Syneresis is commonly believed to be incompatible with the application of polymer gels to reduce the permeability of porous media. In this paper it is shown that although the permeability of a gel-treated porous medium does increase as syneresis proceeds, the degree of permeability reduction in cores remains technologically useful even when 95% syneresis is observed in bulk samples. Arguments are presented for the preferential shrinking of syneresed gel into pore throats; this model explains why permeability reduction is maintained and accounts for several other experimental results. The absence of performance penalties for syneresis has significant implications for the applicability of gels for water shut off treatments in matrix formations. For example, it raises the possibility that polymer gels can be applied successfully in situations previously considered unfeasible because of the difficulty of maintaining gel stability, e.g. at high temperatures or in the presence of hard formation brines.
The controlled transformation from solution to gel is the basis for water and gas shutoff treatments using cross-linked polymers. In these applications, an appropriately formulated cross-linker/polymer solution is pumped into the desired part of the reservoir. After the transition to gel occurs, the treated rock is rendered essentially impermeable, and so subsequently produced or injected fluids will be blocked by or re-directed around the treated rock volume. This technology is becoming increasingly important for reducing the production of water and gas from oil wells and for improving the distribution of injected fluids.
The polymer solution-to-gel transformation is mediated by a chemical cross-linker. As the cross-linker bonds to reactive groups on adjacent polymer chains, the effective molecular weight of the polymer increases. Above some threshold the solution becomes a viscoelastic solid. Where too much cross-linker is present, cross-linking continues well past the point of gelation. This causes the polymer gel to contract in volume, expelling water as it does so. The phenomenon of gel contraction is known as syneresis. Depending on the composition, a syneresed gel may occupy as little as 5% of the initial solution volume.
Syneresis can result not only from excessive cross-linking but also from chemical modification of the polymer during aging. The acrylamide-based polymers employed for water shut-off treatments are prone, to a greater or lesser degree, to hydrolysis at elevated temperature. Hydrolysis converts acrylamide groups on the polymer backbone to the acrylate functionality, whose interaction with divalent cations can lead to syneresis of the polymer gel. This form of syneresis is particularly relevant to the use of polymer gels in seawater or hard formation brines at elevated temperature. The search for polymers suitable for such applications has focused primarily on reducing the tendency toward hydrolysis.
It seems obvious that syneresis would fatally compromise the usefulness of a gel for fluid-blocking applications. If the final gel volume were only a fraction of the initial solution volume, then part of the pore space of the treated rock would remain open for fluid flow. The intuitive appeal of this argument perhaps explains why it has scarcely been tested experimentally.