Modeling Gelation Time of Organically Crosslinked Polyacrylamide Gel System for Conformance Control Applications

Abstract

Gel-based conformance control has been successfully applied in both sandstone and carbonate reservoirs. However, deep conformance-control in high-temperature reservoirs is still a challenge due to fast gelation. Gelation time depends on several factors. Successful modeling of the different parameters’ effects on gelation time is invaluable for formulation optimization to realize sufficient gelation-time for deep-diversion. In this work, a laboratory study was conducted to investigate and model gelation-time for organically-crosslinked Polyacrylamide formulations. Sulfonated polyacrylamides with different molecular weights were used. The solutions were organically crosslinked using a polyethylenemine (PEI). The effects of different factors including temperature, brine salinity, pH, and polymer and cross-linker concentrations on gelation time were investigated using bottle tests and rheological measurements. For bottle tests, the solutions are prepared and aged in an oven. The vials are then retrieved and visually examined for gelation before being returned to the oven. For rheological measurement, dedicated solutions are prepared in separate vials and aged in the oven. Each vial is then retrieved where the solutions viscous and elastic moduli are measured. The results indicate that polyacrylamide/PEI gel exhibits good thermal stability and gelation time reaches up to two and a half days at 95°C. The gelation time decreases with increasing temperature, polymer molecular weight, and polymer and crosslinker concentrations. However, there were lower limits for the polymer and crosslinker concentrations below which gels were not observed. Brine salinity and pH showed a wide range of effects on gelation time. Gelation time slightly increases with increasing the solution salinity at a fixed pH of around 8.0. A mathematical model was developed based on the experimental results to capture the effects of the main parameters. The predictable equation of gelation time was constructed using multivariable regression method, and the model successfully predicts the gelation time of the conformance control system at a fixed pH value of around 8.0. The good agreement is illustrated by the R-square value being around 98%. Furthermore, the model shows that temperature posses the highest impact on gelation time followed by the crosslinker concentration and brine salinity. The developed mathematical model can be used to predict the gelation time of a polyacrylamide-PEI gel system. In addition, it can be utilized to optimize a given gel design and further validate the applicability of a given polymer/crosslinker formulation for deep fluid diversion application.