The commonly used logging techniques for cementing quality evaluation include acoustic amplitude logging, acoustic variable density logging and cement evaluation logging.

1. Basic principle of acoustic amplitude logging (CBL)

Acoustic amplitude logging (CBI) uses a single transmitter and single receiver sound system. The sound signal received by the source distance LCM receiver is a sliding longitudinal wave (called casing wave) propagating along the casing. Its amplitude is related to the properties and distribution of media inside and outside the casing. The influence of casing and media inside the casing is a fixed value. Therefore, the amplitude of the acoustic signal received by the receiver mainly depends on the nature and distribution of the medium outside the casing.

The domestic evaluation of cementing quality is divided into three grades: good cementation, general cementation and poor cementation.

2. Acoustic variable density logging (VDI)

In order to record the wave train received by the receiver as a continuous record varying with depth, and the wave trains at each depth point do not interfere with each other, the recording method of variable density logging is adopted, so it is called variable density logging (VD).

Even if the cementing quality of the first interface is good, the cementing quality of the second interface is not necessarily good. When the cement sheath and the formation (second interface) are not well sealed, oil and water channeling is easy to occur in production, affecting the normal use of the well. CBL can only show the cementation of casing and cement, but not the cementation of cement sheath and wellbore formation. However, the cementing quality of cement sheath and wellbore formation is also very important. In order to comprehensively evaluate the cementation of two interfaces of cement sheath, acoustic variable density logging is proposed, which can comprehensively evaluate the cementation quality of cementing cement. In addition to recording the first wave, acoustic variable density logging also records the subsequent wave (casing wave propagating along the casing and formation wave propagating along the formation through the cement sheath). It can not only evaluate the cement cementation quality of the first interface, but also better evaluate the cement cementation quality of the second interface, so as to better meet the needs of cementing quality inspection in oilfield development. It has achieved good results in field application and gradually replaced the traditional acoustic amplitude logging.

3. Cement evaluation logging (CEI)

In order to overcome the disadvantage that CBL and VDL logging can not show poor cementation in a certain direction, people have developed a cement evaluation instrument. It uses the reflected sound wave to determine the average diameter and ovality of the casing and the cement compressive strength reflecting the cement cementation quality. It not only reflects the cementation of the two acoustic interfaces in the cased well, but also has circumferential resolution and longitudinal resolution for the cement cementation outside the pipe, provides the measurement of cement compressive strength, reflects the casing inner diameter in all directions with high-sensitivity acoustic well diameter, and indicates the casing deformation, corrosion and wear.