Abstract
Asphaltene may cause severe challenges during acid stimulation of oil wells. In the course of contact between crude oil and the injected acid (i.e. HCl), dispersed asphaltene molecules in the organic medium chemically adsorb and accumulate onto the surface of acid droplets due to interaction with acid ions. This causes forming a shear-resistance interfacial layer, known as acid-induced sludge, that prevents droplet coalescence and stabilizes the unwanted acid in oil emulsion. In this study, a set of compatibility tests was conducted to discern the emulsion stabilizing affinity of various asphaltene samples and sludge formation as a function of experimental variables including acid-to-mixture volumetric ratio (AMR) and pH, ferric ion, and asphaltene concentrations, aromaticity of the model oil, and shear (mixing speed). The utilization of heptol model oils (a mixture of heptane, toluene, and asphaltene) would help to focus on asphaltene behavior while isolating the other crude oil components. Five asphaltene samples (A1 to A5) having different structures were used in this study. Regardless of asphaltene properties, the emulsion stability was decreased approximately 70 % by increasing AMR. In addition, more aromaticity of the model oil resulted in 80 % less emulsion stability. There is a minimum range for emulsion stability in the interval of 500–1000 ppm asphaltene concentration for all samples. Up to 50 % emulsion stability was detected in the case of asphaltene A3 because of basic properties and higher colloidal instability. The effect of ferric ion concentration on intensifying emulsion stability and sludge formation was noticeable up to 500 ppm. HCl-model oil emulsion and the formed sludge had no sensitivity to shear force in the range of 500–1500 rpm due to the very low viscosity of the model oil. Therefore, in the studied shear range, emulsion and sludge formation have been more chemically controlled instead of being physically limited due to mixing. Moreover, sludge formation is not affected by pH of the HCl solution higher than 2 and mixing speed.
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