Detecting and modeling cement failure in high pressure/ high temperature wells using finite-element method

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. In the event that high-pressure and high-temperature (HPHT) conditions are encountered, we must attempt to achieve permeability in the set cement to prevent gas migration and to prevent any other fluid passing through to collapse the entire structure. Therefore, the design of the cement must be such that it prevents: Micro-annuli formation Stress cracking Corrosive fluid invasion Fluid migration Annular gas pressure In HPHT cases, we need more flexible cement than in conventional wells. This cement expands more at least 2 to 3 times more in some special cases. The stress in the cement is strongly connected with temperature and pressure, as well as lithology and in-situ stress. If we can define a method which connects the higher temperature to the lower stress field, we would have the solution for one side of the equation, and then we could model the pressure (stress principles) at the designated depth and lithology. Since the stress is so dependent on temperature, the temperature variation must be accurately predicted to properly design the fluid and eliminate excessive time spent waiting on cement. In addition, a post-job analysis is necessary to ascertain zonal isolation and avoid unnecessary remedial work. By increasing the flexibility of the set cement (lowering the Young's modulus), we can reduce 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, and to forecast the time of failure and its affect on cement integrity. 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. The results of this study show the relevant dependency of stress principles with temperature and pressure. These results clarify the deformation caused by any disturbance in the system and the behavior of under-stress locations based on their relative solid properties.