Chemical & Biomolecular Engineering
Areas of Expertise
Petroleum thermo-physical and rheological behavior, flow assurance and production chemistry
William W. Akers Chair Professor
Chemical & Biomolecular Engineering
Areas of Expertise
Molecular modeling of complex fluids, polymer systems, confined fluids and natural gas hydrates
Chemical & Biomolecular Engineering
Areas of Expertise
Soft matter, colloidal systems, surfactants and multiphase flow in microfluidic systems.
Both gold and silver members of this consortium will have access to:
Current infrastructure in the Petroleum Thermodynamics and Flow Assurance Laboratory (PTFA Lab):
Besides the instruments currently available in the PTFA Lab, a number of instruments are also available on campus for determination of various properties of interest. For instance Biswal’s Lab has the following capabilities:
Also equipment for fabrication of micro-channels and for determination of contact angle and wettability, interfacial tension, viscosity, and fluorescence intensity of samples are readily available. NMR, SEM, TEM, FTIR-Microscope, ICP-MS, XRD, and Zeta Potential Analyzer are also examples of instruments that are accessible on campus through the Shared Equipment Authority at Rice University.
Future infrastructure for the PTFA Lab:
We have successfully adapted the PC-SAFT equation of state (Gross & Sadowski, 2001; Chapman et al., 1988) for accurate prediction of the phase behavior of crude oils, including the precipitation of asphaltenes. The simulation parameters are fitted to compositional data, experimental values of stock-tank oil density, bubble pressure (BP) and asphaltene onset pressures (AOP) at certain composition. Figure 2 shows the simulation results for the blind prediction of asphaltene onset pressure (AOP) and bubble point for a light crude oil at different lean gas injection percentages. The simulation parameters are fitted to the 5% gas case and the asphaltene stability and bubble point are accurately predicted for 10, 15 and 30 % gas. Figures 3 and 4 show that the liquid density and gas-to-oil ratio, the isothermal compressibility and the gas composition during differential liberation experiments can be accurately predicted using this modeling method.
An important application of our modeling methods based on the PC-SAFT equation of state is for the design of experiments and validation of experimental data. AOP and BP determinations done on bottom-hole samples are time consuming and represent a significant investment. Our simulation tool can assist in the design of such experiments to reduce the execution time and for validation of the results obtained. Figure 5 shows the results of a case study, in which simulation parameters were tuned to compositional data, STO density and bubble pressure of live oil and AOP data. The predictions of AOP and BP (blue lines and red lines, respectively) at 5% and 20% gas injection are acceptable. However, the results at 10% gas injection shows significant discrepancy between the model and the experiments. Repetition of this experiment led to experimental AOP and BP values that were consistent with the modeling predictions. Thus, this tool is very effective for quick data consistency validation.
Other potential applications for the PC-SAFT equation of state are for predicting the compositional grading of asphaltenes in the reservoir, reservoir connectivity, formation of tar-mats, effect of oil-base mud contamination on petroleum phase behavior and asphaltene stability and even for the calculation of derivative properties, such as heat capacity, speed of sound or the Joule-Thomson coefficient. Figure 6 shows the comparison of the modeling results using the Soave-Redlich-Kwong, the Peng-Robinson and the PC-SAFT equations of state with respect to experimental data for the Joule-Thomson coefficient, mJT, of n-decane as a function of pressure. The PC-SAFT equation of state has a clear superior performance in the prediction of this property in a wide range of temperatures (36 to 283 °C, 97 to 541 °F) and pressures (1 to 800 bar, 14.7 to 11,600 psi).
The PC-SAFT Equation of State has been proven to be successful in the prediction of petroleum phase behavior at reservoir conditions, including the precipitation of asphaltenes in a wide range of temperatures, pressures and compositions. The PC-SAFT equation of state is available in commercial software packages such as Infochem’s (KBC) Multiflash, VLXE, Calsep’s PVTsim and Aspen Plus. Nevertheless, the methodologies for parameter estimation vary greatly among the different commercial packages and this leads to significant differences in the predictions. Our methodology for parameter estimation that has been used to obtain the results shown in this document is only available in our own software that is currently being developed, which offers a user-friendly highly automated interface for quick and accurate calculations.
J. Gross & G. Sadowski. Ind. & Eng. Chem. Res. 2001, 40, 1244-1260 | W.G. Chapman, K.E. Gubbins, G. Jackson, M. Radosz, Ind. & Eng. Chem. Res. 1990, 29, 1709-1721 | S. Punnapala & F.M. Vargas. Fuel, 2013,108, pp 417-429. | S.R Panuganti, F.M Vargas, D.L. Gonzalez, W.G Chapman. Fuel, 93, 2012, pp 658–669. | Abutaqiya, M. The Petroleum Institute, MSc Thesis 2013
F.M. Vargas, M. Garcia-Bermudes, M. Boggara, S. Punnapala, M. Abutaqiya, N. Mathew, S. Prasad, A. Khaleel, M. Al Rashed, H. Al Asafen, Offshore Technology Conference, OTC 25294, Houston, TX, May 2014.
