Delayed Crosslinking and Plugging Performance of Polyacrylamide Gel Using CaCl2-Tolerant Delayed-Release Crosslinker in High-Calcium Medium

Lost circulation is a major technical bottleneck restricting safe and efficient while-drilling plugging operations. Polyacrylamide gel has become a widely used plugging material in drilling engineering. Unlike rigid, cement-like plugging materials, the gel system formed in this study does not develop a hardened, consolidated structure capable of anchoring or binding the drill bit during subsequent drilling operations, thereby eliminating the risk of bit-sticking. Nevertheless, the gel possesses sufficient elastic (viscoelastic) structural strength—reflected in its storage modulus (G′)—to effectively resist deformation and displacement under differential pressure, thereby providing reliable fracture-sealing performance, which effectively prevents pipe-sticking risks. However, high-concentration PAM molecular chains easily stretch and entangle in aqueous solution, triggering an abnormal increase in initial viscosity and poor pumpability. Although Ca2+ can inhibit the premature water absorption and thickening of PAM to maintain system fluidity, an excessively high Ca2+ concentration will suppress the hydrolysis of Al3+ and hinder the formation of hydroxyaluminum—the key crosslinking component of the gel system. To solve the above contradiction, a CaCl2-tolerant delayed-release crosslinker was synthesized. ZnO was selected as a carrier to adsorb and immobilize polynuclear hydroxyaluminum complexes hydrolyzed from an inorganic aluminum crosslinker at 70 °C, realizing the controlled delayed release of the crosslinker. The microstructures and chemical bonding were characterized by SEM elemental mapping, FT-IR and 27Al MAS NMR. The results confirm that abundant aluminum species are uniformly loaded on the ZnO surface to form stable Zn–O–Al covalent bonds, and the loaded aluminum exists mainly in the form of hydroxyaluminum. With increasing temperature, the Zn–O–Al bonds gradually break and slowly release hydroxyaluminum species. A novel delayed crosslinking gel system was ultimately optimized, composed of 6% CaCl2, 10.4% PAM and 3% ZnO loaded with polynuclear hydroxyaluminum. The system exhibits excellent delayed gelation behavior, with a fluidity loss time longer than 120 min and a gelation time over 200 min. It maintains favorable fluidity within 30–90 °C, and the formed gel shows a stable elastic modulus (G′) and viscous modulus (G″). Moreover, the system achieves a plugging rate of more than 90% and a breakthrough pressure above 5 MPa, demonstrating superior comprehensive plugging performance for while-drilling plugging applications.