Ultrasonic Logging of Creeping Shale

With the end of oil production, the need for safe and economically efficient plug and abandonment (P&A) operations increases. Due to the high costs related to traditional P&A processes, the possibility of shale bonding to the well casing, forming a natural barrier, is explored. Here, we present a unique laboratory experiment of creeping shale in a pressure cell, which is monitored during the creep and bonding processes using the ultrasound pulse-echo (PE) and pitch-catch (PC) measurement techniques. The combination of continuous PE and PC measurements in one direction, with discrete measurements in 5° intervals, and the logging of confining pressure, temperature, and permeability allows an improved understanding of the bonding processes and the interpretation of parameters derived from PE and PC measurements. We find that the initial creep of the shale occurs when the confining pressure is increased from 5 to 9 MPa, resulting in significant changes observed in the acoustic impedances derived from PE and PC measurements. However, as permeability is still high, we assume that channels are still open. With the increase in confining pressure to 13 and 14 MPa, respectively, we observe a decrease in permeability corresponding to an overall decrease in signal strength for the PC data of up to 25 dB and a decrease in the PE group delay parameter of up to 0.6 μs corresponding to a change in acoustic impedance from about 1 MRayl to about 3 to 3.5 MRayl. Large variations with direction can be observed. Combining the information of PE and PC gives a good impression of the bonding processes.