(146c) Impacts of Relaxation Time and Cohesive Energy Density on Syneresis Time of Hydrogels Under Harsh Reservoir Conditions

Hydrogels have been proven as promising treatment agent for water shutoff and conformance control applications. However, the biggest disadvantage of these materials is in their weak and brittle structures. Hence, improving the performance of hydrogels under harsh reservoir conditions is the objective of this study. Adopting the key thrust in this field, we attempt to optimize the energy dissipating mechanisms within the hydrogel network structure.

Our approach is to create hydrogels with a large number of dynamic network interactions through increasing the density of hydrogen bonds and, thus, improving the cohesive energy density (E_c). The oscillatory sweep and frequency sweep tests were carried out on a series of hydrogels of sulfonated polyacrylamide/chromium triacetate prepared using various initial concentrations of polymer and crosslinker, to derive a coherent hydrogel structure. The characteristic frequency and relaxation time of each hydrogel was determined, based on the modifications of the elastic, viscous and complex moduli. Finally, the performance of the optimal hydrogel was investigated by conducting a core-flooding test.

The cohesive energy density was calculated for all samples and compared with other parameters. A relationship between the cohesive energy density and the relaxation time was established. It was observed that as the cohesive energy density increases, the relaxation time of the network also increases. Our results suggest that the longer the relaxation time of a hydrogel, the longer its syneresis time will become. In some hydrogel networks, with cohesive energy densities in the range of 40´10⁷ to 3´10⁷ J/m³, no volume reduction occurred over a period of three months. However, for another hydrogel sample with a cohesive energy density of 10⁴ J/m³ and the lowest relaxation time of all those samples examined in this study, syneresis was realized only after about 25 days. Based on the experimentally derived hydrogel structures, the optimal composition was identified, and its performance in reduction of water cut was evaluated by conducting a core-flooding test on a fractured core, in the presence of formation water with 200,000 ppm total salinity at 90°C. The optimal hydrogel sample retained its structural and viscoelastic behavior under a pressure differential of (at least) 100 bar while successfully reducing the produced water cut by 85%.

It is concluded that the successful performance of hydrogels under harsh reservoir conditions can be achieved by improving their viscoelastic structures. E_c is a valuable parameter to consider when investigating the structural strength of hydrogels. The performance and lifetime of hydrogels in reservoir environments can be significantly optimized by improving the rheological properties of hydrogels, E_c, and adjusting the relaxation time.