Mobilization of Trapped Oil by Surfactant Injection - An Experimental Study Using Multi-scale Imaging

At the end of the secondary recovery in water-wet reservoirs, the remaining oil saturation might be high due to capillary trapping of the oil ganglia within the micro-structure of the rock. However, this trapped oil can be produced if the capillary barrier is suppressed by surfactant injection. To characterize this phenomenon, the Capillary Desaturation Curve (CDC : the residual oil saturation in a rock sample as a function of the trapping number Nt (sum of the Capillary and Bond numbers)) needs to be evaluated experimentally. This measure constitutes an important input parameter in chemical EOR flooding for screening and simulation. In this work, we investigated experimentally the relation between CDC and microscopic properties at the pore scale : the oil ganglia size distribution and the porous structure. Experiments were performed on a set of water-wet sandstones with different petrophysical properties. Computed Tomography imaging (CT-Scan) was used to accurately measure the mean residual oil saturation and the local saturation distribution at the plug scale (macro scale). Oil ganglia size-distribution as well as pore geometrical properties were also quantified using high resolution micro-CT images at the pore scale. The effect of trapped oil on water relative permeability is also discussed. Results showed how the CDC depends on the pore structure and is strongly related to the oil ganglia size distribution at residual oil saturation. We show that based on those microscopic properties the Capillary Desaturation Curve measured on the macro plugs can be directly predicted. We show also that in this regime of oil ganglia mobilization the water relative permeability has a specific scaling with the trapping number.