Performance Prediction of a Rich Gas/Condensate Reservoir Through Material Balance and PVT Behavior: A Case Study

Abstract

Abstract With a 217 STB/MMSCF liquid-gas ratio (LGR) Sarvak reservoir of CARBONATE field in Iran is one of the richest gas condensate reservoirs in the world. Due to asphaltene precipitation during well shut-in, it is impossible to acquire static pressure data for the reservoir. Since the reservoir pressure is of the main reservoir parameters for performance prediction, material balance calculations was performed based on volumetric and PVT data to determine current reservoir pressure. Furthermore the retrograde condensate deposition (liquid drop-out) is also calculated with the introduction of new parameter of Liquid-Fluid ratio (LFR). The curve of P/z (the ratio of pressure and gas deviation factor) versus cumulative gas production is conventionally utilized to predict the gas reservoir performance. This method is used for gas condensate reservoirs by replacing gas z-factor with two-phase z-factor (zT). Since reservoir pressure data are not available a direct usage of the above method for CARBONATE-Sarvak reservoir is impossible. As such the experimental data of Constant Volume Depletion (CVD) experiment have been used that simulate the gas condensate fluid behavior. The experimental data are simulated using the PVT simulator of Winprop and tuned to Peng-Robinson EOS to provide non-available parameters and to check the validity of the data. Gas deviation factor at the dew point (zd) was determined as 1.2433 based on which the two-phase z-factor was calculated for different pressures below the dew point pressure. Backward calculation was performed to correct the available experimental data of produced gas percentage to reservoir conditions. The percentage of gas produced to reach the dew point was determined as 3.3029 percent of the Gas Initially In Place (GIIP). Utilizing the GIIP of 620 MMMSCF concluded from volumetric calculations the reservoir will reach the dew point after 20.48MMMSCF of gas production. It is estimated that current reservoir pressure after cumulative gas production (Gp) of 60 MMMSCF would be 5819 Psia. Furthermore the retrograde liquid deposition of the reservoir is determined to be 35.31 MMSTB through the plot of liquid-Feed Ratio (LFR) versus Gp/G.