Oil Reservoir Pressure Reduction and Gas Liberation

Pressure and Volume Measurements

• After 4-8 pressure reduction, the cell contains only residual oil at reservoir temperature, which is then cooled to standard temperature and has its volume measured.

Liberation of Gas from Oil

• When reservoir pressure drops below the bubble point, flash liberation dominates.
• Gas comes out of solution and initially remains in the pores in contact with the oil.
• Single-phase oil continues to flow into the well, although it has a slightly different composition than that of the original reservoir fluid.
• When enough gas accumulates to reach the critical gas saturation, it begins to flow.
• Gas liberation occurs in the tubing, considered flash liberation, keeping the two phases in contact with each other.
• Flash liberation also dominates in the separator, where a sudden pressure drop releases more gas.

Petroleum Reservoir Fluids

• Petroleum reservoir fluids can be divided into five general categories in increasing order of chemical complexity:
  • Dry gas
  • Wet gas
  • Gas condensate
  • Volatile oil
  • Black oils
• The phase behaviors of gas condensates and volatile oils are considerably more complex than those of black oils.

Phase Behavior

• Understanding phase behavior is crucial as oil and gas are produced from the reservoir, subjected to pressure, temperature, and compositional changes.
• These changes affect the volumetric and transport behavior of reservoir fluids and the produced oil and gas volumes.
• The type of reservoir fluid determines depletion and production strategies and the design of the surface facilities.
• Except for polymer flooding, all EOR methods rely on the phase behavior of reservoir fluids and fluids injected into the reservoir.

Reservoir Fluids

• Schematic phase diagrams include:
  • Generic (single and multi-component cases)
  • Black Oil
  • Solution-Gas Drive
  • Dry Gas
• Black oil (P>Pb):
  • Properties: Bo, µo, Co (assumed constant)
• Solution-gas drive (P