Borehole Compensated Tool (BHC)

The BHC sonde measures the time required for a compressional sound wave to travel through one foot of formation (see fig. 1a). The BHC consists of an upper and lower transmitter arranged symmetrically on either side of two pair of receivers. The spacings T1-R1 and T1-R3 are 3 and 5 apart as well as the spacings T2 -R4 and T2 -R2. The transit time of the compressional wave in the formation, measured in microseconds per foot is given by:

Æt=1/2 (T1R3-T1R1+T2R2-T2R4)

Long Spacing Sonic (LSS)

The LSS relies on the "depth derived" borehole compensation principle because the sonde would be too long if it used the same configuration as the BHC tool. Two transmitters spaced 2 feet apart are located 8 feet below two receivers which are also 2 feet apart. Hole size compensation is obtained by memorizing the first DT reading and averaging it with a second reading measured after the sonde has been pulled up to a fixed distance along the borehole. The LSS provides an improved measurement of the sonic travel time. Thanks to its longer spacing (10-12 feet) the sonde has a deeper investigation depth and the measurement is not influenced by the altered zone close to the borehole. In fact, drilling operations in the altered zone produce a decrease of acoustic velocity below that of the virgin zone. Full waveforms are always recorded for each receiver. Shear velocity can be recorded with delay beyond P-wave arrival during a separate run.

 

Array Sonic (SDT)

In a fast formation, where shear velocity is faster than the velocity of the drilling fluid, the SDT obtains direct measurements for shear, compressional, and Stoneley wave values. In a slow formation, the SDT obtains real-time measurements of compressional, Stoneley, and mud wave velocities. Shear wave values can be then derived from these velocities. The multireceiver sonic tool, with its linear array of eight receivers, provides more spatial samples of the propagating wavefield for full waveform analysis than the standard two-receiver tools. This arrangement allows measurements of wave components propagating deeper into the formation past the altered zone.

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Applications

Porosity and "pseudodensity" log.

The sonic transit time can be used to compute porosity by using the appropriate transform and to estimate fracture porosity in carbonate rocks. In addition, it can be used to compute a "pseudodensity" log over sections where this log has not been recorded or the response was not satisfactory.

Seismic impedance.

The product of compressional velocity and density is useful in computing synthetic seismograms for time-depth ties of seismic reflectors.

Sonic waveforms analysis.

If a refracted shear arrival is present, its velocity can be computed from the full waveforms, and the frequency content and energy of both compressional and shear arrivals can also be determined.

Fracture porosity.

Variations in energy and frequency content are indicative of changes in fracture density, porosity, and in the material filling the pores. In some cases compressional-wave attenuation can also be computed from the full waveforms.

Limitations

Environmental Effects.

One common problem is cycle skipping: a low signal level, such as that occurring in large holes and soft formations, can cause the far detectors to trigger on the second or later arrivals, causing the recorded Æt to be too high. This probem can also be related to the presence of fractures or gas. Transit time stretching appears when the detection at the further detector occurs later because of a weak signal. Finally, noise peaks are caused by triggering of detectors by mechanically induced noise, which causes the Æt to be too low. Reprocessing programs that can eliminate the aberrations described above are available both at sea and on-shore.

Depth of Investigation and Vertical Resolution

The depth of investigation cannot be easily quantified: it depends on the spacing of the detectors and on the petrophysical characteristics of the rock such as rock type, porosity (granular, vacuolar, fracture porosity), and alteration. For source-detector spacings of 3-5 ft, 8-10 ft, and 10-12 ft the depth of investigation ranges from 2 in to 10 in (altered/invaded and undisturbed formation, respectively), 5 in to 25 in, and 5 in to 30 in. The vertical resolution is 2 ft (61 cm).

Log Presentations

DT and DTL are interval travel-times in microseconds per foot for the near and far receiver pairs, respectively. In very slow formations DTL provides the more reliable measurement as the refracted wave is not seen at the near receivers. The acoustic data is usually presented as compressional (Vp) velocity and, where available, as shear velocity (Vs) in km/s.

Tool Specifications

Temperature Rating	350° F (175° C)
Pressure Rating	20 kpsi (13.8 kPa)
Tool Diameter	3 5/8 in (9.2 cm)
Tool Length	37.9 ft (11.6 m)
Acoustic Bandwidth	5 kHz to 18 kHz
Waveform Duration	5 ms nominally, 10 ms maximum
Sampling Interval	6 in. (15.24 cm)
Max. Logging Speed	1,700 ft/hr for eight-receiver array