Predicting Yield of Revaporized Condensate in Gas Storage
Authors: Katz Donald L. , Herzog Robert A. , Hekim Yusuf 
Published: December 2007

Summary
Phase behavior calculations for gas-condensate systems Phase behavior calculations for gas-condensate systems were used to predict depletion yields of condensates. The same procedures may be used for predicting revaporization and depletion yields of condensates in gas storage cycles. Data on two Niagaran reef storage reservoirs in Michigan show yields less than ideal for complete mixing and revaporization in the reservoir.
Phase Calculations for Gas Phase Calculations for Gas Condensate Reservoir
Depletion of gas-condensate reservoirs of fixed volume is accompanied by retrograde condensation of liquid in the reservoir. Calculations of phase behavior during primary production is needed to verify the surface yield primary production is needed to verify the surface yield of condensate and to obtain the composition and quantity of retrograde liquid in the reservoir. The depletion program used for studying Michigan Silurian reef fields for program used for studying Michigan Silurian reef fields for conversion to storage is that of Firoozabadi et al. based on the Peng-Robinson equation of state and interaction parameters for methane and the C6+ constituents. parameters for methane and the C6+ constituents. A first step in these phase-behavior calculations is to obtain a recombined separator gas and liquid well-stream analysis at an early date in the life of the reservoir along with the reservoir temperature and pressure. The extended analysis basic to these calculations should include constituents with boiling points as high as C13 to C20 groups. The gas and condensate production vs. reservoir pressures is needed for material balance of yields and determination of reservoir size.
Fig. 1 for Cold Springs 12 and Fig. 2 for Rapid River 35 reservoirs show a comparison of primary yield of condensate. The agreement verifies that the phase relationships, composition of well fluid, and depletion balances are reliable for gas-storage vaporization and depletion calculations. The calculated composition of the reservoir gas and retrograde liquid (some 3 to 4% by volume of pore space) are needed in gas-storage calculations. pore space) are needed in gas-storage calculations. Gas Storage
Underground storage fields are equipped to inject gas in summer at periods of low demand and to produce gas at high rates for peak or high-demand periods. With pressures above discovery, storage reservoirs may pressures above discovery, storage reservoirs may deliver 80 to 100% of the primary content in a 100-day withdrawal period. When condensate is present in the reservoir because of retrograde condensation, a second purpose is to produce the condensate by vaporizing it purpose is to produce the condensate by vaporizing it with gas pressure, followed by depletion or displacement.

Modes of Operation
There are alternate modes of operation for injecting and withdrawing gas, three of which are discussed. Mode 1 is to inject into all wells and withdraw gas from all wells. One might assume a completely mixed fluid of dry gas, reservoir gas, and reservoir liquid. In the initial withdrawal, the gas will be leaner than calculated in that the condensate near the wellbore will have been stripped before full pressurization.
JPT
P. 1173