ABSTRACT

Detecting gas in formations is an important task of formation evaluation. Oftentimes, gas saturation is in small pockets or in thin/laminated beds of the formation, which tend to be overlooked by conventional methods that have a lower vertical resolution. Nevertheless, finding such thin gas intervals is quite significant, especially for enhancing production in heavy oil reservoirs. This paper describes the acoustic determination of thin gas intervals using Stoneley wave reflection and high-resolution slowness measurements. A gas-saturated interval has drastically different fluid mobility and compressibility compared to those of the surrounding formation. During acoustic logging across the interval, the significant fluid mobility/compressibility contrast will generate a measurable Stoneley wave reflection and increase the travel time/slowness of compressional waves. Theoretical modeling shows that the Stoneley-wave reflectance is primarily controlled by the fluid property contrast and is sensitive, to a lesser extent, to the interval thickness. Thus the Stoneley reflection measurement can be used for thin gas bed detection. The slowness measurement, however, is controlled by the thickness of the gas bed. With a recently developed thin bed slowness analysis, formation compressional and shear slowness profiles can be determined with a measurement aperture down to 0.5 ft, allowing for the detection of thin gas intervals. Combining the Stoneley and slowness measurements can provide mutual verification of the result and enhance the reliability of the detection. Field examples will be used to demonstrate the application results.

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