Risk reduction and drilling performance are the overarching goals of any drilling campaign. To minimize risks and ensure that a well reaches target depth safely and cost-effectively, drillers increasingly turn to sonic-while-drilling (SWD) technologies. The while-drilling measurement of the acoustic properties of the formation, such as the velocity, delivers information needed to reduce drilling risk, improve well placement and optimize completions design.

The Schlumberger SonicScope multipole SWD service has advanced real-time acoustic measurements for complex reservoirs. Combining high-quality monopole and quadrupole measurements, the service delivers real-time compressional and shear slowness in any downhole environment along with the quality assurance to enable confident drilling.

Multiple measurements provided

Equipped with wideband transmitters, an attenuator section optimized for cleaner acoustics and an array of 48 sensors, the service conducts sonic measurements in three modes. For compressional data in all formations and shear data in fast formations such as consolidated rock, the measurements are obtained from the high-frequency monopole firing mode.

In slower formations such as the soft sediments common in shallow deepwater drilling, the formation shear is obtained from the quadrupole firing mode. Additionally, the low-frequency monopole mode is used for compressional data in extremely slow formations as well as for the measurement of Stoneley mode data.

Taken together, these compressional and shear measurements open the door for many applications. Real-time compressional and shear slownesses are vital to more accurately monitor pore pressure, fracture gradients and wellbore stresses while drilling.

Unlike resistivity measurements, acoustic measurements are unaffected by changes in salinity and temperature. This means that acoustic is a more robust measurement for the geomechanical evaluations required to calculate the mud weight window and to adjust mud weight to mitigate against wellbore instability and kicks.

Because the monopole and quadrupole measurements provide real-time compressional and shear velocities, the operator can more accurately calibrate seismic-time-to-depth processing and interpretation techniques, including amplitude vs. offset (AVO) modeling and identification of fluid in place. When limited to monopole measurements only, the operator is forced to compute a synthetic shear based on historical data and empirical formulas and models, which potentially introduces inaccuracies into the data analysis.

An important completion application for the service is ensuring deepwater well integrity and zonal isolation. The multipole SWD service can locate the top of cement in real time while running in hole to drill out a new hole section or pulling out of the hole as well as estimate the location of free pipe and cemented zones. The data can be recorded in memory mode and also transmitted in real time.

Low-frequency monopole for Stoneley acquisition represents the final firing mode. By evaluating the reflected Stoneley waves, one can identify open permeable fractures that intersect the wellbore, an important parameter for estimating the potential productivity of the well and in completions design. By incorporating any available high-resolution imaging it is possible to characterize these factures, including the orientation and aperture.

Controlling quality with each measurement

Schlumberger conducts its own quality control (QC) analysis during data processing, which is made available to the operator in the form of QC logs. These logs graphically display a comprehensive set of plots that represent the data processing workflow.

Common logs include slowness-time coherence; quadrupole energy spectra, semblance projections and slowness frequency analysis; inversion quality controls; semblance spectra; waveform variable density logs; and slowness/frequency dispersion plots.

All logs are generated in a transparent manner, and operators can use them to validate the quality of the quadrupole data for themselves. This process engenders greater trust in the data and the answers provided.

While the multipole service has been available since 2011, it is steadily expanding its application base. The original 4¾-in. multipole sonic tool was joined in 2013 by a tool with an 8¼-in. outside diameter (OD) and in 2015 with a mid-size 6¾-in. OD tool.

Data acquisition in dual-reamer BHA

The service was recently deployed for an operator in the Gulf of Mexico challenged with drilling, evaluating and completing a well characterized by unconsolidated formations, narrow mud weight windows, shallow geohazards and seismic velocity uncertainties that hindered pore pressure modeling.

Schlumberger collaborated with the operator to develop a compact bottomhole assembly (BHA) containing the 8¼-in. multipole service in combination with two reamers. The sonic tool was placed closer to the bit, which allowed drilling hazards to be detected sooner and helped place the casing at the necessary depth.

The high-quality, real-time sonic data helped the client decide where to stop drilling to deliver a hole size suitable for running the casing string to its desired depth. The ability to simultaneously log and run two reamers saved the company two days of rig time, while past drilling operations required an extra data acquisition trip to avoid the noise of the dual reamers.

Applicability in soft formations

The 8¼-in. service also acquired high-quality compressional and shear data in a soft formation offshore Malaysia. The operator needed these datasets as part of a four-well drilling campaign, where accurate time-depth information was essential to reduce uncertainty in the surface seismic.

The BHA was configured with the multipole SWD service in combination with the Schlumberger TeleScope high-speed telemetry-while-drilling service. This technology combination was run in the shallow 12¼-in. section of each well to acquire data that would complement velocity measurements planned in deeper sections of the wells.

Despite the soft formations, the operator successfully acquired compressional and shear data using the multipole service in all four wells. Compressional slowness was extracted using a unique real-time Leaky-P processing technique, and the quadrupole acquisition provided shear slowness data of up to 960 ms/ft, which marked the slowest shear measured by the tool at the time. The operator used this information to correct the time-depth conversion and therefore update the projected depths of important geological layers and potential hazards.

Reduced drilling risks in HP/HT environment

The SonicScope service also has supported geomechanical analysis aimed at reducing risk in an HP/HT region of the South China Sea. The operator required high-quality SWD measurements to navigate a narrow mud weight window in a high-pressure formation prone to high shock and vibration during drilling.

The service obtained reliable sonic data for both real time and memory. An excellent match was acquired between the real-time and memory compressional and shear slowness in the high stick/slip environment of the HP/HT reservoir.

Schlumberger geomechanics experts in its PetroTechnical Services provided additional insight where time-lapsed logging helped quantify the impact of mud invasion in the high-porosity formations as a reduced slowness was acquired in the gas zone.

These datasets were compared with surface seismic data collected before drilling, and a good match between the two reduced uncertainty and boosted the operator’s confidence in being able to reach total depth with reduced risk. Having proven success after this first-time use of the service, the operator decided to run the sonic service in 14¾-in. and 12½-in. hole sections in future HP/HT deepwater wells.

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