Development and Performance Study of Delayed Swelling Lost Circulation Material for Water‐Based Drilling Fluids

ABSTRACT Fractured formation lost circulation hinders oil‐gas drilling efficiency. Existing bridging materials lack deformability/adaptability, while conventional swelling agents suffer from excessive swelling rates and low mechanical strength. A delayed‐swelling lost circulation material for water‐based drilling fluids has been developed via free radical copolymerization and encapsulation, using acrylamide, dimethyldiallylammonium chloride,1,3‐divinyltetramethyldisiloxane as monomers, plus crosslinking, initiating, toughening (lauryl acrylate) and reinforcing (bentonite, sodium silicate) agents. Performance tests show the encapsulated material (FSJ) exhibits excellent delayed swelling: at 25°C, 30 min swelling ratios are 2.4 times (distilled water) and 1.4 times (drilling fluid) vs. over 3 times for unencapsulated ones, the gel strength reaches 1000 kPa. In a salt solution with a mineralization degree of 2.0 × 10 5 mg/L, the swelling ratio exceeds 4 times, and the temperature resistance is up to 120°C. When compounded with traditional bridging materials, the developed lost circulation material exhibits excellent pressure‐bearing capacity: it withstands a pressure of 13.41 MPa in fractures with an opening of 5 mm, and maintains a pressure‐bearing capacity exceeding 8 MPa in fractures with openings of 3 and 8 mm respectively, outperforming single traditional materials, offering a technical solution for complex formation lost circulation.