Development and Application of Oil-Based Thermo-Responsive Gel Lost Circulation Material SBR−g−PSA

Lost circulation of oil-based drilling fluids (OBDFs) in deep, fractured formations remains a critical challenge, primarily due to the poor retention and low pressure-bearing capacity of conventional materials on oil-wet rock surfaces. To bridge this gap, this study reports the development and field application of a novel thermo-responsive gel (SBR−g−PSA). Synthesized by grafting highly lipophilic stearyl acrylate (SA) onto a flexible styrene–butadiene rubber (SBR) backbone, the microgel exploits a UCST-like (Upper Critical Solution Temperature) phase transition behavior. At surface temperatures, rigid styrene microdomains act as physical crosslinks, maintaining the particles in a compact, highly dispersible state. Under elevated downhole temperatures (120–180 °C), these microdomains relax, triggering the massive unfolding and physical entanglement of the octadecyl side chains. This in situ topological evolution generates a resilient three-dimensional network with a high thermal degradation temperature of 346.6 °C. Rheological and macroscopic sealing evaluations demonstrate that the gel effectively adapts to OBDFs, securely sealing 1–1.5 mm fractures and limiting fluid loss to within 5 mL under a 4 MPa differential pressure. Successful field application in the deep well Dashen-X reduced severe fluid leakage from 2.42 m3/h to zero. This intelligent thermo-responsive platform offers a robust, high-performance technological solution for precise lost circulation control in deep oil and gas exploration.