Predicting water encroachment can be of critical importance in describing and managing a hydrocarbon reservoir.

This study focuses on the use of the generalized material balance equation (GMBE) proposed by Walsh, et al1,2  along with the analytical solution to the diffusivity equation for a constant inner boundary pressure as presented by van Everdingen and Hurst to determine the size of a water-bearing formation in contact with a gas-condensate reservoir.

When the hydrocarbon reservoir fluid is a gas-condensate, it is essential to use the GMBE. Should the conventional material balance equation (CMBE) be used in lieu of the GMBE, considerable errors could be introduced as will be shown.

The main goal of this paper is to illustrate the ability of the GMBE to determine the size of an aquifer encroaching into a gas-condensate reservoir. Once this size is obtained, it could be used as an input parameter to a reservoir simulator to forecast the future expansion of the aquifer as depletion proceeds.

When the CMBE is used to describe black oil reservoirs, certain simplifying assumptions are normally made. Those include neglecting the effect of the compressibilities of the connate water and the reservoir rock, in addition to the volatilized liquid in the gas phase. The effect of these assumptions in gas-condensate reservoirs will be examined.

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