Extensional viscosity aspects of HPAM in porous flow: An experimental and numerical study

Polymer flooding is the most widely used chemical EOR method. Despite being widely used, the apparent shear-thickening behaviour of the polymer solutions in porous flow at high flow rates is poorly understood. One of the supposed mechanisms is the strain-thickening behaviour of polymer solutions. This fluid property will alter the flow dynamics during porous flow compared to Newtonian and purely shear-thinning flow. In this research, the objective is to improve our understanding of polymer flow through a simple single slit geometry (e-VROC) and more complex geometries (porous flow characterised by pore network models) in order to allow oil recovery optimisation for polymer flooding. The intrinsic viscosity of HPAM3630S is investigated experimentally using the Extensional Viscometer/ Rheometer On a Chip (e-VROC). The e-VROC has a microfluidic hyperbolically-shaped contraction-expansion geometry. The water salinity is used as a control parameter to reduce the fluid viscosity. Initial calibration of the device with Newtonian fluids and analytical analysis of the e-VROC geometry indicate that the shear component of the flow is large in the converging section – contrary to the claimed advantages of the hyperbolic geometry. Newtonian flow can therefore not be regarded as extension dominated. Consequently, the provided analysis of the e-VROC pressure data is currently unable to determine the true extensional viscosity of a Newtonian fluid. Therefore the analysis should be regarded as an extensional viscosity indexer in comparing different fluids. For polymer flowthe pressure gradient over the contraction-expansion area increases more than linearlywith increasing flow rate. This indicates strain-thickening behaviour. The salinity highly impacts the amount of strain-thickening; the higher the brine salinity the lower the pressure gradient over the contraction-expansion area. Furthermore, a high noise content in the time-pressure signal is observed together with reproducibility problems regarding polymer flow. Differences upto 30% in pressure gradients between measurements are reported for the same fluid. Both can probably be attributed to elasticity due to the short residence time of the polymer solutions in the contraction-expansion geometry compared to their relaxation times. The fluid flow process is modelled using finite element modelling (GeoDict&COMSOL) and pore network modelling using MATLAB to study the (changed) fluid flow behaviour. It was shown that Newtonian flow through the e-VROC can be modelled using both COMSOL and GeoDict. Furthermore, it was shown that the pressure drop due to pure shear losses in the e-VROC can be significant during polymer flow. The developed Matlab code enables modelling the steady state response of pore network systems. The systems contains more than 10000 non-linear throat equations. This captures both extension-thickening and shear-thinning pressure losses. This successfully demonstrates proof of concept set out at the beginning of this study. Using the pore network model, it is studied how a macroscopic pressure over a rock sample is redistributed in microscopic pressure drops between individual pores. It is shown that the microscopic pressure drop distribution forNewtonian flowpredicted by the pore network model is in good agreementwith the microscopic pressure drop distribution inside the porous medium predicted by GeoDict. However, the corresponding permeability predicted by the pore network modelling is one order of magnitude too low. From this it can be concluded that the resistance to flow between the pores is overestimated by the used transport equations. Nevertheless, a qualitative interpretation of the (changed) microscopic redistribution of pressure for non-Newtonian flow can still be made. Secondly, the microscopic pressure drop distribution within the sample for purely shear-thinning flow is compared to Newtonian flow. The variance and kurtosis of the distribution decrease compared to Newtonian flow. This implies better conformance control during using purely shear-thinning flow. Above a critical flow rate, strain-thickening behaviour reverses this process. The variance and kurtosis of the pressure drop distribution increase. This has a negative impact on conformance control.

Subject

Extensional viscosity
HPAM
e-VROC
Polymer
Extensional rheology
Pore Networks
Pore Network Modelling
in-situ rheology
Pore scale

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