Study on a Cement Slurry for Cementing Uncemented Shallow Section of Old Casing String by Secondary Displacement

In Shengli Oilfield, after more than 20 years’ development, the production casing strings above the top of the set cement in some early development wells have been seriously damaged by corrosion, and this badly hinders the normal production of the oil and gas wells. To deal with this problem, it was proposed to conduct secondary well cementing to the free section of the production casing string in the shallow formations. This secondary well cementing not only repairs the integrity of the casing string, it also protects the casing string from further corrosion. Several factors were considered in developing the required cement slurry, such as the technical characteristics of cement slurry injection into the annulus, the nature of the shallow formations being soft, and the requirements of the secondary cementing operation outside the free section of the production casing string. A low-density low-viscosity strong-thixotropy long-thickening time cement slurry was formulated with carefully selected gel agent, low-viscosity thixotropic agent, fluid loss additive and retarder. The composition of the cement slurry is as follows: 40% class G oil well cement + 30% fly ash + 30% slag + (20% – 25%) low-viscosity thixotropic agent L-TA + 2.5% polyvinyl alcohol fluid loss additive+ (0.16% – 0.32%) ZnO + 1% dispersant + 0.5% defoamer. Water/solids ratio of this cement slurry is 0.56. This cement slurry has a density that is as low as 1.5 – 1.6 g/cm³, viscosity of less than 50 mPa∙s, thickening time of longer than 15 h, difference of average gel strengths of 17.885 N/m², as well as repeatable thixotropy. Field application proved that this cement slurry satisfied the requirements of the secondary cementing outside the free section of the casing string in the shallow formations in Shengli Oilfield.