Analysis of a non-volumetric gas-condensate reservoir using a generalized material balance equation with fluid properties from an equation of state

Abstract

Predicting the expansion of an underlying aquifer can be of paramount importance in certain aspects of petroleum engineering, such as the control of water coning, calculating reserves, designing well completions, designing surface facilities such as separators, pumps, etc. Predicting water encroachment can be of critical importance in describing and managing a hydrocarbon reservoir. This study focuses on the use of a generalized material balance equation (GMBE) along with the analytical solution to the diffusivity equation for a constant inner boundary pressure 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 demonstrated in this paper. 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.