Risk-Based Analysis of Thermal Well Integrity Through Integration of Caprock Geomechanics and Cement Sheath Design

Abstract The integrity of thermal in situ wells is a function of the distinct yet inter-related processes that control the integrity of the reservoir caprock and of the wells themselves. Our objective was to bridge the scale gap between reservoir caprock integrity analysis and thermal well cement sheath design, and in so doing, improve the risk analysis of cement sheath integrity. At the well pad scale, a coupled finite element geomechanical model was used to predict the evolution of thermal and mechanical stresses in the caprock due to thermal in situ operations. The geomechanical models were populated with data from public domain reports from northeastern Alberta thermal in situ projects. These model results were then used to constrain a risk-based analysis of the multiple potential failure mechanisms at the wellbore scale that can compromise cement sheath integrity of thermal wells. Common thermal well and cement blend properties were used to then analyze the probability and magnitude of cement sheath damage during thermal injection operations. Although integrity may be predicted within the caprock for a given operational practice, the integrity of the cement sheath was shown to be less certain. Several factors contributed to this uncertainty. Firstly, the selection of caprock material and plasticity models for caprock stress analysis has significant implications for the risk of cement sheath damage. Secondly, the statistical distribution of caprock mechanical properties is typically quite broad through the overburden and caprock intervals, meaning that average zone mechanical properties may not be sufficient to model and design cement sheath integrity. Thirdly, it is necessary to adequately constrain our knowledge of the initial state of stress within the cement sheath after placement, and before steam injection. Our results show that these three factors can be adequately constrained to reduce well integrity uncertainty through the integration of geomechanical caprock integrity analyses and cement sheath integrity analyses. In this study we combine relatively time-consuming coupled finite element method analyses with rapid semi-analytical system response curve methods for cement sheath integrity. In so doing, we quantify the impact of the uncertainties present in caprock modeling and in the constitutive modeling of thermal cement blends on a risk-based evaluation of cement design requirements.