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A logging tool has been constructed and tested, with the aim of detecting fractures in the wall of a borehole. The novelty of this tool lies in the fact that it is incorporated between the straddle packers of a conventional hydrofrac tool and allows the initiation and response of the fracture to be observed during a fracturing operation. The tool consists of 10 "crowns", each with 16 electrodes, incorporated in a 1.7 m long chamber between packers. The total tool length is 9.2 m, its diameter is 160 mm and its total weight 350 kg. It is run on conventional 7-core logging cable. The tool also incorporates a pressure sensor. Current injected into the formation from each electrode is measured, at a sampling rate of 300 Hz. A complete image of the resistivity of the borehole wall can thus be obtained in about 0.5 second. The data are multiplexed downhole, and displayed on a computer screen at the surface. The tool is orientated by means of two orthogonal fluxgate magnetometers. Tests in France and the USA have shown that the tool can detect the formation of a hydraulically-induced fracture and indicate its size and alignment. Two aspects need further attention, however: reduction of electromagnetic noise, and "hardening" of the tool to withstand temperatures up to 150 C.

Additional information

Authors: LABORATOIRE DE GÉOPHYSIQUE APPLIQUÉE DU CNRS, Laboratoire de Géophysique Appliquée du CNRS, Orléans (FR);LABORATOIRE DE GEOPHYSIQUE APPLIQUEE DU CNRS, Laboratoire de Geophysique Appliquee du CNRS, Orleans (FR)
Bibliographic Reference: EUR 12855 FR (1990) 66 pp., MF, ECU 4
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