Numerical Study of Polymer Flow in Porous Media using Dynamic Pore Network Modelling

Polymer flooding is a mature EOR technology, in which polymer is generally used to modify mobility ratio to improve volumetric sweep efficiency. Some experimental and numerical studies have revealed that beside volumetric sweep efficiency, polymer is also able to increase microscopic sweep efficiency by mobilizing trapped and diverting fluid towards by-passed oil. Polymer solution is a non-Newtonian fluid, meaning that its viscosity may change at different flow conditions; it may show both shear thinning as well as “flow thickening” behaviour when the extensional viscosity increases sharply at higher flow rates. Both experimental and numerical studies confirm that microstructure properties of rock samples such as pore aspect ratio and connectivity play important role on in-situ rheological properties of polymer solution, especially on the onset of extensional viscosity. Onset of extensional viscosity is an important factor for two reasons: (I) its impact on polymer solution injectivity and (II) its role in potential oil mobilization. Therefore, for an efficient polymer flooding design, several parameters of both rock and polymer properties should be considered, and if possible optimised.

Traditional pore network models use an invasion-percolation approach, which causes some limitations to include EOR methods, since this describes a purely drainage process. However, dynamic pore network modelling of imbibition is more relevant for including EOR processes. In this study, we have developed a new dynamic imbibition approach for pore network model (based on Li et al., 2017 ) for polymer flow, for both single and two-phase flow. Rheological properties such as shear-thinning, shear thickening and a complex rheological model, (includes both shear thinning and shear thickening behaviour) are included in the code. We have studied effect of porous media properties on the onset of extensional viscosity and the code has been validated by comparing with Chauveteau's experimental results and results from the Navier-Stokes approach ( Zamani et al., 2015 ). It is shown that by increasing the aspect ratio, onset of extensional viscosity happens at lower injection rate, which is consistent with experimental and numerical studies.

In addition, effect of polymer solution rheology on fluid distribution at different mobility ratios and initial water saturations are studied. The results show that, at adverse mobility ratio, the more viscous polymer makes thicker fingers and sweep more oil in domain and more injecting fluid is diverted into the bonds perpendicular to the main flow. Meanwhile, higher initial water saturation significantly reduces the sweep efficiency at different mobility ratios.