Simple Asphaltene Thermodynamics, Oilfield Reservoir Evaluation, and Reservoir Fluid Geodynamics

Abstract

Reservoir crude oils consist of dissolved gases, liquids, and dissolved solids, the asphaltenes. For 40 years, gas-liquid equilibria of crude oils have been treated with cubic equations of state, variants of the van der Waals EoS. However, asphaltenes had eluded a similar thermodynamic treatment. Recently, the molecular and nanocolloidal structures of asphaltenes in crude oil have been developed and codified in the Yen-Mullins model. With sizes known, the gravity term clarifies and can be added to the Flory-Huggins polymer solution theory, yielding the Flory-Huggins-Zuo (FHZ) EoS. Measurement of gradients of crude oils vertically and laterally in reservoirs is best accomplished with “downhole fluid analysis” (DFA). Interpretation of asphaltene gradients with the FHZ EoS allows determination of the extent of thermodynamic equilibrium of reservoir crude oils. Equilibrated crude oils imply flow connectivity in reservoirs, addressing one of the most important uncertainties in reservoirs. Disequilibrium fluid gradients in reservoirs imply ongoing fluid processes in geologic time. Specification of many of these processes in reservoir case studies has led to a newly formalized discipline, “reservoir fluid geodynamics” (RFG). Many reservoir concerns are addressed utilizing RFG analyses including reservoir connectivity, viscosity gradients, solution gas gradients, variable productivity index, tar mat formation, and sluggish aquifer support. The combination of new science embodied in the Flory-Huggins-Zuo EoS and the Yen-Mullins model and new technology, DFA, has led to an explosion of applications through reservoir fluid geodynamics.