An Ultra-High Temperature High Density Corrosion Inhibitive Anti-Channeling Cement Slurry

The oilfield in the west of the South China Sea, in which high temperature high pressure gas reservoirs are drilled, is a main gas production area. In this oilfield the formation temperature of the target zone in the Ledong area is 210 ℃, the formation pressure coefficient 2.20-2.29 and the safe pressure window only 0.04-0.05. These formation characteristics together with the existence of CO₂ in the same target zone make it difficult for the energy to be explored and developed. One of the key technologies for the gas development in this area is the development of a corrosion-resistant cement slurry for use in ultra-high temperature high pressure environment. To efficiently develop the gas resource, a cement slurry, having its densities ranging between 2.40 g/cm³ and 2.60 g/cm³ and with CO₂ corrosion resistance is developed. In this cement slurry, a reduced iron powder, a manganese ore powder and a silica of different particle sizes are sized and used as the solid material, a low activity MgO as the lattice swelling agent to prevent shrinkage of the cement sheath, a special latex as the corrosion inhibitor and anti-channeling agent, and an organic and an inorganic intercalated polymers as the high temperature suspending stabilizer. Laboratory evaluation experiment shows that under CO₂ partial pressure of 50%, aging pressure of 100 MPa and aging temperature of 180-220 ℃, the compressive strength of the set cement after being corroded for 180 d is three times of the compressive strength of the set cement before being corroded. At the same time, the permeability of the set cement is only slightly increased (<0.01 mD). The hydration products of the cement change from C₆S₆H, an xonotlite before corrosion of the set cement, to CaCO₃ and SiO₂ after corrosion. The set cement, after being aged at 220 ℃ for 24 h, has compressive strength of greater than 25 MPa, and SPN value of less than or equal to 0.5. The thickening time of the cement slurry is easy to adjust, the cement is easy to flow, and the difference between the density of the slurry at the bottom and that of the slurry at the top is less than 0.01. These properties of the cement satisfy the requirements of the field operation.