A Study of the Information Potential in Electrical Image Logs: Analysis and Calculations performed on FMI Curves
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This project focuses on measurements from the electrical image tool Fullbore Formation MicroImager (FMI). High resolution electrical image logs can determine individual layers in thinly bedded sand and clay sections that cannot be detected by other well logs. Normally, image logs are interpreted qualitatively, however in this project the 192 curves that comprise the FMI image log were extracted and analysed individually. Each curve reflects resistivity contrasts in the formation, but does not measure the true resistivity like standard resistivity tools do. Nevertheless, the individual FMI curves have been compared with the shallow resistivity logs. Several examples from hydrocarbon and water-saturated sand zones show that the FMI curves are similar to the shallow resistivity logs. The FMI curves seem to be affected by changes in the formation in a similar way as the resistivity logs, suggesting that the FMI log could give an even better description and numerical analysis of thin layers of sand and clay than conventional logs like the resistivity and gamma ray. Typically the clay volume is estimated from the gamma ray log. The resistivity logs can also give an estimate of clay content in a formation. In this project clay volume in a thinly bedded interval was estimated based on the gamma ray log, the shallow resistivity logs and two FMI curves. The resolution of the gamma ray log and the resistivity logs are too low to reflect changes in very thin layers and therefore give an average measurement. The volume of clay estimated from the FMI curve shows more variation. The various clay volume curves were overlaid on core photos to determine correlation with clay and sand layers visible on the photos. To calculate the amount of clay in an interval a clean value (0% clay) and a clay value (100% clay) need to be found in two representative layers. Values representing clean sand were first found in a 2 meter thick sand zone. Then the clean values were found in two thinner sand layers. The clay volume calculated in the interval between 1595.2-1595.6 m showed that the clay volume changed when the thickness of the zone representing the clean value was reduced from 2 m to 0.5 m. The clay volume estimated from the gamma ray log reduced by 17 %, while the FMI clay volume curve changed by only 1 %. The high resolution FMI tool has the potential to more accurately measure true values in thin layers compared to, for example the gamma ray log which only measures an average value over a larger interval. This indicates that the FMI log could estimate the volume of clay in layers that are too thin for the gamma ray log to be used.