Successful Application of High Temperature Multi-Functional Gas Channeling Preventing Cement Slurry in Marine Ultra Deep Wells in Sichuan Basin

In cementing ultra-deep gas wells with high temperature, high pressure and high sulfide content in marine Sichuan basin, settling of cement slurry at high temperatures, gas channeling and decline of the strength of set cement were the main technical problems that need to be addressed. In the development of a latex cement slurry with high elasticity, high toughness, low permeability and anti-channeling performance, different methods were used to optimize the cementing additives. The stabilizing agent was selected by settling stability evaluation performed on an HTHP thickening instrument and by particle sizing principles. By directly measuring the gas channeling permeability and the permeability of the set cement under plastic conditions, the gas channeling agent was selected. The high temperature retarder was selected using Ubbelohde viscometer which evaluates the thermal stability of polymers. The particle size and concentration of a quartz sand as high temperature stabilizer were optimized by analyzing the compressive strength of the set cement cured at 180 ℃ through orthogonal test method and thermogravimetry. It was found in these experiments that: 1) at temperatures between 150 ℃ and 180 ℃, the cement slurries of 1.90 g/cm³-2.30 g/cm³ all had good engineering performance, laying the foundation of solving the well cementing difficulties as mentioned above. 2) Adding sand into the cement slurry can mitigate strength decline of the set cement. At elevated temperatures, sand concentration has a positive relationship with compressive strength of the set cement. At the same sand concentration, sand of finer particle sizes is better than sand of coarse particles, but from the long run, addition of coarse sand is more beneficial to the strength development of the set cement. It was thus suggested that at temperatures between 150 ℃ and 180 ℃, the silica sand concentration should be greater than 35% and 45%, respectively. 3) Ubbelohde viscometer can be used to evaluate the high temperature resistance of filter loss reducer and retarder, and is therefore an important auxiliary means to fast screening of key additives for high temperature wells. 4) Further evaluation on the corrosion by acidic gases such as H₂S, to set cement should be carried out, and this will be beneficial to the design of cement slurries with anti-gas-channeling and corrosion resistance capacities.