Unexpected complications occurred while producing several wells drilled by OMV AUT in the area north-east of Vienna from about 2006 to 2009. On some wells the watercut increased much faster than predicted, on other wells cross flow was proven. Zonal isolation of these wells, which should be provided by the cement behind the annulus, could be compromised. The initial task of this thesis was to research how compromised zonal isolation can be identified in a reliable manner and if/how they can be related to the cementing practices. The current cementing practices as performed by AUT SOB were evaluated and compared to the best practices recommended by the oil industry. The following objectives are covered by this thesis •At the very beginning there is a short summary to gain a quick overview about this work the results of the research. This section is followed by an introduction which previous work was done related to this topic by OMV AUT •An extensive theoretical overview is given about all the factors that influence the quality of a cement job •Relevant data of cement jobs from selected wells were collected and displayed in a big matrix poster. A spreadsheet was created to display which practices were performed while cementing these wells •The cementing practices of OMV AUT are examined and described in detail. •The 9 5/8” and 7” cement job on the Husky 1 was monitored on location in April 2010. The process of this cementations was recorded in detail, the real data is compared with previously simulated data •The cement jobs on several other wells were examined in detail, casing rotation and different flow rates were simulated to evaluate if the guidelines were applicable •A detailed conclusion and recommendations are given at the end