Gravity-Stable Nitrogen Displacement of Oil

Abstract

Abstract A series of experiments to investigate secondary and tertiary, gravity stable, immiscible, nitrogen displacement of oil in the presence of connate water is presented. Both high and low pressure presented. Both high and low pressure experiments have been conducted using water wet sandstone cores and dead oils. Effluent production was recorded and the spatial distribution of oil along the core was determined using a radioactive tracer technique. Experimental results are presented for secondary nitrogen injection at different flow rates, connate water saturations and core orientations. In-situ saturation measurements in a low pressure experiment have revealed a residual oil saturation to gas of 0.15 after 68 days of drainage. The results of a tertiary nitrogen injection experiment at low pressure are presented. The residual oil saturation to presented. The residual oil saturation to water flooding was 0.47. During the tertiary gas flood, an oil bank was observed to develop and the oil saturation decreased to 0.08 after 86 days of gas flooding. This experiment demonstrates that tertiary gas injection can result in significant oil production, even at low pressures where the process is expected to pressures where the process is expected to be least efficient. Oil relative permeability data have been derived from the in-situ saturation measurements and permeabilities down to 10(-5) observed. Introduction Gravity stabilised injection of miscible or immiscible gas into steeply dipping, high permeability reservoirs can recover substantial quantities of oil which are not recoverable by water flooding. Gravity stable gas displacement presents a complicated problem in reservoir engineering. The economic optimisation of potential projects requires a detailed potential projects requires a detailed understanding of both relative permeability and the thermodynamic permeability and the thermodynamic behaviour of the reservoir fluids. Experiments with water wet micromodels indicate that gas injection can transform trapped oil ganglia into an oil film on top of residual water. Oil can then move in a film flow regime under the influence of gravity hence recovering water flood residual oil. Component exchange may reduce the gas/oil interfacial tension sufficiently to improve the oil relative permeability but there is some disagreement in the literature concerning this effect. Low interfacial tension may influence both the transition of oil from discrete ganglia to a film and pore-to-pore 'dripping', but it is pore-to-pore 'dripping', but it is unlikely to have a strong effect on film flow. Dumore and Schols have reported an air-kerosene drainage capillary pressure experiment using Bentheim sandstone containing connate water. A low oil saturation of 0.03 was observed at the end of the experiment but the duration was not reported. Initially, this result was thought to be caused by spreading of the oil on the water in the presence of gas. P. 155