A successful cement job results in complete zonal isolation while saving time and money. To achieve these goals, various factors such as well security, casing centralization, effective mud removal, and gas migration must be considered in the design. For the cement job to be successful, the permeability of the set cement must be low enough to prevent any fluid flow through the cement, possibly damaging the casing. Therefore, the design of the cement must be such that it prevents:
Corrosive fluid invasion
Annular gas pressure
In High Pressure/High Temperature (HPHT), cases, a more flexible cement which expands more than conventional cement may prevent cement failure.
The stress in the cement is strongly dependent on temperature and pressure as well as the lithology and in-situ stress. Since the stress is so dependent on temperature, the temperature variation must be precisely predicted to achieve the proper design of the cement slurry and eliminate excessive time spent waiting on cement. In addition, a post-job analysis is necessary to ascertain zonal complete isolation and avoid unnecessary remedial work.
By increasing the flexibility of the set cement (lowering the Young's modulus) we could reduced the tensile stress in the cement sheath during thermal expansion. This could be a solution to the problem of cement stability in high temperature cases.
Here we report the use of the Finite Element Method, (FEM), to investigate the stress fields around and inside the cement in and to forecast the condition of cement failure. This study mostly focuses on the cement-casing boundary. This method is more powerful than conventional stability methods since complex boundary conditions are involved as initial conditions and are investigated simultaneously to more accurately predict cement failure.
There are many of challenges dealing with cementing wells under High Pressure High Temperature (HPHT) conditions. Cement design and placement are very critical under these conditions. Since the casing stability is very dependent on cement integrity, and bonding with the casing and formation, operators must do everything feasible in order to insure a successful cement job.