Modeling of Large Hydrocarbon Compositional Gradient

Abstract

Abstract The paper presents fluid compositions sampled in 6 different depths in a North Sea petroleum reservoir over a vertical interval of 44 m. The fluid column consists of an upper gas condensate zone and a lower volatile oil zone connected by a gas-oil-contact (GOC). The data reveals large vertical variations in the fluid composition. It is shown that the compositional variation with depth is much higher that what can be explained solely by the action of gravitational forces. The major contribution to the compositional variation originates from the influence of the vertical temperature gradient, which is of the order of 0.026 °C/m. The influence on the compositional gradient from the temperature variation is successfully described using the theory of irreversible thermodynamics. Introduction In a system without significant height differences each component will at thermodynamic equilibrium have the same chemical potential no matter where in the system it is located. For a system with considerable height differences an additional height potential term will have to be considered. For an isothermal reservoir this results in an equilibrium state where the concentration of high molecular weight components increases with depth, while on the other hand low molecular weight components like methane is found in higher concentrations in the upper zones than in the deeper ones. Schulte1 outlined procedure for simulating compositional variations with depth for an isothermal petroleum reservoir. In most reservoirs the temperature increases with depth. A typical vertical temperature gradient is 0.02 °C/m. A petroleum reservoir with a thermal gradient will not be at thermodynamic equilibrium. There will be a transport of heat and material between the upper and lower regions. At stationary conditions the compositional variation with depth is determined by component fugacities, molecular weights and heat content (absolute enthalpy per mass unit). Pedersen and Lindeloff 2 have set up relations based on irreversible thermodynamics for the compositional variation with depth in a stationary reservoir with a temperature gradient. Ignoring gravity effects and assuming stationary conditions the compositional variation with temperature is determined by the absolute enthalpy per mass unit (the so-called specific enthalpy) of each component. A component with a higher specific enthalpy than the average for the mixture will in a system with a temperature gradient have a preference for the warmer zone. At typical reservoir conditions high molecular weight components will have a higher specific enthalpies than low molecular ones. This is compliant with experimental observations showing that the compositional variation with depth in reservoirs with a positive vertical temperature gradient is higher than what can be explained gravity segregation alone2,3. Experimental data Table 1 shows compositions from 6 different depths in a petroleum reservoir. The compositions are average values of a total of 17 compositions sampled in these 6 depths. The reservoir fluid consists of a gas cap on top with an oil column beneath. A gas-oil contact (GOC) is found to be in a depth of 3647 meter. The two compositions in Table 1 from depths above 3647 m are from the gas zone and the remaining 4 samples are from the oil zone. Table 2 shows some key data for the reservoir and for the sampled compositions including pressure and temperatures variations with depth and variations in saturation point, gas-oil ratio (GOR) and fluid density with depth. The shown numbers are average values from a total of 17 samples. The temperature increases by approximately 0.026 °C per meter vertical depth. Figure 1 to Figure 4 shows plots of the variation with depth in reservoir pressure, saturation pressure, methane mol%, GOR and reservoir fluid density. Data is plotted for all 17 samples. It is obvious from Figures 2–4 that there a distinct change in properties at of depth of around 3647 m consistent with the observation that a GOC should exist at this depth. Considering that the total depth of the sampled zone is only 44 m, the observed compositional variations are quite considerable. At all depths the fluid is close to its saturation point at which state the compositional variations with depth will always be more pronounced than for a more undersaturated fluid. The compositional change at the GOC further contributes to the picture of a strong compositional gradient.