Question 2 of 10: Borehole Cross-Section from a Two-Arm Caliper Log

Question 2: Borehole Cross-Section from a Two-Arm Caliper Log (6 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. A two-arm (orthogonal-axis) caliper log, Hole Diameter (inch) track, with one arm drawn solid and the other dashed. Where only the solid line is visible the dashed arm lies on top of it, i.e. both arms read the same diameter. Hole diameter increases to the right.

Find. The cross-sectional borehole shape at the depths where the hole is irregular.

Approach. A genuine two-independent-arm caliper (as opposed to a single-arm or averaged 4-arm tool) reads TWO orthogonal diameters at every depth; where the two traces coincide the hole is circular (in gauge or uniformly enlarged), and wherever they separate, the hole cross-section is elongated (non-circular) along the axis of the arm reading the larger value — a classic borehole-breakout / key-seating signature.

1. Depth interval 1 (near top of the shown log, dashed arm reads a smaller diameter than the solid arm). The two traces diverge with the dashed-arm reading BELOW the solid-arm reading — the borehole is elongated along the solid arm's azimuth and compressed along the dashed arm's azimuth: an elliptical (ovalized) cross-section, typically from stress-induced breakout aligned with the minimum horizontal stress direction.
2. Depth interval 2 (mid-log, the dashed arm swings to a LARGE diameter loop while the solid arm stays near gauge). One arm reads a pronounced washout excursion while the orthogonal arm barely moves — a keyhole/keyseat shape: a narrow, deep slot cut in one azimuth (commonly where the drillstring has worn a groove on the low side of a deviated hole) superimposed on an otherwise near-gauge circular hole.
3. Depth interval 3 (about two-thirds of the way down, the sharp excursion). Here the dashed arm follows the solid arm exactly: both swing sharply to a LARGER diameter and then, a short distance lower, both swing sharply to a SMALLER diameter before returning to gauge. Because the two orthogonal arms read the same value throughout, the hole stays circular but changes size: a short circular washout (cave) directly above a circular undergauge (tight) section, such as a ledge, swelling shale or thick mud cake. This is a size irregularity, not a shape irregularity.
4. Remaining depths (both arms track together with low-amplitude serration). The two arms read essentially the same diameter — a circular, near-gauge but rugose borehole. One isolated spike of the solid arm alone toward a smaller diameter, in the lower third, marks a very short, slightly oval (elliptical) spot.