In Naturally Fractured Reservoirs (NFRs), production rate quickly drops once the oil in the fractures is produced. Indeed, high connectivity of the fracture network soon requires the application of IOR/EOR methods to unlock production from the low permeability matrix blocks which bear the major part of the oil in most carbonate-NFRs. This paper is intended as a thorough review of the EOR field tests in such reservoirs and discussion of the physical mechanisms at stakes.
To the best of our knowledge there has never been any review dedicated to EOR methods specifically applied to NFRs. After a description of the main mechanisms involved in these processes this paper proposes to revisit the EOR field tests performed in carbonate-NFRs based on the following technologies: Gas injection, thermal- and solvent-assisted production, and finally chemical EOR, the latter being seen as a way to improve the kinetic of oil recovery from the matrix: foam, surfactant-polymer, low-salinity and hybrid processes, including a discussion of the remaining challenges to successfully apply these more recent technologies in the field.
Beyond the fact that NFRs are quite interesting because of their complexity, they account for a large part of the world's oil proven reserves, approximatively 20%. Even though this value should be taken with caution, as it depends on the NFRs definitions and their characterizations, there are clearly significant amounts of oil trapped inside these reservoirs. For that reason, they are of great interest, both in terms of understanding and economical consideration for oil companies willing to get the most out of their fractured fields. Even when based on sound static and dynamic characterizations, production optimization of NFRs remains challenging: classical flooding mainly produces the oil contained in the fractures during the early stages of the field development. The high connectivity of the different levels of the fracture network along with the low permeability of the oil-bearing matrix blocks, combined with the often oil-wet nature of carbonate formations, are the three main factors at plays preventing the production of the remaining oil from the matrix blocks. Design of dedicated EOR solutions are needed and learning from past experiences through the review and analyze of the past field trials is of great value to that purpose.
This review and discussion will allow engineers to get an extended and up to date understanding of the features and drawbacks of the main EOR methods which could be applied to NFRs, and also provide some guidance in the selection or design of the best suited EOR solution to a specific naturally fracture carbonate field.