Compositional Mixing Characteristics of Separate Gas and Oil Charges into Oil Field Reservoirs

Abstract

Many reservoirs worldwide received separate charges of primary biogenic gas and oil. In high pressure provinces such as the Gulf of Mexico, the added gas should be in solution if thoroughly mixed. In addition, this biogenic gas is easily differentiated from thermogenic gas by measurement of methane carbon isotopes and gas composition. Consequently, the extent of mixing can be determined with accuracy, and the gas and oil mixing processes depend on specifics of reservoir charging and can lead to multiple outcomes. The conventional expectation is that the biogenic gas always precedes oil entry into the reservoir. Here, chemical evaluation of reservoir fluids coupled with reservoir understanding of several reservoirs establishes that the oil can arrive prior to biogenic gas. Seismic imaging of gas chimneys provides a plausible explanation for how this can occur. In addition, thermodynamic and geochemical evaluation of the reservoir fluids across the reservoir tightly constrains possible mixing processes. Simple reservoir simulation shows that excellent gas and oil mixing can occur over a wide range of conditions provided that the gas charges into the oil column at its bottom (at the oil–water contact). This modeling exhibits simple fluid mechanics expectations for this reservoir charging process. In addition, since the increase of solution gas destabilizes asphaltenes, fluid geodynamic processes are indicated that can lead to redistributions of asphaltenes. Evaluation of gas and oil mixing processes in reservoirs fits within the framework of the powerful new technical discipline, reservoir fluid geodynamics, for subsurface characterization.