Simulation of asphaltene precipitation during gas injection using PC-SAFT EOS

摘要

AbstractOilfield problems owing to asphaltene precipitation are well known. Asphaltenes can block pore throats or change the formation wettability and thereby reduce the hydrocarbon mobility. Simulation of asphaltene precipitation during gas injection requires a comprehensive thermodynamic model, which accounts for the complex phase behavior of asphaltenes. In this paper, PC-SAFT EOS is implemented for the first time in a compositional reservoir simulator to model asphaltene precipitation. The additional computational time of PC-SAFT compared to the cubic equations-of-state such as Pen-Robinson (PR EOS) is decreased by improving its root finding algorithm. A deposition and wettability alteration model is then integrated with the thermodynamic model to simulate the dynamics of precipitated asphaltenes. Different gas injection scenarios are modeled to show the effect of gas injection on apshaltene precipitation and deposition.Simulation results show that the profile of the damaged area by asphaltene deposition is governed by the shape of the asphaltene precipitation envelope for the reservoir fluid. The damage caused by asphaltene deposition, through plugging and wettability alteration, was revealed as a decline in productivity index curves. Results indicate that a reservoir fluid can have precipitation risk during gas injection even if it does not exhibit precipitation in the primary recovery life of a field. The computational time of the simulations using PC-SAFT EOS were compared to those using PR EOS for different number of components. Results of these comparisons show the feasibility of using PC-SAFT in compositional simulations.Highlights?The PC-SAFT EOS was implemented in a compositional reservoir simulator to model asphaltene precipitation. The additional computational time of PC-SAFT was decreased by improving the root finding algorithm and calculating the derivatives of the model analytically.?A deposition and wettability alteration model was integrated with the thermodynamic model to simulate the dynamics of precipitated asphaltenes.?The reservoir fluids can be categorized into two classes: (I) oils with inherent potential of asphaltene precipitation, and (II) oils without initial asphaltene precipitation risk. For type-I fluids, a persistent window between the upper onset and lower onset pressures exists regardless of the temperature. For type-II fluids, the upper and lower onset pressures coincide over a certain temperature range.?Introduction of a gas to the fluid type (II) can create a precipitation window leading to asphaltene deposition. Pore throat plugging around the production well is more severe in fluid type-I because of the contribution of pressure depletion to asphaltene deposition before gas breakthrough.?