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The time spent on developing a common EoS model for reservoir fluids with gas injection can be reduced considerably by following the simple recipe given below. It is a requirement that swelling data exists and the swelling experiments have been continued until the saturation point changes from a bubble point to a dew point as sketched in Figure 1.

The saturation point of the reservoir fluid can generally be matched by adjusting the plus molecular weight by 5-10% while keeping the weight% composition constant. Such adjustment is justified because of the experimental uncertainty on the plus molecular weight. The STO oil density can be matched by adjustment of the Peneloux volume correction parameter of the C 7+ components.

The critical point of a reservoir fluid will not in general be known, but it can be determined in an indirect manner. bp gas prices akron ohio Figure 2 shows two phase envelopes. They are for the same oil mixture, but using two different EoS models (EoS 1 and EoS 2). The saturation pressure at the reservoir temperature is almost the same, but with EoS 1 the critical temperature is 409 oC (768 oF), while it is only 360 oC (680 oF) with EoS 2.

Figure 3 shows experimental and simulated liquid dropout curves for swollen mixtures of the oil in question using EoS 1 and EoS 2. With 150 mole% gas added (grey points), the saturation point is a bubble point. o goshi With 400 mole% gas added (dark blue points), it has shifted to a dew point. gas vs electric heat The fluid composition becomes critical somewhere in that interval. At near critical conditions the liquid dropout curve would start out almost vertical from either 100% liquid volume (T Tcrit). The experimental liquid dropout points for 150 mole% gas added in Figure 3 shows a fairly steep decrease in liquid volume with decreasing pressure signaling a fluid approaching criticality. The liquid dropout points for 400 mole% gas added on the other hand has a relatively moderate increase in liquid volume with decreasing pressure. The simulation results with EoS 2 reflects the experimental behavior, but EoS 1 does not.

Figure 4 shows experimental and simulated swelling saturation pressures. With both EoS models the experimental saturation pressures are matched well, but 328 mole% of gas must be added per initial mole oil to create a critical composition with EoS 1, while only 244 mole% gas is required with EoS 2. gasbuddy This is reflected in the simulated liquid dropout curves in Figure 3. With EoS 1 the liquid dropout curve starts out relatively steep after 400 mole% gas has been added, which is an indication of near-critical behavior. gas pedal lyrics With EoS 2 the simulated liquid dropout curve after addition of 400 mole% gas is relatively flat, signaling some distance from a critical composition.

The observed difference in critical point on the swelling curve in Figure 4 leads back to the difference in critical point in Figure 1. The higher the difference between the critical temperature of a reservoir fluid and the reservoir temperature, the more gas is required to shift the critical temperature to that of the reservoir. As can be seen from the right hand plot in Figure 3, EoS 2 provides as almost perfect match of the liquid dropout curves of the swollen mixtures. The good match is achieved solely by tuning to a critical temperature of 360 oC and to the saturation pressure at the reservoir temperature. With EoS 1 too much gas is to be added before a critical composition is reached. This is due to the fact that EoS 1 predicts a too high a critical temperature of the reservoir fluid.

An alternative to matching the critical point of the reservoir fluid itself is to match the critical composition on the swelling curve. For the case studied, the critical composition is in the interval from 150 to 400 mole% gas added per initial mole oil, and therefore relatively undefined. In such case it can be easier to determine the critical point in an indirect manner as outlined above.