The Use of Two Phase Compressibility Factors in Predicting Gas Condensate Performance

Abstract

Abstract Gas condensate reserves are typically underestimated when single phase compressibility factors are applied to two-phase systems. The effect is worse in rich gas condensate system with adverse implications for gas initially in place (GIIP) estimation, gas reserves, petroleum accounting, and pipeline designs. Accurate prediction of GIIP early in the life of a reservoir is therefore crucial in reaching development decisions and contractual agreements. Underestimation of reserves often results from approximations based on single phase (Standing and Katz) compressibility factors that do not consider two-phase regions that retrograde condensate reservoirs exhibit at pressures below the dew point. During isothermal pressure reduction below the dew point phase changes occur leading to liquid and gaseous phase separation in the reservoir. Consequently, there is need to introduce a two phase Z factor in material balance calculations to account for the phase changes in the reservoir. Ideally, Z-factors for gas reservoirs are determined from constant volume depletion (CVD) tests that simulate compositional changes during reservoir depletion. Sometimes the CVD test reports are not readily available or plans are non-existent for PVT data acquisition campaign. A methodology that employs a suitable correlation developed using fluid molar composition or gas gravity of reservoir fluid to obtain two-phase Z-factors (Z2p) is described. This compares quite well with actual data for retrograde condensate systems. This paper describes a correlational research design with two-phase Z-factor derived from the real gas law equation and single phase Z factor from Rayes et al (1992) to determine GIIP for gas condensate reservoirs. Z-factors were also determined from Dranchuk Abu-Kassem (1975) correlations using pseudo-reduced properties presented by Sutton (1985) and Kay (1936) mixing rules. The paper compares Z2p, Z-factors, GIIP values obtained from the CVD test and correlations and establishes that the two-phase Z-factor should be used for more precise estimation of GIIP in gas condensate reservoirs.