Behavior in Gas and Oil Systems

Four types of reservoirs have been identified depending upon the temperature and pressure in the wellbore  

A black oil reservoir is one whose temperature is less than the critical temperature and is undersaturated. Gas will form at the wellbore in this reservoir only when the pressure reaches the bubble point. 

A volatile oil reservoir also occurs before the critical temperature however, the gas-oil ratio is higher than in black oil reservoir. As mentioned earlier, even though gas forms at the wellhead normally, the bubble point pressure has to be reached for gas to form in the reservoir. 

A gas condensate reservoir occurs between the critical temperature and cricodentherm (the maximum temperature at which the vapor and liquid 

A gas reservoir occurs at temperatures above the cricodentherm and no liquid forms at any pressure in this well. However, if the wellhead conditions prevail in the phase envelope, some liquid may form in the wellbore and appear at the surface. 

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