音波検層活用法

―Sonic Scannerアプリケーションレビュー―

抄録

The latest generation of sonic logging tool (Sonic Scanner) can record monopole and dipole waveforms data in wider range of formation types and logging condition than the previous generation of the sonic tool could through the advance in tool design and associated technology such as electronics, sensor and so on. Wider frequency bands, better modal purity, increased signal-to-noise of the recorded sonic waveforms and the configuration of the transmitters and receivers enable to measure formation acoustic characteristic around the borehole, which was not possible before. While more measurements became possible, the most important outputs from the sonic logging are P and S wave velocities (V_p and V_s). To obtain V_p and V_s from array waveform data, slowness-time-coherence (STC) processing technique is commonly used. With appropriate processing parameters, this processing method results in accurate and robust slownesses (reciprocal of velocity) for non-dispersive coherent arrivals, which is usually the case of P and S waves in monopole data. When the dipole waveform data are processed for formation shear slowness, dispersive STC method is used to take dispersive nature of flexural mode into account before calculating semblance. For quality control, the shear slowness estimated by dispersive STC is overlaid on slowness-frequency-analysis (SFA) projection log, and if the estimated slowness matches the low-frequency limit of the SFA projection, the quality of the estimated slowness is high.
Acoustic logging data were conventionally used for seismic tie, velocity modeling, porosity evaluation, lithology analyses, gas detections, cased-hole analyses and so on. The shear anisotropy analysis using cross-dipole data had been available with the previous generation of the sonic tool and has been used for characterization of fractures and the stress field around the borehole. Clean dispersion curves of monopole and dipole data in wide frequency band from the latest sonic tool further enable identifying the dominant mechanism of observed elastic anisotropy as well as near wellbore formation alteration. V_p, V_s, anisotropy parameters and fast shear azimuth are important input for mechanical earth model (MEM) to better understand the rock mechanical properties and assess the state of the stress. Using MEM, the stability of the wellbore can be modeled, and mud weight and the design of well trajectory can be optimized for safe and cost effective drilling. This would be particularly important for unconventional explorations like gas hydrates and CO₂ geosequestration due to special concerns about wellbore and formation stability.