The cement sheath serves several purposes, which are to provide (1) zonal isolation and segregation, (2) corrosion control, and (3) reinforcing formation stability and casing strength. The failure that is of most concern is the failure to provide zone isolation and segregation. Loss of zonal isolation can have catastrophic consequences, including loss of well control (API 65-2). In gas plays, such as the Marcellus Shale, there is a high probability that failure of the cement sheath will result in annular gas migration
(Halliburton, 2013a). Failures of the cement sheath can be divided into two primary categories, failures that occur during the primary cementing process and those that occur after the cement has set. Again, this distinction is largely arbitrary, as defects in the cement sheath caused by deficiencies in the primary cementing process can lead to failure of the cement sheath during operation.
During the execution of the primary cement job, there are many potential causes of failure. One failure mode during cement placement is channeling. Channeling is a complex phenomenon, but a major cause is the development of gel strength and resulting loss of hydrostatic pressure. Undisplaced mud during slurry placement can result in a channel of high permeability mud, compromising the integrity of the cement sheath. Channeling can also be caused by bridging and poor centralization. If the casing is poorly centralized, the cement slurry will travel the path of least resistance in the wellbore, leaving the side of the casing close to the borehole wall with little or no cement sheath (Transocean, 2013). If the average fluid velocity of the cement slurry in the annulus is not above the velocity required to initiate flow in the narrowest part of the annulus, mud will not be displaced, causing channeling and possible contamination of the cement, which could result in failure to attain desired set properties. A diagram contrasting the effects on cement placement of poorly centralized casing to centralized casing is shown in Figure 34.
Figure 34: Poor Centralization versus Good Centralization of Casing (Transocean, 2013)
Excessive loading of the cement sheath during operations can cause multiple types of failures. This loading may be pressure induced during operations such as hydraulic fracturing, or induced due to temperature changes. These temperature or pressure fluctuations within the well can cause debonding of the cement sheath from the casing. This results in the formation of a small annular void (a microannulus) between the casing and cement sheath. In severe cases, the microannulus may encircle the entire casing circumference, possibly compromising hydraulic sealing ability of the cement sheath (Schlumberger, 2013). Varying degrees of microannulus formation can be seen in Figure 35.
Figure 35: Varying Degrees of Microannulus (Schlumberger, 2013)
Research conducted on steel to cement bonding in structural applications has high relevance to casing to cement sheath bonding. In a steel-reinforced concrete structure, load transfer between the steel cement occurs through the bond between the two materials. In structural applications, the three mechanisms of bond response are
mechanical interaction, chemical adhesion, and friction, with mechanical interaction the dominant mechanism for load transfer. It has been shown that during cyclic shear loading, degradation of cement-steel bond occurs at small fractions of the fatigue life (Cao and Chung, 2001). Similarly, one would expect that in well applications that operations inducing cyclic shear loading on the casing to steel bond (such as hydraulic fracturing) would cause a decrease in bond strength and increased probability of
microannulus formation. Bond strength degradation may also be caused by corrosion of the casing. The effect of cyclic loading and corrosion on micro-annulus formation is an area that could benefit from further research and investigation.
Fractures in the cement result from loading occurring in the downhole
environment in excess of the material strength. The fracture mechanics of concrete are best described as progressing through four stages: (1) the linear elastic regime, (2) pre- critical (stable) crack growth, (3) critical (unstable) crack growth, and (4) the bridging stage. Within stage 1, the linear elastic regime, Hooke’s law is valid and can be used to model the behavior of the material. Hooke’s law can be expressed as follows:
(8)
Where the stress ( ), is equal to the elongation (ε) multiplied by Young’s Modulus (E), which is a material property that depends on cement composition.
For materials with small amounts of crack bridging, such as cement without aggregates, stages 2 and 3 of pre-critical and critical crack growth can be described using linear elastic fracture mechanics (LEFM):
( ) √ (9)
Where is the critical stress intensity factor, ( ) is the crack shape factor as a function of and , and is the length of a surface crack or half the length of an interior crack. The critical stress intensity factor, , is a material property that is dependent on composition of the cement.
In concrete, crack bridging by the aggregate has the effect of stabilizing the macro-crack growth to some extent, and must be modeled and included in analysis. However, for cement without aggregates, such as oil well cement, the bridging stage consists of the interaction of ligaments between overlapping crack tips and is typically negligible (Van Mier, 1997).
The stress-strain behavior of brittle and quasi-brittle materials is shown in Figure 36. Typical oil well cement behaves in a brittle manner, while concrete displays quasi- brittle behavior. Important features to note in Figure 36 include the extremely small region of stable crack growth for brittle materials, and the almost complete lack of
bridging. As aggregates are added, the region of stable crack growth and the effect of crack bridging increases.
Figure 36: Brittle and Quasi-brittle Crack Growth (Adapted from Van Mier, 1997)
Cracks that can be arrested by elements in the material structure are termed micro-cracks, while cracks that can only be delayed or arrested through structural
measures at a larger scale than the material structure are termed macro-cracks (Van Mier, 1997). Under confined external compression, micro-cracking can be severe, but the increase in applied stress to cause crack growth criticality is typically large. Micro- cracking may increase the permeability of cement and enable gas migration through the cement sheath.
A summary of cementing failure modes and the recommended methods of
mitigation is presented in Table 5. The relative risk of each failure mode was ranked on a scale from 1-5 depending on the frequency of occurrence and the ease of mitigation. Less qualitative data was available for raking the relative risk of cementing failure modes, so a risk value of 3 was assigned unless specific information was available.
Table 5: Risk Ranking of Cementing Failure Modes
Cementing Failure
Modes Methods of Mitigation
Relative Risk
Corrosion Proper slurry design 4
Channeling Proper slurry design, adequate centralization 3 Contamination Adequate pre-treatment of the borehole 3 Microannulus formation Design for actual loading conditions 3
Fracture Design for actual loading conditions 2
Similar to the failures presented in the casing section, for each cementing failure mode there is a wealth of information on mitigation methods and techniques. The cementing process has developed to satisfy the needs of wells with more challenging downhole conditions than those found in the Marcellus Shale, and slurry properties can be tailored to address problems that may arise.