Practical Drilling Technology Sonic logging-while-drilling tool produces wire line quality data

Dale R. Heysse
Halliburton Energy Services
Houston

A new logging-while-drilling (LWD) tool offers high-quality sonic measurements over a wide range of formation types. The compressional wave slowness measurement provided through the service approaches or exceeds wire line quality and allows more complete formation evaluation.

LWD sonic data are available at the surface only a few minutes after the bit penetrates a formation, enabling timely analysis of formation properties without having to wait hours, or sometimes days, for the acquisition of similar data by wire line logging.

This long-spaced sonic tool has a rugged construction and special electronics which contribute to reliable operation and a wide measurement range. More than 49,000 ft of hole have been logged and over 1,400 hr of sonic waveforms have been recorded with the new tool worldwide. The tool has produced logs in the acoustically fast, hard-rock formations of the North Sea and the slow, soft-rock formations of the Gulf of Mexico.

Acoustic amplitude and slowness measurements provided through sonic logging have a wide variety of applications, including the following:

• Correlation of log depths with seismic times for geologic mapping

• Calculation of formation porosity for reserves estimates

• Evaluation of rock mechanical properties for stimulation design

• Detection of natural fractures and evaluation of their extent for use in drilling programs.

Sonic logging has become an important part of many operators' exploration and development programs.

Sonic measurements

With sonic logging tools, an acoustic signal is generated by a tool transmitter, then travels through the formation, and subsequently arrives at a tool receiver (Fig. 1 [29689 bytes]). Modern logging tools record acoustic waveforms, which are the amplitudes of the pressure signals arriving at the receiver as a function of time. The waveforms are processed to determine compressional wave slowness (Dtc), which is the time required for compressional waves to travel a certain distance through the formation, usually 1 ft or 1 m.

Acoustic slowness is actually the inverse of acoustic velocity and is usually expressed in msec/ft or msec/m. In subsurface formations, Dtc most commonly ranges from 45 to 170 msec/ft.

Early sonic logging

The first attempt at sonic logging was made in the 1930s when the horn of a Model A Ford was used as the acoustic source.1 However, the development of a more suitable source, along with small receivers, associated downhole electronics, and signal processing techniques, delayed the introduction of a working tool until 1955.

In that year, a successful wire line tool was built, based on a design patented by Humble Oil. Forty years of technological advances since then have resulted in very reliable wire line sonic tools with wide-ranging capabilities and applications. Today's wire line tools use multiple transmitters and receivers to record acoustic waveforms digitally, which are then electronically telemetered to surface equipment for processing and Dt determinations.

LWD sonic challenges

Although a wealth of technological information has been accumulated on wire line sonic logging, developing LWD sonic tools has presented unique challenges. The foremost challenges have been related to tool strength, drilling noise, and data retrieval.

• Tool strength

  Sonic logging tools must be constructed so that emitted acoustic signals traveling directly along the tool body are sufficiently impeded that they arrive at the receivers at low amplitude and after the emitted signals traveling through the formation have arrived at the receivers. Otherwise, the direct signals can interfere with the formation signals and corrupt the critical waveform recordings.

  To avoid this problem, the bodies of wire line sonic tools are manufactured with numerous elongated open slots transverse to the tool axis. Such construction, however, is not feasible with LWD tools; such slots in LWD tool collars could weaken the collars too much to withstand the stresses of drilling, fishing, and jarring.

• Drilling noise

  In wire line logging, downhole noises are generated by the movement of the tools through the well bore, but these noises usually do not interfere significantly with normal sonic logging. During LWD operations, however, the downhole noise level from drillstring rotation, axial tool movement through the borehole, and mud flow through and around the tool can be considerably louder and may mask formation signals.

• Data retrieval

  During wire line sonic logging, waveform data are transmitted uphole, where they are processed to obtain the Dt measurements. The electronic telemetry of wire line systems can operate at the high transmission rates needed to transfer the high-density waveform data to the surface.

  Even the fastest mud-pulse telemetry systems used in LWD operations are unable to transfer the waveform data in a reasonable time frame, however. Thus, if a real-time log is desired, waveform processing must be performed downhole in the tool during drilling.

New LWD sonic technology

A new compensated long spaced sonic (CLSS) tool, recently introduced by Halliburton Energy Services as part of the scout service, contains dual transmitters, one above and one below the four-receiver array, to record waveforms from which Dtc is determined (Figs. 2 [24870 bytes] and 3 [49583 bytes]).2 The use of multiple transmitters improves measurements by reducing the effects of noise, tool tilt, tool eccentering in the borehole, and borehole enlargement. A special ultrasonic transducer measures standoff and indicates borehole rugosity. The tool is available in either 63/4-in. or 8-in. OD for use in 81/2-in. or 121/4-in. boreholes, respectively.

In developing the new tool, the design team was able to overcome the challenges of producing an LWD sonic tool. The team discovered a unique, proprietary means to attenuate the direct acoustic signal traveling through the tool collar while maintaining sufficient mechanical strength. The acoustic isolator of the tool, which contains the transmitters and receivers, is stronger than the standard box/pin connections for conventional collars of the same size.

Field tests demonstrated the strength of the new acoustic isolator. In one case, the CLSS tool was part of a bottom hole assembly (BHA) that became stuck when the drill pipe twisted off above. After several days of fishing, during which hydraulic jars were used, the BHA was recovered. The logging tool required no repairs. The memory was dumped, and a log was delivered.

