The Challenges of Designing Enhanced Oil Recovery Methods in Large Gas Cap Fractured Reservoirs and their Solutions by the Approach of Advanced Screening

One of the current challenges in fractured reservoirs, especially those with a large gas cap, is the production of oil with a high gas-to-oil ratio, coupled with the entrapment of oil in the matrix. The lack of a correct understanding of the fracture distribution network and its impact on the performance of enhanced oil recovery methods can pose numerous challenges in designing these methods. This study aims to investigate the effect of fracture geometry and its network on the performance of enhanced oil recovery methods, as well as to determine the optimal amount of oil remaining in the matrix, by classifying fractured reservoirs into water and gas-invaded regions. Initially, inefficient water and gas-based enhanced oil recovery methods were eliminated based on preliminary screening. Subsequently, the study investigated the influenced parameters in the field-scale simulation of enhanced oil recovery methods using conceptual modeling. Additionally, a new algorithm for fuzzy multi-criteria decision-making was employed to rank and select pilot areas. Consequently, the field-scale simulation of enhanced oil recovery methods was conducted in pilot areas using an advanced screening approach. The results reveal that one of the most significant challenges in numerical simulation in fractured reservoirs with an initial large gas cap is determining a representative PVT model for both areas near the gas/oil contact and the interval of the oil datum. To address this challenge, a two-fluid PVT model was used. Another challenge involves the integration of static data with production and operational data for pilot area selection, considering reservoir properties. This challenge was addressed using a new fuzzy multi-criteria decision-making algorithm. Conceptual modeling results show that carbon dioxide injection is the top-ranked method technically. However, field-scale results of the pilot model demonstrate that nitrogen flooding is the best techno-economic method. The primary reason is the higher rate of the gravity drainage mechanism, and the lower cost of nitrogen compared to other candidate gases. Additionally, low salinity water flooding with a dilution ratio of 100 is the most effective water-based enhanced oil recovery method. Nevertheless, field-scale simulation results indicate that in reservoirs with a large gas cap, the efficiency of water or gas-based enhanced oil recovery methods is evident only in sectors with growing fracture networks. Conversely, in areas with a poor fracture network, other improved recovery methods such as workover of high gas-to-oil ratio wells can be promising candidates.