 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 inches), 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 at 35 Hz for the deep readings and 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.
Fractures
The response of each of the 12 electrodes is strongly influenced by conductive fluid-filled fractures, and each log trace is affected by its position and orientation in relation to the fractures. Deep fractures can be clearly identified and are differentiated from the shallow drilling-induced cracks to which the tool is insensitive.
Formation heterogeneity
Average resistivity can be strongly affected by formation heterogeneities. In such cases, the azimuthal images from the ARI tool help interpret the resistivity log.
Formation dip
ARI images can give a good estimate of formation dip, although they cannot provide dipmeter accuracy. They may detect unexpected structural features such as unconformities and faults, and they help confirm expected features.
Resistivity in dipping beds
ARI electrodes facing along the strike of the formation dip are barely affected by anistropy of the apparently dipping layers. Selecting the readings from these electrodes gives a much more accurate resistivity in thin dipping formations.
|
Temperature Rating:
|
350° F (175° C) |
|
Pressure Rating:
|
20 kpsi (13.8 kPa) |
|
Tool Diameter:
|
3.625 in (9.21 cm) |
|
Tool Length:
|
33.2 ft (10.12 m) |
|
Sampling Interval:
|
6 in (15.24 cm) |
|
Max. Logging Speed:
|
1800 ft/hr (550 m/hr) |
|
Resistivity Range:
|
0.2 to 100,000 ohm-m |
|
Vertical Resolution:
|
8 in. |
The ARI may be deployed in the Triple Combo, where it replaces the Dual Induction Tool (DIT-E), in several other combinations, or deployed independently. However the ARI must be used with the GPIT for image orientation, as is the case for the FMS tool. Repeat passes of the ARI may be useful to obtain consistent azimuth measurements.
Stuck/lost tool information
* ®trademark of Schlumberger
|
