| Abstract |
Cement sheath, placed between casing and formation, is an important barrier to provide a hydraulic seal and establish zonal isolation, preventing fluid communication in the wellbore. In geothermal wells, the temperature at the bottom hole is high. Thermal stress induced by temperature is one of the major considerations to trigger failure in the cement sheath. In this study, a numerical model is created based on the FORGE geothermal well in Utah to investigate the effects of casing pressure, temperature and thermal related parameters on cement integrity of casing-cement-formation systems. To achieve the object, a 3D finite element numerical model consisting of casing-cement-formation was developed. The model assumed that the cement-pipe and cement-formation interfaces are bonded. Maximum and minimum horizontal stresses from FORGE geothermal well drilling reports were applied as far-field tectonic stresses. Elastic modulus and mechanical strength used in the model were obtained through laboratory measurements and research literature. Cylindrical mechanical stresses in the cement (i.e. radial and hoop stresses) were analyzed against respective limiting strength to identify failure modes of cement. Because of the symmetric nature of the problem, only one quarter of the well model (0° to 90°) was examined. Sensitivity analysis was conducted to understand the effects of cement properties (expansion coefficient and conductivity), temperature difference between casing and formation (ðš«T), and casing pressure on cement integrity. Results from sensitivity analysis revealed that for similar percentage change, temperature difference could have more influence on cement integrity compared to wellbore pressure. For the same differential temperature (ðš«T) between the wellbore and formation, heat transfer from formation to pipe (temperature of pipe is less than formation) is more detrimental to cement integrity than the opposite scenario (temperature of pipe is higher than formation). In the case of heat flow away from the wellbore, hoop stress in cement is mainly dependent on temperature difference while radial stress is primarily dependent on temperature difference as well as wellbore pressure. In the case of heat flow towards the wellbore, radial stress in cement is primarily a function of temperature difference and wellbore pressure. Hoop stress on the other hand, is mainly dependent on temperature difference and cement’s thermal expansion coefficient. This paper adds novel information to research literature by presenting sensitivity of cement mechanical stresses to thermal parameters. Useful information generated from this work would be applicable to both injection and production wells in geothermal applications. Better understanding of influence of thermal parameters will help improve the cement design and structural integrity. |