Cement Integrity Assessment Using a Hydration-Coupled Thermo-Mechanical Model

Abstract

Abstract Portland cement is commonly used in wells to provide zonal isolation in the annulus. A damaged cement sheath can expose the casing to corrosive fluids and open a leakage pathway to shallow freshwater aquifers and atmosphere. The leakage can manifest itself as sustained casing pressure (SCP) or lead to gas accumulation in shallower formations. The impact of pressure and temperature variation on cement stress has been widely studied in the literature. However, the hydration reactions of cement are not usually included in the mechanical models. This leads to incorrect assumptions about the initial state of stress in cement immediately after curing. In this work, we have developed a 3D well integrity model that incorporates the cement hydration process. The model is verified using laboratory experiments on cement stress evolution. The model calculates the water consumption during the hydration reactions to predict the pore pressure change in cement. The evolution of cement's mechanical properties with the hydration degree is captured using a homogenization model. A case study is designed to represent a typical low-enthalpy geothermal well in the Netherlands, using well designs and inputs from publicly available data. The cement stresses are tracked over the life of the well, to understand the magnitude of the stress cycles and to assess the potential long-term damage to the cement sheath. The results show that the pore pressure drop due to cement hydration causes an increase in shear stress in the cement sheath. The pore pressure drop during hydration can debond the cement from the formation. The level of destressing in cement is a function of cement properties, formation stiffness, and the depth of the top of cement. When placed against softer formations, the stress drop in cement is more muted leading to a better seal. During the temperature cycles, the shear stress in cement changes in a cyclical manner. Depending on the magnitude of the stress cycles, damage can be accumulated in the cement sheath. The stress evolution in cement can also vary depending on the presence of external water (formation permeability). The modelling technique presented in this work provides a robust methodology to estimate the magnitude of cyclical stresses in the cement sheath. This is a critical input to design cement recipes that can withstand load cycles throughout the lifetime of the well. The results of this work indicate the need to assess the integrity of cement at various depths and against various formations. It may not be possible to guarantee the seal efficiency against all formations, however risk analysis can be conducted using the presented model to assess the seal integrity of critical locations in the well profile.