Check and Modify Equilibrium Contact Mixing in Gas Condensate System with Case Studies of Iranian Reservoirs

The significance of obtaining representative samples in optimizing reservoir management is clearly obvious for petroleum experts. Always, representative samples is tried to collect in preliminary time of production by standard methods. In the basis of valid theories, sampling operation must be done before reduction of bulk reservoir pressure below initial dew point pressure for retrograde gas reservoirs. Because representative samples can be collected just in this limited time. However Equilibrium Contact Mixing (ECM) method claims to obtain original in-situ fluid composition from non-representative samples without time limitation, so it contravenes valid theories. Huge differences between governed standards of sampling that are in the basis of valid theories and the ECM method make us to test ECM in two aspects and compare it to the recombination method. Case studies of Iranian gas condensate reservoirs and synthetic models in the basis of real PVT data are our aspects. For simulating ECM method, detailed characterization of Equation Of State (EOS) in a dynamic process is used for the first time. Also synthetic models are constructed by compositional simulator and sampling is performed in depleted conditions. Then ECM and recombination methods are performed on obtained samples. In conclusion, we find that ECM in a current form is not performable in all gas condensate systems and it needs a modification. Therefore, we propose modified ECM method as an accurate and performable method in every condition. Also, we conclude that accuracy of modified ECM method is reduced by increasing the richness of reservoir fluid. In addition, ECM is investigated in two retrograde gas fields from center and south of Iran as case studies. Therefore accuracy of ECM is checked and endorsed in the basis of rock and fluid properties of Iranian reservoirs. In conclusion, current standards and theories of sampling are contravened by ECM and modified ECM methods. By comparing results of ECM, modified ECM and recombination methods, it is understood that the modified ECM and ECM methods are more accurate than recombination. Hence useless but accurate ECM and modified ECM techniques must be replaced to not accurate but utilized recombination.