Thursday, November 26, 2015

Dip Distortion in Geosteering Due to Stratigraphic Thinning or Thickening Relative to Control Wells

Dip Distortion in Geosteering Due to Stratigraphic Thinning or Thickening Relative to Control Well(s)

Dip distortion due to stratigraphic thinning or thickening relative to control well(s) is common when geosteering wells. Distorted dip may be misinterpreted as a structural anomaly. It may look like a small fault or fold in graphic format when matching up gamma. A tell-tale sign of thinning is dip distortion every time the particular section is cut. The direction of dip change (up or down) should be opposite when cutting down section compared to the direction when cutting up section.

Geosteering programs will distort the dip automatically with thickness differences relative to control well(s) unless the dips are manually adjusted. If dips are not manually adjusted, the TVDs given at the MDs in those sections will not be correct. Such TVDs should not be used in subsequent structure mapping.

Thickening relative to control well(s) is harder to detect and may manifest as more section running along with dip in the particular section than otherwise would be the case.

Deflection of the bit off of hard zones can also cause dip distortion, often of a similar magnitude. However, this is not accompanied by cutting through section but by changing inclination due to deflecting. In deflection the inclination is often unexpected and it is likely that the inclination is distorted rather than the dip, although it is typically assumed that it is dip distortion since that is how the geosteering program must interpret it. On this basis it should be relatively easy to distinguish between the two. Of course, sometimes there may be genuine structural changes due to small faults so that the expressed dip is real. Folds typically show as folds with no dip distortion.

Below is an SES example of dip distortion. Here it looks like the clean limey section in the target interval is thinned relative to the control well. It will be noted that when cutting down through that section the dip was distorted up and when cutting up through the section the dip was distorted down. This occurred multiple times.

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