Carbonate reservoirs under harsh conditions of high temperature and high salinity (HTHS) have been exploited through primary and secondary recovery methods. This leaves substantial untapped reserves that require the use of enhanced oil recovery (EOR) techniques. Chemical EOR (CEOR) applications, particularly surfactants, in improving recovery under these HTHS conditions are challenging. Developing suitable surfactants that withstand these conditions can improve water imbibition into the low permeability rock matrix, alter the rock wettability, and significantly lower the interfacial tension. The assessment and evaluation of potential surfactants as EOR agents is of great interest and has a strategic role in unlocking further reserves from the vast accumulations of light oil in low permeability carbonates. However, the implementation of surfactants under these conditions faces various challenges, such as stability, compatibility, and high retention values, which need to be overcome for successful applications. This paper provides comparative review analyses and critical discussions on the recent developments to overcome these obstacles and the promising potential for successful surfactant flooding implementations in carbonates.

Surfactant selection is a complicated process, where the surfactant formulation needs to pass several screening techniques. In this paper, limitations, requirements, and aspects affecting the IFT, microemulsion phase behavior, and retention were thoroughly reviewed. Surfactant retention remains the primary factor limiting the implementation of surfactants in carbonate reservoirs under harsh conditions. Nevertheless, recent laboratory studies (screening and corefloods) showed that chemical formulations, including new classes of surfactants with suitable solvents and alkalis, showed excellent performance with minimal retention values under these conditions. Field studies and pilots of surfactant EOR in carbonate reservoirs were also reviewed, highlighting procedures, achievements, challenges, and the way forward to successful applications. A list of recommendations and conclusions is provided at the end of the study based on the literature and our expertise in this area. Surfactant EOR has long been considered impractical in the high temperature and high salinity conditions present in carbonate reservoirs. This study reviews the latest developments and positive outcomes that change this perception and aid in unlocking these reserves. The study is also considered a guide to starting surfactant flooding projects in carbonates under harsh conditions in the Middle East region and elsewhere.

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