Secondary and tertiary gas injection in fractured carbonate rock: Experimental study

The use of CO2 has received considerable interest as a method of EOR but a major drawback is its availability and increasing cost. Therefore, as the number of CO2 injection projects increase, an alternative must be considered to meet the economic considerations. For this reason attention has been directed to nitrogen injection which may be a good substitute for CO2. The purpose of the experiments described in this paper was to investigate the efficiency of oil recovery by CO2 and N2 in fractured carbonate rock. The combined effects of gravity drainage and component exchange between gas in fracture and oil in matrix on oil recovery in fractured reservoirs subjected to CO2 or nitrogen gas injection are experimentally studied. Laboratory experiments have been carried out on a low permeable outcrop chalk, as an analogue to a North Sea reservoir rock. This was surrounded by a fracture, established with a novel experimental set-up. The experiments aimed to investigate the potential of oil recovery by secondary and tertiary CO2 and nitrogen gas injection at high pressure high temperature condition. The matrix block was saturated using recombined binary mixture live oil (C1–C7), while the fracture was filled with a sealing material to obtain a homogeneous saturation. The sealing material was then removed by increasing the temperature which in turn creates the fracture surrounding the core. Gas was injected into the fracture at pressures above the bubble point of the oil. Oil recovery as a function of time was monitored during the experiments.Results from secondary gas injection experiments indicate that CO2 injection at elevated pressure and temperature is more efficient than N2 injection.Results from tertiary gas injection experiments also show that injection of CO2 could significantly recover the oil, even after waterflooding, compared to N2 injection.