In a series of experiments we studied the reversibility of asphaltene precipitation and the morphology of the precipitate obtained. Fig. 7 shows that precipitation of asphaltenes upon addition of iso-octane is a reversible process after the iso-octane is evaporated from the sample. In Fig. 8a details of micro-structure of asphaltene aggregates are observed using Scanning Electron Microscopy. Asphaltene aggregates are porous materials formed by particles of fairly uniform size distribution (average diameter of about 350 nm in this case). In Figs. 8b and c asphaltenes that were separated from oil by addition of n-heptane are dried at ambient temperature and 120°C (248°F), respectively, and put in contact with dead oil. Asphaltenes that were dried at ambient conditions are partially re-dissolved, but not the asphaltenes that were dried at high temperature. These observations suggest chemical and physical changes of the asphaltenes at elevated temperatures. By combining the experimental observations, a conceptual mechanism for asphaltene precipitation, aggregation and aging is proposed, which is depicted in Figure 9. Asphaltene nano-aggregates, which are present even in good solvents (Mullins, 2010) upon precipitation form aggregates of less than 1 mm (which are not easily detected by commercial methods), and in turn further aggregate and eventually modify their structure to form solid-like materials. Some steps are reversible (represented by double green arrows in Fig. 9). However, once a solid-like structure is formed re-dissolution is not easily achieved. Finally, Fig. 10a shows a graphical representation of the multi-step mechanism for asphaltene precipitation, aggregation and deposition in the wellbore. The multi-step process is partially confirmed experimentally according to Fig. 10b. From this mechanism it is clear that asphaltene aggregation and deposition are two competing phenomena. Based on the mechanisms described in Figs. 9 and 10, novel asphaltene deposition inhibitors are being developed in our lab, which disrupt the aging process of asphaltene aggregates and help maintaining a soft asphaltene structure that is easier to re-dissolve and/or remove.
F.M. Vargas et al., OTC 25294. | O.C. Mullins. Energy & Fuels, 2010, 24, 2179–2207. | F.M. Vargas, J.L. Creek & W.G. Chapman, Energy & Fuels, 2010, 24 (4), 2294-2299
A large body of the available viscosity data for asphaltene suspensions was reinterpreted in terms of the Krieger–Dougherty model. Based on the analysis carried out in this work, the following model is proposed for accurate estimation and correlation of the viscosity of asphaltene suspensions:
where: ηr and η are the relative and intrinsic viscosities, respectively, and ϕ is the volume fraction of asphaltenes.
Ref: R. Pal & F.M. Vargas. Can. J. of Chem. Eng., 2013, 92, 573-577.
Based on the conceptual mechanism described in Fig. 10, thermodynamic and transport equations have been developed and solved to predict the occurrence and the magnitude of asphaltene deposition in wellbores. This simulation tool has been successfully used to predict asphaltene deposit thickness as a function of well depth. The example shown below is for an oil well from the Marrat field in Kuwait.
Ref: A.S. Kurup, F.M. Vargas, J. Wang, J. Buckley, J.L. Creek, H.J. Subramani, and W.G. Chapman. Energy & Fuels, 2011, 25, 4506–4516.
Micro-models with various geometries, permeabilities and surface specifications can be created. These micro-models can mimic contrast of permeability and fractures in rocks. A special cell is currently being developed for high pressure experiments.
Surface alteration is possible and wettability can be studied via contact angle measurements.
Left: The mechanisms of asphaltene deposition in porous media can be studied using micro-models. Fluorescence of asphaltenes present in crude oil can be observed by using confocal microscopy. This technique is also useful to study remediation strategies for asphaltene deposition.
Ref: Y.J. Lin, F.M. Vargas & S.L. Biswal, in preparation.
A predictive model has been developed for the saturated water concentration in n-alkanes based on a theoretical equation of state for the hydrocarbon rich phase and a water equation of state for the aqueous phase. Excellent qualitative and good quantitative agreement is exhibited without fitting a binary interaction parameter. The model is then extrapolated to predict water solubility in n-alkanes as a function of temperature, pressure, and carbon number for conditions where experimental data is of questionable validity or unavailable.
Ref:C.P. Emborsky, K.R. Cox, and W.G. Chapman. Ind. & Eng. Chem. Res. 2011, 50, 7791-7799 | R.H. Olds, B. H. Sage & W. N. Lacey, Ind. Eng. Chem. 1942, 34, 1223.
For smaller companies interested in keeping up with the dynamic research and continuous development of experimental methods and simulation tools in the areas of petroleum thermodynamics and flow assurance. Silver members receive:
For larger corporations that want to steer the research direction of the consortium and lead the exploration of problems and proof of concepts and the development of state-of-the-art experimental techniques and modeling tools for petroleum phase behavior and flow assurance applications. Gold members receive all the benefits of silver members plus: