We are studying the two-phase flow in porous media with the MRI techniques (Magnetic Resonance Imaging), which allows to visualize the internal fluid distribution in 2D or 3D mapping. We use an NMR spectrometer operating at 14 T (corresponding to a 600 MHz 1H resonance) equipped with an imaging device. With this technique we can measure and visualize the two-phase flow in a vertical model of porous medium under ambient conditions.

To perform the experimental study, the porous medium consists of packed beads or sand grains, with a diameter ranging from 0.6 mm to 0.1 mm. The porous model is initially saturated only by the oil. The water is injected with a syringe pump into the model from the bottom inlet with the objective to reduce the deformation of water-oil interface because of gravity effects.

The MRI visualization technique allows for non-invasive measurements of the concentration of hydrogen nuclei in the liquid phase contained in the porous matrix. These hydrogen nuclei can belong to molecules of water or oil. The 1H species associated with water or hydrocarbon provide the phase distribution images, and it is also possible to distinguish the solid and liquid phases.

For the displacing fluid we use an aqueous solution of MnCl2 (~0.02 mol/l). The addition of Mn2+ paramagnetic ions affected the relaxation decay of the water phase: T2 of water (~3 s for pure water) can be shorted to below 1 ms. With such a short relaxation time, the signal of water disappears so quickly that it is not possible to image it. In addition, as MnCl2 is not soluble in organic compounds, there is no effect on the relaxation time of oil (~1 s). Therefore, the signal of oil is the only one to be measured, without any overlap from the water signal.

The slow flow in porous media makes it possible to take an image of the phase distribution each 15 minutes and construct the residual oil saturation as the function of space and time.

The combined effects of the superficial tension, viscosity and gravity have an impact on the oil trapping process and the front stability during the oil-water displacement within the porous media.

The obtained results have shown that the oil saturation profile is strongly influenced by the material properties such as the phase wetting, the sample porosity and permeability and also by the injection rate. The experiments of displacement of oil by water (µw/µo = 0.345) in packed sands show that the advance of the displacement front happens in the form of the casual saltatory movements and it is controlled by the gravity and capillary forces. In spite of the fact that under the vertical direction of flow the viscous instability is partially compensated by the gravity and capillary forces, the displacing phase forms micro-fingers. The period of the alternation between the capillary penetration and appearance of a new viscous finger depends on the injection rate and model permeability. The loss of connectivity of the oil phase between the water fingers leads to the formation of trapped oil areas. The residual oil saturation after injection of 5 PV (porous volume) of water was obtained for oil-wetting and water-wetting porous solid structures (polystyrene beads or sand grains).

The influence of flow velocity on the residual oil saturation was studied. The numerical simulations based on a macroscopic description of the flow model, confirm qualitatively the experimental results. Finally, the closing relations for the two-phase model were obtained from the results of the experiments.