Investigation of the rheological properties of nanosilica-reinforced polyacrylamide/polyethyleneimine gels for wellbore strengthening at high reservoir temperatures

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 Mw 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.