A new logging-while-drilling (LWD) sonic tool provides compressional and shear logs in fast and slow formations.

LWD triple-combo services have developed in recent years to the point of replacing equivalent wireline services, especially when rig time and cost considerations make LWD economically attractive.
Real-time applications such as geosteering or pore pressure determination also require the use of measurement-while-drilling (MWD) and LWD sensors. While LWD triple-combo logs often are adequate for evaluating porosity, water saturation and fluid type, no sonic sensor is yet capable of providing compressional and shear velocity data in fast and slow formations. For example, if compressional and shear ?t logs were required for geophysical applications such as time-depth correlation and AVO analysis, the operator would be required to make a wireline logging run to obtain these data, often negating the cost and time savings associated with LWD.
The BAT (Bi-modal AcousTic) tool is a new LWD sonic tool capable of serving as a replacement for wireline dipole sonic tools while providing the added benefits of real-time measurements. Other LWD sonic tools can provide compressional and shear ?t logs in so-called fast formations, where the formation shear velocity is greater then the compressional velocity in the borehole fluid (?ts < 200 µsec/ft). However, in slow formations, where the formation shear velocity is slower than the fluid velocity (?ts > 200 µsec/ft), those tools can only provide compressional data. The BAT sensor is the first LWD sonic tool capable of providing compressional and shear logs in both fast and slow formations.
The sensor comprises two radially opposed, dual-frequency transmitters and 14 receivers arranged into two radially opposed arrays. Programmable firing control allows the two opposed transmitters to be fired in phase in a pseudomonopole mode or out of phase in a pseudodipole mode. The pseudodipole transmitter-firing mode enhances formation shear measurements, particularly in slow formations.
Transmitter and receiver configuration
The transmitters also operate at two frequencies. The low-frequency firing mode emits acoustic energy in the 6- to 8-kHz range, and the high-frequency mode operates in the 12- to 15-kHz range. Refracted compressional and shear waves have a relatively high natural frequency in typical formations and are optimally excited by the 12- to 15-kHz transmitter firing. However, Snell's Law of Refraction dictates that in slow formations (?ts > ?tfluid) a refracted shear wave cannot be measured. Thus, the low-frequency pseudodipole firing of the transmitters excites lower-frequency modes, such as flexural body waves and interface modes, which facilitate the measurement of formation shear velocity in slow formations.
Another significant technology advancement is the use of dual opposed receiver arrays, each consisting of seven receiver elements spaced 6in. apart. These two large receiver arrays provide a significant improvement in signal-to-noise ratio and measurement reliability compared to the single four-receiver array found in previous LWD sonic tools. Although the tool's receiver isolation and acoustic attenuation techniques effectively eliminate noise from the drill bit, the receiver signals are still subject to more random noise sources such as mud circulation swirling around the receivers, cuttings impacting the receivers and drill collar contact with the borehole wall. The processing software automatically checks each channel and eliminates bad or noisy receiver signals from the slowness (?t) cross-correlation calculation. Thus, in a worst-case situation, the tool could suffer excessive noise contamination of one complete receiver array and up to three channels of the second array and still provide a four-receiver slowness calculation.
Another advantage of the dual receiver array configuration is the ability to obtain valid ?t measurements when the drillstring is lying against the low side of a deviated wellbore. For example, when drilling in a nonrotating or sliding mode with a steerable mud motor assembly, if one receiver array is buried in a cuttings bed on the low side of the hole, the opposed receiver array will be well-positioned to acquire accurate formation ?t measurements. The waveform data from the front and back receiver arrays from the high- and low-frequency transmitter firings provide four practically simultaneous data sets for ?t calculations. This has resulted in exceptional tool and measurement reliability.
An additional benefit of the receiver array design is the ability to use a subset of the full array to enhance the vertical resolution of the ?t logs. The full seven-receiver array is 3ft (1m) long. However, a four-receiver subset array could be used to produce compressional and shear ?t logs with 18in. vertical resolution for evaluation of thinly bedded formations.
Drilling noise and tool mode
Among the key challenges in designing an LWD sonic tool are drilling noise and the acoustic signal traveling from the transmitter to the receivers directly through the steel drill collar - a phenomenon called "tool mode."
The design employs multiple techniques to eliminate drilling noise and tool mode effects from the measured signal. First, the transmitters were designed to maximize the energy transmitted axially into the formation while minimizing the transverse energy transmitted into the tool. An axial-to-transverse ratio of almost 20 dB was achieved for the transmitters, with a similar axial sensitivity bias for the receivers.
The second step in eliminating the tool mode was to mount the transmitter and receiver transducers on an insert assembly in the bore of the drill collar, rather than mount them directly to the drill collar itself. Elastomeric compounds, air gaps and other sound-absorbing materials are used to acoustically isolate the transducers from the insert and the insert from the drill collar.
