Optimization of Lean Gas Injection in Gas-Condensate Reservoirs

Abstract

Abstract In gas-condensate reservoirs, a major challenge is to avoid the drop-out of the heavier fractions, which represent the rich part of the gas, responsible for great part of its heat capacity and, consequently, its price. A widely used practice to keep the reservoir fluid above its dew point pressure is the injection of produced lean gas into the reservoir. This technique is known as gas cycling. As the lean gas is injected in the reservoir, the overall fluid composition is changed and so does its dew point and phase envelope. Furthermore, the rate of pressure decline in the reservoir depends on the fraction of the produced gas being injected. We present a coherent thermodynamic analysis of the effects of lean gas injection in two Brazilian gas/condensate reservoirs through the determination of the phase behavior of injected/original gas mixtures. The accuracy of the thermodynamic model employed is verified through the comparison of the model predictions with laboratory data. The results obtained from this analysis are used to determine the maximum amount and optimal composition of lean gas to be injected in a gas/condensate reservoir in order to change its composition and shift the phase envelope to a point where the liquid drop-out can be eliminated or minimized during the isothermal depletion of the reservoir. The results can also be used to optimize the operation of gas pipelines and production equipment and facilities. Introduction Lean gas injection is widely used in gas/condensate reservoirs to maintain the reservoir pressure above the dew point pressure of the fluid system and avoid the condensation of the heavy fraction. The injected gas can also modify the phase behavior of the reservoir fluids leading to positive or negative implications with regard to the recovery of the heavy fraction. The injection can cause an increase in the dew point pressure of the reservoir fluid and leads consequently to the retrograde condensation of the heavy fraction at the dew point pressure of the original reservoir fluid. The injected gas can, on the other hand, result in the revaporization of the liquid previously formed in the reservoir", The ultimate recovery will strongly depend on the phase behavior of the mixture reservoir/injected fluid. The design of a gas cycling project involves the determination of the type and quantity of fluid to be injected. The increasIng price and demand for methane in the international market can make methane gas cycling economically impractical. The use of nonhydrocarbon gases has been suggested earlier in the literature. The decision should be based on an economical analysis of the project. The project design requires correct and reliable information of the thermodynamic properties and phase behavior of the reservoir fluid and mixtures generated during the cycling process. Simple cubic equations of state, such as the Peng-Robinson or the Soave-Redlich-Kwong have long been used to predict the phase behavior of reservoir fluids. Once adjusted to some experimental data, they are found to predict the phase behavior of petroleum fluids with reasonable accuracy. The difficult and challenging retrograde condensation problems have required the application of a more rigorous analysis that included the use of volume translation correction near the critical point. In this work, we report our investigation of the gas cycling projects proposed for two reservoir fluids. We analyzed the effect of lean gas injection on the condensate recovery of two Brazilian gas/condensate reservoirs. We also evaluated the effect of using an equimolar lean gas/nitrogen mixture as the injection fluid. The phase behavior of the reservoir fluid and reservoir fluid/injected gas mixtures was calculated using a thermodynamic model tuned using reservoir fluid liquid drop-out laboratory data. Thermodynamic Model and Fluid Systems The Peng-Robinson equation of state was used for all the calculations carried out in this investigation due to its simplicity and good accuracy in the prediction of natural gas systems. The volume translation as suggested by Peneloux and Rauzy was employed with the objective of obtaining a better prediction of liquid densities. Properties of the heavier fractions were obtained using experimental data, the Katz-Firoozabadi correlation and the Lee-Kesler critical properties correlation. P. 209