The strongly oil wet nature of tight carbonate or shale reservoirs leads to rapidly declining oil rates during production stages. For fractured tight carbonate reservoirs, chemical flooding with surfactants has been shown to enhance oil production from water flooding (huff and puff method) due to spontaneous imbibition of water into oil-rich matrix, leading to oil displacement from tight pores and increased recovery. Success of such surfactant based EOR technology depends heavily on developing the optimal surfactant formulation for the field. The objective of this work is to identify the key mechanisms responsible for the improved performance of surfactant formulations and to characterize the interactions that can induce imbibition into an originally mixed-wet, tight, hydraulically fractured shale reservoir via laboratory evaluations. Wettability alteration formulations are first developed and screened for aqueous stability and phase behavior at reservoir conditions. Tailored hydrophobic surfaces are used to evaluate selected formulations for their potential to shift rock wettability through contact angle measurements. Water-oil interfacial tension behavior is investigated to elucidate the mechanism of oil recovery. Adsorption behavior of the chemicals is investigated on various rock minerals to identify formulations with minimal retention and component separation under static conditions. Finally, the candidates are comparatively tested for oil recovery performance in spontaneous imbibition experiments at simulated reservoir conditions.

The key factors considered in developing an optimal surfactant formulation are favorable phase behavior in reservoir fluids at reservoir conditions, the ability to shift the rock surface wettability to a more water-wet state, and minimal chemical losses on reservoir minerals. Also, the formulation should not form a stable emulsion which may pose an oil-water separation difficulty in oil production. The designed formulation was tested using forced imbibition test (core flood) and show significant incremental oil recovery. As part of our rigorous approach to design optimal surfactant formulations for improving oil recovery, we perform surfactant phase stability, surfactant retention and rock-surfactant interaction related to wettability alteration. Surfactant formulations designed in this work are unique and provide high temperature stability (above 70 °C and up to 120 °C) and high salinity tolerance (> 12% TDS and up to 22%). This study also highlights the underlying mechanism of surfactant induced wettability alteration of tight carbonate rock due to novel surfactant formulations.

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