Enhanced oil recovery techniques are having a growing interest for unconventional reservoirs to attack the low primary recovery factor issue, especially in a low oil price environment. Tight oil behaves a quick decline without much pressure support and thus jeopardies project profitability given a large initial capital requirement for horizontal drilling and massive multi-stage hydraulic fracturing operations. However, over 90% oil is still left behind in stimulated reservoir volume. It is imperative to understand the differences in production drivers in unconventional reservoirs, especially using EOR techniques, for example, how capillarity and adsorption may greatly alter phase behavior in nano-size pores.

Previous studies showed that phase behavior and production mechanisms might be significantly different under nano-scale pore confinement in organic matter in Eagle Ford formation. This research was to shed light to quantify such differences under the influce of large capillary forces in organic and inorganic pores integrating petrographic and/or petrophysical measurements, for example, mercury injection capillary pressure (MICP) experiments. Further, this study developed a new methodology to consistently evaluate the effect of adsorption at the same pore size distribution as capillary evolved.

Results showed that a large difference of phase and adsorption behavior existed for unconventional reservoirs in the oil window in the Eagle Ford formation, and pore size distribution played an important role in quantifying such differences. This may lead to additional rock-typing techniques to better characterize the Eagle Ford oil play and improve well expected ultimate recovery (EUR) and economics.

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