Characterization and Modeling of Asphaltenes for Complex Reservoirs in Venezuela: State of the Art

Asphaltenes are complex hydrocarbon molecules that are in suspension in the oil, stabilized by resins, which may cause severe production issues at reservoir and surface conditions. High asphaltene and resin contents is one of the main characteristics of the Venezuelan unconventional oils (highly viscous oils) in the Orinoco Oil Belt. This high concentration of resins in the oil maintains the aggregates of asphaltenes dissolved in the continue oil phase avoiding asphaltene precipitation/ flocculation/deposition issues at field conditions as some Venezuelan conventional oil reservoirs located in northern Monagas State in which unfavorable resins/asphaltene (R/A) ratios promote the precipitation of asphaltenes. Conventional oil reservoirs in northern Monagas show gravitational segregation, this is the case of Carito-Mulata and Santa Barbara Field, varying from an upper zone of critical fluid behavior to a black oil zone in the lowest part of the structure in which the current pressure levels induce asphaltene precipitation, causing problems by plugging reservoirs, wells and pipelines, severely affecting oil and gas production. This causes increased production costs (chemical cleaning) and/or irreversible formation damage when reservoir pressures are less than asphaltene precipitation/flocculation onset pressures. Therefore it is necessary to characterize the asphaltene thermodynamic behavior and include this in reservoir numerical-simulation models, with the aim of increasing the reliability of the results and optimizing production strategies. Reproducing the thermodynamic behavior of asphaltenes is very complex, both experimentally and in numerical simulation, especially in terms of description and measurement of the degree of asphaltene-porous media interaction and the effect of injected fluids into the reservoir (EOR methods such as miscible/non-miscible gas injection or chemical flooding). Nevertheless, efforts have been done by the Venezuelan National Oil Company and collaborators, both at laboratory and simulation scales, to study the asphaltene thermodynamic behavior and the effect of permeability reduction in the porous media and its impact on the production profiles for complex Venezuelan reservoirs. This article presents a literature review of the Venezuelan experience for the characterization and modeling of asphaltenes for conventional and heavy oil reservoirs.