The Dual Laterolog (DLL) provides two resistivity measurements with different depths of investigation into the formation: deep (LLd) and shallow (LLs). In both devices, a current beam 2 ft-thick (Ao) is forced horizontally into the formation by using focusing (also called bucking) currents (A1-A2, A'1-A'2); two monitoring electrodes (M1, M2, M'1, M'2) are part of a loop that adjusts the focusing currents so that no current flows in the borehole between the two electrodes. For the deep measurement both measure and focusing currents return to a remote electrode on the surface; thus the depth of investigation is greatly improved, and the effect of borehole conductivity and of adjacent formations is reduced. In the shallow laterolog, instead, the return electrodes which measure the bucking currents are located on the sonde, and therefore the current sheet retains focus over a shorter distance than the deep laterolog.

The Dual Laterolog response ranges from 0.2 to 40,000 ohm.m, thus permitting a good characterization of highly resistive rocks such as oceanic basalts and gabbros.

The DLL is usually run in combination with the Natural Gamma Ray spectrometry tool (NGT).

Applications

Porosity estimate

Because of the inverse relationship between resistivity and porosity, the dual laterolog can be used to compute the porosity of the rock from Archie's equation if the sediments/rocks do not contain any clay or if the contribution of surface conduction to the signal is negligible.

Fracture porosity estimate

This can be estimated from the separation between the deep and shallow measurements based on the observation that the former is sensitive to the presence of horizontal conductive features only, while the latter responds to both horizontal and vertical conductive structures.

Limitations

Environmental Effects:

For the LLd the borehole effect is small for hole diameters up to 16 in, while the LLs provides good readings in holes not exceeding 12 in. Corrections are available for holes up to 20 ft in diameter.

Depth of Investigation and Vertical Resolution:

The depth of investigation of the laterolog depends on the resistivity of the rock and on the resistivity contrast between the zone invaded by the drilling fluid and the virgin (uninvaded) zone. The vertical resolution of both LLd and LLs depends on the geometry defined by the focusing electrodes: this is about 2 ft (61 cm).

Log Presentation

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

Tool Specifications

Temperature Rating	350°F (175° C)
Pressure Rating	20 kpsi (13.8 kPa)
Tool Diameter	3 5/8 in (9.2 cm)
Tool Length	30.6 ft ( 9.35 m)
Sampling Interval	6 in ( 15.24 cm)
Max. Logging Speed	10,000 ft/hr
Depth of Investigation	see "Limitations"
Vertical Resolution	2 ft (61 cm)

The Azimuthal Resistivity Imager (ARI) is a new generation of laterolog tool which 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 may be deployed in the Triple Combo, where it replaces the dual induction tool (DITE), 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.

The ARI electrode array operates at 35 Hz for the deep readings and focuses currents which flow from the 12 electrodes to the grounded logging cable. The sum of these 12 readings produces a high-resolution measurement, equilivant 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 in a similar manner to FMS image processing. Comparison of image data from different logging tools can also be dislpayed using this software, which may provide information about fracture and fault orientation and aperature, 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 fractues, the origin and lateral extent of such features may be determined from the comparison of FMS and ARI images.

Applications

Fractures

The response of each of the 12 electrodes is strongly influenced by conductive fluid-filled fractures. And each log trace is affected according to 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 igve 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.

Tool Specifications

Temperature Rating	350°F (175° C)
Pressure Rating	20 kpsi (13.8 kPa)
Tool Diameter	3 5/8 in (9.2 cm)
Tool Length	33.2 ft
Sampling Interval	6 in ( 15.24 cm)
Max. Logging Speed	1800 ft/hr (550 m/hr)
Resistivity Range	0.2 to 100,000 ohn-m
Vertical Resolution	8 in