Gas exsolution during fluid migration and its relation to overpressure and petroleum accumulation

Abstract

Petroleum systems in the Tertiary sections of the Gulf of Mexico and Niger Delta, among other basins, are characterized by the close association between petroleum accumulation and over-pressure. Although overpressure is generated mainly by compaction disequilibrium of the low-permeability sediments, exsolution of gases from water and petroleum during migration over great vertical distance may create overpressure or modify the existing overpressure. In this paper, the author combined relative permeability data and petroleum phase behavior modelling to explain and predict the presence of an overpressure and petroleum ‘baffle’ due to gas exsolution. Generalized isoperm diagrams for oil, gas and water phases in sandy and silty/shaly sediments were created based on published data. Aqueous solubility of methane and carbon dioxide as a function of temperature, pressure, and salinity is summarized based on literature data. Phase diagrams of petroleum with a wide range of composition, API gravity, and GOR were produced using phase behavior software with calibration using field data. These tools were integrated to model overpressure and petroleum phase variations in two oil/gas fields in the Gulf of Mexico. The pressure gradients in these fields are likely modified by gas exsolution from the petroleum which migrated and accumulated within and below the pressure transition zone (PTZ). Petroleum trapped within PTZ tends to be fractionated, whereas that accumulated below PTZ tends to be single phase with high API gravity. These results provide a model which explains both the effect of gas exsolution on PTZ and the occurrence of different types of petroleum in these fields. These tools may be integrated with geological and seismic data to assess more accurately the economic value of petroleum accumulations in petroleum systems governed by overpressure and petroleum phase separation and fractionation.