Recovery methods and EOR screening criteria for naturally fractured reservoirs

Extensive studies focused on fractured carbonate reservoirs show that there is currently a high level of interest in this type of reservoir, especially in Middle East and also Mediterranean region. It is evident that the economical driving force in these regions is stronger than in North America. It would be reasonable to wait for EOR technologies to be developed and tested first in the Middle Eastern and Mediterranean area, and then apply the relevant techniques to North American resources.Fractured reservoirs constitute a large percentage of the world’s hydrocarbon reserves. However, the ultimate oil recovery from most of these reservoirs is extremely low. This can be attributed to their poor response to both secondary and tertiary recovery operations. In a fractured system, the displacement process depends on the fracture-matrix geometry, size, and interaction apart from other physical properties. As most of the oil is stored in the matrix due to its higher storage capacity than in fracture network, reservoir development plans and EOR methods will aim at maximizing the matrix oil recovery. An Enhanced Oil Recovery (EOR) process should have these two criteria, in order to be applicable for fractured reservoirs: (a) Minimize the residual oil saturation in matrix, depleting the matrix as effective as possible, and, (b) Accelerate the recovery rate for rapid and cost efficient oil production.In order to select the relevant EOR method for a particular fractured reservoir, it is important to critically analyze the past production performance, including recovery mechanism, daily fluid (oil, gas and water) production as well as cumulative fluid production, and economical aspects of the project. For example in fractured reservoirs with high overall recovery factor, minimizing matrix residual oil saturation is a critical issue to extend the life of the reservoir, however in case of the fractured reservoirs with 1low recovery factor, accelerating the production rate is more vital. For each of these two fractured reservoir types, different EOR methods should be considered and implemented accordingly. Screening criteria are useful for cursory examination of many candidate reservoirs before expensive reservoir descriptions and economic evaluations are done.In this paper, the main recovery processes for naturally fractured reservoirs are discussed first, followed by proposing some screening criteria for EOR/IOR methods selection