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 crosslinking 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 3 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/phenolformaldehyde/HPAM secondary cross-linking gel system can be used for water shutoff in fractured reservoirs.

Using gel as the water shutoff agent has been widely reported in the aspect of enhanced oil recovery (EOR) of the development of mature oil fields. Cross-linkers such as chromium(III) acetate and phenol−formaldehyde are commonly used, but they are toxic. Today, use of low toxic polyethyleneimine (PEI) cross-linking a copolymer of acrylamide and tert-butyl acrylate (PAtBA) as the water shutoff gel is reported more but is mainly focused on its theoretical research and applications in high temperature reservoirs. Because of the defects of the chromium(III) acetate and phenol−formaldehyde cross-linkers, the gelation performance of PEI/partially hydrolyzed polyacrylamide (PHPAM) gel systems at low temperature (40 °C) is discussed in this paper. The results show that the gelation time and strength of the PEI/PHPAM gel system can be perfectly controlled by adjusting the PHPAM concentration, the molecular weight, the PEI concentration, and the total dissolved solids. The polymer concentration and molecular weight can affect not only the gelation time and the gel strength but also its stability. Among them the polymer concentration is the most important factor that affects the gel stability. For the low temperature reservoirs, the most common used chromium(III) acetate/PHPAM gel system has only around 5 h of gelation time at 40 °C to get a high gel strength. However, the PEI/PHPAM gel system can achieve a much longer gelation time. Polymer gels formulated with a combination of 1.5 wt % PHPAM and added 0.3−0.8 wt % PEI provide a gelation time between 15 h and 9 days, and the maximum gel strength could reach code I. This study suggests that the low toxic PEI/PHPAM gel system can be used in low temperature reservoirs.

Using polymer gel is still the mainstream technique of the chemical method for water shut-off in mature oilfield development. The gelation performance of polyethyleneimine (PEI) cross-linking partially hydrolyzed polyacrylamide (HPAM) gel was systematically investigated in this paper by using three types of molecular weight (M w ) HPAM. Results show that the gelant solution (the fluid solution of cross-linker and polymer that exists before gelation) can slightly gel even at room temperature and form a rudimentary 3D network structure for protecting the subsequent gelling. The main factors, including HPAM, PEI concentrations, HPAM M w , and total dissolved solids (TDS) to affect the gelation performance are also discussed. The gelation time is around 18−72 h at 65 °C, which is much longer than the commonly used chromium(III)-acetate crosslinking HPAM gel system. The gelation performance of the PEI/HPAM gel system is improved after flowing through porous media and shows the adverse law for a porous media sheared chromium(III)-acetate or phenol-formaldehyde cross-linking HPAM gel system. Atomic force microscopy (AFM) scanning results show that the microstructure of PEI/HPAM gel distributes a lot of cavities. Compared to the original premature gel (code B), the cavities are more uniform and regular after flowing through porous media. In addition, this study revealed that the presence of free oxygen in the uniform cavities of the weak crosslinked gel after flowing through porous media can accelerate the oxidation reaction to produce a darker brown gel. It also discusses how these new findings will affect the application of the gel system in the oil field.

A category of gemini surfactants with novel structures were designed and synthesized as thickening agents of clean fracturing fluids with good temperature stability in this research.