Development and Validation of a Two-Stage Kinetic Sorption Model for Polymer and Surfactant Transport in Porous Media

Understanding the sorption processes is critical to the successful design and implementation of a variety of technologies in subsurface application. Most transport models assume minimal interactions between adsorbed species and, thus, are unable to accurately describe the formation of adsorbed bilayers. To address this limitation, a two-stage kinetic sorption model is developed and incorporated into a one-dimensional advective–dispersive–reactive transport simulator. The model is evaluated using data obtained from column experiments conducted with a representative polymer [gum arabic (GA)] and a nonionic surfactant [Witconol 2722 (WT)] under a range of experimental conditions. Model simulations demonstrate that the first-stage polymer/surfactant-surface sorption rate is at least 1 order of magnitude greater than the second-stage rate, associated with bilayer formation, indicating that the first-stage reaction is more favorable. The reversibility of the second-stage sorption process is found to be compound-specific, with irreversible sorption observed for GA and prolonged tailing observed for WT. This study demonstrates that the developed two-stage kinetic model is superior to a two-stage equilibrium-based model in its replication of two-leg breakthrough curves observed in core flood experiments; the normalized root-mean-square error between measurement and regressed model simulations was reduced by an average of 41% with the kinetic approach

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

Influence of Residual Nonaqueous-Phase Liquids (NAPLs) on the Transport and Retention of Perfluoroalkyl Substances

Per- and polyfluoralkyl substances (PFAS) are known to accumulate at interfaces, and the presence of nonaqueous-phase liquids (NAPLs) could influence the PFAS fate in the subsurface. Experimental and mathematical modeling studies were conducted to investigate the effect of a representative NAPL, tetrachloroethene (PCE), on the transport behavior of PFAS in a quartz sand. Perfluorooctanesulfonate (PFOS), perfluorononanoic acid (PFNA), a 1:1 mixture of PFOS and PFNA, and a mixture of six PFAS (PFOS, PFNA, perfluorooctanoic acid (PFOA), perfluoroheptanoic acid (PFHpA), perfluorohexanesulfonate (PFHxS), and perfluorobutanesulfonate (PFBS)) were used to assess PFAS interactions with PCE-NAPL. Batch studies indicated that PFAS partitioning into PCE-NAPL (Knw < 0.1) and adsorption on 60–80 mesh Ottawa sand (Kd < 6 × 10–5 L/g) were minimal. Column studies demonstrated that the presence of residual PCE-NAPL (∼16% saturation) delayed the breakthrough of PFOS and PFNA, with minimal effects on the mobility of PFBS, PFHpA, PFHxS, and PFOA. Breakthrough curves (BTCs) obtained for PFNA and PFOS alone and in mixtures were nearly identical, indicating the absence of competitive adsorption effects. A mathematical model that accounts for NAPL–water interfacial sorption accurately reproduced PFAS BTCs, providing a tool to predict PFAS fate and transport in co-contaminated subsurface environments