A new copolymer based on hydrophobically modified polyacrylamide was characterized by its rheological behavior. Viscosities were measured at various temperatures and salinities in comparison to a regular acrylamide/sodium acrylate copolymer (HPAM). It is proven that the viscosity of these new associative polymers increases with temperature, while the viscosity of HPAM is decreasing.
This effect was further explored in oscillatory rheology and porous media studies. Polymer solution was injected into a Bentheimer sandstone with a permeability of around 2 Darcy at 20°C and the pressure drop was measured along the core. From the pressure drop the resistance factor, RF, was derived, which is a measure of the in-situ viscosity in the porous medium.
Next, the temperature was increased to 45°C. This resulted in an increase of the RF from 15 to 94. At 60°C a RF of even 152 was observed. By reducing the flow rate from 0.5 ml/min to 0.1 ml/min the RF could be further increased to 562. Finally, the flow rate and the temperature were set to the initial values and a RF of 21 was measured, which shows that the thermothickening behavior of the novel polymer is reversible.
Monitoring the effluent viscosity indicated that the increase of the RF / in-situ viscosity is due to polymer being retained in the porous media. This retained polymer lowers the permeability of the rock pores and thereby increases RF. By lowering the temperature, the properties are switched back to the original state and the polymer is released from the rock matrix.
The thermothickening behavior of the discussed copolymers can be quite beneficial for applications like polymer flooding or in-depth conformance control. During injection at surface temperature the viscosity of the fluid is low and therefore it can be injected at high rates. Once the polymer solution migrates deeper into the reservoir formation, the temperature of the fluid will rise gradually; in-situ viscosity will increase and simultaneously the flow rate will decrease. Both effects will help to improve sweep efficiency.