Predicting gas-cap and tar-mat formation conditions in hydrocarbon reservoirs. Application of Continuous Thermodynamics

A method to predict the gas/oil (GOC) and oil/tar (OTC) fluid contacts of reservoir fluid columns containing heavy fractions and asphaltenes is presented. A new formulation and solution to the gravitational equilibrium equations in terms of the method of moments, and the use of an equation of state for semicontinuous mixtures are shown to provide a robust and consistent method to describe the variation of pressure, reservoir fluid composition and heavy-ends molecular weights with depth, as well as the location of the fluid contacts of a petroleum reservoir. Use of the new method is first illustrated by locating the GOC of a (continuous) ideal reservoir described by Raoult´s law, and by predicting the GOC and OTC of two reservoir systems where measured data are available. The good agreement of the new method with measured gradients shows the adequacy of the proposed approach.