Shear Sonic Interpretation in Gas-Bearing Sands

Abstract

fax 01-214-952-9435. ' AbstractThe 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 i1tc 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 VpNs crossplot, gas-bearing formations clearly differentiate from liquid tilled 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 VpNs 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.

Estimation of rock physics properties from seismic attributes — Part 1: Strategy and sensitivity analysis

“…For water saturation less than 99.9%, the harmonic average gives underestimated results. For this reason, Brie et al (1995) formulate the effective K f as…”

“…4) The pores are saturated with one single (or one effective) fluid phase. Domenico (1976) and Brie et al (1995), for instance, use simple averages, partly empirical, to compute the properties of the effective fluid. However, the patchy saturation theory (White, 1975;Dutta and Odé, 1979) makes it possible to take into account the spatial distribution of each fluid phase, which modifies seismic waveforms (dispersion and attenuation).…”

Petrophysical evaluation of well log data and rock physics modeling for characterization of Eocene reservoir in Chandmari oil field of Assam-Arakan basin, India

“…The density mix is straightforward using the volume The acoustic properties are mixed using Brie's formula (Brie et al 1995) to account for the variation in distribution of the different fluids within the pore space,…”

Co2 Capture Project - An Integrated, Collaborative Technology Development Project for Next Generation Co2 Separation, Capture and Geologic Sequestration

“…However, we found that to fit the observed velocity in areas where the gas saturation was non-zero, the gas effect had to be reduced. The overestimation of the gas effect on fluid bulk modulus by the Wood's mixing law, Equation (6), has been observed by Brie et al (1995). A better match between predicted and observed velocity could be achieved by a simple correction to the gas term in Equation (6) (1)- (5), (7), (8), and (10), with constants listed in Table 2, to calculate time-lapse changes in reservoir parameters.…”