A treatment fluid can include: a base fluid, wherein the base fluid comprises water; and a lost-circulation material, wherein the lost-circulation material comprises a polymer network having at least one branching point formed with a monomer and a cross-linking agent that comprises at least three active functional groups. The treatment fluid can maintain a pressure differential of at least 1 psi when tested with a cylindrical void that has a diameter of 0.02 inches. The monomer can be a vinyl ester-based monomer that is polymerized with the cross-linking agent to form the polymer network. The treatment fluid can be used in an oil and gas operation.