The Use of Coring-Induced Petal Fractures in Coal to Supplement and Ground Truth the Interpretation of Image Logs

Abstract

Resistivity and acoustic scanner image logs, in both the CSG and coal-mining industries, are the preferred means of determining azimuths of joints and cleat in coal. This paper indicates the need for care when interpreting cleat azimuths from image logs. Image logs of the bore wall often exhibit large fractures (joints) that intersect the entirety of the bore wall. They are visible as sinusoidal traces. Those fractures that have low height (cleat), and intersect the bore wall in one or two places, are represented by vertical to sub-vertical linear traces on an image log. These lineations can be represented on a stereo- or polar net as a point with a plunge and trend. Both joints and cleat generally have high dips (greater than 75°). Resistivity image logs detect both joints and cleat. Acoustic image logs often only record joints. The value of obtaining cleat, joint and horizontal stress azimuths, is in planning the optimal orientation of deviated in-seam (lateral) production wells. An image log of a cleat lineation records the bore wall intersection azimuth (BIAZ), that is an apparent azimuth, as well as the apparent dip (or plunge) of the lineation. The best way to determine true azimuth of cleat from lineations, on an image log, is from a statistical weighted mean of numerous BIAZ measurements. In some instances, cleat azimuth has been calculated as 90° to BIAZ. Some interpreters view the dispersion of azimuths as a natural feature, when the dispersion is an artefact arising from the intersection of cleat of the same azimuth, at varying distances from the centre of a bore wall. Petal fractures (PF) in coal, when combined with breakout information, can also be used to determine or ground truth joint/cleat azimuths of both large and small scale fractures. Bedding-plane observations of core also provides cleat/fracture information not obtainable from an image log.