Ultrasonic scanner tools measure the cement acoustic impedance at higher radial and vertical resolution than standard cement bond and sector bond tools. The impedance is affected by variations of the borehole fluid contained in the well. This is not problematic for most wells as the tool is typically calibrated in the same fluid as the main logging section. Also, small variations in the well fluid between the bottom and the top of the logging section can go undetected in the computed cement impedance as these variations are continuous and not abrupt.
In the past, ultrasonic tools estimated the value of the fluid impedance by measuring the slowness and multiplying this value by the density. The fluid density is taken as a constant reading at the surface. Sometimes the slowness measurement is done during a logging down pass. An error is introduced when the fluid changes from one type to another because both slowness and density changes; however, density changes in larger percentages than slowness, causing the fluid impedance to have a substantially different value than the estimated one. This difference in fluid impedance causes the cement evaluation to be different also.
In the present paper we introduce a modified mud chamber for the ultrasonic radial scanner tool that measures the fluid impedance directly, in situ, while logging and with greater accuracy. In addition, we present an algorithm to compute the influence of changes in the cement impedance due to variations of the impedance in the fluid column. Laboratory tests are presented using static logs and logs obtained during field testing, corroborating the methodology and algorithm. Most of the time the tools give a correct reading but in instances where stratigraphic fluid columns with vastly different densities are present, if no correction is applied; it can mean the difference in detecting the presence or the absence of cement.