Water flooding and gas injection into a primary gas cap,where this exists,provide alternative strategies for the development of viscous oil fields.Most UKCS viscous oil reservoirs are of excellent quality,comprised of unconsolidated sands with high horizontal and vertical permeability.Gas injection may give lower effective residual oil saturations due to the much greater density contrast between oil and gas compared to that between water and oil.A comparison of gas and water gravity drainage over a range of viscosities allows the potential benefits from gas injection and water flooding to be assessed.Measurements of gas/oil relative permeabilities are typically made under viscous dominated flooding conditions,which are unrepresentative of gravity drainage conditions in the field,and without the benefit of in-situ saturation monitoring which is essential to remove laboratory artefacts arising from capillary end effects.To address these issues we performed a series of both gas/oil and water/oil gravity drainage experiments in three sandpacks,with permeabilities representative of UKCS viscous oil fields.Oils with viscosities varying from 2 cp to 210 cp have been used.Oil drainage was measured using in-situ saturation monitoring.The in-situ saturation data has been analysed to provide fractional flow and relative permeability data.For the gas gravity drainage,oil relative permeabilities were found to be independent of viscosity and significantly higher than data in consolidated sandstone.Calculation of effective residual saturations at a range of reservoir viscosities and displacement rates,show that under appropriate conditions,very low oil saturations can be achieved.Similarly,fractional flow curves derived from the water flood experiments did not exhibit any significant dependence on oil viscosity.A comparison of oil recoveries for waterflooding and gas injection shows gas injection can be preferable for high viscosity oils – particularly in more permeable reservoirs.
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