Abstract
Logging-While-Drilling (LWD) acoustic tools encounter interference from collar arrivals when attempting quantitative cement evaluation. In low-bonding condition, the interference is not significant, an amplitude-based bond index determination is reliable with the limitation in higher bonding conditions under most scenarios (Kinoshita, T. et al. 2013). However, using the sensitivity of the measured acoustic attenuation to cement bond quality, then LWD based quantitative cement evaluation can be expanded to cover the full range of bonding (Pistre, V. et al. 2014). The combination of the two methodologies into a hybrid method has allowed quantitative cement evaluation to be delivered on LWD acoustic tools and so offer significant time saving advantages over conventional methods (Feng, Y.W. et al. 2016). In this reference example, a LWD acoustic tool was run in a deviated well combined with a casing scraper in the casing cleaning run. The LWD cement quality data were converted to a synthetic Cement Bond Log (CBL) curve expressed in millivolt (mV) units and compared with traditional CBL data. The final processed result is later interpreted as being good bonding, medium bonding and poor bonding as per the operators’ standard.
Overall, the LWD cement evaluation results showed good agreement with the traditional CBL data in almost all intervals where cement quality ranged from good to poor. However, the acoustic data were affected by noise in part of the section as indicated by the noisy variable density log (VDL) in the example well. The LWD cement evaluation delivered very positive results but it is also noticed that the mechanical casing scraper could potentially present a significant source of acoustic noise. The severity of this noise varied with logging speed. Although the downhole noise generation cannot be eliminated, through detailed prejob planning, its effects were mitigated and the results obtained met the logging expectations despite the challenging environment.
As a counterpart measurement of conventional CBL logging, utilizing the novel LWD cement evaluation methodology brings a new method to the industry for quantitatively logging the cement quality of complex wells, while at the same time, running the cement logging tool together with a casing scraper provides a more efficient and economical method to acquire the data.
