Effect of gravity drainage models on production efficiency of naturally fractured reservoirs

The gravity drainage process is one of the essential recovery mechanisms in the naturally fractured reservoirs. The contribution of the process to the ultimate oil recovery is quite uncertain, and it highly depends on the mathematical models that used in representing the process besides matrix characteristics such as shape factor and matrix block dimensions in addition to the matrix permeability. The fluid exchange rate between the matrix and fractures is the main controlling factor on the oil recovery, as most of the oil reserve stored in the matrix. Therefore, appropriate gravity model selection supported by accurate matrix characterizations can enhance the simulation accuracy and to avoid an overestimation to the oil recovery. In this work, an outcrop-based model was used to provide a realistic representation of the fracture network in a dual-porosity model. The constructed fracture model was employed to assess the impact of the gravity drainage mechanism. The investigation comprises several sensitivity scenarios and cases to evaluate the influence of both mathematical models and matrix properties using an intermediate resolution model with a single producer located the grid centre and a natural depletion scenario. The simulation results indicated remarkable differences in the producer's performance and productivity. The variation in performance is purely mathematical and related only to the gravity drainage options. Furthermore, the sensitivity results highlighted the significant impact of the matrix characteristics on the fluid exchange between the matrix and fractures, hence oil recovery. Therefore, misunderstand the impact of the mathematical models, and the influence of the matrix properties could result in a compound error in predicting the reservoir performance and its recovery, hence making an inappropriate development decision.