The acoustic isolator reduces much of the drilling noise. During some of the early developmental testing of the CLSS tool, transmitters were disabled so that the tool would record only drilling noise (Fig. 4 [45471 bytes]).

The data showed that most of the drilling noise occurred at frequencies below 12 khz and could be eliminated from the recorded signals by placing frequency-based filters in the tool's electronic circuitry. The resulting reductions in drilling noise were significant and enhanced the tool's ability to sense acoustic signals in slow formations, where acoustic attenuation is high and the signals arriving at the receivers are generally of low amplitude.

The identification and elimination of most drilling noise also allowed the new tool to be designed to operate in essentially the same frequency range as wire line sonic tools, namely 10-20 khz. Thus, CLSS tool responses and depth of investigation are very similar to wire line tools, allowing these LWD sonic logs to be easily correlated with wire line logs and permitting well-documented acoustic processing and analysis techniques to be applied to the LWD sonic waveform data.

To address the data transmission issue, CLSS tool designers embedded appropriate algorithms into the tool's circuitry so that waveform processing and Dtc determination could be accomplished downhole. Thus, only Dtc values, rather than complete waveforms, need to be transmitted to the surface in real time. During normal operation, the tool alternately fires the transmitters at a programmable rate, typically every 10-30 sec.

The waveforms recorded at the four receivers are correlated to obtain a Dtc value from each of the two transmitter firings. The two Dtc values, along with a standoff measurement, are then available to be telemetered uphole in real time. Waveforms, as well as higher-resolution calculated Dtc values and standoff measurements, are stored in the tool's 68-Mb memory and can be recovered between bit runs when the tool is returned to the surface. The memory is sufficient for the tool to record data at a rate of two samples/ft at 60 ft/hr for 187 hr. Recovered waveforms can be further evaluated on a workstation.

Field examples

These LWD sonic logs have been run in a wide variety of field situations. The extremes of formation types were represented by the hard-rock formations of the North Sea and the soft-rock formations of the Gulf of Mexico basin.

Hard rock

The sonic log of Fig. 5 [58817 bytes] was obtained in a field test in the North Sea. An 8-in. CLSS tool was run during drilling of the 121/4-in. hole section, where the operator set intermediate casing. Then a 63/4-in. CLSS tool was run while drilling out of the 95/8-in. casing through the sandstone and shale, and into a limestone below.

A 13.7-ppg water-based mud containing 24% solids was used throughout. The tool was situated above a tandem drilling motor, and the drilling rate of penetration varied from 75 to 150 ft/hr, rotating. Following drilling, a full set of wire line logs was run.

The wire line sonic Dtc curve is superimposed on the LWD sonic log. The agreement between the wire line and LWD measurements is excellent, except in the shale section where it is believed that borehole rugosity adversely affected wire line sonic responses. The CLSS transducer yielded a usable standoff measurement.

Soft rock

The sonic log of Fig. 6 [71140 bytes] was obtained in a shallow field test in the U.S. A 63/4-in. CLSS tool was used to log this hole while drilling at over 200 ft/hr with water-based mud. The formations were slow (Dtc ranged from 130 to 170 msec/ft) and significantly attenuated the acoustic signals. In addition, the borehole was enlarged and irregular, which led to further signal attenuation.

A subsequent wire line sonic log was in general agreement with the LWD sonic log and is shown in Fig. 6 [71140 bytes].

Future developments

Workstation processing of waveform data is being further developed to determine shear slowness (Dts), as well as an improved Dtc, and also to estimate rock mechanical properties such as Young's modulus, shear modulus, and Poisson's ratio.

Additional work in workstation processing of compressional and shear wave amplitudes is aimed at enhancing the identification of natural fractures.

A slim version of the CLSS tool is being tested for logging while drilling 6-in. boreholes. Combining this tool with an existing slim resistivity tool will offer operators the opportunity to simultaneously obtain resistivity and sonic porosity logs in a 6-in. hole without using a radioactive source for the porosity determination.3

The introduction of LWD sonic services makes LWD quad-combo logging available to operators. Quad-combo logs contain resistivity, density, neutron, and sonic data along with auxiliary gamma ray and caliper data. Besides receiving timely information with LWD logs, operators are also able to save rig time that would later need to be dedicated to wire line logging.4 5

References

1. Ellis, D.V., Well Logging for Earth Scientists, Elsevier, New York, 1987.

2. Minear, J.W., et al., "Compressional Wave Slowness Measurement While Drilling," Transactions of the 1995 Annual Logging Symposium of the Society of Professional Well Log Analysts, Paris, June 26-29, 1995.

3. Heysse, D.R., et al., "Field Tests of a New 2-MHz Resistivity Tool for Slimhole Formation Evaluation While Drilling," paper SPE 30548 presented at the Society of Petroleum Engineers Annual Technical Conference and Exhibition, Dallas, Oct. 22-25, 1995.

4. Cantrell, L., et al., "Case Histories of MWD as Wireline Replacement: An Evolution of Formation Evaluation Philosophy," paper SPE 24673 presented at the SPE Annual Technical Conference and Exhibition, Washington, D.C., Oct. 4-7, 1992.

5. Moake, G.L., et al., "Improved Measurement Quality and Reliability in a Formation Evaluation LWD System," paper SPE 28429 presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Sept. 25-28, 1994.

The Authors