The objective of this study was to evaluate the effect of thermal conductivities and Young's moduli of damaged materials (high-porosity material with the lowest absorption of X-rays in computed tomography images) on tensile stresses and tensile damage evolution during heating and cooling of a downscaled casing-cement-rock assembly. Tensile failure was predicted during thermal cycling of the casing-cement-rock assembly at both heating and cooling stages. The failure occurred predominantly in damaged cement and damaged rock. The simulation results suggested that cement immediately adjacent to the casing pipe is most prone to tensile cracking during both heating and cooling. Reducing stiffness of damaged materials reduced the overall number of tensile cracks, and introduced a small amount of cracks in the intact cement. Reducing thermal conductivity of damaged materials increased the overall number of tensile cracks, and introduced a small amount of cracks in the intact rock and a significant number of tensile cracks in intact cement. The results indicate that tensile cracks may be generated even in the intact cement during heating, if there is sufficient contrast in thermal conductivity between damaged and undamaged materials.
Injection of relatively cold or hot fluids into cased wells is not uncommon in oil and gas industry. Injection of hot fluids takes place e.g. under huff-and-puff stimulation and during steam-assisted gravity drainage. Cooling of a cased well can occur during injection of CO2 into underground reservoirs for permanent storage. Heating or cooling may have an adverse effect on well integrity. In particular, tensile thermal stresses can be induced in cement around casing. Cement is a brittle material. Its tensile strength is typically much lower than its compressive strength. Tensile thermal stresses can therefore produce cracks in bulk cement or at an interface between cement and casing or rock. Such interfacial cracks, once joined together, can make up a conductive fracture network and act as a conduit along the well. This can enable fluid flow between geological strata and thereby violate zonal isolation principles. This will call for remedial measures which can be expensive. Therefore, being able to predict mechanical stability around wells subject to heating, cooling or thermal cycling is crucial for safe and environmentally-conscious operation of wells.