Sealing of new wells, and the increasing need of plug and abandonment (P&A) of old wells, has triggered research and development of new methods for saving time at this operation. Shale has proven to seal off the annulus in longer well sections; however, the detection and qualification of the sealing ability can still be improved. As a response to this, a laboratory setup has been built to perform pressurized measurements on shale cores. The laboratory setup consists of a steel casing inside a vessel, where the tool is inside the casing and locked at a rotation board at top for steadily handling the logging procedure. The measurements are carried out using the pulse-echo method with a 5MHz focused transducer, which is around 20 times lower than the center frequency used when logging in the field. The setup is downscaled approximately 20 times compared to a scenario using a 9 5/8 inch casing, so the frequency was picked to simulate a real well. The casing can be cast with cement under pressure to differentiate good and bad bonding, or the vessel can be filled with a rock sample, a second casing, or any fluid.

A detailed study of this new laboratory setup has been performed. The focusing of the transducer and the high frequency is used to make the laboratory setup realistic and similar as in the field, however, the high frequency also increases the resolution for improved evaluation of debonding and determination of the material behind the casing. The laboratory setup has been consistently tested with different materials behind the casing, and the results has been compared and verified with numerical simulations. The laboratory results and the numerical simulations show good correlation. Further, the sensitivity of the laboratory setup has been investigated, and different attenuation parameters are used to develop a model for quantifying the effect of the different artefacts. Observations have shown how the eccentricity of the tool, varying casing conditions, and the distance of the transducer relative to the casing affects the decaying reverberation ringdown.

This content is only available via PDF.
You can access this article if you purchase or spend a download.