The acoustic properties of rock formations, such as the velocity and rate of attenuation of different modes of sound waves, are closely related to rock type and formation fluid and can provide useful information for reservoir evaluation. This information can be provided using sonic logging. Obtaining acoustic measurements while drilling, combined with real-time drilling geomechanics evaluation, provides additional benefits, including correlation with surface seismic and monitoring of pore pressure anomalies. Sonic LWD was introduced in the 1990s, enhancing real-time support of the drilling process and also making sonic data more available in highly deviated wells where wireline logs can be difficult to obtain.

Multipole sonic measurements

Early sonic LWD tools used omni-directional monopole transmitters and receivers to measure compressional (P-wave) slowness. Monopole sonic tools also can deliver shear (S-wave) slowness data in a fast formation but are unable to make such measurements when formation slowness is lower than mud slowness, as is often the case when drilling soft sediments in deep water. In such situations, a quadrupole tool is needed to provide shear measurements while drilling. The SonicScope multipole sonic-while-drilling service from Schlumberger combines high-quality monopole and quadrupole measurements to deliver robust compressional and shear slownesses in any environment.

Robust real-time compressional and shear slownesses are fundamental to pore pressure monitoring and wellbore stability evaluation, especially important for deepwater drilling. This real-time geomechanical evaluation helps to properly evaluate the pore pressure window so that mud weight can be adjusted to mitigate the risks of wellbore instability and kicks. The same robust real-time data can be used for sonic-to-seismic ties (if the seismicVISION tool is in the same BHA) that allow bit-on-seismic positioning and minimize well placement and casing positioning uncertainty. In addition, a while-tripping mode provides multi-pass analysis and top-of-cement (ToC) evaluation. Reliable measurements of compressional and shear velocities (Vp and Vs) are useful in calibrating advanced seismic processing and interpretation techniques, such as amplitude versus offset (AVO) modeling and estimating Vp/Vs and Poission’s ratio, which can indicate the presence of gas.

The use of quadrupole acoustic logging to determine shear velocities in slow formations uses an inversion method that takes into account the dispersion of the quadrupole mode, including the effects of the tool presence, which are largest in smaller boreholes where the tool takes up the largest percentage of hole volume. The quadrupole shear inversion involves careful and well-validated modeling of the tool acoustics.

At work in the GoM

The 4.75-in. multipole sonic tool was used while drilling a deepwater US Gulf of Mexico (GoM) well where shear slowness ranged from 150 to above 250 ?s/ft (Micro seconds/feet) and, consequently, monopole shear measurements were only possible in limited depth intervals. The well was drilled using the PowerDrive rotary steerable system with a 5.75-in. PDC bit and heavy (15.7 ppg) oil-based mud to mitigate a challenging pore pressure environment. The well had 7-in. liner to 15,073 ft (4,597 m) measured depth (MD). ROP was generally 20-40 ft/hr (6-12 m/hr), and drillstring rotation was 90-120 rpm. The logged interval started at an inclination of 23 degrees, dropping to 9 degrees at TD.

In addition to the SonicScope 475 tool, the bottomhole assembly included an ImPulse integrated survey, gamma ray, resistivity, and telemetry MWD tool; VISION pressure-while-drilling (VPWD) sub; and a 7-in. reamer to increase the well control. The reamer was placed just above the multipole sonic-while-drilling tool, which increased noise in the logging environment; however, proper modeling of the drilling assembly and parameters mitigated the effects of this noise and enabled high-quality P and S data to be measured. The target was a Lower Pleistocene delta-sourced sand that had re-deposits into previously cut channels or depressions on the outer slope. The predominant reservoir rock types were laminated, moderately well sorted, and characterized by inter-bedded silty-sandstones facies. The interval above the reservoir was predominantly shale.

Data acquisition

The multipole sonic-while-drilling tool had a single wideband transmitter in four quadrants to enable monopole or quadrupole firing. The receiver array consisted of 48 sensors in a 4 x 12 arrangement with four sensors around the tool 90 degrees apart at each of the 12 spacings. The nearest receiver station was 7 ft (2 m) from the transmitter, and the inter-receiver spacing was 4 in. To provide the best acoustic quality, the collar structure was designed to provide as much uniformity and azimuthal symmetry as possible between the transmitter and receivers and along the receiver array.

Received signals were digitized at the receiver sensors to reduce electronic noise and crosstalk. Small inter-receiver spacing, clean acoustics, and electronic noise reduction all enhance the ability to provide real-time monopole compressional and shear slowness. The monopole slowness-time-coherence (STC) plane peaks were sent uphole, providing a real-time QC capability. These continuous compressional data were used to enable pore pressure monitoring and remote support by a team of drilling and engineering experts in a Schlumberger Operations Support Center (OSC.) The downhole system included 1 GB of waveform recording memory. After more than 10 days of operations, drilling more than 1,800 ft (548.5 m) in one bit run, the recorded multipole data were analyzed and processed by data services experts.

P and S slowness

The monopole compressional data were clear and coherent, ranging from 90 to 110 ?s/ft. By contrast, the monopole shear data were weak and highly attenuated across the array, even in the fastest intervals near 165 ?s/ft and missing in much of the upper section of the log. Shear slowness was accessible for only 25% of the 1,800-ft interval with monopole mode.

Quadrupole data showed high-signal quality, and the shear measurements – as slow as 250 ?s/ft – were more continuous and coherent than from the monopole. The quadrupole shear slowness compared well to the monopole where monopole shear was available. The combination of high-fidelity monopole and quadrupole LWD sonic acquisition and dispersive processing produced a continuous shear log where the monopole shear was weak or absent. An important component of the quadrupole processing was a suite of QC logs that allowed aspects of the dispersive processing results to be critically evaluated. In addition to helping to evaluate the slowness estimates, the dispersion plots helped provide evidence of tool eccentering and near-borehole slowness alteration when present in isolated intervals. The real-time compressional data were used for real-time pore pressure monitoring, well correlation, and quick-look formation evaluation analysis. P and S memory slowness were used to calibrate seismic AVO analysis and help compute the mechanical earth model (MEM) to assist completions design.

ToC evaluation

In addition to delivering reliable P and S data, the multipole sonic-while-drilling service has a fast-logging multimode for multipass analysis and ToC evaluation. It can be run while drilling and pulling out of hole in all acquisition modes at up to 1,800 ft/h (548.5 ft/h) with 6-in. sampling. ToC identification is achieved by qualitatively processing the casing arrival amplitude. If the casing or liner is not properly cemented, the poor bonding results in acoustic ringing of the casing, causing higher amplitude. Under correct bonding conditions, the casing vibration is acoustically transparent, and the formation compressional data is measurable. While not as accurate as wireline cement bond logging quantitative services, this LWD technology allows extreme conditions such as free or well-bonded pipe to be logged qualitatively.

ToC identification was provided for the GoM well at no additional rig time cost while pulling out to surface after drilling the openhole section. The tool was switched to a fast acquisition rate using a dedicated downlink command. The data were then analyzed to provide casing amplitude and associated interpretation.

New insights

The new multipole sonic-while-drilling service provides multipole measurements to deliver compressional and shear data. Robust strategies for handling and processing acoustic data, coupled with extensive modeling, ensure a predictable response in the sand-shale formations. The information delivered by the new service provides insights for improving drilling optimization, formation evaluation, and completion design.

Acknowledgements

This article is based on a paper presentation at the SPWLA 51st Annual Logging Symposium held in Perth, Australia, June 19-23, 2010. The authors thank Eni for permission to publish the data used in this study.