The Hostile Environment Litho-Density Sonde (HLDS) uses a pad that is mechanically identical to that of the Hostile Environment Litho-Density tool (HLDT), but includes magnetic shielding and high-speed electronics. The entire pulse-height spectra from both detectors are recorded and processed into windows identical to those of the conventional Litho-Density measurement. Bulk density and photoelectric factor formation is conventionally derived; therefore, the tool response remains unchanged. The available spectral information is used for improved log and calibration quality control.
HLDS consists of a 137Cs radioactive source and two detectors mounted on a shielded skid
which is pressed against the formation by a hydraulically activated
eccentering arm. The 662 keV gamma rays emitted by the source
into the formation experience two types of interaction with the
electrons in the formation -- Compton scattering and photoelectric
Compton scattering is an elastic collision by which energy is
transferred from the gamma ray to the electrons in the formation.
This interaction forms the basis of the density measurement;
in fact, because the number of scattered gamma rays which reach
the detectors is directly related to the number of electrons
in the formation, the tool responds to the electron density
of the rocks, which is in turn related to the bulk density.
Photoelectric absorption occurs when the gamma rays reach a
low energy (<150 keV) level after being repeatedly scattered
by the electrons in the formation. The photoelectric effect
index is determined by comparing the counts from the far detector
in the high energy region, where only Compton scattering occurs,
with those in the low energy region, where the count rates depend
on both reactions. The far detector is used because it has a
greater depth of investigation. The response of the short-spacing
detector, which is mostly influenced by mudcake (not present
in IODP boreholes where a seawater-based drilling fluid is used)
and borehole rugosity is used to correct the density measurement
for these effects.
As with the case of the sonic tool, the depth of investigation
of the lithodensity tool cannot be easily quantified; it is
in the range of tens of centimeters, depending on the density
of the rock. The vertical resolution is 16 in (38 cm).
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.
The product of velocity and density can be utilized as input to
synthetic seismogram computations.
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.
A reliable density measurement requires good contact between
pad and formation. Because a caliper measurement is made during
the recording, it is possible to check the quality of the contact.
In the lithodensity tool the presence of mudcake and hole irregularities
are automatically accounted for using a "spine and ribs"
chart based on a series of laboratory measurements. The "spine"
is the locus of the two counting rates (short and long spacing)
without mudcake and the "ribs" trace out the counting
rates for the presence of mudcake at a fixed formation density.
The short and long spacing readings are automatically plotted
on this chart and corrected for their departure from true value.
These corrected data are typically located in the DRHO data column.
The primary curves are: bulk density (RHOM, in g/cm³),
photoelectric effect (PEFL, in barns/electron) density correction
(DRH, in g/cm³), and caliper (LCAL, in in.). They are
usually displayed along with the neutron curve APLC. Also, DPHI
(density porosity) can be computed and displayed by assuming
a constant grain density of the matrix. DRH is useful for quality
control of the data; if the tool is operating correctly it should
be less than 0.1 g/cm³.
|500° F (260° C )
|25 kpsi (17.25 kPa)
|3.5 in (8.9 cm)
|12.58 ft (3.83 m)
||402 lbs (182 kg)
|Standard: 1800 ft/hr (549 m/hr)
High speed: 3600 ft/hr (1097 m/hr)
||Density, porosity, and PEF 15: in (38.1 cm)
||Density: ± 0.01 g/cm³
PEF: ± 6%
Caliper: 16 in. (40.64 cm)
|Depth of investigation:
||Density, porosity, and PEF: 6 in.
||Bulk density (g/cm³)
||Bulk density correction (g/cm³)
||Long-spaced photoelectric effect (barns/e-)
||Enhanced bulk density (g/cm³)
|| Caliper (in)
||High res. bulk density (g/cm³)
||High res. bulk density correction (g/cm³)
||High res. photoelectric effect (barns/e-)
||High res. enhanced bulk density (g/cm³)
* ®trademark of Schlumberger