Laboratory evaluation of nitrogen injection for enhanced oil recovery: Effects of pressure and induced fractures

摘要

Nitrogen has emerged as a suitable alternative to carbon dioxide for injection into hydrocarbon reservoirs worldwide to enhance the recovery of subsurface energy. Nitrogen typically costs less than CO2 and natural gas, and has the added benefit of being widely available and non-corrosive. However, the underlying mechanisms of recovery following N-2 injection into fractured reservoirs that make up a large portion of the world's oil and gas reserves are not well understood. Here we present the laboratory results of N-2 injection into carbonate rocks acquired from a newly developed oil reservoir in Iran with a huge N-2 -containing natural gas reservoir nearby. We investigate the effectiveness of N-2 injection for enhanced oil recovery in immiscible conditions before and after gas breakthrough under a low and high differential pressures across the core. In addition to the effects of pressure, we further illuminate the impacts of fractures-induced on the cores-to assess the displacement behavior and oil recovery factor. Our findings show that an ultimate oil recovery factor of more than 40% can be achieved by N-2 injection in non-fractured cores even at immiscible conditions. The ultimate recovery and the onset times for oil production and gas breakthrough are found to be lowered by increasing differential pressures as well as inducing fractures (e.g., 17% reduction in ultimate recovery due to fracturing). However, at a given time when gas-oil interface has not yet reached the production zone (outlet), both increasing differential pressures and fractures transiently enhance the recovery efficiency. As a result, the impact of fractures is more pronounced in lower differential pressures, while the impact of differential pressures is stronger in the absence of fractures. Interestingly, our results attest to the role of molecular diffusion across fracture-matrix interface as the main recovery mechanism in fractured media, which controls the system dynamics before and after breakthroughs. The results not only provide a new perspective into how differential pressures and fractures fundamentally control the effectiveness of N-2 flooding but also further show the promising prospects of N-2 injection for FOR even at immiscible conditions.