Shear sonic interpretation in gas-bearing sands

Conference · Sun Dec 31 04:00:00 EST 1995

OSTI ID:207300

Brie, A; Marsala, A F ^[1]

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1. SPE, Richardson, TX (United States); and others

The introduction, a few years ago, of shear dipole sonic logs gave the industry the possibility to record high-quality shear aid compressional slownesses in soft formations. Data sets were acquired and analyzed on Vp/Vs versus {Delta}tc crossplots. Trends were identified in sands and shales and were matched with semi-empirical correlations based on the Gassmann formalism. These trends can be used to quality control shear logs and for quicklook lithology interpretation. The presence of gas in soft formations makes the interpretation more complicated as it can affect the sonic slownesses significantly, in particular the compressional. On the Vp/Vs crossplot, gas-bearing formations clearly differentiate from liquid filled formations. However, quantitative interpretation of the gas effect with the Gassmann equation gives deceptive results, although this model is successfully used in geophysics interpretation at a lower frequency. We indicate that the Gassmann model itself is not at fault. The responsibility is with the pore fluids mixture law used to compute the average fluid properties. We therefore propose a new empirical mixture law that better fits laboratory measurements and field observations. Using this revised model realistic gas trends can be identified on the Vp/Vs crossplot. The model can be solved to evaluate gas volume from compressional and shear slownesses. Additionally, the effect of shaliness can be accounted for. The results agree well, in most instances, with flushed-zone saturation obtained from resistivity measurements and provide another opinion on gas volume. An additional product of the interpretation is to provide reliable values of dry-frame dynamic elastic constants of the rock for possible subsequent use in a rock mechanics evaluation.

• OSTI ID: 207300
• Report Number(s): CONF-951002--
• Country of Publication: United States
• Language: English

Related Subjects

03 NATURAL GAS
ACOUSTIC MEASUREMENTS
GEOLOGIC MODELS
NATURAL GAS WELLS
RESERVOIR ROCK
SAND
SONIC LOGGING