Image artefact feature of the borehole wall observed as arising from the development of a spiral shape to the borehole and/or as helical markings on the borehole wall; it occurs across a range of scales (as varying amplitude and wavelength) with varying severity. Spiral hole often results from drilling and reaming operations including bit-whirl, anti-whirl, reaming and wiper trips. Tools run in spiral-shaped holes experience a wobble associated with its cork-screw passage during logging. Well-developed hole spiral can be detected with the caliper logs as regular undulations that show offset between sets of pad calipers from a pad device. On borehole images, it is observed as regular and inclined, en-echelon, non-planar features on the borehole wall. We have observed up to four sets of spirals on any one borehole image.
Spiral hole can result in anomalous automatic dips and fully degrade an image to prevent the extraction of confident manual dips. Spiral hole can also lead to problems with petrophysical calculations. Images affected by spiral hole and lead to the impression of common inclined dipping surfaces. In many instances, in particular with well bore images, the data can be seen to be influenced by spiralling and can be ignored. Manual picking of dipmeter curves, however, should always be associated with examination of calipers to detect any spiralling of the well bore. Surfaces picked on only three points (curves) will be suspect within regions of spiral hole. Knowing the mean amplitude of the spiral (from the calipers) and the borehole size (as bit size) it is possible to calculate the theoretical dip of a planar surface calculated from a three-curve pick along a spiral feature. Such surfaces, when plotted on a Wulff stereonet, will form a cloud of points along small circles around the hole axis with an apical angle equal to the dip of these surfaces.
See also calipers, rugose hole, washout, borehole breakout and bit size.