The Effect of Gas Injection on the Enhancement of Condensate Recovery in Gas Condensate Reservoirs: A Comparison Between a Synthetic Model and PVT Cell Results

Abstract

Gas condensate reservoirs have complicated fluid flow and thermodynamic processes. As reservoir pressure decreased, the gas condensate system exhibits a dew point. As pressure is reduced, liquid condenses from the gas to form free liquid in the reservoir. This liquid phase develops with pressure reduction and the composition of gas phase is changing due to pressure reduction. The condensate is deposited at such low liquid saturation that is usually trapped by surface tension forces, is therefore immobile, and cannot be produced. Condensate liquid builds up near wellbore causing a reduction in gas permeability and gas productivity. In order to prevent the condensate formation in reservoir, different scenarios have been proposed in which the most important method is gas injection to maintain the pressure or to revaporize the condensed liquid. In other words, the injection of dry gas into a retrograde gas condensate reservoir helps in vaporizing the condensate and increases its dew point. This study prepared a comparison between gas injection scenarios in cells (PVT studies) and a synthetic model of the reservoir. The main goal of this work is to investigate the lean gas recycling into the PVT cell and the synthetic model to enhance condensate recovery. In this work, these subjects were studied: the effect of CO2, separator gas, and N2 on condensate recovery enhancement and comparison of these scenarios with CO2, separator gas, and N2 injection and discussion on why the results are different in these two approaches. This study shows that when CO2, separator gas, and N2 are injected in PVT cell, deduction of condensed liquid during the pressure reduction of simulated constant volume depletion test at different injection volumes related to CO2 is more than separator gas and separator gas scenario is more than N2. In other words CO2 is the most effective and N2 is the least effective scenarios in reduction of condensate in the simulated constant volume depletion. But in the synthetic model, the results are completely different with the simulated PVT cell. In the synthetic model, deduction of condensed liquid during the different injection volumes related to N2 is more than separator gas and separator gas scenario is more than CO2. In other words N2 is the most effective and CO2 is the least effective scenarios in reduction of condensate in the synthetic model. Based on the type of condensate deduction mechanism, the three mentioned gases presented different results. N2 acts as the best gas when the decrease of liquid dropout is aimed to evaporate existing since at the same injection rate for all injected gases, N2 has increased the reservoir pressure more than the other gases and this reservoir pressure increase causes more re-vaporization at the synthetic model. In fact, the injection gas affects the decrease of liquid dropout formation indirectly. But if the injection gas affects directly on the reservoir fluid, CO2 acts as the best injection gas regarding the reservoir composition change. In this mechanism, the injection gas affects the liquid dropout formation directly.