Effect of Including Composition-Dependent K Values To Generate Data for a Modified Beta-Type Simulator (includes associated papers 6418 and 6419)
- Richard L. Henry (Amoco Production Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- October 1976
- Document Type
- Journal Paper
- 1,180 - 1,183
- 1976. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 5.4.3 Gas Cycling, 4.1.2 Separation and Treating, 5.4.2 Gas Injection Methods, 4.6 Natural Gas, 4.1.5 Processing Equipment, 4.3.4 Scale, 5.5 Reservoir Simulation, 5.8.8 Gas-condensate reservoirs, 5.2.2 Fluid Modeling, Equations of State, 5.2 Reservoir Fluid Dynamics
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Cook et al. presented a modified beta-type reservoir simulatorforapproximatingcompositionaleffectsduring simulation of dry-gas injection into an oil or rich-gas-condensate reservoir. A composition parameter representing the cumulative volume of dry gas per volume of oil (G ) was incorporated into a standard beta-type simulator to make that simulator behave as a compositional model with a reported substantial reduction in computer time. The composition parameter and fluid properties required by the beta-type simulator were obtained from a linear cell-to-cell model. The parameter was plotted vs the fluid properties, and the result was a set of curves for each fluid property that was independent of cell number in the linear model. Therefore, the curves would be independent of the space dimension in a reservoir model. This was important because incorporating position-dependent curves into a beta-type model would significantly complicate this model, resulting in a loss of model simplicity and computer-time advantage.
The cell-to-cell model used by Cook et al. incorporated K values that were a function of pressure only; the K values were independent of fluid composition. The authors suggested their model for simulation of drygas injection. However, the results presented are for a relatively rich gas (9.3 1-percent C -C ). Composition dependence of K values would be minimal for this gas, which supports the fair agreement between performance predicted by the modified beta-type model and the compositional model.
For complex fluid systems, such as dry-gas injection into a volatile oil or a rich-gas condensate, K values are highly dependent on fluid composition. This paper presents results demonstrating that composition-dependent K values yield fluid properties that are dependent on cell number or location within the reservoir. Consequently, the modified beta-type model should not be used for reservoir simulations where large changes in fluid composition are anticipated.
A study was initiated to determine the effects of composition-dependent K values on reservoir properties during dry-gas injection into a volatile oil reservoir. The gas and oil compositions and their physical properties are given in Table 1. A cell-to-cell model, similar to the one used by Cook et al., was used to obtain the composition parameter. Gi, and the phase properties. This model differed from Cook et al.'s, however, in that it used the Redlich-Kwong equation of state matched experimentally equilibria. The equation of state matched experimentally determined volumetric data of the volatile oil and of mixtures of a lean gas with the gas condensate that is in equilibrium with this oil at reservoir conditions, The matching of these volumetric data results in composition-dependent K values; and to match these data, composition-dependent K values were predicted by the equation of state. These results suggest the equation of state can be used to simulate dry-gas injection into the volatile oil.
The computation logic of the cell-to-cell model has been presented in Refs. 4 and 5. Each cell of the model used in this study contained 1 bbl of equilibrated reservoir oil at the reservoir temperature of 265 deg. F and a pressure of 4,000 psia.
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