A new type of casing plug effectively preventing the backflow of cement slurry

Abstract

For mitigating the hazards of slurry flowing back into the casing after liner cementing, the deficiency of slurry backflow prevention structure of current liner hangers at home and aboard was analyzed in this paper. Then, a structurally new casing plug, i.e., a semi-hollow casing plug was designed and developed. And it, combined with the hanger, was tested on site. And the following research results were obtained. First, the deficiency of the slurry backflow prevention structure of liner hanger, is that the locking mechanism on the drill pipe plug is damaged before it reaches the casing plug and matches with each other. Second, the inner bore of the semi-hollow casing plug is designed as a bend hole, which has a hollow round hole in its upper part and a solid column in its lower part. Third, during the cementing operation, the drill pipe plug enters into the semi-hollow casing plug once it gets to the position of hanger. And it is stuck at the bend and cannot go down anymore. Consequently it is integrated with the casing plug as a whole part. Fourth, when the shear pin goes down to the setting seat through the casing string under the force of drilling fluid, the solid column at the lower part of the semi-hollow casing plug is squeezed into the inner hole of the seat. In this situation, the backflow of cement slurry can be prevented only by using sealing elements and locking device on the casing plug instead of by using drill pipe plug. Fifth, field tests show that the cement top of test wells is only 24 m higher than the designed value while that of offset wells is 59 m higher. Obviously, the former is 50% less than the latter. In conclusion, this newly developed semi-hollow casing plug matches well with hangers, having no negative effects on cementing operation, so the cement top after cementing can be well controlled.