Introduction General The prediction of reservoir performance is generally based on a combination of laboratory tests and numerical simulation. The validity of laboratory scale experiments and, hence, numerical simulations is the subject of debate and criticism. Small sample size, end effects, sampling bias, and measurement error all contribute to difficulty in interpreting laboratory results. It is unlikely there will be a significant change in the manner in which data is obtained. Therefore, an understanding of simultaneous flow of water and oil at the laboratory scale and the significance of the above-mentioned test impediments will assist in prediction of reservoir performance. Recently CAT Scanners have been used to visualize the simultaneous flow of oil/gas/water during corefloods(1,2,3), helped to resolve some of the problems associated with coreflood experiments, and helped to better understand displacement mechanisms. CAT Scan devices measure X-ray attenuation and reconstruct a pseudo-density map of the object. The relationship between density and phase displacement is not always readily apparent and scan images require additional data manipulation prior to interpretation. A method has been developed to calculate oil/water saturation from CAT Scan images taken during a displacement experiment. Heavy oil displacement by brine is used as an example to demonstrate the power of the technique. Computer graphics have been used to generate a saturation map as the flood progresses. The flood was subsequently analyzed using a two-phase numerical simulator to establish a link between the physical measurements of the CAT Scan and the predictive power of numerical modelling. The application of CAT Scan techniques to understanding oil recovery mechanisms is still at a research stage. However, the technique does show promise and should prove to be a valuable tool in modelling multi-phase flow and ultimately improving recovery of oil. Abstract An investigation has been carried out to study the displacement of viscous oil by water in a linear core flood. This paper describes the experimental procedures, graphical display of CAT Scan data, a method to determine saturation from CAT Scan measurements, and a comparison of saturation profiles from the CAT Scan with those of a numerical model. The CAT Scan images provide a unique opportunity to view viscous fingering while a flood is in progress. Silica sand was packed into a lucite cylinder 10 cm in diameter and 30 cm in length. The sand was flooded with a NaCl and KI brine followed by a 3000 cp viscous gear oil. The oil was subsequently displaced by brine. CAT Scan images were produced at regular time intervals every 2 cm along the core. The change in oil saturation along the core as a junction of time is clearly visible and demonstrates the power of the CAT Scan in visualizing multi-phase flow in porous media. Principle of CAT Scan Tomography (CT, CAT) is a relatively old concept. Radon(4) derived the mathematics required for image reconstruction in 1917. CAT Scans for medical use were introduced in 1972(5) and since that time the subject has received considerable treatment in the literature.