The critical salinity concentration (CSC) was used to compare the ability of nonionic polyacrylamides (PAM) and cationic polyacrylamides (CPAM) to stabilize montmorillonite clay dispersed in sandpacks. The method consists of injecting brine at decreasing salinity levels until clay release is detected by a continuous increase in pressure drop.
As expected, because of the neutralization of negative clay surface charges by adsorbed macromolecules, CPAM have a higher stabilizing power than PAM, lowering the CSC of NaCl from 27 500 to only 500 ppm, and the CSC of KC1 from 5000 to 1000 ppm. However, because of their ionicity and very high adsorption level, CPAM strongly reduce sandpack permeability. Likewise, alow-molecular-weight PAM may be preferred because of its good stabilizing power associated with a minimal loss of core permeability.
In a preceding paper, we evaluated the ability of different polymers widely used in the oilfield industry to stabilize montmorillonite clay dispersed in sandpacks. The method designed to run the stability tests, namely the critical salinity concentration (CSC) method, consisted of injecting brine at a wide range of injection rates with decreasing salinity levels until pressure did not stabilize anymore because of clay release in the sandpack. The CSC is the lowest salinity tolerated before clay release. The tests were run first without any polymer, then, in the same experimental conditions, after polymeradsorption. The stabilizing power of a given polymer was quantified by the reduction in the CSC value obtained after polymer adsorption in the pack.
Among the different polymers tested, xanthan gum and carboxymethylcellulose were found to have almost no stabilizing power. Scleroglucan was found to have some stabilizing properties, although much less than nonionic polyacrylamides PAM). Indeed, after PAM adsorption in the sandpack, the CSC of NaCl dropped from 27 500 to 6500 ppm and the CSC of KC1 dropped from 5000 to 1500 ppm.
The mechanism involved in the polymer stabilizing effect is the coating of pore walls by adsorbed macromolecules, which both inhibits clay swelling to some extent and prevents clay release and migration (Fig. 1).