Models for Asphaltene Phase Behavior and Deposition Evolve
Asphaltene deposits in wellbores and pipelines are an expensive and challenging problem for operators, and finding the source of this problem could have positive effects on both production planning and improved designs, said Walter Chapman, a professor of chemical and biomolecular engineering at Rice University.
In a presentation hosted by the SPE Flow Assurance Technical Section, Chapman said that the key to finding that solution was to develop an effective model of phase behavior and deposition in crude oil systems.
He focused on two main models developed at Rice: the perturbed chain form of the statistical associating fluid theory equation of state (PC-SAFT) and the asphaltene deposition tool (ADEPT).
PC-SAFT is a useful tool in describing the phase behavior in a given system because it works well with the polydisperse, polyatomic nature of asphaltenes, Chapman said.
The model allows engineers to form any desired shape or sized molecule, which can then associate with each other. Commercially available, the model has physical parameters for a range of hydrocarbon components: The number of beads that make up a chain, the diameter of a bead, and the van der Waals attraction between beads can be used to characterize a norm.
Chapman said that phase behavior is also well modeled by polymer solution models such as Flory‑Huggins, in part because any equation of state is able to handle large sizes and asymmetry. “Particle phase behavior is just regular solution theory. Regular solution theory is actually pretty good, and it allows us to interpret those things,” he said.
Unlike PC-SAFT, ADEPT is a simulator that predicts the occurrence and magnitude of asphaltene deposition in the wellbore that helps engineers better understand the rates of precipitation, aggregation, and deposition. Chapman called it a straightforward mathematical model that describes the changing composition of asphaltenes over time.
Chapman said there are other opportunities for improving the modeling process. He said ADEPT must be further tested and case studies should be done so engineers can determine how well the test worked. He is conducting a study of asphaltene plugging with DeepStar, a joint industry technology development project focused on advancing deepwater technologies. Other challenges are interfacial properties and better predictions at higher temperatures and pressures.
Chapman said a consortium on petroleum thermodynamics and flow assurance is currently active at Rice. Led by Chapman and his fellow chemical and biomolecular engineering professors, Francisco Vargas and Sibani Lisa Biswal, the consortium aims to establish a forum for oil and gas companies to exchange ideas, conduct research, and develop more modeling tools and experimental procedures to help mitigate flow assurance problems.
“Rice is somewhat unique in that we have researchers in virtually all areas of flow assurance,” Chapman said. “What’s interesting about flow assurance is that there’s a lot of interaction between these areas, so having researchers that encompass this entire area allows us to better look at control strategies that can address some of these cross interactions.”
For Further Reading
OTC 23347 Asphaltene Deposition Tool: Field Case Application Protocol by A.S. Kurup, Rice University, J.S. Buckley, New Mexico Institute of Mining and Technology, and J. Wang, Chevron Energy Technology Co., et al.
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