The modified black-oil model (MBO) was tested against the fully compositional model and performances of both models were compared using various production and injection scenarios for a rich gas condensate reservoir.
We evaluated the performance of MBO model by investigating: the effects of black-oil PVT table generation methods from a tuned equation-of-state, oil-gas ratio (OGR) and saturation pressure versus depth as initialization methods, uniform composition versus compositional gradient with depth, location of the completions, production and injection rates, kv/kh ratios, and vertical wells versus horizontal wells.
Contrary to the common belief that OGR versus depth initialization gives better representation of original fluids in place, initializations with saturation pressure versus depth gave closer original fluids in place considering the true initial fluidsin place are given by the fully compositional model initialized with compositional gradient.
Unrealistic vaporization in the MBO model was encountered in both, production by natural depletion and gascycling. The changes in oil-gas ratio of the recyled gas showed that, it is not possible to accurately represent the changing PVT properties of the recycled gas with a single PVT table. Unrealistic vaporization also led to different arrival times for the displacement fronts and different saturation profiles for the near wellbore area and for the entire reservoir for the two models even though the production performance of the models was in good agreement.
The MBO model representation of compositional phenomena for a gas condensate reservoir proved to be adequate for full pressure maintenance, reduced vertical communication, vertical well with upper completions, and for horizontal well producers.
Black-oil simulators represent a high percentage of all simulation applications and they can model immiscible flowunder conditions such that fluid properties can be treated as functions of pressure only.
However, gas condensate reservoirs exhibit a complex thermodynamic behavior that cannot be described by simple pressure dependent functional relations. Compositions change continuously during production by natural depletion, or by cycling above and below dew point pressures.
In another black-oil modeling approach reservoir fluid consists of a gas component and vaporized oil which allows the use of a simple and less expensive model. According to this modified black-oil approach liquid condenses from a condensate gas by retrograde condensation when the pressure is reduced isothermally from the dew point, and retrograde liquid is vaporized by dry gas.
Coats1 presented radial well simulations of a gas condensate that showed a modified black-oil PVT formulation giving the same results as a fully compositional EOS PVT formulation for natural depletion above and below dew point. Under certain conditions, he found that the modified black-oil model could reproduce the results of compositional simulation for cycling above the dew point. For cycling below the dew point, the two component simulation gave results that were quite inaccurate.
According to Fevang and Whitson2, results from Coats' example should be used with caution as EOS characterization uses seven components with one C7+ fraction. With a more detailed C7+ split, oil viscosity differences between black-oil and compositional formulations often yield noticeable differences in well deliverability.
