What are the main factors affecting the cement sheath stress and strain behavior?

1 Cementing operation

The cementing operation involves pumping a slurry of cement and water into the annulus between the casing and the formation, displacing the drilling fluid, and allowing the cement to set and harden. The cementing operation can generate thermal and hydraulic loads on the cement sheath, as well as mechanical loads from the casing and the formation. These loads can cause the cement sheath to deform, crack, or debond from the casing or the formation, compromising its integrity and isolation.

2 Temperature changes

One of the main factors affecting the cement sheath stress and strain behavior is the temperature change during and after the cementing operation. The cement slurry is usually pumped at a lower temperature than the formation temperature, and it heats up as it sets and hardens. This causes thermal expansion of the cement and the casing, which can induce tensile or compressive stresses in the cement sheath, depending on the thermal properties and the geometry of the system. Moreover, the temperature can fluctuate during the production or injection phases, creating additional thermal cycles and stresses.

3 Pressure changes

Another factor affecting the cement sheath stress and strain behavior is the pressure change during and after the cementing operation. The cement slurry is usually pumped at a higher pressure than the formation pressure, and it decreases as it sets and hardens. This causes hydraulic contraction of the cement and the casing, which can induce compressive or tensile stresses in the cement sheath, depending on the hydraulic properties and the geometry of the system. Furthermore, the pressure can vary during the production or injection phases, creating additional hydraulic cycles and stresses.

4 Casing movement

A third factor affecting the cement sheath stress and strain behavior is the casing movement during and after the cementing operation. The casing can move due to various reasons, such as axial loading from the weight of the string, bending from the curvature of the wellbore, or buckling from the thermal or hydraulic loads. These movements can cause shear or radial stresses in the cement sheath, which can lead to cracking or debonding of the cement from the casing or the formation.

5 Formation movement

A fourth factor affecting the cement sheath stress and strain behavior is the formation movement during and after the cementing operation. The formation can move due to various reasons, such as elastic or plastic deformation from the pore pressure or the overburden stress, or creep or shrinkage from the temperature or the fluid flow. These movements can cause shear or radial stresses in the cement sheath, which can lead to cracking or debonding of the cement from the formation or the casing.

6 Cement properties

A fifth factor affecting the cement sheath stress and strain behavior is the cement properties during and after the cementing operation. The cement properties can vary due to various reasons, such as chemical or physical reactions of the cement components, hydration and dehydration processes, or aging and degradation effects. These variations can affect the mechanical properties and the performance of the cement, such as its strength, elasticity, ductility, permeability, or durability.

7 Mitigation strategies

To mitigate the risks of cement failure due to stress and strain behavior, several strategies can be adopted. This includes designing the cement slurry and casing with appropriate thermal and hydraulic properties, controlling the temperature and pressure profiles, selecting optimal casing centralization and cement placement methods, monitoring and evaluating the cement quality and integrity, as well as repairing or remediation of the damaged cement.