 The Resistivity-at-the-Bit (LWD-RAB) tool makes lateral resistivity measurements. As a formation evaluation tool, its application is limited to conductive muds. It may be run in several configurations and provides up to five resistivity measurements. The RAB tool contains a scintillation gamma ray detector which supplies a total gamma ray measurement. An azimuthal positioning system allows the gamma ray measurement and certain resistivity measurements to be acquired around the borehole. Additional measurements are chassis temperature and radial and longitudinal shocks.
The RAB tool has a nominal 6.75-in. diameter; it is meant to be run in 8.5-in. holes. Designed to be a flexible component of the bottomhole assembly, the RAB tool may be connected directly behind the bit or further back in the bottomhole assembly. The tool may be configured for packed or pendulum assemblies.
 When connected directly to the bit, the RAB tool uses the lower portion (8-in.) of the tool and the bit as a measure electrode. In this configuration, it provides a bit resistivity measurement, RBIT, with a vertical resolution just a few inches longer than the length of the bit. A 1.5-in. tall cylindrical electrode, located 3 feet from the bottom of the tool, provides a focused lateral resistivity measurement, RING, with a 2-in. vertical resolution, independent of the location of the RAB tool in the bottomhole assembly. In addition, the RAB sub has three longitudinally spaced button electrodes that provide staggered depths of investigation. As the tool rotates, azimuthal measurements are acquired from the button electrodes.
The RAB measurements have a high vertical and azimuthal resolution. To make the most of the vertical resolution, the optimal sampling density is greater than one sample every inch. At the maximum sampling interval of 10 sec, the optimal sampling density can be achieved for rates of penetration up to 30 ft/hr. Achieving this vertical sampling is most important when imaging.
The RAB tool provides four depth of investigation measurements to detect early invasion of borehole fluids into the formation, a sensor at the bit to ensure minimum invasion, azimuthal resistivity images of the borehole to detect resistivity heterogeneity, and a gamma-ray sensor for lithology characterization.
The RAB tool can also provide a close look at structural information within a fault zone or an active tectonic are with a resolution of 15-30 cm. The RAB measures oriented resistivity images of the borehole wall, similar to an FMI or FMS wireline tools. These fracture orientations and distributions can be observed as resistivity contrasts in the image logs and are critical to recognize the extent of the deformation front along a tectonic front. Conversion of RAB images into relative porosity using Archie’s equation (Archie, 1942) can be used in combination with density and porosity data to help define the azimuthal distribution of porosity and overpressurized zones which may contribute to fluid flow along planes of structural weakness.
The button measurements have a shallow depth of investigation by design, in order to be sensitive to shallow invasion. When the RAB tool is centralized in a 8.5-in. hole, the buttons are 0.1875 in. from the formation. Controlling this standoff insures correct measurements. Therefore, proper centralization is recommended.
The RAB processing automatically corrects the resistivity measurements for frequency effects and the effects of the borehole. Routines to derive Rt from the multidepth measurements are being developed.
In impermeable zones, such as shales, and zones where insufficient time has passed for any significant invasion to take place, the measurements from the Ring, Deep Button and Medium Button will match, all reading Rt.
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Tool weight:
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1200 lbm |
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Tool length:
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10.1 ft. |
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Min. - Max. temp:
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-13° - 300°F (-25° - 150°C) |
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Drill collar nominal outside diameter:
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6.75 in. |
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Drill collar maximum outside diameter (slick):
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7.5 in. |
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Drill collar maximum outside diameter (azimuthal):
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8.125 in. |
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Maximum flow rate:
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800 gal/min |
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Maximum operating pressure:
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18,000 psi |
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Maximum weight on bit:
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F = 74,000,000/L2 lbm (where L is the distance between stabilizers in feet) |
| Maximum jarring load: |
330,000 lbf |
| PGRD |
Gamma ray average (API) |
| PGR_UP |
Gamma ray up quadrant (API) |
| PGR_RT |
Gamma ray right quadrant (API) |
| PGR_DN |
Gamma ray down quadrant (API) |
| PGR_LT |
Gamma ray left quadrant (API) |
| GTCK |
Gamma ray tick |
| RTCK |
Resistivity tick |
| RPM |
RAB rotational speed (rpm) |
| ROP5 |
Rate of penetration (ft/hr) |
| RTAB |
Ring time after bit (hr or min) |
| RTMP |
RAB chassis temperature (°F / °C) |
| RB3 |
Relative bearing (deg.) |
| P1NO3 |
P1 north (deg) |
| P1AZ |
P1 azimuth (deg) |
| HAZI |
Azimuth (deg) |
* ®trademark of Schlumberger |
