Investigation of Polymer Enhanced Oil Recovery (EOR) in Microfluidic Devices that Resemble Porous Media - An Experimental and Numerical Approach

摘要

After water flooding, oil is left behind either because it is trapped by capillary forces (residual oil) or because it is bypassed in some other way. The bypassed oil may arise because of an unfavorable viscosity ratio between the aqueous and oleic phase or because of large-scale heterogeneities present in a reservoir. The residual oil on the other hand is made up of discrete nodular blobs (ganglia) that are produced when a finger-like protrusion of the oleic mass forms a narrow neck by the combined effects of local pressure gradient and interfacial tension. In order to re-mobilize the residual oil usually a significant increase in the viscous to capillary force balance between the aqueous and oleic phase is required. Polymer flooding is an Enhanced Oil Recovery (EOR) technology that consists of adding high molecular weight polymers to the injection water. This results in an increase of the viscosity of the injected aqueous phase and leads to an improved recovery of the bypassed oil. Nevertheless, the increase in viscous forces encountered during polymer flooding is not considered to be sufficient to mobilize the residual oil. However, literature review suggests that viscoelasticity of aqueous polymer solutions can considerably reduce the residual oil saturation compared to water flooding without an increase of the viscous to capillary force balance (capillary number). Different working hypotheses have been proposed in the literature to explain this effect, including (I) pulling, (2) stripping, (3) oil thread flow and' (4) shear thickening. All of these mechanisms relate to the percolation characterise tics of viscoelastic polymers inporous media and usually correlate to the degree of polymer viscoelasticity reflected in the magnitude of the longest relaxation time determined in oscillatory rheology. In this article oil displacement experiments performed in water-wet micromodels that resemble porous media having different average reservoir qualities (k, φ) are presented. Silicon-edged micromodels provide visual access to the displacement process, hence, can lead to a more detailed displacement process description compared to experiments performed in cores. The experiments aim to verify theproposedviscoelastic oil displacement mechanisms and investigate if polymer viscoelasticity has a considerable impact on the residual oil mobilization, percolation and recovery in "secondary (at initial oil saturation) " and "tertiary (after extensive waterflooding) " modes at typical reservoir shear rates. In addition, results obtained from micro-model flooding experiments will be compared to results from numerical simulation in order to test the validity of existing mathematical models to history-match micro-model flooding experiments. To match the simulation to the experimental results the oil saturation distribution is used, which is obtained for each cell based on image analysis of flooding experiments. Comparison of experimental results obtained during viscoelastic and non-viscoelastic oil displacement in "secondary " and "tertiary " mode at similar capillary number reveal, that polymer solution viscoelasticity does not have a significant impact on the residual oil saturation in the micromodels used under the experimental conditions used in this work. Therefore, a simple relation between polymer solution viscoelasticity and residual oil saturation cannot be generalized.