After the oil and gas wells are inserted into the casing and cemented in the cementing operation, the cementing quality logging must be performed. Field experience shows that there are multiple solutions for using cement bond logging and variable density logging. In most cases, the logging response is also affected by other factors than cementing quality.
Instrument calibration: Before the CBL/VDL logging, it is necessary to scale the free casing of the same type in the measuring well section. The quality of the calibration directly affects the interpretation of the logging data and the determination of the cementing quality.
Instrument eccentricity: Instrument eccentricity has a great influence on the sound amplitude. When the eccentricity is measured, the sound amplitude measurement value is smaller than the value when the instrument is centered, which affects the reliability of cementing quality of the first interface by CBL.
Logging time: When the logging time is too early, the cement waiting time is not enough, the cement is not completely solidified, the nature of the cement ring is unstable, the strength is not enough, and it is easy to make a low evaluation; on the contrary, when the cement waiting time is too long, it is easy to be overrated.
Formation lithology: The lithology of different formations has different wave impedance and permeability, and the degree of action is different from that of cement slurry. Formation with high porosity and high permeability is not easy to form small gaps. Sound waves travel at different speeds in fast and slow formations, affecting the measured amplitude of the sound, which in turn affects the determination of cementing quality.
Cement slurry ratio: Different formula design of cement slurry will have a significant impact on the hardening and crystallization morphology of the material, thus affecting the propagation speed of sound waves. The impact of cement slurry formulation on cementing quality evaluation is mainly in two aspects: one is the impact on the waiting time, and the other is the impact on the evaluation criteria.
Micro-annular gap: chemical reaction occurs during cement solidification. Cement shrinkage may cause the cementation strength between cement and casing, cement and wellbore to decrease, forming a micro-annulus, which makes the amplitude of sound increase, and VDL graphics cannot accurately identify formation waves.
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