Preparation and High-Temperature Fracture-Sealing Performance of an Oil-Absorbing Swelling Gel for Lost-Circulation Control in Oil-Based Drilling Fluids

Lost circulation in fractured formations remains challenging for oil-based drilling fluids because conventional rigid bridging materials can be sensitive to fracture geometry and form porous seals. In this study, an oil-absorbing swelling gel plugging agent (OSGP) was prepared by water-in-oil emulsion free-radical polymerization. Its oil-induced swelling, strain-dependent viscoelasticity, and bidirectional fracture-sealing behavior were evaluated together with morphology and thermal mass-loss behavior. After 12 h in white oil at 120 °C, the particle volume increased by 19.2%. At 25 °C, OSGP showed an elastic-dominant response with strain-induced softening. In independent fracture-sealing tests at 120 °C, OSGP formed pressure-bearing seals in both 3 mm parallel and 3 mm-to-1 mm wedge-shaped steel fractures, with a maximum forward breakthrough pressure of 8.06 MPa. Comparison of forward and reverse tests showed that fracture geometry and flow direction strongly affected pressure-bearing behavior. The combined results support a proposed sealing process involving particle bridging, oil-induced swelling, deformation, and compaction. OSGP therefore shows potential for further evaluation as a lost-circulation material in fully formulated oil-based drilling fluids and rock fractures.