Surfactant　injection　technology　has　recently　been　proposed　as　an　alternative　to　mechanical　pigging　for　liquid　unloading　from　the　wellhead　to　the　surface　gas　gathering　system.　This　study　aimed　to　verify　the　effectiveness　of　the　technology　by　experimentally　evaluating　the　liquid　loading　characteristics　of　gas–liquid　flow　with　a　surfactant　at　the　undeveloped　and　developed　flow　positions　of　the　uphill　test　section　(10°)　of　a　hilly　terrain　pipeline.　The　inner　diameter　and　length　of　the　uphill　test　section　were　50.0　mm　and　16.0　m,　respectively.　The　liquid　and　gas　working　fluids　were　deionized　water　or　250　ppm　sodium　dodecyl　sulfate　(SDS)　surfactant　solution　and　air　with　the　superficial　velocities　of　0.001　to　0.010　m/s　and　2.0　to　14.0　m/s.　The　liquid　volume　fraction　and　liquid　holdup　were　measured　using　the　image-quick　closing　valve　method　and　wire　mesh　sensor　method,　respectively,　and　the　liquid　holdup　data　were　visualized　using　the　MATLAB　program　and　Tecplot　software.　Owing　to　the　effect　of　the　surfactant,　the　flow　range　of　low　liquid　loading　increased　significantly　with　the　flow　regime　transition　from　intermittent　flow　to　segregated　flow.　Because　of　the　complex　interaction　between　the　morphology　of　the　slug　body　and　the　foam　formation,　the　fluctuation　range　and　peak　of　liquid　holdup　and　the　liquid　volume　fraction　decreased　significantly　in　the　intermittent　flow.　The　liquid　holdup　fluctuation　range　and　liquid　volume　fraction　decreased　to　nearly　zero,　and　a　foam　layer　was　formed　and　climbed　up　along　the　pipe　wall　in　the　segregated　flow.　The　results　and　analyses　are　beneficial　for　improving　the　surfactant　injection　scheme　and　establishing　a　prediction　model.　©　2022　Elsevier　Inc.