Assessment of the cement failure probability using statistical characterization of the strength data

Cement sheath failure may occur under the influence of induced stresses during service life of the well. For many years, analytical or numerical models have been developed to study the probability of the cement sheath failure under different circumstances. Cement compressive strength is considered as a valuable input parameter during the assessment of the cement failure probability. Usually the compressive strength data are obtained by conducting several direct laboratory experiments on cement samples. The average value of these experiments are reported and used in different cement failure models. However, this approach may results in underestimating the cement failure probability. As cement have heterogeneous structures, therefore their strength variations have a statistical character. In this work, the compressive strength data are represented by the Weibull distribution function, which then will be used in the cement failure probability calculations. Results show that using the statistically distributed compressive strength data in calculations, leads to more accurate outputs of the failure model.