Delineation of Gravitational Instability Induced by Gas Charges into Oil Reservoirs Using Diffusion and Flory-Huggins-Zuo Equations

Abstract

Abstract A reservoir crude oil contains dissolved gas, liquid and solids (asphaltenes). Together with the Yen-Mullins model and the Flory-Huggins-Zuo equation of state (FHZ EOS), Downhole fluid analysis (DFA) has been successfully employed to delineate asphaltene gradients and reservoir connectivity. Because solubility of asphaltenes in crude oil decreases with increasing gas/oil ratio (GOR) and asphaltene instability, tar mats may be formed by a late gas or light hydrocarbon charge into an oil reservoir. This tar mat is frequently far away from the location of the asphaltene instability, thereby implying asphaltene migration in the formation at much faster rates than diffusive velocities. Therefore, a dynamic mechanism is required to take this phenomenon into account. A dynamic mechanism with gravitational instability induced by gas charges into an oil reservoir is proposed for this purpose. A late gas charge into an oil reservoir can yield a gas cap above initially undersaturated oil. Gas diffusion into the oil column then increases solution gas (GOR) which has the effect of decreasing asphaltene solubility in the crude oil. This can cause asphaltenes to diffuse away from the region of increasing solution gas. Diffusion equations that couple methane, maltene and asphaltene diffusion are consistent with this description. This process can lead to a build-up of asphaltenes at a certain position of the diffusive solution gas front. This increase in asphaltenes, which have been expelled from the high GOR regions, leads to higher density oil than the original oil below this perturbed section of the oil column. In turn, this ‘fluid density inversion’ in the oil column gives rise to gravity currents (gravitational instability – diffusion induced convection) that enable the movement of asphaltenes in porous media over large distances. To validate the proposed mechanism, a diffusive model with moving boundary conditions was developed in conjunction with the FHZ EOS and was applied to gas, maltene and asphaltene multicomponent systems. In addition, this diffusive model is compared with the simplified diffusive model developed by Zuo et al. (2016) and the rigorous diffusive model developed by Shu et al. (2016). The three models predict that the gas/oil contact (GOC) moves up with time due to the swelling effect of a gas charge into a crude oil reservoir and to the loss of gas from the gas cap diffusing into the oil. Density inversion can be produced by the three models by the coupled gas and asphaltene diffusion equations in a relatively wide range of conditions. Significant fluid density inversion is generated at the conditions close to the asphaltene phase instability boundary. This is consistent with the frequent observation of some asphaltene deposition at the gas-oil contact and some asphaltene deposition at the oil-water contact.