Phase Changes: A Major Aspect of Deep Water Hydrocarbon Migration

Gatenby, Glenn, Maxus (U.S.) Exploration Co., The Woodlands, TX

Abstract

Major oil fields in the deep water Gulf of Mexico are found at subsurface depths coinciding with significant changes in the solubility of oil in gas – suggesting that oil and gas phase separation is a pervasive step in petroleum migration. Additionally, changes in the solubility of methane in water coincide with the depth of gas reservoirs – suggesting that gas exsolving from water is also a process to consider in hydrocarbon migration.

The solubility of oil in gas is calculated from Price’s experimental data and plotted versus depth. Increasing temperature and pressure with depth increases the solubility of oil in gas and it eventually becomes asymptotic leading to the two becoming cosoluble. Vertically migrating hydrocarbons are most likely to phase separate at this depth. The importance of this depth of phase separation cannot be overstated as the largest fields are found proximally above this depth. Generally, as the magnitude of the change in solubility increases, the accumulation size also increases. Conversely, no commercial pays are found beneath the depth where oil and gas are cosoluble (an economic basement).

The solubility of methane in water is calculated using Haas’ program and when plotted versus depth also reveals that major gas reservoirs are commonly found at the same depth where vertically migrating water would experience an abrupt decrease in methane solubility. Results from onshore GOM wells confirm methane saturation at high temperature and pressure and these waters would exsolve gas if the water migrated vertically.

The solubility versus pay relationship is an improvement over the existing pressure versus pay relationship, and, this important new step in the hydrocarbon migration process fortunately can be modeled ahead of the bit thus leading to the development of a new exploration strategy.