Hanssen, J.E., Rogaland Research Inst. SPE Member
As part of R and D efforts to evaluate nitrogen for pressure maintenance and EOR offshore North 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 slime-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.
In a number of offshore fields on the Norwegian UK Continental Shelf, produced gas is being reinjected or may be so at one stage. The motivation for reinjection may be pressure maintenance, enhanced recovery or simply market demand fluctuations. Even in a generally glutted market, regional shortages of natural gas may occur due to pipeline and transport conditions, so that a replacement injectant may be needed. Nitrogen has been suggested, apparently well-suited to the generally deep and hot reservoirs light volatile oils, and especially considering the importance of gas sales.
Miscibility with reservoir oil is one important criterion for choosing an injectant. Nitrogen may develop miscibility by the vaporizing-drive mechanism, but a widely held opinion has been that this will be unlikely to occur below 41 MPa [6000 psi] which is in excess of many North Sea reservoir pressures. Research has been directed towards improving the understanding and modelling of nitrogen/reservoir fluid interaction, obtaining experimental minimum miscibility pressures for fields of interest, and developing screening tools for field evaluation.