Cement is critical to well integrity. It provides hydraulic isolation, preventing fluid flow between producing zones, ground water aquifers, and the surface. In steam stimulated wells, such as for Steam Assisted Gravity Drainage (SAGD) or Cyclic Steam Stimulation (CSS), the heat-up period places severe mechanical loading on the cement sheath. Heat is transferred from high temperature steam, through completion strings and annular fluids, to the casing, cement and formation. Thermal expansion of the casing combined with axial constraint make radial expansion of the casing the most severe in the energy industry. Furthermore, with constrained expansion of the cement, the range of deformations that must be accommodated by the cement sheath while maintaining isolation is challenging. These deformations can cause shear or tensile failure of the cement and result in leakage paths through the cement (e.g. cracks or global changes in cement permeability), or leave a micro-annulus when recovery has been completed and thermal operations halt.
This paper describes the impact of key thermal and mechanical properties on the structural performance of cement blends in thermally stimulated wells. The work is based on laboratory testing of thermal cement blends and the use of Finite Element Analysis (FEA) to examine cement performance under operating conditions. The findings provide insight into important cement behaviours that impact longterm integrity of cement and highlight the significance of conducting tests under field representative conditions. Results indicate the importance of compressive strengths, flexibility, and shrinkage/expansion characteristics to ensure the cement sheath remains structurally intact during initial heat-up.