Well Log Conditioning for Quantitative Seismic Interpretation

Abstract Reliable sonic and density logs are needed for calibration of seismic data for use in reservoir characterization, and for specialized AVO and 4-D analysis. Due to the small depths of investigation of sonic and density logging tools, measured logs are affected by borehole washout and mud filtrate invasion. In many wells, particularly older ones, density and shear sonic logs were acquired within limited depth sections, or not at all. To address these various log quality issues and make the logs suitable for use with seismic data, appropriate log editing must be done, and missing logs or log sections must be synthesized. In addition, complex waveforms measured by modern sonic logging tools must be processed before they can be used for geophysical analysis, a step often ignored by geophysicists. Our paper presents a sequential workflow to address sonic and density well log quality issues for approximately one hundred wells penetrating multiple carbonate reservoirs within a supergiant field from the Middle East. Borehole washout problems are frequent in the shales that overlie the main reservoir of the field, resulting in poor sonic and density log measurements. Within the reservoirs, high salinity filtrate invasion that displaces light oil affects both sonic and density log measurements. To address these issues, we developed a well log conditioning workflow that involves the following steps: - Processing of sonic logs from wells with dipole sonic measurement - QC of all well data, and selection of wells with optimal logs for building a training data base - Generation of missing data using multi-variate statistical analysis - Correction of borehole-related anomalies - Correction for invasion effects - Validation against VSP and seismic data. Results of log conditioning have improved well-seismic ties. In many cases these ties confirm the presence of residual interbed multiples within the 3D seismic data that covers the field. Modeling of fluid effects during invasion correction shows acoustic impedance to be sensitive to both porosity and fluid effects: up to 9% change occurs in acoustic impedance due to invasion-related saturation changes. Such saturation-related effects suggest the feasibility of 4D seismic as a surveillance tool in future FDP of the carbonate reservoirs in this field.