Analysis of Full-Waveform Sonic Data Acquired in Unconsolidated Gas Sands

摘要

Experimental full-waveform sonic data were collected in an exploration well penetrating unconsolidated sand-shale sequences. Using a low frequency (2,000 Hz) compressional wave source drive, formation compressional (P) wave arrivals were recorded in acoustically slow, gas bearing sands. In some intervals formation velocities were slower than the borehole fluid speed. Standard (10,000 Hz) P-wave acquisition failed to detect formation signals across the same intervals. Shear wave acquisition was accomplished using dipole transducers. Interpretation of the measured compressional to shear wave velocity ratio (V_p/V_s) high-lighted gas-bearing intervals where the V_p/V_s ratio dropped below the background compaction trend.One of these gas-bearing intervals had commercial saturations of gas while the other zones were both sands and shales with low, non-commercial gas saturations. The low velocity zones appeared to be caused by a variety of mechanisms. Very slow P-wave velocities (borehole fluid speed or slower), positively indicating the presence of gas, were noted in several shale formations. Using a model of thin, gas charged silts or sand embedded in the shale matrix, effective medium modeling indicated that the recorded slowness could be caused by a concentration of less than 20 percent gas charged silt layers. This is significant because it indicates that gas charged shales can give amplitude anomalies, indicating hydrocarbons, on the seismic section.Slow P-wave velocities indicating the presence of gas were also noted in high-porosity sands that appeared wet, with the eater saturation approximately 90 percent, using conventional log interpretation. Pressure profiling gave a water gradient, confirming that water was the dominant phase in those intervals and indicating only a low gas saturation. However, this low gas saturation was sufficient to cause a significant slowing of the P-wave velocity and a corresponding amplitude anomaly on the seismic section.