Asphaltene precipitation and deposition is a serious problem in many Iranian fields. The deposited asphaltene results in partial or total blockage of the wellbore, reducing or completely seizing oil production. This paper presents a new approach, based on PVT data, for thermodynamic modeling of asphaltene precipitation. The developed model has been combined with temperature and pressure modeling of the wellbore. The combined model has been used to predict the depth of asphaltene precipitation in the wellbore in several fields in an Iranian field (Kupal), as case study. The results are compared with the field data of asphaltene problems in various wells in Kupal field, as well as the measured depth of obstruction when applicable. There is a good agreement between the predictions and the field data. Finally, a sensitivity analysis has been conducted to simulate the effect of various operational parameters, including; flow rate, wellhead pressure and tubing diameter. The results provide important guidelines in minimizing the risks associated with asphaltene deposition.
Asphaltenes are the heaviest fraction of crude oils which are polyaromatic structures or molecules, containing heteroatom (i.e. S, O, N) and metals (e.g. Va, Ni) that exist in petroleum fluids in an aggregated state1. These aggregates are stabilized in solution by resins and aromatics, which act as peptize agents2. Asphaltenes and resins, are in the thermodynamic equilibrium at static reservoir condition. However changes in thermodynamic condition such as pressure, temperature or compositions during oil production may cause stabilized asphaltenes precipitate out of fluid and could deposit in reservoir, wellbore, wellstring, transport pipeline or surface processing facilities.
Since pressure and temperature changes extremely through production string, this area is the most susceptible for asphaltene precipitation.
Deposition of asphaltene on the wall of production string reduces available diameter to oil flow, subsequently, oil production rate decreases. Gradually, oil flow path is plugged by increasing the thickness of deposited asphaltenes. In addition to economical damages as result of seizing oil production, the costs could increase for removing asphaltene obstruction plug by chemical or mechanical treatments.
The mechanical treatment of asphaltene removal do not use practically. Because this process is time consuming and sometimes impractical owing to some parts of production system are not accessible. Therefore the most of asphaltene depositions are removed by chemical treatments.
It is essential to know the depth of asphaltene deposition in order to implement a successful asphaltene removal operation. Because by knowing the depth of asphaltene deposition, the amount of chemical could be determined. Also, the risk of stuck reduces for coil tubing tool operation. Therefore, asphaltene removal treatment could be faster.
The operational production condition affects on the asphaltene deposition in wellstring. Therefore, operational parameters should be known to prevent or eliminate the asphaltenes deposition damages in well string. Thus damages could be reduced by alerting operating conditions.
This paper tries to find an approach that is capable to predict asphaltene deposition in well string and studies the effect of different factors on the process of asphaltene deposition including; flow rate, wellhead pressure and tubing diameter.