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

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.

Single-Core PAHs in Petroleum- and Coal-Derived Asphaltenes: Size and Distribution from Solid-State NMR Spectroscopy and Optical Absorption Measurements

“…38 The cluster size used for this application sets constraints to what can be obtained experimentally and was subsequently validated in NMR studies, which explicitly obtained an aggregation number of 6−7 nanoaggregates. 19 This first-principles modeling of asphaltene gradients in oil reservoirs has enabled understanding of many diverse reservoir concerns such as reservoir connectivity, 36,39,40 baffling, 41 heavy oil and tar mat formation 38,41,42 (related to gradients of dissolved gas 43 ), and viscosity gradients from diffusive processes. 44 In a similar manner, the Yen−Mullins model has been combined with the Langmuir EoS to model three separate universal curves obtained in oil−water interfacial studies.…”

Reservoir Fluid Geodynamics: The Chemistry and Physics of Oilfield Reservoir Fluids after Trap Filling

“…6 In addition, a comparison of the three diffusion models found that similar results were obtained in all cases. 6 More importantly, density inversion can be simulated by the three models. 6,34,35 As gas diffuses into the oil, the asphaltenes accumulate at the base of the gas front.…”

“…Nevertheless, this transformation is difficult to extend to two-dimensional/three-dimensional (2D/3D) problems. To overcome the deficiency of a presumption of constant total molarity, a formula has been derived to obtain component mole fractions based on component concentrations . In addition, a comparison of the three diffusion models found that similar results were obtained in all cases …”

“…6 More importantly, density inversion can be simulated by the three models. 6,34,35 As gas diffuses into the oil, the asphaltenes accumulate at the base of the gas front. An accumulation of asphaltene here can increase density of the oil beyond the original concentration, increasing the density likewise beyond the original density.…”

“…The color difference reflects large differences in asphaltene content of the fluids. In addition, gas–oil ratios (GORs) of the corresponding live oil samples vary enormously from 8000 scf/bbl to 1800 scf/bbl (where the unit scf/bbl represents standard cubic feet of gas per barrel of oil at 1 atm and 60 °F), further demonstrating the wide range of fluid composition encountered in a single reservoir. , (Dead crude oils have been flashed to 1 atm, thereby losing dissolved gases; live crude oil contains its dissolved gases as in the reservoir at elevated pressures. )…”