Acoustic Evolution Brings Full Characterization, Near-Deep Profiling
Authors: JPT staff _ 
Published: February 2015

The evolution of sonic capabilities has now taken the form of a modular platform from which advanced geomechanical, petrophysical, geophysical, and reservoir measurements can be obtained, leading to actionable data about the drilling environment and the reservoir. Both short and long transmitter-to-receiver spacing and crossed dipole measurements, along with enhanced acoustic data for cement evaluation, are being obtained with the new Schlumberger sonic service.

Currently in field-test, the new modular sonic imaging service, fully combinable with other wireline services, was developed to provide comprehensive, robust measurements in rock for incorporation with other formation measurements for use in reducing drilling risk, improving recovery, and maximizing productivity.

Properties of elastic materials are well defined, but rock properties in formations vary with pressure and exhibit variations that are not easily predictable because of plastic and nonlinear deformation. Measurements at sonic frequencies continue to dominate evaluation efforts, with significant improvements being made since the introduction of wireline sonic tools in the 1950s. These improvements have culminated in a new sonic platform that can obtain stress information critical to wellbore placement and maximum reservoir drainage. The service provides a comprehensive, mechanical characterization of the wellbore, including 3D characterization of the compressional- and shear-wave slowness in terms of their axial, azimuthal, and radial variations that arise from nonuniform stress distributions, mechanical or near-wellbore-region alteration caused by the drilling process, and from formation intrinsic anisotropy related to the in-situ properties of the formation.

The platform can be leveraged in a geomechanical application to deliver mechanical-property measurements that are derived from radial profiling using a dipole source that not only provides a large amount of energy over a wide frequency band, but also is capable of compensating for the effect of the sonic tool in the borehole (full characterization for acoustic predictability). Dipole radial profiling inverts the slowness response vs. frequency into a slowness response vs. depth into the formation. Radial profiling has not been previously available from either monopole or dipole tools. From a petrophysical perspective, monopole, dipole, and Stoneley acoustic measurements are applicable for gas detection, input to volumetrics, and permeability calculations. The new telemetry and optimized hardware and software can provide slowness measurements with enhanced reliability over an increased envelope of hole size and slowness, offering measurements from total depth to the mudline and at higher logging speeds.

Benefits in Geomechanics

Because shear-wave velocity responds to the solid framework of the rock and is not sensitive to fluid effects, it is important from a rock-mechanics perspective to evaluate shear velocity in both the far field and the near-wellbore region. Able to remain steady even in low-anisotropy conditions, new tool functions can discern the maximum-stress direction to as little as 2%. The importance of being able to profile shear slowness for the near-wellbore region and deep into the formation is seen when determining changes caused by drilling-induced stresses or stresses caused by intrinsic anisotropy (such as shales and fractures). It is also apparent when evaluating minimum/maximum stress direction for wellbore placement, during optimized drilling, and in oriented perforating. This is accomplished by rotating the measured waveforms into the fast (maximum stress) and slow (minimum stress) shear directions. The shear- and compression-al-wave velocity is also used as an input for calculating rock properties such as Poisson’s ratio, Young’s modulus, bulk modulus, and shear modulus. These properties, which control the physical behavior of the formation, are then used to calculate a safe mud-weight window or to reduce the uncertainty of well-bore instability ahead of the drill bit. They are also used in hydraulic-fracture design.