In the design of oil and gas well barriers, the quality of the cemented casing must be assessed to optimize the cost of the operation. Borehole cement evaluation logs are used to verify the quality of the cement bond to the casing and formation and identify any defects that could compromise the quality of the annular seal. This study used ultrasonic logging to evaluate reference barrier cells constructed with known defects. In this joint project between industry representatives and a research institute, reference barrier cells were created to simulate downhole oil well conditions, such as free/cemented pipe, sagged barite, gas environment, mud channel, microannulus, axial hole, and cemented control line. The cell concept was developed with an emphasis on low cost and ease of use. Different cell configurations could be quickly coupled to provide the required tool lengths and measurement points for logging experiments. The tubing and annulus pressures (maximum 150 bar) and fluids can be varied according to the test requirement.
Each reference cell was constructed to highlight a condition that is typically encountered in an oil well. Full-scale logging was carried out in which multiple reference cells were coupled vertically on a drilling rig. Data were acquired at multiple resolutions, while applying pressure (0, 100, and 150 bar) to the outer annulus, thereby varying microannulus size. Data from the pulse echo and flexural sensors provided a detailed image highlighting the condition of the bond behind the inner casing. Laboratory experiments were performed to characterize the reference cells, including measurements of strain and infrared thermography to visualize the leakage paths. The log response was found to be consistent with the physical observation and laboratory experiments.
During the cement evaluation of a tubular casing, interpretation is made based on log response under certain well conditions. Inferences are based on fundamental sensor physics, laboratory experiments, common knowledge, and contemporary understanding of the technology. Comparison of ultrasonic tool response against reference barrier cells validates the measurements, which are crucial in the decision-making process during well construction and abandonment. The paper describes the full-scale test performed at the research institute, including the laboratory experiments and log data acquired.