Elastic log editing and alternative invasion correction methods

Abstract

Recent applications of seismic data in reservoir characterization, direct hydrocarbon indication, and production monitoring rely on the accuracy of elastic logs (density and sonic). Trying to develop (semi) quantitative seismic-based studies requires the use of well-to-seismic calibration; the well information is used as a bridge between the geology and the surface seismic data. Nevertheless, we must take into account that well logs are also indirect measurements of rock properties and subject to various sources of errors. Borehole wall rugosity due to washouts (or even threaded well bore) and mud filtrate invasion are the main sources of uncertainty on elastic logs. Velocity dispersion and shale alteration can also be very important. It is commonly believed that the correction of elastic logs is important only in refined studies, but sometimes even a simple acoustic well-to-seismic tie can be drastically improved with such corrections. Figure 1 presents two wavelets extracted from the same well based on raw and invasion-corrected acoustic impedance. Some amplitude and phase differences in the estimated wavelets can be seen. Even though these changes seem subtle, they can be important in quantitative processes like inversion and AVO interpretation. Figure 1. Wavelets extracted with raw (blue) and with invasion-corrected (red) acoustic impedance data. Figure 2 shows synthetic seismograms for four different wells from the same field. The synthetics based on the original logs are blue and those calculated with the corrected logs are red. The synthetics for three wells seem virtually unaffected by the correction but the synthetics for well 2 present significant differences near the objective. Figure 2. Synthetic zero-offset seismograms based on raw logs (blue) and after correction (red). Although elastic-log corrections can be done deterministically, based on rock physics theories and models (e.g., fluid substitution for invasion correction), there are some issues that we must take into consideration. In the case of …