High-resolution x-ray imaging was used in combination with differential pressure measurements to measure
relative permeability and capillary pressure simultaneously during a steady-state waterflood experiment on a
sample of Bentheimer sandstone 51.6 mm long and 6.1 mm in diameter. After prolonged contact with crude oil to
alter the surface wettability, a refined oil and formation brine were injected through the sample at a fixed total flow
rate but in a sequence of increasing brine fractional flows. When the pressure across the system stabilized, x-ray
tomographic images were taken. The images were used to compute saturation, interfacial area, curvature, and
contact angle. From this information relative permeability and capillary pressure were determined as functions
of saturation. We compare our results with a previously published experiment under water-wet conditions. The
oil relative permeability was lower than in the water-wet case, although a smaller residual oil saturation, of
approximately 0.11, was obtained, since the oil remained connected in layers in the altered wettability rock.
The capillary pressure was slightly negative and 10 times smaller in magnitude than for the water-wet rock,
and approximately constant over a wide range of intermediate saturation. The oil-brine interfacial area was
also largely constant in this saturation range. The measured static contact angles had an average of 80◦ with a
standard deviation of 17◦. We observed that the oil-brine interfaces were not flat, as may be expected for a very
low mean curvature, but had two approximately equal, but opposite, curvatures in orthogonal directions. These
interfaces were approximately minimal surfaces, which implies well-connected phases. Saddle-shaped menisci
swept through the pore space at a constant capillary pressure and with an almost fixed area, removing most of
the oil.