An assessment of non-wetting phase relative permeability in water-wet sandstones based on quantitative MRI of capillary end effects

• Low field MRI is a practical alternative to X-ray CT in core analysis applications.
• Relative permeability is assessed from the capillary end effect saturation profiles.
• Capillary end effect is enforced with a brine reservoir at the core holder outlet.
• The Brooks–Corey model is a good description of end effects in sandstone cores.

The capillary end effect quantified by means of magnetic resonance imaging (MRI) is exploited for assessment of non-wetting phase relative permeability. The steady state gradient of the wetting phase saturation developed in the primary drainage process can be quantitatively measured with low field spin-echo single point imaging technique. This phase-encode MRI method provides more reliable and accurate measurements as compared to conventional X-ray CT applications. The capillary end effect is enforced with a wetting phase reservoir inside the outlet end plug of the core holder. This design ensures that the wetting phase saturation at the outlet end of the core is maintained close to 100%. Using D2O based brine as a wetting phase allows one to completely separate oil and water in MRI measurements. A relative permeability function for the non-wetting phase can be reconstructed based on observation of the capillary end effect alone. This approach employs the Brooks–Corey capillary pressure model and a differential form of the Darcy equation. The robust experiment design and straightforward data processing are advantageous for routine core analysis applications. The method is illustrated through measurements and analysis of the capillary end saturation profiles in Berea and Bentheimer sandstone core plugs.