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Downhole logging tools

Triple Combo Tool String

High-Resolution Laterolog Array Tool (HRLA*)


The High-Resolution Laterolog Array (HRLA) tool provides six resistivity measurements with different depths of investigation (including the borehole, or mud resistivity, and five measurements of formation resistivity with increasing penetration into the formation). The sonde sends a focused current into the formation and measures the intensity necessary to maintain a constant drop in voltage across a fixed interval, providing direct resistivity measurement. The array has one central (source) electrode and six electrodes above and below it, which serve alternately as focusing and returning current electrodes. By rapidly changing the role of these electrodes, a simultaneous resistivity measurement at 6 penetration depths is achieved. The tool is designed to ensure that all signals are measured at exactly the same time and same tool position, and to reduce the sensitivity to "shoulder bed" effects when crossing sharp beds thinner than the electrode spacing.


Porosity estimate

Because of the inverse relationship between resistivity and porosity, the HRLA 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.

Log Presentation

Sample image of HRLA data

Tool Specifications

Temperature rating: 300° F (150° C)
Pressure rating: 15 kpsi (10.4 kPa)
Diameter: 3.625 in (9.21 cm)
Length: 24.1 ft (7.35 m)
Weight: 394 lbs (178.7 kg)

Measurement Specifications

Vertical resolution: 12 in. (30.48 cm)
Depth of investigation: 50 in.
Sampling rate: 2 in. at 3600 ft/hr logging speed

Major Outputs

RLA1: Apparent resistivity from Computed Focusing Mode 1
RLA2: Apparent resistivity from Computed Focusing Mode 2
RLA3: Apparent resistivity from Computed Focusing Mode 3
RLA4: Apparent resistivity from Computed Focusing Mode 4
RLA5: Apparent resistivity from Computed Focusing Mode 5
RM_HRLT: Computed Mud Resistivity
RT_HRLT: True Formation Resistivity

Deployment Notes

The HRLA is typically deployed on the triple combo tool string in high-resistivity environments. It is compatible with virtually all other tools commonly used on the JOIDES Resolution except the FMS, and unlike the bottom-only DIT, it can be run higher in the string should it be necessary to combine it with a bottom-only tool such as the MSS.

* ®trademark of Schlumberger