The effect of clay and salinity on asphaltene stability
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Copyright 2016, Society of Petroleum Engineers. Destabilized asphaltenes cause serious problems by precipitating in the reservoir pores which reduces the oil flow by decreasing the reservoir porosity and permeability significantly. This study investigates the impact of clays and salinity on the stability of asphaltenes for five different crude oil samples. First, both n-pentane and n-heptane insoluble fractions of five crude oils were examined under microscope after the interaction with water, brine, and porous media. The porous media was prepared with clay only, sand only, or sand-clay mixture. A monovalent (NaCl) and a divalent (CaCl2) salts at 0.2% and 4% concentrations were used to prepare brine samples. The systematic microscopic imaging conducted on asphaltenes show that both n-pentane and n-heptane asphaltenes are getting dispersed in water phase. However, this dispersion is hampered with the existence of ions in water and this impact increases with the increase in brine concentration. Nevertheless, the destabilization of asphaltenes is more pronounced with the presence of clays. Because the destabilized asphaltenes are precipitated in porous media, asphaltenes precipitation impacts the wettability of porous media. Hence, the wettability of oil-water-rock and oil-brine-rock systems was determined through water-air contact angle measurements. The pseudo reservoir rock-oil-water/brine systems were prepared by keeping constant the weight of each component in the system for five crude oil samples. Reservoir rocks were prepared by mixing sand and clay. The brine was prepared either with NaCl or with CaCl2. The impact of salt concentration on wettability was tested for both 0.2% and 4% brine solutions. It has been observed that the asphaltenes fraction has an important role on wettability determination, however, the interaction of asphaltenes with deasphalted oil is the main contributor which defines the wettability of the system. Thus, our systematic analyses on asphaltenes should be extended and similar analyses should be conducted on the deasphalted oil fractions.
