There are two types of cross-linked reactions to form polymer gels, intra-molecular cross-linked reaction that takes place within a polymer molecule, and inter-molecular cross-linked reaction that takes place among different polymer molecules. Viscosity measurements and flow performance tests can be used to differentiate between intra-molecular and inter-molecular cross-linked reactions. Viscosity increases with time if inter-molecular cross-linked reaction occurs in a polymer solution, but stays constant or slightly decreases with time if intra-molecular cross-linked reaction occurs. Flow resistance and residual flow resistance increase when intra-molecular cross-linked polymer gel is injected into the porous media. This phenomenon is not observed with polymer injection.
Cross-linked reaction in a polymer solution is affected by the concentrations of the polymer and the coagulant, electrolyte type and concentration, temperature, and pore size and structure. Laboratory results show that high electrolyte concentration and temperature (in a reasonable range) can increase the tendency for intra-molecular cross-linked reaction. The higher the concentration of electrolytes, the more the polymer molecular clews curl and contract, improving the probability for intra-molecular cross-linked reaction to occur. For the three studied electrolytes, Ca2+ is more likely to enhance the intra-molecular cross-linked reaction, Mg2+ is not as good as Ca2+ but better than Na+. Three of the possible intra-molecular cross-linked polymer gels that are verified by the viscosity tests are injected into the artificial cores for flow performance tests. The results confirmed the intra-molecular cross-linked reaction in the tested polymer gels. The mechanisms of curling and stretching of polymer molecular clews may be explained by Stern-Grahame double layer theory.
Commercial-scale polymer flooding has been successfully implemented in Daqing oil fields since the early 1990s (Chang et al, 2006). High viscosity polymer and gel have been injected into formations to improve sweep efficiency. Those operations have gained great success in high permeability reservoirs with high water cuts. However, for low permeability, heterogeneous, and thin pay formations, excessive polymer injection did not improve oil recovery. In one case, polymer with high concentration was produced from production wells after injection of over 50% pore volume of polymer in a reservoir. Polymer flooding had to be stopped and oil production declined dramatically (Chang et al., 2006). In those poor quality reservoirs oil was left un-swept with the traditional polymer flooding. To improve the sweep efficiency of those reservoirs, pilot tests have been performed by injecting intra-molecular cross-linked (InMC) polymers into various reservoirs in Daqing oil fields for profile modification since 1999. Tests show that InMC polymers performed better than traditional polymers in both profile modification and sweep efficiency. For example, an InMC polymer with Al3+ as the coagulant was injected into Reservoir Pu I1–4, sweep thickness improved by 26 to 45%, oil recovery increased by 6% of the original oil in place.
Cross-linked reactions can take place in polymers with the help of high valence metal coagulants, such as Al3+ and Cr3+. The molecular structures of polymers and cross-linked polymer gels can be observed with an optical microscope (Fig.1). Polymer molecular cross-linked reactions that take place in partially hydyolyzed polyacrylamide can be classified as two types. One is InMC reaction that takes place in different chains within the same polymer molecules, the other is inter-molecular cross-linked (ItMC) reaction that takes place among different polymer molecules. The ItMC polymer has "local" network molecular structures which result in increase in viscosity. ItMC polymer solution cannot be injected into pore throats under regular pressures because of the size of the molecule structure. If excess pressure is used, the molecular structures can be damaged and become less effective on profile modification. In the contrary, the InMC polymer forms "regional" network molecular structures, the viscosity stays the same or even decreases slightly after gelation. Although the viscosity is almost the same as the ordinary polymer, the pliability of the InMC polymer molecular chains decreases, the trapping probability and flow resistance increase in porous media. Those features make it desirable to inject InMC polymers in reservoirs for profile modification and sweep efficiency (Lu et al., 2006, 2007).