In hot, high pressure gas condensate reservoirs significant vaporization of connate water can occur during depletion. As the reservoir pressure decreases, the equilibrium water vapor content of the reservoir gas increases, causing connate water within the gas-bearing portion of the reservoir to vaporize. This both increases the vapor phase pore volume of the reservoir, and decreases the hydrocarbon content of the reservoir gas. Also, condensation of liquid hydrocarbons below the dewpoint will decrease the vapor phase pore volume of the reservoir, invalidating assumptions made in a simple material balance calculation.
A modified form of the material balance equation has been developed to account for these effects. Failure to account for them- can result in erroneous predictions of gas-initially-in-place, and hence reserves. It may also lead to incorrect identification of reservoir drive mechanisms.
Field examples are presented for a gas condensate reservoir below the dewpoint, and for a gas reservoir where water vaporization effects are significant.
As targets for gas reservoirs become progressively deeper, and hotter, the significance of the effects noted in this paper increases.