Research and testing of gas injection gravity drainage of enriched oil in buried hill reservoirs

The Huabei Oilfield has performed enhanced oil recovery testing based on nitrogen injection gravity drainage in the buried hill oil reservoir since the 1990s. The pattern composed of the gas cap, oil-enriched zone and bottom water was formed. The existing wells were used for production, in which pay zones were drilled-in and left openhole after the upper wellbore was cased. However, these wells presented only gas production, with no oil production, Given this, the investigation integrating oil reservoir engineering and production engineering was performed and the production engineering design idea combining isolation, control, and selective production was proposed. It is considered that the key to preventing gas and water from directly flowing into the wellbore is to overcome the effects of gravitational differentiation and density difference to facilitate effective containment, pressure-bearing, and plugging. Inspired by the natural process of lava eruption, cooling and accumulation, the pressure-bearable channel plugging technology of fluid retention and temperature-controlled fast setting was developed, with the help of the unique thermosensitive rheology of thermosensitive resins to overcome gravitational differentiation. Reservoirs developed high-angle large fractures and caverns suffer from severe leakoff, which impacts the quality of cementing and well completion. Yet, the presented technology overcomes such challenges. Moreover, the presented technology combined with the control of drawdown pressure during artificial lifting realizes effective recovery of enriched oil zones with limited thickness. The application performance of this technology strengthens the confidence in applying gas injection gravity drainage in buried hill reservoirs.