Evaluating Cement Bond Quality and Casing Integrity in Well P&A Operations: A Case Study From a Depleted Oil Field, Germany

The paper delineates a case study utilizing an enhanced cement bond evaluation workflow tailored for an ongoing Plug and Abandonment (P&A) campaign involving approximately 70 wells in an onshore oil field in Germany from the 1950s. The results from the cement bond and casing integrity assessments were crucial for facilitating prompt operational decisions including cementing unit mobilization, cement plugs pressure tests, depth determination for cutting and recovering free pipe, and ultimately deciding on partial P&A or permanent P&A to surface. The primary Jurassic- reservoirs in this field are stacked and have been produced through multiple sets of perforations over time. Shallower Cretaceous- sand layers are sporadically distributed in the field, which may or may not be charged with oil necessitating considerations of its flow potential. Accurate assessment of primary cement quality and general well integrity is essential to ensure adequate isolation in accordance with regulatory standards for P&A decisions. To ensure the acquisition of good quality data, removal of paraffin deposits has proven essential to prevent false interpretation. We implemented standard quality control measures during both acquisition and processing to ensure accurate interpretation. This cement evaluation workflow has been evolved from visual inspection and manual assignment of qualitative cement bond evaluation terms like ‘good’, ‘moderate’, and ‘poor’, to a data-driven approach that consistently derives a combined cement score using sonic and ultrasonic measurements in a consistent fashion. Another key input of the evaluation workflow is the Geological data from the wells has been a key input for the evaluation workflow. Integrating cement score and geological factors, we establish the zonal isolation criteria, ensuring presence of rock-to-rock barriers, thereby completely preventing any fluid migration. Additionally, the assessment of casing integrity using ultrasonic pulse echo measurements help determining the suitability and potential risks associated with the planned operations. The wells in this field are relatively shallow, with typical total measured depths around 500- 600 meters Occurrences of micro-annulus were not common making CBL protocols of single passes under normal pressure sufficient. Standard composite log plot integrating key well integrity parameters proved to be essential for enhancing time efficiency and ensuring uniformity throughout the P&A campaign. Integrated interpretation of sonic and ultrasonic logs using the new workflow, combined with other relevant data proved invaluable for the P&A calls. Identifying free pipe within double casing is challenging, as ultrasonic measurements are less effective and influenced by multiple tubulars. As the P&A campaign advanced, successful identification and extraction of free pipes yielded valuable insights, enhancing workflow effectiveness. These experiences bolstered confidence in the application of CBL and VDL techniques. In some cases, sector bond log measurements performed in double casing environments yielded results consistent with standard CBL/VDL outcomes. In summary, the current approach has led to more reliable and consistent results, overcoming initial challenges through continuous learning and effective integration.