Evaluation of Polyelectrolyte Complex Nanoparticles for Prolonged Scale Inhibitor Release in Porous Media

The formation of inorganic scales on the surfaces of porous media, production wells, and pipelines can substantially reduce the efficiency of oil production and damage reservoir formations. Scale inhibitors (SIs) are often applied to prevent or mitigate scale formation using a "squeeze treatment", where the SI is injected into a formation and allowed to equilibrate, and then the flow is reversed (return phase). Although organic polymers, such as poly(vinyl sulfonic acid) (PVS), can tolerate high temperatures and have been effective for scale control, repeated applications may be required because they exhibit weak adsorption (retention) in most reservoir formations. To address this limitation, the release performance of a polyelectrolyte complex nanoparticle (PECNP) loaded with PVS was evaluated in laboratory-scale squeeze tests and compared to PVS alone. After injection of the PECNP into a Berea sandstone core and a 24 h shut-in period, a brine solution was introduced to the core. Following injection, the free or "active" PVS concentration in the effluent spiked to approximately 600 mg/L, decreased to 10 mg/L after 10 pore volumes (PVs), and then gradually declined to concentrations between 1 and 3 mg/L over the remaining 450 PVs of the test. Minimal PECNPs were detected in effluent samples during the return phase, indicating that PECNP attachment was irreversible under these experimental conditions. In contrast, the PVS-only squeeze test exhibited elevated PVS concentrations that approached the applied concentration immediately after a brine solution was introduced during the return phase, and the PVS return concentration decreased to below the detection limit (0.5 mg/L) after only 70 PVs. A mathematical model that incorporated nanoparticle attachment and rate-limited release of the SI successfully reproduced the experimental results and can be used to predict PECNP squeeze lifetime. These findings demonstrate the potential application of PECNPs for scale control in reservoir formations.