Application of association theory to the prediction of asphaltene deposition: Deposition due to natural depletion and miscible gas injection processes in petroleum reservoirs

Asphaltene flocculation and deposition during natural depletion and/or miscible gas injection in enhanced oil recovery (EOR) processes is a common problem in oilfields throughout the world. The complexity of the asphaltene structure and the high affinity of these molecules to absorb on surfaces, such as rock surfaces, creates difficulties in deposition prediction. There are numerous models to predict the various mechanisms related to asphaltene precipitation, flocculation, aggregation or deposition in static state. Each model can predict only a part of the asphaltene deposition phenomenon due to parameter limitations, i.e. temperature, pressure and miscible gas injection. This study modifies previous attempts at association modeling of asphaltene deposition, with the use of applied thermodynamic principles conjugated to other assumptions, resulting in a simplified, applicable model for prediction of asphaltene deposition due to pressure and miscible gas injection. An association term has been incorporated in an equation of state (EOS) by a simple association factors obtained from molecular weight distribution (fractional molecular weight) and average asphaltene molecular weight. Using some linear relations for EOS parameters of asphaltene fractions as a function of fractional molecular weight, there was no need to define asphaltene critical properties, such as critical temperature and pressure. This model is able to predict asphaltene deposition and the amount of deposition in terms of pressure variations, miscible gas injection and some specific scenarios, such as CO 2 flooding. The results of the proposed model are in good agreement with experimental data and other previous attempts.