Institute of Petroleum Engineering

Project: Depressurisation (DP)

When the water-cut is too high for economical production, the field is usually abandoned with significant quantity of oil and gas still remaining in the reservoir. It is likely that in some cases a significant amount of gas and residual oil can be produced by the process of depressurisation.

The depressurisation process includes several stages: supersaturation, gas nucleation, bubble growth, critical gas saturation and finally oil and gas production. When the pressure of saturated oil falls below the bubble point pressure, there comes a point at which stable nuclei are formed in the supersaturated oil. The difference of pressure at which the first bubble is formed during depressurisation and the saturation pressure is called the critical supersaturation.

Formation of a new gas/liquid interface requires energy; therefore the liquid has to be at a pressure lower than the bubble point pressure, before a nucleus can form. The required work to create a gas bubble in the saturated oil phase is a function of the interfacial tension between gas and liquid. When the gas/oil interfacial tension is high the required work to create a gas nucleus will also be high. The extent of critical supersaturation is affected by dynamic conditions as well as departure from equilibrium conditions. Hence increasing the depletion rate, that is, lowering the time at which a certain level of supersaturation is maintained will increase the value of critical supersaturation. The limit of supersaturation may be determined by thermodynamic considerations. Once gas bubbles are formed, and depressurisation is continued, bubbles keep growing and displacing oil and water. As the nucleation mechanism has a direct impact on the gas evolution and hydrocarbon recovery during the depressurisation process, the nucleation phenomenon is of great interest.

The underlying physics of the above complex three-phase flow is not well understood to allow reliable predictions to be made for economic evaluation of the process.