State of the Art Development in Annulus Evaluation

In an oilfield well, when the annulus contents behind the casing are evaluated using ultrasonic measurements, the properties of the borehole mud, such as acoustic impedance and fluid velocity, are critical input for the accurate determination of acoustic impedance of annulus material and its subsequent bond quality. In deviated or horizontal wells, mud settling, and subsequent segregation leads to azimuthal and depth uncertainties in annulus evaluation. Typically, due to gravity, mud segregates, with the light component at the top and heavier component at the lower side of the well. In a non-homogeneous mud, using a single mud impedance value for computing acoustic impedance of the annulus can lead to ambiguous answers with uncertainties. Traditionally, it has been a challenge to accurately measure and apply these variations in acoustic impedance of the mud to precisely interpret the bond quality in the annulus. A novel pulse-echo processing scheme called R+ inversion, based on a 3-parameter inversion approach, eliminates, to a great extent, the dependence on prior knowledge of the borehole mud. The 3-parameter inversion can also reveal conditions such as mud deposition and segregation in deviated pipes. This new processing enables easier and accurate interpretation of the annular content together with essential information about the logging fluid. Four case studies established the successful implementation of R+ inversion in deviated wells in the Norwegian Continental Shelf (NCS) with azimuthal uncertainties in the mud acoustic impedance to provide reliable annulus interpretation. These measurements correlate and are validated using sonic logs as well as flexural attenuation measurements, thus providing confidence in the results and decisions. The case studies compare acoustic impedance results using legacy processing and R+ inversion processing. The limitation to use the azimuthal variations of mud in the traditional processing sometimes leave unanswered questions related to the bond quality affecting the intervention decisions expected from the bond log. With the help of R+ inversion, the operator managed to take informed intervention decisions faster, thus saving rig time and cost. Four case studies are explained in the details that demonstrate and validate the importance of R+ inversion when borehole mud settling occurs azimuthally, thus overcoming previous limitations of mud impedance computation and usage. Cement evaluation using R+ inversion enables accurate and critical decision making during new well construction, intervention, plugging, and abandonment in all conditions, irrespective of the casing sizes and cement types.