Cementing is an essential part of well construction as it supports the casing and provides hydraulic sealing. Quantitative cement evaluation is becoming increasingly important in the industry to verify well integrity and zonal isolation. Such information has historically been obtained with a wireline tool, acquiring data at ultrasonic or sonic frequencies. The most common log providing information on cement bond quality is the cement bond log (CBL), which relies on the amplitude of the casing arrival to determine the quality of cement bonding to the casing and in some applications to the formation.
Although logging while drilling (LWD) sonic technology has progressed rapidly in recent years the established service from an LWD sonic tool that provides cement information is only top of cement (TOC), and it indicates only the presence or absence of cement behind the casing. Since the major structural part of the tool is a rigid steel collar, there is significant acoustic propagation along the collar. It is technically difficult to separate the casing signal from the one traveling in the collar as these two propagations have similar speed and frequency content. When the cement is poorly bonded to the casing, the casing amplitude can be far larger than the collar arrival. However, if the cement is well-bonded, most of the casing signal energy is dissipated into the cement, and the casing amplitude at a receiver a certain distance from the transmitter tends to be similar or even smaller than the collar arrival. In such cases, the measured signal loses sensitivity to cement bonding. However, it is challenging to establish a relationship between the respective amplitudes of the collar and casing as it depends upon many factors such as the acoustic impedance of the cement and the casing size.
We explore the feasibility of quantitative cement evaluation with a conventional LWD sonic tool. Finite-difference method (FDM) modeling over a wide range of conditions was conducted to quantify the relationship between casing amplitude and collar arrival. Results indicate that quantitative cement evaluation is feasible with existing LWD sonic tools, depending upon the bonding value. This study also provided information for comparisons between conventional wireline cement evaluation methods and LWD-derived CBL.