Downhole logging tools

The Azimuthal Resistivity Imager (ARI) is a new generation of laterolog tool that makes deep measurements and azimuthal resistivity images around the borehole. Using these data it is possible to analyze features and details that escape conventional resistivity measurements: thin beds (down to 8 in, 20 cm), borehole formation heterogeneity, formation dip, resistivity in dipping beds, and fracture position and orientation. The ARI produces images similar to the FMS with coarser vertical resolution, but complete azimuthal coverage. Whereas FMS electrodes are pad-mounted and in contact with the borehole surface, the ARI provides a remote image of the formation in a similar way to that of the BHTV.

The ARI electrode array operates simultaneously at two frequencies: 35 Hz for the deep readings and 280 Hz for shallow readings. It focuses currents that flow from the 12 electrodes to the grounded logging cable. The sum of these 12 readings produces a high-resolution measurement, equivalent to a single laterolog electrode of the same height. To correct for tool eccentralization and variations in borehole shape, a shallow auxiliary measurement of electrical resistivities is performed at a much higher frequency of 71 kHz. This measurement responds primarily to the volume of borehole fluid affecting each electrode. If the borehole fluid resistivity is independently measured, then borehole size and shape can be deduced from the auxiliary array measurements. While the vertical resolution of the standard laterolog readings is about 0.60 m, the high-resolution array can reduce this by up to a factor of 6, depending on the formation resistivity.

Preliminary processing of ARI images may be accomplished using GeoFrame, a software package developed by Schlumberger and GeoQuest, in a similar manner to FMS image processing. Comparison of image data from different logging tools can also be displayed using this software, which may provide information about fracture and fault orientation and aperture, formation dip and heterogeneity, and borehole shape. As the FMS is less sensitive to features near the borehole than the FMS, such as drilling-induced fractures, the origin and lateral extent of such features may be determined from the comparison of FMS and ARI images.

Log Presentation

The LLd and LLs curves are usually displayed on a resistivity logarithmic scale, along with the gamma ray log.

Output plot of DLL data

Specifications

Major Outputs

CHRA	Coherence at Compressional Peak, from Receiver Array
CHTA	Coherence at Compressional Peak, from Transmitter Array
DTBC	Compressional, Borehole Compensated (μsec/ft)
DTRA	Compressional, from Receiver Array (μsec/ft)
DTTA	Compressional, from Transmitter Array (μsec/ft)

 

Deployment Notes

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