 The physics of the measurements made by the LWD-CDN tool are similar to those of corresponding wireline services. For the neutron porosity measurement, fast neutrons are emitted from a 7.5-curie (Ci) americium-beryllium (Am-Be) source. The quantities of hydrogen in the formation, in the form of water- or oil-filled porosity, primarily control the rate at which the neutrons slow down to epithermal and thermal energies. Neutrons are detected in near- and far-spacing detectors, and ratio processing is used for borehole compensation. The energy of the detected neutrons has an epithermal component because a high percentage of the incoming thermal neutron flux is absorbed as it passes through the 1-in. (2.5 cm) steel wall of the drill collar. Also, a wrap of cadmium under the detector banks shields them from thermal neutrons arriving from the inner mud channel. This mainly epithermal detection practically eliminates adverse effects caused by thermal absorbers in the borehole or formation.
The density section of the tool uses a 1.7-Ci 137 Cesium (Ce) gamma ray source in conjunction with two gain-stabilized scintillation detectors to provide a high-quality, borehole-compensated density measurement. The tool also measures the photoelectric effect, Pe, for lithology identification.
The density source and detectors are positioned behind a full-gauge clamp-on stabilizer, which excludes mud from the path of the gamma rays, greatly reducing borehole effect. In deviated and horizontal wells, the stabilizer may be run under gauge for directional drilling purposes. Rotational processing provides an important correction in oval holes and yields a differential caliper.
Porosity estimate
If grain density is known, porosity can be calculated from the density log. Alternatively, porosity and density logs can together be used to calculate grain density.
Seismic impedance calculation
The product of velocity and density can be utilized as input to synthetic seismogram computations.
Lithology and rock chemistry definition
In combination with the neutron log, the density log allows for the definition of the lithology and of lithologic boundaries. Because each element is characterized by a different photoelectric factor, this can be used, alone or in combination with other logs, to determine the lithologic type. Both density and photoelectric effect index are input parameters to some of the geochemical processing algorithms used onshore.
Porosity
In reservoir engineering its importance is quite evident; in the study of the volcanic rocks that make up the upper oceanic crust, a good in-situ porosity measurement is most important to the correct understanding of the crustal structure. First, because it samples both the small-scale (microcrack, vesicle) porosity seen in the cores and large-scale fractures not sampled by drilling, and secondly because other properties such as density, seismic velocity, and permeability depend strictly on porosity variations and on the geometry of the pore space. In the presence of clays or hydrous alteration minerals a correction is required to account for the presence of bound water.
Lithologic determination
Because the hydrogen measured by the tool is present not only as free water but also as bound water in clay minerals, the porosity curve, often combined with the density log, can be used to detect shaly intervals, or minerals such as gypsum, which have a high hydrogen index due to its water of crystallization. Conversely, the neutron curve can be used to identify anhydrite and salt layers (which are both characterized by low neutron readings and by high and low bulk density readings, respectively).
A reliable density measurement requires good contact between stabilizer and formation. Because a statistical caliper measurement is made during the recording, it is possible to check the quality of the contact. Contact also affects the neutron log response; the formation signal, particularly for the epithermal count rates, tends to be masked by the borehole signal with increasing hole size.
The primary curves are: bulk density (ROMT, in g/cc), photoelectric effect (PEF, in barns/electron) density correction (DRHO, in g/cc), and caliper (DCAL, in in.). They are usually displayed along with the neutron curve TNPH in porosity units. DRHO and DCAL are useful for quality control of the data; if the tool is operating correctly they should be less than 0.1 g/cc and 1 in., respectively. Gamma ray (GR) log in API units is also plotted.
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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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30.6 ft (9.3 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. |
| DCAL |
Differential Caliper (in.) |
| DRHO |
Bulk Density Correction (g/cm3) |
| PEF |
Photoelectric Effect (barns/e-) |
| ROMT |
Max. Density Total (g/cm3) from rotational processing |
| TNPH |
Thermal Neutron Porosity (%) |
| DTAB |
CDN Density Time after Bit (hr) |
| NTAB |
CDN Neutron Time after Bit (hr) |
* ®trademark of Schlumberger |
