The gradual depletion of conventional oil reserves and the growing demand for hydrocarbon feedstock have led to shale deposits development necessity, which are characterized by high reservoir temperatures and very low permeabilites. One of the methods proposed for unconventional reservoirs development is surfactant injection in huff-n-puff mode. Unlike surfactant flooding, where the main effect is achieved through the displacement mechanism, the huff-n-puff method is based on capillary imbibition. Surfactant solutions decrease oil-water interfacial tension, change rock surface wettability to water-wet, lead to desorption of adsorbed hydrocarbons and increase relative permeability to water, thus increasing oil production.

A number of commercially available anionic and nonionic surfactants were selected for laboratory investigation. Compatibility with reservoir fluids and thermal stability were tested for 14 days. For the stable compositions, the interfacial tension at the boundary with oil was measured with the spinning drop method. Special attention was paid to the study of initial reservoir wettability and the ability of the selected surfactants to shift it towards water-wet. Wettability at the macro level was determined by the "sessile" drop method.

As a result of the screening, two surfactant compositions capable to alter the wettability of the rock surface to strictly water-wet were selected, as this is the key point when selecting surfactant compositions for low-permeable reservoirs. The optimum operating concentrations were selected to avoid the formation of a Winsor III microemulsion, which can lead to plugging of narrow channels and fluid flow blockage in the formation. Values of static adsorption onto crushed rock were also evaluated. The most effective composition was investigated in a huff-n-puff filtration experiment and positive results were obtained.

Nanoparticles were also screened as potential components of surfactant compositions. It was found that nanoparticles forming stable dispersions in surfactant solutions improve their ability to change the wettability to a water-wet state.

As a result of the laboratory work performed, it can be concluded that the huff-n-puff technology is applicable in unconventional reservoirs with very low permeabilities. The huff-n-puff mode for surfactant solutions injection is preferable due to lower surfactant consumption, targeted effect, and shortened well response time. Successful implementation of this technology on an industrial scale can improve the efficiency of shale oil production.

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