True Vertical Depth, True Vertical Thickness And True Stratigraphic Thickness Logs
Holt, O.R.; Schoonover, L.G.; Wichmann, P.A.
Transactions of the SPWLA Eighteenth Annual Logging Symposium: 1-19
The increased numbers of wells both onshore and offshore which must be directionally drilled to reach their intended objective has increased significantly in the past ten years. These directionally drilled wells show bed thickness to be too great, the amount depending upon the direction of dip of the measured formation and the drift angle and direction of the borehole. To rectify these differences in thickness for mapping and reserves estimation, differentapproaches have been used. The most frequently used method is the True Vertical Depth log which computes each point to the depth it would appear if the hole were drilled vertically. For mapping purposes, this approach is better than the information derived from the original open hole log, but is not accurate enough in highly deviated wells. The True Vertical Thickness log is derived by computing each bed as though the borehole passed through it in a vertical direction. This method results in improved data for reserves determination and also better precision for subsurface mapping points. The True Stratigraphic Thickness log is computed to provide the actual thickness of a given formation perpendicular to the bedding planes. The True Vertical Thickness, True Stratigraphic Thickness, and True Vertical Depth logs can be presented on the same plot. However, it is usually best to plot each as an individual log to reduce the amount of information which must be evaluated. The dip angle and direction of target formations must be known before the True Stratigraphic Thickness log can be run. This information can be most reliably derived from Diplog data. However, dip- data from other sources can be used in the program if a Diplog is not available. A review of the mathematics involved in the computation and some computed examples are included in the paper.