Effect of combining rules on modeling of asphaltene precipitation

Abstract

Asphaltene deposition causes serious problems in the oil industry and reduces oil recovery. Deposition happens as a consequence of asphaltene precipitation which is a process as a result of a change in thermodynamic stability. Thus, prediction and preventing of the precipitation condition are the first step of preventing asphaltene precipitation and deposition. In this study, a thermodynamic model for asphaltene precipitation has been developed using Peng–Robinson (PR), Soave–Redlich–Kwong (SRK), and a new modification on SRK [modified-SRK equations of state (EOS)]. To modify EOS for non-pure sample (oil sample), van der Waals mixing rule with three types of combining rule containing conventional, Margules, and van Laar type was used. In addition, to verify the derived model, the experiments were conducted on a live oil sample to investigate the effect of pressure reduction and gas injection [nitrogen (0.1, 0.2, and 0.4 mol fraction) and first stage gas (0.2, 0.4 and 0.6 mol fraction)] on asphaltene precipitation. The results show that at low pressures (pressures below 5000 psia), nitrogen is not soluble in oil and the injection of nitrogen reduces asphaltene precipitation because of the liberation of the light component from crude oil; however, increasing the pressure (pressures above 6000 psia) increases the solubility of nitrogen and increases the asphaltene precipitation. For the first stage gas injection, asphaltene precipitation increases because of its high solubility in crude oil at any pressure. The amount of asphaltene precipitation due to first stage gas injection is higher than nitrogen injection except at nitrogen concentration and pressures near the bubble point (pressure of 7000 psia and nitrogen injection of 0.1 mol fraction). According to the modeling results, van Laar type combining rule in conjugated with modified-SRK-EOS predicts the amount of asphaltene precipitation very well at all situations of pressures and different gas injections, and has the least deviation from experimental data rather than the other two types of combining rules; and using mentioned combining formula, the RMSE value decreases to about 50% of the conventional combining rule. It is because of the accurate and distinct interaction parameters of each pair of components in van Laar equation.