Reservoir Fluid Geodynamics, a New Way to Evaluate the Reservoir Connectivity and Crude Oil Alteration with Late Gas Charge
Authors: Chen Li , Gan Yunyan , Gao Bei , Chen Jichao , Canas Jesus A. , Jackson Richard , EI-Khoury Jules , Mullins Oliver C. 
Published: April 2019

Reservoir connectivity is always the major concern for reservoir evaluation. In addition, reservoirs exhibit all manners of complexities that introduce many other production concerns such as aquifer support, viscous oil, low productivity index, and high AOP. Improved methods of reservoir evaluation are needed. A new discipline Reservoir Fluid Geodynamics (RFG) provides a powerful framework to significantly improve reservoir understanding and naturally allows substantial data integration across many discipline. Many reservoir fluids have been altered by reservoir fluid geodynamic (RFG) processes such as a late gas charge, asphaltene migration, biodegradation, water washing, which complicate fluid distributions and produces extra challenges to understand connectivity and other reservoir concerns. Identifying and quantifying the alteration processes will be key issues addressed herein. This RFG approach also helps to clarify complexities associated with reservoir concerns such as asphaltene stability, composition gradients, viscosity variations, tar mat formation, and fault block migration.

This paper describes an integrated approach within an RFG framework to understand reservoir connectivity and fluid alteration processes. This approach is founded on simple asphaltene thermodynamics and the ability to identify fluid equilibration utilizing the Flory-Huggins-Zuo Equation of State (FHZ EoS) with its reliance on the Yen-Mullins model. This model classifies the asphaltene species dispersed in crude oil in 3 different forms: molecules, nanoaggregates (of molecules) and clusters (of nanoaggregates). For low concentrations of asphaltenes, they are present in crude oils as a true molecular solution and corresponds to the Light Oil Model. At higher asphaltene concentrations, they are present as nanoaggregates giving the Black Oil Model and at even higher concentrations, they are present as clusters giving the Heavy Oil Model. Gas charge into reservoirs can destabilized asphaltenes and can change their colloidal description causing a transition in the appropriate model for describing asphaltene (and viscosity) gradients. Continued asphaltene instability can yield tar mat or local asphaltene deposition. The Flory-Huggins-Zuo Equation of State (FHZ EoS) is best used in conjunction with Downhole Fluid Analysis (DFA) to delineate these asphaltene gradients. This thermodynamic analysis of asphaltene gradient and GOR gradient using the Cubic EoS is best linked with high resolution analytical chemistry such as two-dimensional gas chromatography (GCxGC) and GC compositional analysis with geochemical interpretation. Fluid inclusion analysis is particularly useful to identify fluid type that occupied the reservoir in the geologic past. The stable isotope analysis of methane and other gases helps identify specific fluids that entered the reservoir.

The paper presents a case study from Gulf of Mexico and include data from three stacked reservoirs in a single reservoir. Each sand received two charges, an initial (light) black oil charge and a subsequent primary biogenic charge. Even with this simple scenario, three totally different fluids are found at the well location in each of the three sands. One sand currently contains a black oil with moderate GOR having received limited biogenic gas, a second sand contains a near critical fluid with both gas and oil phases, and the third sand contains a dry gas, the gas having blown out all the oil. The FHZ EoS is shown to apply for connectivity analysis as validated by many other methods. In the black oil column, some asphaltene instability caused by the late gas charge created a heavy oil at the oil-water contact. Lab measurements of Asphaltene Onset Pressure (AOP) confirmed this evaluation. The integration of asphaltene gradient modeling, DFA, gas isotopes, fluid inclusions, Cubic EoS, GCxGC with geochemical analysis provides a novel and systematic approach and should be considered for most conventional reservoirs.