We report on three dimensional pore scale medium characterization and fluid displacement in a Berea (water wet) core. X-ray computed microtomography combined with X-ray attenuating dopants is used to obtain three dimensional images of two phase distributions at residual fluid conditions in the pore space. Employing numerical image processing algorithms based upon computational geometry to identify individual pores, throats, and fluid "blobs", we extract water and oil distributions at the level of individual pores in imbibition and drainage displacements.We present results on the pore space characterization including distributions for pore volume, pore surface area, throat surface area, and principal direction diameters for pores and throats. We present results on oil and water distribution in the pore space at residual oil and water conditions. We also consider the effects on residual fluid distribution due to the injection and gelation of a water-based gel.In extensive studies of Berea cores it has been observed that introducing water-based gels in the displacement process (for example, in water shut-off treatment of production wells) reduces permeability to water more than to oil. A number of micro-scale mechanisms  have been proposed for this disproportional permeability reduction. Our results provide supporting evidence for the involvement of gel dehydration and oil trapping while discounting gel blockage in throats as contributing mechanisms.