Polymer mechanical degradation can be induced by high flow rate during the injection in subterranean formations, especially near-wellbore regions. This work presents a study on the polymer injection in carbonate core plugs and capillary tubes at different injection rates in order to assess the critical shear rates beyond which significant polymer degradation takes place. The polymer degradation process in capillary tubes is correlated with that in the carbonate core plugs, which facilitates the degradation assessment.
A semi-dilute polymer solution in synthetic injection water was injected into carbonate and sandstone core plugs with different permeabilities and length. The collected effluent solutions were monitored by viscosity measurement using a rheometer and molecular weight distribution measurement using gel permeation chromatography (GPC). Similar procedures were followed for the polymer mechanical degradation using capillary tubes with inner diameters of 0.12, 0.254 and 0.508 mm.
Flow in porous media induced severe polymer degradation at a flow rate above the critical shear rate. The carbonate cores showed lower critical shear rates than the sandstone cores. For carbonate core plugs with permeability 390md and 60md, the critical flow rate was 20mL/min corresponding to a shear rate of 4402.1 s-1 and 2mL/min corresponding to a shear rate of 1122.2 s-1. For the sandstone core plugs with permeability 490md and 40md, the critical flow rate was 10mL/min corresponding to a shear rate of 2198.0 s-1 and 1mL/min corresponding to a shear rate of 771.5 s-1. It was observed that greater polymer degradation appeared in the flow through the lower permeability core plugs. Core length had limited effect on the degradation. The average molecular weight became smaller while the molecular weight distribution became wider for the polymer solutions after the mechanical degradation, indicating the irreversible fragmentation of polymer molecules. In comparison, polymer degradation in capillary tubes appeared at a critical shear rate in the magnitude of 200,000 s-1. Similar with the scenarios of polymer injection in core plugs, greater viscosity loss happens in the flow through smaller capillary diameter. By the correlation, the polymer degradation in carbonate core plugs can be predicted by the measurement in the capillary tubes.
This work provides the insight of polymer mechanical degradation in carbonate matrix. An easy-to-operate method on the evaluation of polymer mechanical degradation was developed to assist in the operation of the polymer injections.