Thus, in heavy oil reservoirs, the reservoir rock particles may physically be attached to asphaltene clusters and make the asphaltene surfaces charged. 20 , 32 Although this interaction is physical and not chemical, since this interaction is in micro- and nanoscales, it is irreversible. 33 − 35 Therefore, the interaction between the charged polar head of surfactants with charged asphaltene surfaces may be inevitable.…”