Abstract: :In order to conduct water shutoff effectively in high water cut horizontal wells with known water points,especially for wells completed with screens,the fixed-point water shutoff technology for horizontal wells with low-dose modified cyanogen coagulation was proposed by drawing on the experience of leakage prevention in dam crack treatment and waterproofing engineering.Through the chemical modification of viscosity and foaming reaction of the slurry,a modified chemical self-swelling cyanogen coagulation was developed,so as to make it injectable and obtain a controllable reaction time.The compressive strength,pressure resistance and influencing factors of the solidified body were investigated;meanwhile,the matched piston forwarding and bridging residence process were developed.Test results demonstrated that the modified self-swelling cyanogen coagulation as a plugging agent could delay the controllable reaction at 20-90℃,its compressive strength is over 12 MPa,and its pressure resistance is up to 20 MPa.Field tests were conducted in 3 wells,including the first fixed-point water shutoff in horizontal well completed with L-type screen pipe in China.The results demonstrated that the fixed-point water shutoff technology based on modified cyanogen coagulation in horizontal wells could solve the fixed-point plugging problems in extended reach horizontal wells.They could also provide new technical means for water producing treatment in horizontal wells.