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Wellbore strengthening has been introduced recently to resolve lost circulation problems by improving the fracture gradient and hence extending the mud window. Polymeric cross-linkable solutions showed outstanding strength with high thermal stability at elevated temperatures. In this study, silica with different sizes is used to reinforce the polymer solutions. The objective of this work is to investigate the effect of nanosilica sizes (8, 20, 50, and 85 nm) and concentrations (0.1–2 wt %) on the stability and viscoelastic behavior of polyacrylamide (PAM) cross-linked with polyethyleneimine (PEI) at 130 °C. Moreover, the effect of PAM M w is also studied. The results have shown that nanosilica has reinforced the PAM/PEI solution; the gel of the base polymeric solution has upgraded from code “F” to codes “G” and “I” based on the Sydansk coding system. The strongest gel was formed with the addition of 2 wt % of 50 nm silica to the polymer base solution which enhanced the gel strength by more than 300%. Zeta potential confirmed that 50 nm silica was the most stable among the other sizes. Gel strength was observed to increase upon increasing the size of nanosilica initially, and then, it has decreased which gave an optimum nanosilica size of 50 nm. The stability of silica particles in the system is suggested as an explanation for this drop in strength. The interaction between silanol and carbonyl groups via hydrogen bonding is proposed as the controlling mechanism of gel formation. The results suggest the importance of selecting the proper size and content of nanosilica for reinforcing PAM/PEI gels.

Excessive water production from natural gas reservoirs is a main challenge facing the industry nowadays. Polymeric gelants have been widely applied to seal the water production zones, leading to a more feasible production operation. Nevertheless, conventional treatments fail in reservoirs characterized with the presence of sour gases. In this paper, aluminum-based salts are investigated as potential replacement for the conventional chromium acetate as crosslinkers for polyacrylamide (PAM), where aluminum has the advantage of being more environment-friendly besides its abundance. The investigation covers the whole pH range and examines the rheological behavior of the mature gels in the temperature range between 25 and 100 °C. While chromium acetate was proven to be sensitive to the presence of sour gases, namely, CO 2 and H 2 S, because of the inability to produce a stable gel at the acidic conditions, this paper presents aluminum-based crosslinkers that are more tolerable toward high acidity. Unlike the conventional crosslinkers, the gelation rate in aluminum acetate and aluminum aminoacetate systems was found to decrease with the increase in pH. Both the crosslinkers succeeded in producing a strong gel with a storage modulus of more than 2000 Pa. Moreover, this study relates the physical stability of the colloidal aluminum crosslinkers with the viscoelastic behavior of the mature gel. The results reveal that aluminum acetate, among the screened salts, has a controllable gelation time at pH conditions between 3.5 and 8.5 and is the most stable in the temperature range 25–100 °C. PAM/ AlAc system has a gelation time of around 50 min at 75 °C making it suitable for near-wellbore treatments, while the gelation time increased to 80 min upon increasing the pH of the system from 4.1 to 4.6. Moreover, the system showed good stability in saline conditions with NaCl concentration of up to 50,000 ppm. Scanning electron microscopy of freeze-dried samples proved the uniform distribution of colloidal crosslinkers and showed sheets wrapping around the colloidal particles. The performance of the new crosslinker is compared with available commercial crosslinkers.

Recently, cross-linked polymers such as polyacrylamide (PAM)/polyethylenimine (PEI) have been used in conformance control applications to replace conventional lost circulation materials (LCMs). The objective of this work is to introduce functionalized silica to reinforce PAM and enhance gel performance under high temperature high pressure (HTHP) conditions. A system was developed using PAM/triamine silica (TAS) combined with nanosilica (NS) to replace the conventional PEI as a cross-linker. Polymer systems consisting of 9 wt % PAM and polyacrylamide tert-butyl (PAtBA) with various concentrations of PEI, TAS, and NS (50 nm) were used to produce mature gels at 130 °C. The results have shown that TAS has formed a gel with code “E” on the Sydansk coding system. Adding a low concentration (1–2 wt %) of 50 nm silica has eliminated the settling of microsized TAS and produced a reinforced gelant system with code “I”. The cross-linking was accomplished via an amine group similar to the PEI mechanism with the advantage of strength and stability due to the presence of silica. The increase in the gel strength of the PAM/TAS formulation depends on the amount of NS; the gel strength of the PAM/PEI system has increased from 1670 Pa to more than 9000 Pa for the PAM/TAS/NS system. The new formulation is compatible with high salinity water, and a chelating agent could be introduced to prevent formation damage.

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