Well Interference Test in Naturally Fractured Gas Reservoir: Numerical Simulation and Impact on Reservoir Performance Forecasting

Abstract The paper presents the evolution of the simulation approach for a green gas field in the Republic of Uzbekistan. The field is currently being appraised as part of the pilot development design process. The reservoir is composed of intensely fractured carbonate rock. Appraisal results that explicitly required an immediate transition from single porosity (SP) simulation to the dual porosity (DP) approach are described along with the resulting impact on production forecasting and reservoir development design. During an appraisal campaign, a well interference test intended for a dual media study has been conducted. The SP simulation model, which was used as a main RE tool for designing the field development, failed to simulate it. This required the implementation of the DP approach. The DP model predicts a shorter period of gas production plateau and earlier water breakthrough compared to the SP model. The uncertainty most impacting on cumulative production is revealed to be the aquifer volume. In the DP model case, the larger the aquifer, the shorter the production plateau and the sooner water breakthrough occurs. Conversely, the SP model case establishes the positive impact of the aquifer volume on cumulative production. Thus, the SP approach conceals significant RE risks, leading to suboptimal field development decisions. The appraisal program and the pilot development project for the field have been updated to ensure they remain optimal, considering the new simulation approach. Therefore, for the fractured reservoir – where reservoir permeability is provided by fractures and porosity is attributed to matrix – it is crucial to conduct fracture studies and implement the appropriate simulation approach from the very first steps of field exploration. Matching a DP model to an interference test data results in determining storage and flow capacity parameters: porosity, permeability and compressibility for the matrix and fractures separately, whereas the analytical model's interpretation provides only integrated parameters of dual media – interporosity flow coefficient and storativity ratio. Furthermore, at the early stages of a field development the fracture compressibility, which critically impacts reservoir performance, can only be evaluated using the approach presented.