Study of Class G Cement Fatigue using Experimental Investigations

This paper presents the results of laboratory investigation about fatigue of Class G wellbore cement exposed to radial loads under room temperature conditions. While fatigue is well described for metals, wellbore cement fatigue is a rather unknown field. As well cements can be exposed to cyclic loading situations like in new oilfield technologies e.g. enhanced oil recovery by steam injection or geothermal applications using single-well-solutions, cement damage by fatigue becomes a more important issue. Cement integrity is crucial environmental and economic issue and no risk on cement failure should be accepted. The thermal induced loads in the reality were replaced by mechanical loads that recreates the stress situation on the cement. In order to evaluate the behavior of the cement, experiments were performed to investigate how cement reacts to cyclic loadings. A low number of cycles mean up to 100 loadings sequences. Samples of a pipe-cement-compound recreating the wellbore geometry are tested in a hydraulic press under axial loads. The stress situation in the cement sheath of the tested samples was calculated using a combination of numerical and analytical methods. Failure criteria were used to evaluate these calculations and could be used to predict future failure behavior. It has been found that fatigue of cement is rather similar for metal and cement at least in the low cycle range. For metals there is a specific stress limit where failure or significant damage to the material occurs within several cycles. This means, if the limit is exceeded the material will fail, maybe not at the first cycle, but it will fail over the cycles. Cement shows that this behavior is similar to metals, no other fatigue mechanisms like damage accumulation were observed, just a straight load limit.