Finally, it was recognized that drilling noise and residual tool mode energy are manifested in several acoustic propagation modes in the drill collar, and no single attenuation scheme could adequately suppress these various distinct modes. 3D modeling and laboratory experiments were used to develop several attenuation techniques, each designed to attenuate a specific tool or drilling noise propagation mode. The result is that the tool mode is rendered inconsequential to the waveform processing and ?t calculation. Thus, it is not necessary to subtract a tool mode signal from the raw waveforms, and there are no restrictions on where the tool is placed in the drill string. Additionally, this noise attenuation of more than 34 dB was achieved without compromising the strength of the drill collar. The tool body is roughly 20% stronger in all failure modes than the standard API tool joint used on all Sperry-Sun LWD tools.
Operating parameters and data processing
The new tool is available in 6¾in. and 8in. tool sizes, with a 4¾in. tool design being field-tested. It is 21ft (6.4m) long and can be run in real time in conjunction with other Sperry-Sun MWD/LWD sensors or in standalone recorded mode. Internal battery power allows the tool to log while tripping. Typical battery operating life is 500 hours, and the battery can be replaced at the well site. Downhole memory capacity of 256 MB supports full waveform storage from all 14 receivers for a minimum of 150 hours at a 30-second sampling rate. (Memory capacity soon will be increased to 1 GB.) The tool is capable of measuring compressional ?t values ranging from 40 to 190 µsec/ft and shear ?t values from 60 to 500 µsec/ft.
Compressional ?t is computed by a downhole processor and transmitted via real-time mud pulse telemetry. At the end of each bit run, improved wellsite logs with higher sampling density are produced from the recorded data. Final ?tc and ?ts logs are produced from post-well processing in the local computing center.
Applications
Applications of the log data are essentially the same as those for wireline dipole sonic logs, with the additional advantage of the data being available as drilling progresses for real-time drilling, petrophysical and geophysical applications.
Drilling applications
real-time pore pressure determination;
rock strength calculations and bit wear predictions;
borehole stability analysis; and
rig time and cost savings by replacing wireline logging in high-angle wells.
Petrophysical applications
porosity determination;
gas detection from Vp/Vs ratio;
complex lithology and porosity determination (in combination with LWD neutron porosity and spectral density measurements); and
formation mechanical properties (bulk modulus, shear modulus, Young's modulus, Poisson's ratio) for completion and stimulation applications.
Geophysical applications
time-to-depth seismic correlation while drilling;
real-time synthetic seismograms;
correlation of log features and seismic attributes; and
AVO analysis.
Case histories
An operator drilling a high-angle exploration well in the deepwater Gulf of Mexico required log data for petrophysical evaluation of potential reservoirs, as well as compressional and shear sonic logs for seismic interpretation applications.
An LWD tool string including directional, gamma ray, resistivity, neutron porosity and density sensors was used along with the sonic tool for directional control, real-time correlation and formation evaluation. The availability of the tool with its capability of measuring compressional and shear ?t in this slow formation allowed the operator to eliminate a costly drillpipe-deployed wireline logging run, with an estimated cost savings in rig time and logging charges of more than US $250,000.
Another deepwater Gulf of Mexico operator is using real-time LWD data, including the real-time compressional ?t log, to compute a synthetic seismogram to correlate the actual well with seismic data. The LWD data increased the certainty of the geological and geophysical positioning of the well and also was useful for detecting a pore pressure increase where pore pressure determination from resistivity measurements alone can be uncertain.
Apache in Australia is continuing a long-term multiprospect exploration, appraisal and development program offshore in the Asia-Pacific region with a historical exploration success rate of 20% (one well in five). The use of LWD "quad-combo," including the sonic tool, provided the necessary data for geological, petrophysical and geophysical evaluation of each well, while eliminating the cost and risk of acquiring conventional wireline measurements. When discoveries are made, the LWD data typically are supplemented by running a wireline formation tester and electrical imaging log for further evaluation of the reservoir intervals. LWD logging also ensures that the client acquires excellent quality log measurements in every well without the risk of subsequent hole problems, precluding the acquisition of acceptable quality wireline log data.
"As long-time users of quad-combo LWD, we have logged many more meters and hours of the BAT sonic tool than any other operator since the tool's release. To date the reliability has been an enviable 100%, and the data quality and implementation of the technology have been excellent. It's a winning combination," said Colin Higgins, senior operations geologist with Apache in Australia.
The BAT sonic tool represents a significant step in the development of LWD technology and applications. The traditional basic open-hole wireline logging measurements can be acquired with LWD tools. With its ability to provide accurate compressional and shear logs in fast and slow formations, the sonic tool can serve as an LWD replacement for wireline dipole sonic tools. The BAT service also extends the range of LWD real-time applications from traditional petrophysical applications into new areas such as geophysical and rock mechanics applications.