Thermodynamic investigation of asphaltene precipitation and deposition profile in wellbore: A case study

Abstract

Asphaltene precipitation and deposition are considered major problems in primary, secondary and tertiary phases of oil production. These phenomena can cause partial or complete blockage in the near-wellbore region, production string and surface facilities. To study the extent of asphaltene precipitation and deposition, the wellbore pressure and temperature profiles are determined and vapor-liquid flash calculations are conducted with the aid of Peng-Robinson equation of state for a well in one of the southwest fields of Iran. Subsequently, the matching parameters of the solubility model are optimized with genetic algorithm as the search engine so that the model is tuned with experimental data of asphaltene precipitation. Finally, the rate of deposition and the resulting changes in tubing inner diameter are determined at each depth versus time. The results showed that as the fluid moves upwards from the bottom-hole until the bubble point is reached, the liquid expands and its solubility parameter slightly decreases. This leads to a discrepancy between the solubility parameter of liquid and asphaltene and the asphaltene starts to precipitate. On the other hand, in the two-phase region, liquid solubility parameter increases considerably due to gas evolution. Hence, the difference between solubility parameter of liquid and asphaltene significantly drops, leading to a considerable decrement in asphaltene precipitation. Fluid temperature can be considered as an acceptable approximation for tubing surface temperature. As long as the tubing diameter remains constant, by travelling upwards in the well, the rate of deposition per unit area decreases. At points where the tubing diameter changes, a change in liquid velocity inversely affects the rate of deposition. Well tubing blockage is more likely to occur at points where the rate of deposition per unit area has a maximum value.