Summary

This paper describes two- and three-phase relative permeability concepts important for Prudhoe Bay. It includes a three-phase relative permeability correlation that incorporates hysteresis in gas, oil, and water relative permeability as well as the dependence of relative permeability on composition and gas/oil interfacial tension (IFT). The functional forms chosen to correlate the relative permeability data were based on interpretation of the pore-level mechanisms that determine fluid flow. The three-phase correlation reduces to traditional models in various limits and is more consistent with available data and trends in the literature than previous correlations. Although this correlation was developed for Prudhoe Bay, it can be and has been applied to other mixed-wet reservoirs with changes in the input parameters. The correlation is particularly useful in situations where both compositional effects and hysteresis are important.

Introduction

The ultimate use of relative permeability models is to help design, optimize, and analyze oil-displacement processes. Clearly, relative permeability is just one part of the recovery picture; reservoir characterization, gravitational effects, phase behavior, and mass transfer processes among other factors all interact to determine the amount of oil that can be recovered economically. Nevertheless, relative permeability plays a central role. The primary impact of relative permeability on process design is through fluid mobilities and fractional flows. Total fluid mobilities determine the resistance to flow of the fluids and hence affect (1) injectivity and the overall timing of the process and (2) the severity of viscous fingering or channeling and the "robustness" of a process to heterogeneities in general. The fractional flows impact producing-water/-oil ratio, producing-gas/oil ratio, breakthrough timing, and ultimate and incremental recoveries. The magnitude and location of waterflood residual oil, the target for the enhanced oil recovery (EOR) process, is impacted by low-capillary-number relative permeability. Waterflood oil recovery is also affected by the presence of gas through its impact on water/oil relative permeability ratio as in immiscible water-alternating-gas (WAG)-processes or in waterflooding where oil was forced into regions previously invaded by an expanded gas cap. Residual oil to gas in gravity drainage is determined primarily by oil/gas relative permeability, which in turn is impacted by the level of initial water saturation. Although phase-behavior mechanisms (attainment of miscibility, stripping of oil by gas, or swelling of oil by gas) are important mechanisms in developed miscible flooding, capillary number effects could also play a role because the IFT between the oil and gas becomes low as miscibility is approached.

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