Although borehole image logs may be the best approach for characterizing relative bedding dips in near horizontal well-bores, issues still exist in obtaining sufficiently accurate dips, particularly when wellbore deviation exceeds 80 degrees. A recent SPWLA paper (Passey et al, 2005) concluded that true stratigraphic thickness can be off by as much as 200 to 300% because of 2 to 3 degrees uncertainty in apparent dip in highly deviated wells.
A problem with measuring relative dip with any image log is that the apparent dip on the image is affected by the depth of investigation and vertical resolution of the measurement. Passey et al. (2005) demonstrates that the depth of investigation can be a function of the formation constituents, and that the apparent image dip from LWD density can be different by 2 to 3 degrees when going from shale to sand than by going from sand to shale.
This paper investigates and summarizes factors that affect wireline and LWD borehole image log dip and azimuth uncertainty. Factors investigated include error in depth, uncertainty in borehole azimuth and inclination, uncertainty attributable to borehole geometry (non-circular) and tool eccentricity in the borehole, errors in assumed depth of investigation, density of data samples azimuthally and along the borehole direction, resolution of the actual measurements, and quality and location of formation boundaries used for placing sinusoids.
The basic premise made in this paper is that all uncertainties must initially be modeled and expressed in the local borehole coordinate system because this is where the data is recorded. This premise is simply the geometry defined by the intersection of the bedding plane, assumed to be planar, with the borehole, which manifests primarily as the uncertainty inherent in the sine wave observed and selected by the analyst. The angle between the borehole axis and a line perpendicular to the bedding plane, measured in their common plane, is called relative dip and can be caused by dip, well deviation, or a combination of the two. The local uncertainty in the tool measured relative dip and relative dip azimuth is then transformed to the reference coordinate system by accounting for the borehole deviation and azimuth, and the uncertainty in the bedding plane's predicted "true" dip and azimuth can be summarized with Schmidt plots, histogram displays, or 3D surface plots.
Strike is the direction of a line formed by the intersection of the surface of an inclined bed with a horizontal plane. Dip (true dip) refers to the angle of slope of an inclined bed measured perpendicular to the strike and in the vertical plane referenced to the horizontal plane. Note that the geologists define an apparent dip as a dip measured in a direction other than perpendicular to strike. Dip azimuth is the direction of the dip measured perpendicular to the strike and referenced to north.
