Investigation of density inversion induced by gas charges into oil reservoirs using diffusion equations

A one dimension (1-D) diffusive model with a moving boundary is developed in this paper coupled with the FHZ EOS(Flory-Huggins-Zuo equation of state ) and applied to three component (gas, maltene and asphaltene) systems. The gas/oil contact moves up with time due to the swelling effect of a gas charge into a crude oil reservoir. Density inversion can be produced mainly by the cross-term effect in diffusion (i.e., the impact of the presence of charging gas on the chemical potential of asphaltenes or gas expulsion) in a relatively wide range of conditions. Parameter sensitivity analyses demonstrate that significant fluid density inversion is generated at the conditions close to the asphaltene phase instability boundary, which is not surprising because the cross-term effect in diffusion becomes significant at those conditions. In addition, the diffusion model can be used to identify asphaltene phase instability directly. Application of this diffusive model to oilfield cases is in progress.