Thermo-Activated Strengthening Effect in Dynamic Covalent Polymer Gels Based on Boron-Nitrogen Bonds for Lost Circulation Control in Deep Coal Measures

Lost circulation in fractured deep coal-measure strata is a major drilling challenge that severely impedes unconventional gas development. This issue is acute in the Hannan Block, where natural fractures with 1–5 mm apertures abound. Conventional gel-based lost-circulation materials (LCMs) often degrade thermally and fail to reconcile injectability with downhole retention. We developed a multiply cross-linked polymer gel with dynamic boron–nitrogen (B–N) bonds, synthesized via in situ free-radical copolymerization of poly(vinyl alcohol) (PVA), acrylamide (AM), 3-acrylamidophenylboronic acid (3-APBA), polyethyleneimine (PEI), and N,N-dimethylaminoethyl methacrylate (DMAEMA). Structural analyses were consistent with dynamic B–N coordination as the primary reversible cross-linking site. Rheological tests at 25 °C showed pronounced shear-thinning: over a shear-rate range of 0.1–1000 s−1, the apparent viscosity dropped from 86,500 to 115 mPa·s, i.e., by more than two orders of magnitude. After severe structural disruption at 150% strain, the storage modulus recovered to 6880 Pa within 120 s, i.e., 83.9% of its initial value (8200 Pa), demonstrating excellent thixotropic recovery. In contrast to conventional gels, this material exhibited a distinctive thermo-activated strengthening effect: after hydrothermal aging at 205 °C for 16.5 h (the upper bound of the target downhole temperature range), tensile strength reached 3.68 MPa and elongation at break rose to 720%, compared with 1.85 MPa and 320% in the unaged state. Plugging experiments performed at 140 °C (the representative bottomhole temperature of the Hannan Block) yielded breakthrough pressures of 7.3 MPa (3 mm parallel fracture) and 7.9 MPa (3 mm wedge fracture), and exceeded 11 MPa in a 30 cm sand-packed tube. Encapsulation of rigid CaCO3 particles further raised the breakthrough pressure in the 3 mm parallel fracture to 8.8 MPa, a 20.5% improvement over the gel alone. Microstructural imaging revealed strong adhesion to fracture surfaces via mechanical interlocking and cationic electrostatic interactions, together with the formation of an integrated gel–particle plug. Collectively, these results suggest that dynamic B–N bond-mediated network rearrangement offers a viable route to achieve both pumpability and high-temperature plugging, underscoring its potential for combating severe lost circulation in deep coal-measure formations.