Abstract
Abstract
The introduction, a few years ago, of shear dipole sonic logs gave the industry the possibility to record high-quality shear and compressional slownesses in soft formations. Data sets were acquired and analyzed on Vp/Vs versus Atc 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.
Introduction
With monopole sonic measurements, shear slowness could only be recorded in fast formations, where the wave in the formation is faster than the compressional wave in the mud. This limitation was removed with the commercial introduction of dipole shear logging (Harrison et al., Ref. 1). Since then, many high-quality shear and compressional logs were acquired worldwide, and our knowledge of acoustic propagation in slow formations has increased substantially. Trends have been identified on Vp/Vs versus Atc crossplots for all lithology types, in particular in unconsolidated shaly sands sequences. We can match these lithology trends with interpretation models, and use them for quality control and quicklook interpretation.
In unconsolidated sands, the presence of gas in the rock is known to have a strong influence on acoustic slownesses (Domenico, Ref. 2). The Vp/Vs ratio is often used qualitatively to detect the presence of gas from sonic logs. Yet, until now, attempts have not been successful at relating quantitatively the gas fraction in the pores to the magnitude of the effect on sonic slownesses. Although work done by Gassmann (Ref. 3) for low frequencies, and Biot (Ref. 4) for all frequencies fits well the data from fully saturated rocks, no established model exists for partially saturated rocks, Fluid distribution in the pore system at the microscopic level, and frequency, have a strong influence on the gas effect at partial saturations. In this paper we will study the gas effect on sonic data from two wells in Italy, and propose a practical interpretation technique. The objective is to evaluate the gas volume from compressional and shear logs in shaly sands, and to provide additional information on gas presence in the reservoir.
Analysis of shear data in shaly sands
To characterize shear data it is convenient to consider the ratio of shear to compressional slownesses. This ratio is identical to the Vp/Vs ratio, a formation parameter that geophysicists widely use. Correlations of shear versus compressional slowness logs were first made by Pickett (Ref. 5) and were confirmed by Leslie and Mons (Ref. 6) on computer-processed sonic waveforms data. They showed that data from carbonates exhibit a constant ts/ tc ratio, with values around 1.9 in limestones and 1.8 in dolomites.
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