Asphaltene precipitation and subsequent deposition is a flow assurance problem that, unlike wax and gas hydrate formation, is not fully understood and available mitigation strategies are not well established and in many cases are performed on a trial and error basis. Also, unlike other flow assurance issues, simulation tools for asphaltene deposition are not commercially available and current commercial packagesthat are used to predict the precipitation of asphaltenes lack the predictive capabilities that are required to distinguish between problematic and non-problematic wells.
In the last ten years, a modeling technique to predict the occurrence and the magnitude of asphaltene precipitation at high pressure and temperature based on the Perturbed Chain version of the Statistical Associating Fluid Theory (PC-SAFT) has been well established and extensively tested. This modeling technique can simulatestandard PVT experiments as well as the asphaltene onset pressure (AOP) for live oils and its blends with hydrocarbon gas (lean and rich gases) and carbon dioxide. Furthermore, the effect of oil based mud contamination, commingling of oils, and even the compositional grading produced in reservoirs that can lead to the formation of tar-mats can be quantitatively predicted with this modeling approach.
Figure 1 shows the prediction of bubble pressure and asphaltene precipitation curves for a light fluid upon injection of various amounts of lean gas.
Figure 1.Prediction of AOP for light crude oil at different temperatures and gas injections.
The modeling method using the PC-SAFT EOS was successful in correlating data at different conditions and predicting potential experimental discrepancies. In this case study, the oil operator was able to successfully reduce the number of experiments required for a complete understanding of the effect of temperature and composition on the AOP and identify and correct experimental shortcomings that otherwise could have been easily missed. For instance, in this case, the simulation parameters were tuned to data obtained for the live oil sample with no gas added, and predictions for 5%, 10% and 20% gas injection were made. The simulation results for the 10% injection case show a significant deviation from experimental values of AOP and BUBP at 255°F, while the simulation results for the 20% case show an excellent agreement. Therefore, the experimental results for the 10% case are deemed inconsistent. After the service laboratory repeated the experiments for 10% gas injection, it was possible to confirm that the first experiment was indeed flawed. Thus, once again, the PC-SAFT EOS, in conjunction with a minimum set of reliable experimental data, can be a powerful tool to design and validate expensive and time-consuming high temperature AOP experiments.
Once the right set of simulation parameters is established, not only AOP and BP curves can be determined but also all kind of PVT properties from conventional experiments such as constant composition expansion, differential liberation, separator test and swell-test.
Contact us for more information about the capabilities of our simulation tools to perform predictions of PVT and asphaltene precipitation at high pressures and temperatures.
References
- M. Abutaqiya, S. Panuganti, F.M. Vargas. “Efficient Algorithm for the Prediction of PVT Properties of Crude Oils using the PC-SAFT EoS”. Ind. Eng. Chem. Res., 2017, 56 (20), 6088–6102.
- M. Tavakkoli, A. Chen, F.M. Vargas. “Rethinking the modeling approach for asphaltene precipitation using the PC-SAFT Equation of State”, Fluid Phase Equil., 2016, 416, 120-129.
- S.R. Panuganti, M. Tavakkoli, F.M. Vargas, D.L. Gonzalez and W.G. Chapman. “SAFT Model for Upstream Asphaltene Applications.”Fluid Phase Equil.,2013, 359, 2-16.
- S. Punnapala, F.M. Vargas. “Revisiting the PC-SAFT Characterization Procedure for an Improved Asphaltene Precipitation Prediction.” Fuel, 2013,108, 417-429.
- S.R Panuganti, F.M Vargas, D.L. Gonzalez, W.G Chapman. “PC-SAFT Characterization of Crude Oils and Modeling of Asphaltene Phase Behavior.”Fuel, 93, 2012, 658–669.
- F.M. Vargas, D.L. Gonzalez, G.J. Hirasaki, and W.G. Chapman, “Modeling Asphaltene Phase Behavior in Crude Oil Systems Using the Perturbed Chain Form of the Statistical Associating Fluid Theory (PC-SAFT) Equation of State,”Energy & Fuels, 2009, 23 (3), 1140–1146.
