A196

For an oil/gas well to maintain its integrity and be produced effectively and economically, it is pertinent that a complete zonal isolation is achieved during the life of the well. This complete zonal isolation, however, can be compromised due to factors that come into play during the operative life of the completed well. Such factors may come in the form of thermal or pressure loads generally regarded as HTHP (High Temperature-High Pressure) loads which can manifest themselves as a static/cyclic load or both depending on how they are exerted. Cement structures experiencing cyclic loading conditions can fail as a result of fatigue due to extensive degradation of the microstructure of the cement material depending on stress levels and number of cycles.
There has been a lot of research and experimental investigations on the mechanism of fatigue failure of structures like buildings, bridges etc. but the fatigue behavior of well cement is still relatively unknown to engineers. Research in the area of well cement design has led to improved cement designs and cementing practices but yet many cement integrity problems persist and this further strengthens the need to understand the mechanism of cement fatigue. Even though most structural failures are as a result of fatigue rather than static loading, insights on the role of both static and fatigue loading conditions on the failure of cement sheath would hopefully lead to improvements in well design.
Project Objective
The objective of this project is to develop a better understanding of the performance of the casing–cement bond under HPHT well conditions, leading to a model to predict well life. This would entail two major tasks:

• to identify the factors that affect the casing–cement integrity under HPHT conditions
• to understand better the fatigue of well cement.

Based on the knowledge acquired from completing these tasks, it will be possible to analyze the mechanics of casing–cement systems under HPHT conditions for the long term integrity of the system.
Approach
The first approach taken in this project was a comprehensive literature review on the fatigue of construction cement/concrete since no literature presently exists on the fatigue of well cement and understanding of the differences between both kinds of cement. The literature review was focused on:

• the state of the art in the fatigue of cement,
• casing cement interaction models currently being utilized by operators, and
• casing–cement integrity under HPHT conditions.

The findings here are discussed in chapters 1 and 2. The knowledge from the literature review was then applied in modeling and understanding the mechanism of fatigue in well cement.

Furthermore, an analytical model of wellbore stresses based on wellbore parameters was developed and used in addition to finite element analysis to conduct stress analysis on a casing–cement–formation model in order to study the integrity of the cement sheath under different HPHT loading scenarios. This is presented in chapters 3 and 4 of this report. This model can be integrated in a simple-to-use computer software package capable of recording the history of the well and predicting its fatigue life.
Finally, recommendations for future work are given in chapter 5.