Experimental Investigation of the Novel Phenol-Formaldehyde Cross-Linking HPAM Gel System: Based on the Secondary Cross-Linking Method of Organic Cross-Linkers and Its Gelation Performance Study after Flowing through Porous Media

The use of a polymer gel is an effective method for water shutoff in mature oilfield development. As for fractured reservoirs, in order to mitigate the filtration of gelant (fluid solution of cross-linker and polymer that exists before gelation) to matrix and increase the enduring erosion ability of mature gel, chromium(III) acetate, and phenol formaldehyde cross-linking, the HPAM gel system of a secondary cross-linking method is used more often. Chromium(III) salt is often used as the first cross-linker. However, the cross-linking mechanism is achieved by an ion bond, which is less stable than a covalent bond when used as an organic cross-linker. Resorcinol and phenol formaldehyde used as the first and secondary cross-linker, respectively, are discussed in this paper. Results showed that resorcinol can quickly cross-link with HPAM at room temperature. Gelant formulated with a combination of 0.3 wt % HPAM added to 10-30 mg/L resorcinol can increase its viscosity from 10.2 to 150 mPa . s within 2 h. SEM results show that the microstructure of the first cross-linking gel appears in typical dendritic shape, with branched chains diffused in arbitrary directions. The high shearing tolerant ability of the first cross-linking gel can be achieved by these branched chains. However, a tight 3-D network structure is formed in the microstructure of the secondary cross-linking gel. This is the benefit of the stability of the skeleton structure of gel enhancing. The main factors, including temperature and total dissolved solids (TDS), to affect the gelation performance of this secondary cross-linking gel are also discussed. Results show that gelation time decreased and gel strength increased with increasing temperature and TDS. Especially for TDS, the adverse law of the gelation performance with PEI/PAtBA or PEI/HPAM gel systems is shown. The gelation performance of a resorcinol/phenol-formaldehyde/HPAM gel system of a first cross-linking state after flowing through porous media is studied. Atomic force microscopy (AFM) scanning results show that in comparison to the original gel, the structure of the weak cross-linking (code B) gels has a certain degree of damage after flowing through porous media. However, the final gel strength of both gels do not show an apparent difference. This demonstrated that the first cross-linking achieved by resorcinol can guarantee the effectiveness of secondary cross-linking. This study suggests that a resorcinol/phenol-formaldehyde/HPAM secondary cross-linking gel system can be used for water shutoff in fractured reservoirs.