 The LWD-CDR is an electromagnetic propagation and spectral gamma ray tool built into a drill collar. It has many similarities to dual induction tools: it responds to conductivity rather than to resistivity, operates in water- or oil-base muds, and provides two depths of investigation. It has better vertical resolution but a shallower depth of investigation than dual induction tools.
The tool broadcasts a 2-Mhz electromagnetic wave and measures the phase shift and the attenuation of the wave between two receivers. These quantities are transformed into two independent resistivities that provide the two depths of investigation. The phase shift is transformed into a shallow resistivity (Rps, for resistivity from phase shift-shallow); the attenuation is transformed into a deep resistivity (Rad, for resistivity from attenuation-deep).
The LWD-CDR has upper and lower transmitters that fire alternately. The average of these phase shifts and attenuations for the upward and downward propagating waves provides a measurement with borehole compensation similar in principle to that of the Borehole-Compensated Sonic Tool (BHC). Borehole compensation reduces borehole effects in rugose holes, improves the vertical response, increases measurement accuracy and provides quality control for the log. An electrical hole diameter is computed from the CDR data and is used as an input to hole size corrections.
Detection of 3 in. (7.5 cm) beds is possible with the CDR tool. However, because of shoulder bed effects, Rps and Rad will read too low in a thin, resistive bed with conductive shoulder beds, and a small correction for bed thickness is required to obtain true resistivity, Rt. A major advantage of the CDR tool is its ability to measure Rt in thin beds before invasion occurs. Once thin beds are deeply invaded, there is no reliable method for obtaining true resistivity.
Porosity estimate
In sediments that do not contain clay or other conductive minerals, the relationship between resistivity and porosity has been quantified by Archie's Law. Archie's Law relates the resistivity to the inverse power of porosity. This relationship has also been used to estimate apparent porosity in oceanic basalts.
Density and velocity reconstruction
Archie's equation has been used effectively to create "pseudodensity" and/or "pseudovelocity" logs from porosity over intervals where no such logs were recorded or were totally unreliable. In some instances velocities derived from resistivity logs can be used to depth-tie seismic reflectors.
Lithologic boundary definition and textural changes
Resistivity, along with acoustic and velocity logs, is a very valuable tool in defining lithologic boundaries over intervals of poor core recovery. In a particular example, the decrease in resistivity towards the top of a carbonate unit, coupled with a decrease in velocity, allowed one to interpret this unit as a fining-upward sequence in mostly carbonatic sediments. Similar saw-tooth patterns in the resistivity response can also be observed in oceanic basalt units where they are related to porosity changes towards the top of each unit.
Clay typing
Potassium and thorium are the primary radioactive elements present in clays; because the result is sometimes ambiguous, it can help combining these curves or the ratios of the radioactive elements with the photoelectric effect from the lithodensity tool.
Mineralogy
Carbonates usually display a low gamma ray signature; an increase of potassium can be related to an algal origin or to the presence of glauconite, while the presence of uranium is often associated with organic matter.
Ash layer detection
Thorium is frequently found in ash layers. The ratio of Th/U can also help detect these ash layers.
The CDR tool provides a set of corrections for different environmental effects. These include corrections for adjacent formations, borehole signal, and invasion. Differences in the temperature of drilling fluid compared to undisturbed formation temperatures can also generate environmental effects, as conductivity in ionic fluids such as seawater is strongly temperature dependent.
Attenuation Resistivity (ATR) and Phase Shift Resistivity (PSR) are usually plotted in ohm-m on a logarithmic scale along with gamma ray (GR) log in API units.
A full display of the Natural Gamma Spectroscopy data with SGR (total gamma ray in CPS), CGR (computed gamma ray -- SGR minus Uranium component -- in CPS), and THOR (in ppm), URAN (in ppm), and POTA (in wet wt%) is usually provided separately.
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Tool weight:
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2000 lb (907 kg) |
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Tool length (with savers):
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22 ft (6.7 m) |
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Min. - Max. temp:
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-13° - 300°F (-25° - 150°C) |
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Maximum weight on bit:
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F = 63,000,000/L2 lbm (where L is the distance between stabilizers in feet) |
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Maximum flow rate:
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600 gal/min |
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Maximum operating pressure:
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18,000 psi (12.4 kPa) |
| Available collar sizes: |
6.75 in., 8.25 in. |
| Available stabilizers: |
8.50 in., 9.75 in. |
| GR |
Gamma Ray (API Units) |
| SGR |
Total Gamma Ray (API units) |
| CGR |
Computed Gamma Ray (API units) |
| POTA |
Potassium (wet wt. %) |
| THOR |
Thorium (ppm) |
| URAN |
Uranium (ppm) |
| ATR |
Attenuation Resistivity (deep; ohm-m) |
| PSR |
Phase Shift Resistivity (shallow; ohm-m) |
| GTIM |
CDR Gamma Ray Time after Bit (sec) |
| RTIM |
CDR Resistivity Time after Bit (hr) |
* ®trademark of Schlumberger |
