Several authors have shown the applicability of modified black oil (MBO) approach for modeling gas condensate and volatile oil reservoirs. MBO approach could adequately replace compositional simulation in many applications (including water influx or water injection applications). Changing separator conditions during the history of the simulation run still needs compositional simulation, since PVT properties are calculated for specific separator conditions in black-oil and MBO approaches. Some commercial simulators have treated changes in separator conditions for black-oil applications. However, and to the best of our knowledge, no such treatment exists for the MBO approach. In field operations of volatile oils and gas condensates, however, separator conditions often change because these volatile fluids are usually separated through three and more separation stages, when wellhead pressure allows. We often see that the well stream is usually rerouted to lower pressure separators at later stage of the field life.

In this work, we derived two new equations that can be easily programmed into existing simulators and will allow accurate calculations of oil (or condensate) and gas rates when separator conditions change. These equations extend the use of MBO approach and provide an alternative to compositional simulation when field operations undergo changing separator conditions. The inputs to our equations can be easily obtained from the EOS model used to generate the PVT tables for MBO simulation runs.

To validate our work, we generated many MBO PVT tables using Whitson and Torp method for a variety of fluids covering wide range of gas condensates and volatile oils. We applied our new equations to calculate oil and gas rates when separator conditions change for all these fluids. For validation of our approach, we then used fully compositional simulation and applied the same changing separator conditions and compared the rates, which compared very well. In the paper, we also show several field examples exhibiting large gas-oil ratio (GOR) and condensate-gas ratio (CGR) variations when separator conditions change, and the applicability of our technique on field data.

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