The production of tight oil in North America has been increased dramatically since the past decade. According to crude oil projection in the Annual Energy Outlook 2014, it will become dominant in crude oil production in the near future. Besides operating new drilling projects, operators need to employ effective secondary or tertiary oil recovery on the current low recovery efficiency shale resources.
Gas injection is a common and widely used EOR method for increasing oil recovery factor. It is an economical method, especially in cases where the injection gas for such gas injection project is easily available. Although gas-flooding recovery process has been successfully applied to conventional reservoirs, almost no literature discussed or relative experiment operated that employing such way to improve oil recovery from shale reservoirs. Therefore, the purpose of this work is to evaluate the EOR performance of gas flooding on shale core plugs.
In this study, Nitrogen flooding was applied to three outcrop Eagle Ford core plugs with same sizes and rock properties. All samples were cleaned and dried, then saturated with crude oil from field before conducting the flooding process. Three series of experiments were performed under different flooding periods of 1-day, 3-day, and 5-day, respectively. Other key operating parameters were kept the same with inlet pressure in 1,000 psi, outlet pressure in atmosphere pressure, and confining pressure in 1,500 psi. The experimental setup was monitored using X-ray CT that helped to visualize phase flow and distribution during the processes. Porosity of core sample was measured and recovery performance was evaluated in two ways: core weight difference and analysis of CT number. Experimental results show that the average gas breakthrough time was 10 hours, and recovery factor of samples under the conditions of 1-day, 3-day and 5-day flooding period were 30.0%, 26.9%, and 39.4%, respectively. It means that most of saturated oil was produced around the gas breakthrough period. Once the fluid flow path was established, the injected gas would flow through the limited communication channels thus no extra more oil could be extracted without increasing injection pressure or adjusting other conditions. The laboratory-scale experimental study demonstrates that gas flooding has an excellent shale oil recovery potential in the early stage of EOR process.
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