Use of equivalent single porosity medium and automated lumped fluid composition simulation in naturally fractured gas condensate reservoirs

Ertürk, Mehmet Cihan
Each naturally fractured gas condensate reservoir is unique and needs special interests for an accurate modelling study. Ordinarily, it is a very difficult task to conduct a fast and well-characterized simulation study and predict the performance of such reservoirs in view of the complicated thermodynamic behavior, the complex fluid composition, dual porosity behavior and significant computational time requirement. The numerical simulation of fractured gas condensate reservoirs offer remarkable potential for understanding of the field development strategies even though it is a challenging process due to the aforementioned reasons. The conventional simulation study of naturally fractured gas reservoirs is carried out with dual porosity and/or permeability models and the compositional simulation methodology. The main issues concerning the fractured gas condensate reservoirs are dealing with a large number of components that form the condensate fluid composition and also the need for a large amount of grid cells because of the nature of dual medium approach. They both have adverse impact on the execution time and give rise to various instabilities and convergence problems; hence the efficiency of the simulation study is affected in a negative manner. vi In this work, it is aimed to attain not only a physically representative but also numerically time-efficient novel modeling approach that is required especially for the time consuming studies such as the sensitivity, uncertainty and optimization analysis. By using the proposed systematic lumping methodology based on the phase diagram comparison considering all the available schemes under the physical constraints and then to calculate the RMS error of each scenario compared to that of original quality lines of phase plot with isothermal depletion assumption, one can reduce the number of the components that represents the original fluid composition. Construction of an equivalent single porosity medium approach, which is not used for this type of reservoirs before, by averaging of variables such as porosity and permeability and weighting of some curves such as relative permeability curves instead of a traditional dual media technique reduces the number of cells, hence the simulation run time. In an attempt to validate the proposed lumping methodology and the equivalent single porosity technique, a naturally fractured gas condensate reservoir is evaluated by the traditional dual media and compositional simulation technique at first and then, additional near wellbore modeling approaches such as velocity dependent relative permeability and generalized pseudo pressure methods are incorporated into the model. The weaknesses and strengths of modeling approaches are assessed. Finally, the proposed new concepts are compared with the results of the conventional cases with respect to the accuracy and the run time of simulation. As a result, it is shown that the proposed lumping methodology and the equivalent single porosity are beneficial and adequate tools to be used for the modeling of naturally fractured gascondensate reservoirs by saving considerable time.