The increasing need for plugging and abandoning old oil and gas wells has triggered research on the development of new time-efficient methods for these operations. It has been proven that shale can seal off the annulus and eliminate the need for a traditional cement barrier or can help close the annulus space if the cement was originally poorly bonded to the casing. The challenge, however, is to distinguish heavy fluids from light solids and identify and qualify if the solid material has bonded to the casing, sealing off the zone. Therefore, a laboratory setup was built to reproduce ultrasonic field logging and to improve the understanding of parameters affecting the following interpretation.
In this study, experiments using the ultrasonic flexural technique (flexural attenuation) were conducted. The experiments involve measuring the attenuation of the flexural wave using different materials outside the casing and performing numerical simulations to compare and verify the measurements. The results are consistent with earlier scientific studies but also provide new insight into the impact of introducing shear coupling on the outside of the casing. A solid bonding to the casing is shown to affect the particle displacement of the flexural wave, thus changing the flexural velocity used in the interpretation. This can provide additional information in the gray zone between heavy mud and light cement or low-density shale. Furthermore, this study demonstrates how partially bonded cement can be distinguished from perfectly bonded cement. With our laboratory setup, it seems to be possible to distinguish if the cast casing includes a local microannulus or a continuous channel.
In addition, it should be recognized that this laboratory setup was proven to be reliable in detecting an annulus, and through simple processing, it was possible to estimate the annulus thickness. In one experiment, a second casing was decentralized from the first casing. By detecting an echo from the outer casing, the complete azimuth log estimated the eccentricity between the two casings, even at the smallest annular width of only 0.1 mm. The outcome of the paper is promising for the use of the flexural technique as a candidate for identifying and qualifying shale as a barrier.
