Bridging Adsorption of Cationic Polyacrylamides in Porous Media

Abstract

Abstract Recently, cationic polyacrylamides (CPAM) have been successfully applied in water shutoff treatments of oil and gas wells. These polymers adsorb strongly on reservoir rocks, building up an adsorbed layer of significant thickness. Moreover, under high rates, the coiled macromolecules stretch and can bridge large pore throats. This so-called bridging adsorption mechanism has been described previously for high-molecular-weight non-ionic polyacrylamides (PAM). CPAM rheology and retention behavior has been studied in unconsolidated SiC packs and in Berea sandstones in a permeability range 0.1-1 D with CPAM solutions having a cationicity between 0 and 50%. Due to the attraction between the positive charges carried by the polymer chain and the negative surface charges of the rock, both CPAM adsorption and bridging adsorption are higher than for PAM having the same molecular weight. A maximum adsorption is found around 10-15% cationicity. This maximum, observed both in SiC and in Berea, is due to the competition between adsorption energy and pore wall accessibility. Although the permeability to water drops considerably after CPAM bridging adsorption, the permeability to oil is remarkably preserved, which makes CPAM attractive for water shutoff applications.