The aim of this master thesis is to investigate well integrity in a high pressure and high-temperature environment. The primary focus lies in the analysis of the cement formation bond and the impact it has on well integrity. The expanding demand for hydrocarbons puts the oil and gas industry in need of additional capacities. This request forces the oil and gas companies to explore and produce from deeper and more hazardous reservoirs. With increasing exploration depth, the environment becomes more hostile regarding pressure and temperature conditions. The HPHT environment experiences temperatures of at least 300°F and pressures of 10,000psi. These extreme conditions present challenges during drilling operations concerning well integrity. Since drilling the HPHT zones is mainly done offshore, wellbore stability is a major factor in the cost of the entire project and non-productive time must be kept to a minimum. For proper structural stability of the well, it is vital to ensure a high quality cement job. With the high-temperature certain factors need to be considered such as the increase of chemical reactions and interactions of the drilling fluid, the cement sheath, the casing and the formation. This study is going to investigate the current problems in well integrity from an HPHT point of view. It will use an experimental approach to demonstrate the effect of HPHT on the cement sheath and the cement formation bond. Chalk formation from Denmark is chosen for the geological setting of this study since it is an analogue to the North Sea HPHT chalk sequence. The workflow of the thesis is subdivided into three different stages. The first stage represents a literature review to define the HPHT environment and determine the challenges for well integrity. In this stage a detailed research on chalk formations and their importance for HPHT operations in the North Sea is carried out. A case study is presented to show the impact of loss in well integrity at the cement formation bond. The second stage in the workflow describes the field trip to Denmark to get chalk samples for this study and analysing the encountered geological sequences. In this stage, the cementing of the samples is described with additional information on HPHT cementing issues. The third stage is dedicated to the laboratory analysis of the samples regarding petrophysical properties, elemental composition, microscopical analysis of thin sections and element distribution mapping.