Underground Gas Storage in an Extra-Heavy Oil Reservoir in the Orinoco Oil Belt-Eastern Venezuela Basin: Modeling Phase Behavior and Main Recovery Mechanisms

The management and storage of gases are essential to produce energy with low environmental impact from the different hydrocarbon basins in Venezuela. In particular, Eastern Venezuela Basin, where a wide variety of reservoirs with different types of complex mixtures are found (including the highly viscous oil reservoirs of the Orinoco Oil Belt), encompasses potential structures for management and storage of produced gases due to proven reservoir/seal integrity and existing infrastructure in many cases. This article presents a methodology for the storage of gases (i.e., carbon dioxide, methane, nitrogen, hydrogen) in an extra-heavy crude oil field in the Eastern Venezuela Basin, with special emphasis on the analyses of phase behavior, role of cushion and type of gas, flow and recovery mechanisms, as well as prospective additional recovery of hydrocarbons. To reproduce the phase behavior and flow/recovery mechanisms, it was necessary to build a fluid model by tuning an equation of state using a PVT analysis representative of the area under study (Junín Block of the Orinoco Belt), specifically, lab tests such as Constant Composition Expansion, Differential Liberation, separator tests, together with viscosity measurements. This fluid model was based on a tailored lumping scheme that allowed the evaluation of the injection of CO₂, methane, nitrogen, and hydrogen. Additionally, miscibility slim tube experiments by injecting each of these gases (as solvents) are utilized and matched accordingly. Finally, multiple scenarios of gas injections were simulated in a cluster of wells in the field by evaluating gas injection time, injection and production rates, effect of pressure on the integrity of the reservoir, along with the possible impact on the production of hydrogen and additional recovery of hydrocarbons. Results of this study show the effect of the type of gas injected on the hydrocarbon storage and recovery capabilities in a real reservoir model, where the injected gases are immiscible with the selected extra-heavy crude oil. Different predictions of gas injection in both the crude oil and water zones, as well as the main mechanisms involved in the process are discussed in this article. Based on the complexity of current reservoir fluids (free gas, oil, water, emulsions, etc.), a program of fluid sampling and laboratory analyses are recommended, including pore scale analysis for the quantification of dissolution, trapping and wettability of the injected gases, among others. This article may serve as a basis for feasibility studies of gas storage in extra-heavy crude oil reservoirs in Venezuela and worldwide for the acquisition of experimental data and reservoir monitoring, which in turn would allow a successful implementation of this technology in a full-field scale.