As part of R&D efforts to evaluate nitrogen for pressure maintenance and EOR offshore Northern Europe, nitrogen miscibility with selected reservoir fluids as well as its diffusion properties have been investigated and compared to those of natural gas. Custom-built equipment includes slim-tube and cell-to-cell miscibility apparatus and a diffusion apparatus, all operating at typical reservoir conditions. Measured minimum miscibility pressures (MMP's) range from 32 to 55 MPa [4600 to 8000 psi]. With near-critical volatile oils from one reservoir, both nitrogen and methane were found to develop miscibility at saturation pressure. This apparently is the first such case known for nitrogen, and the lowest MMP value reported for nitrogen with a North Sea fluid. For another North Sea reservoir fluid, MMP was found to depend strongly on the amount of intermediates in the oil, and a significantly higher MMP measured for nitrogen than for methane. The new data were used to improve a previous MMP correlation for vaporizing gas drives. Predictions by this correlation indicate possible nitrogen miscibility with several North Sea reservoir fluids. Hitherto unavailable diffusivity measurements for the nitrogen-methane binary have been performed at reservoir conditions. Results indicate that serious errors in diffusivity may be introduced if lower-pressure data are used to extrapolate to reservoir conditions.

To complement the experimental work, a critical review of previous onshore nitrogen field application and various offshore process suggestions has been performed, in the perspective of nitrogen replacing produced-gas reinjection.

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