Gas, Bg, and the Phase Envelope

The Two-Phase Loop

On a pressure-temperature diagram the region where oil and gas coexist is a loop. Its left edge is the bubble-point curve and its right edge the dew-point curve; they meet at the critical point. The warmest point on the loop is the cricondentherm.

Reservoir Temperature Sets the Fluid

Where the reservoir temperature sits decides the fluid type. Cooler than the critical point gives an oil, whose depletion crosses the bubble point and evolves gas. Between the critical point and the cricondentherm gives a gas condensate. Warmer than the cricondentherm gives a single-phase gas that never drops liquid in the reservoir.

Retrograde Condensation

The gas condensate is the surprising case: as pressure falls below the dew point, liquid drops out in the reservoir rather than vaporizing, the opposite of intuition, which is why it is called retrograde. That liquid is valuable and can be lost in the pores, so condensate fields are often produced with pressure maintenance. Gas itself expands many-fold from reservoir to surface, so its formation volume factor Bg is tiny and grows as pressure falls.