Sonic logging

Abstract: None of the standard porosity-velocity models (e.g. the time-average equation, Raymer's equations) is satisfactory for interpreting well-logging data over a broad depth range. Clays in the section are the usual source of the difficulty through the bias and scatter that they introduce into the relationship between porosity and P-wave transit time. Because clays are composed of fine sheet-like particles, they normally form pores with much smaller aspect ratios than those associated with sand grains. This difference in pore geometry provides the key to obtaining more consistent resistivity and sonic log interpretations. A velocity model for Clay–sand mixtures has been developed in terms of the Kuster and Toksoz, effective medium and Gassmann theories. In this model, the total pore space is assumed to consist of two parts: (1) pores associated with sand grains and (2) pores associated with clays (including bound water). The essential feature of the model is the assumption that the geometry of pores associated with sand grains is significantly different from that associated with clays. Because of this, porosity in shales affects elastic compliance differently from porosity in sand-Stones. The predictive power of the model is demonstrated by the agreement between its predictions and laboratory measurements and by its ability to predict sonic logs from other logs over large depth intervals where formations vary from unconsolidated to consolidated sandstones and shales.

Abstract: 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 frommore » 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.« less

Abstract: A series of experiments to determine the elastic properties of a sequence of saturated sedimentary rocks over as wide a frequency range as possible was carried out at the Imperial College borehole test site. These experiments fall into four categories: vertical seismic profiles (VSPs) within the frequency range 30-280 Hz, crosshole surveys (0.2-2.3 kHz), sonic logging (8-24 kHz), and laboratory measurements (300-900 kHz). The intrinsic attenuation and velocity of compressional and shear waves were measured whenever possible. Velocity dispersion is observed for both compressional and shear waves. The intrinsic attenuation of compressional waves is frequency dependent with a peak in the attenuation in the sonic frequency band. The data were modeled assuming the attenuation is caused by local fluid flow in pores of small aspect ratio. The modeling indicates that the intrinsic attenuation may be dominated by cracks with aspect ratios of around 10 (super -3) to 10 (super -4) .

Abstract: The velocity-deviation log, which is calculated by combining the sonic log with the neutron-porosity or density log, provides a tool to obtain downhole information on the predominant pore type in carbonates. The log can be used to trace the downhole distribution of diagenetic processes and to estimate trends in permeability. Laboratory measurements on over 300 discrete carbonate samples reveal that sonic velocity is a function not only of total porosity, but also of the predominant pore type. In general, there is an inverse porosity-velocity correlation, but significant deviations occur from this relationship for certain pore types. Frame-forming pore types, such as moldic or intrafossil porosity, result in significantly higher velocity values at equal total porosities than do pore types that are not embedded in a rigid rock frame, such as interparticle porosity or microporosity. The results of the laboratory measurements can be applied to expand interpretations of standard wireline-log data, as shown in this study on two drill holes through Neogene carbonates from the Great Bahama Bank. The velocity-deviation log is calculated by first converting porosity-log data to a synthetic velocity log using a time-average equation. The difference between the real sonic log and the synthetic sonic log can then be plotted as a velocity-deviation log. Because deviations are the result of the variability of velocity at a certain porosity, the deviation log reflects the different rock-physical signatures of the different pore types. Positive velocity deviations mark zones where velocity is higher than expected from the porosity values, such as zones where frame-forming pore types dominate. Zero deviations show intervals where the rock lacks a rigid frame, such as in carbonates with high interparticle porosity or microporosity. Negative deviations mark zones in which sonic log velocities are unusually low, caused, for instance, by a cavernous bore-hole wall, fracturing, or possibly by a high content of free gas. By tracing the velocity deviations continuously downhole, one can identify diagenetic zones that are characterized by these different pore types. In addition, this method can be used to observe permeability trends because pore types influence the permeability of the rock.