Study and Application of a Large Temperature Difference Cement Slurry with Good Elasticity and Toughness

Cementing of a gas well with long section to be cemented and big temperature difference has been challenged with difficulties such as ultra-long time of retarding of the top cement column and retained pressure in annular space resulted from failure of cement sheath. Studies have been conducted on the selection of retarding agents and filter loss reducers that can satisfy the requirements of cementing at 170 ℃ with temperature difference of about 100 ℃ and the strength development of set cement. High temperature additives with good toughness were also selected and used in laboratory studies to reduce the brittleness and to enhance the elasticity and toughness of the set cement. Using these selected additives a large temperature difference cement slurry with good elasticity and toughness was developed. The cement slurry had densities adjustable between 1.50 g/cm³ and 2.20 g/cm³, good rheology, API fluid loss of less than 50 mL, 72 h compressive strength of 11.5 MPa of a 1.50 g/cm³ low density set cement at temperature difference of 50 ℃, 72 h compressive strength of 17.7 MPa of a 1.88 g/cm³ set cement at temperature difference of 70 ℃, and the set cement had elastic modulus of less than 7 GPa and flexural strength greater than 3.5 MPa. Inspection of the integrity of the cement sheath showed that the cement sheath was able to undergo 30 turns of loading and offloading of 90 MPa. This cement slurry was successfully used in the tieback of φ193.7 mm and φ206.4 mm liners in the well Shunbei-4 drilled by the Northwest Oilfield Division of Sinopec, with 5693 m open hole cemented with high quality in one try. The difference of the temperatures between the bottom and top was 105 ℃. The cementing operation of the well Shunbei-4 has set up an example of successfully cementing long section of a gas well