The specific molecular structure of Hydrophobically Modified Water Soluble Polymers (HMWSP, also called hydrophobically associative polymers) gives them both superior thickening and surface adsorption abilities as compared to classical Water Soluble Polymers (WSP) and makes them attractive for both polymer flooding and well treatment operations (water shutoff, conformance control). However, their strong adsorption can obviously impair their injectivity and, conversely, the shear-sensitivity of their gels can be detrimental to well treatments. Determining for which IOR application HMWSP are best suited remains therefore difficult. The aim of this work was to bring new insights regarding the interaction mechanisms between HMWSP and rock matrix and their consequences concerning their propagation in reservoirs.
A consistent set of HMWSP with sulfonated polyacrylamide backbones and alkyl hydrophobic side-chains together with an equivalent WSP were synthesized and fully characterized. HMWSP and WSP solutions were then injected in model granular packs. As expected, with HMWSP, high resistance factors (or mobility reductions, Rm) were observed. Yet, no core-plugging occurred, since stabilization trends in the Rm curves and no loss of viscosity in the effluents were observed. A first significant outcome concerns the specificities of the Rm curves during HMWSP injections. Rm increases took place in two steps: the first corresponded to the propagation of the viscous front, as observed with WSP, whereas the second was markedly delayed, occurring several pore volumes after the breakthrough. This result is not compatible with the classical picture of multilayers adsorption of HMWSP, but suggests that injectivity is controlled by the sole adsorption of minority polymeric species. This hypothesis was confirmed by re-injecting the collected effluents into fresh cores: no second-step Rm increases were observed.
Brine injections in HMWSP treated cores revealed high residual resistance factors (or irreversible permeability reductions, Rk) which can be attributed to the presence of thick polymer adsorbed layers on the pore surface. Nevertheless, Rk values strongly decreased when increasing the brine flow rate. This second significant outcome shows that the adsorbed layers thickness is shear-controlled.
These new results should lead to proposing new adapted filtration and injection procedures for HMWSP, aimed in particular at improving their injectivity.