Since the sonic data is an important input for different disciplines, from petrophysics to seismic, the main aim of this paper is to underline the impact of a proper Quality Control (QC) on sonic log acquisition in order to obtain reliable results. A specific quality control workflow is proposed, and the principal steps will be explained by means of some impacting examples.
Generally speaking, the QC on log data is the fundamental starting point to obtain reliable curves and, consequently, interpretation. For the acoustic measurements is even more important, since the acquired data are very complex: the tool is multi-samples, multi-sensors and it is able to acquire several set of waveforms for a single depth.
The sonic log is the result of a complex processing of a huge amount of raw data. A specific workflow to approach the sonic QC has been defined and structured as follow:
complete data collection from the field;
check of the acquisition/interpretation parameters;
reprocessing when it is necessary;
comparison and integration with other logs or data.
The described approach required a specific know-how and a fit-for purpose software. In Eni HQ, the sonic QC is performed by means of two software packages: Techlog® by Schlumberger and Geolog® by Paradigm. The sonic data are a common input for different disciplines and, if they are not reliable, can adversely affect the further analysis. In particular:
Petrophysics: (porosity from sonic can produce misleading reservoir characterization);
Geomechanics (since shear and compressional velocities are the main input for dynamic elastic parameters calculation, wrong DTs can have detrimental effect on geomechanical and wellbore stability analysis);
Seismic (sonic log is used for well to seismic tie, synthetic seismogram etc… therefore the sonic reliability is strongly affecting).
Different key examples will be presented in order to quantify the impact of a sonic log not properly controlled. A sonic QC workflow has been developed and applied as a mandatory step for any type of sonic acquisition and tool. The availability of this approach inside Eni is a major breakthrough for the whole disciplines where the sonic log is an input, greatly reducing the dependence on the service companies.