A Technique to Estimate P-wave and S-wave Q from Full Waveform Sonic Logs with Application to Acoustic/Seismic Data from South Florida, 14-9345Printer Friendly Version
Inclusive Dates: 08/15/02 - 02/15/04
Background - Wave attenuation is a power attribute that can be used as indicators of lithology, pore structure, fractures, and clay and fluid content in a reservoir characterization program. Because acoustic attributes from full waveform sonic logs can be easily related to petrophysics and core data in a single borehole, the intrinsic and scattering effects at the borehole scale can be determined. Since the early 1980s, various researchers have attempted to develop techniques to determine Q from single-hole full waveform acoustic log data. It is, however, commonly recognized that existing Q processing techniques have major limitations and pitfalls. In particular, these techniques have serious difficulties when the lithology and acoustic properties are highly heterogeneous in the depth direction. Unfortunately, in the sonic signals, the influence of the formation heterogeneity is coupled with the interaction among internally reflected rays of the borehole head wave. In the Hillsboro site aquifer, South Florida, significant changes are observed often in the scale of a foot. This formation is carbonate rock with separated and interconnected vugs. It is desirable to obtain Q from the sonic log of this formation and determine whether Q can be used to assess the pore structure of the vuggy carbonate aquifer. The objective of this work is to develop a processing technique to obtain depth profiles of intrinsic Q from full waveform borehole monopole and dipole sonic logs. The processing package is aimed to be automatic, user-friendly, efficient, and reasonably accurate.
Approach - The technique currently makes use of the head P wave but can be extended to head S wave and guided waves of the sonic log. We require that the borehole sonic data be obtained with controlled gains in a multi-offset source-receiver configuration. The offsets between receivers range from half a foot to several feet, which allow us to achieve low, medium, or high spatial resolution. We use relatively short time windows in which the first one to two wavelets of the head P waves is gated. In particular, we use two different but related ray models to analyze and invert sonic signals. One is a modified ray tracer for layered formation. A conventional ray tracer takes care of the amplification, reduction, and scattering due to heterogeneity. Our ray tracer additionally takes into account geometrical spreading of the head wave. The other is a borehole ray model where the interaction among internally reflected rays with various arrival times is considered. As a result, we are able to separate, correct for and normalize the effects of borehole, geometrical spreading and layering simultaneously. Finally, we calculate the Amplitude Spectral Ratio at properly defined peak areas of the spectrum of the corrected and normalized data to obtain Q for each specified depth.
Accomplishments - The algorithm was tested using sonic data from a south Florida aquifer. The QP log extracted from this data shows that attenuation is sensitive to primary and secondary porosities of a limestone zone in the aquifer. The attenuation (QP-1) relates to crack, vuggy, and matrix porosities, a concept that is supported by statistical correlations and rock physics (see Figure below). The matrix porosity, with the Vp and Vs matrix properties obtained from core data, allows us to calculate the matrix bulk and shear moduli. Analysis of the bulk modulus, primary and secondary porosities, and attenuation provides a framework with which to characterize the pore structures of the different petrophysical units. The vuggy carbonate interval has zones with quality factor Q = 25-50, which correlates with the high matrix bulk modulus that represents the more stiff pore structure. The high permeability stiffer porous matrix allows water to flow between the interconnected vugs and the matrix. The chalky carbonate unit below the vuggy carbonate has a Q = 12.5 that is associated with soft matrix pores and cracks. A similar low Q is observed in the sandy limestone unit above the vuggy carbonate zone. The physical characteristics of chalky and sandy carbonates, which correspond to high matrix porosity and low density, yield lower matrix bulk modulus. A low permeability, muddy, porous matrix with soft pores forms the chalky limestone, and a soft porous matrix forms the sandy carbonate. The significance of this application is the characterization of flow units using Q logs. This information at the borehole scale can be useful for defining flow units at the field scale using seismic techniques.