Elastic Impedance Inversion in Practice男性心理学
*Milos Savic, ARCO British Ltd., Bruce VerWest, ARCO British Ltd., Ron Masters, ARCO Exploration, Arcangelo Sena, ARCO Exploration, and Dean Gingrich, ARCO Alaska Inc
Rervoir characterization should integrate all available ismic, petrophysical and geological information into the volumetric distribution of rervoir properties like porosity, permeability and saturation. Each contributing piece of evidence is incomplete alone. Wells can measure many rervoir properties at high vertical resolution, but offer only spar sampling laterally, often at considerable expen. Seismic data provides nearly continuous lateral sampling at relatively low cost, but with much less vertical resolution. It also measures key rervoir properties less directly. Inversion for elastic impedance is the latest improvement in a continuing process of integrating ismic and well data for rervoir characterization.计算机系统的组成
Seismic inversion for acoustic impedance has been widely practiced in the industry. By "inversion", we mean converting ismic reflection amplitudes into impedance profiles. This involves removing the bandpass filter ("wavelet") impod by ismic acquisition and processing. Well control is ud to calibrate the scaling of the wavelet from ismic to well log units, and also to restore the low frequency
三年级手抄报component of the profile. Thus we recover an absolute measurement of impedance throughout the ismic data t, usually a 3D volume. The benefits are numerous:
•The broader bandwidth of the impedance data maximizes vertical resolution and minimizes tuning effects.
•Interpreting volumes rather than surfaces is more geologically intuitive. It simplifies lithologic and stratigraphic identification, and supports static rervoir models of any complexity.•Since the data is no longer zero-mean, the dynamic range in any given color display scale is more than doubled, increasing confidence in relatively subtle features.
•Calibrated ismic impedance predicts correlative petrophysical properties like porosity, clay content, and net/gross, throughout the ismic data volume.
入党志愿Before the advent of elastic impedance, this last benefit was diluted by a caveat. Seismic reflection amplitudes are dominantly, but not exclusively, generated by contrasts in acoustic impedance. The standard practice of calibrating stacked ismic data to an acoustic impedance log ignores AvO effects, risking failure and missing an opportunity. Normal incidence reflection of compressional plane waves depends only on contrasts in acoustic impedance--density times velocity. Conventional
multi-channel ismic data also records oblique reflections, which have been influenced by concurrent mode conversion from compressional to shear waves. Thus the amplitude variation with offt (AvO) exhibited by ismic reflection events depends on contrasts in shear velocity (Vs) as well as compressional velocity (Vp) and density. AvO is widely exploited as a direct hydrocarbon indicator and/or a rervoir prediction tool, becau rervoirs often have anomalous Vp/Vs ratios, and conquently anomalous AvO behavior. Failure to account for the effects can result in significant miss-ties between ismic and log data, and inaccurate impedance estimation, especially in rervoirs. Extracting both an acoustic impedance volume from normal incidence data and an analogous elastic impedance volume from wide-angle data offers an opportunity for an additional measurement, which can improve prediction of porosity and lithology. Elastic impedance is intuitively obvious, but elusive in practice. Just as contrasts in an acoustic impedance profile, convolved with a wavelet, generate normal incidence ismic data, there should be an "elastic impedance" profile who contrasts would generate wide-angle reflections. The problem is that there is no simple clod form expression for this quantity. There are different ways to approximate it. One is to integrate Aki and Richards' (1980) approximation for the Zoeppritz equation (Connolly, 1998 and Sena, 1997). The result is uable but unttling, with angle-dependent (or even depth-dependent) fractional units. We u an approximation derived from a different ries expansion of the Zoeppritz
equations (VerWest 1998, VerWest et al, 2000 this issue), which always has the same units as acoustic impedance.
Figure 1 shows a cross-plot of elastic impedance (vertical axis) vs. acoustic impedance (horizontal axis) for well A1. The color bar gives Gamma Ray values where blue and green colors reprent shales while red and yellow reprent sands (yellow - the cleanest sand). One can obrve that the cleanest rervoir sands have the same acoustic impedance (AI) range as some shales (around 22000 on horizontal axis). However, the same sands on elastic impedance (EI) axes have consistently lower value
than the corresponding shales. This means that one cannot distinguish shales from sands using only acoustic impedance (or equivalently the near reflectivity) but if one us both AI and EI simultaneously one can design a cut-off trend line that can help distinguish sands from shales.
Figure 1 Cross-Plot of Acoustic Impedance versus Elastic Impedance for well A1. Acoustic Impedance is on horizontal axis (acimp) and Elastic Impedance is on vertical axis (Impedance). The log display on the right features Gamma Ray log in blue, AI log in dark grey, and EI log in red.
Once the elastic well logs are available for well ties and wavelet estimation, ismic inversion of far a
ngle data can be carried out with any standard commercial tool available.
Figure 2 shows result of Acoustic Impedance inversion around one of the wells from Alpine field. In this ca near ismic data is tied to the acoustic impedance log and resulting wavelet ud for inversion.
Figure 3 shows result of elastic impedance inversion. In this ca far ismic data is tied to elastic impedance at 30degrees.
Figure 4 shows the Net Pay map estimated from elastic impedance data. More about u of elastic impedance for Net Pay prediction can be found in Gingrich et al, 2000 (this
issue).
Figure 2 Detail of Acoustic Impedance volume.
Figure 3 Detail of Elastic Impedance volume at the same location.
Conclusions
•Inversion of near-angle ismic data
to Acoustic Impedance and
inversion of far-angle data to Elastic
Impedance was successfully carried
带洋的成语
out.
•Inverted Acoustic Impedance as
well as Elastic Impedance gives
very good tie to the well log derived
Acoustic Impedance and Elastic
Impedance.
•The net-pay map bad on Elastic
平阴玫瑰花推进项目Impedance ties most of the wells.
The net-pay prediction deteriorates
in the vicinity of faults.
•This new integrated ismic
rervoir study will help reduce
production drilling costs by an
estimated $30,000,000 at Alpine.References
Aki, K., and Richards, P.G., 1980, Quantitative Seismology: Theory and Methods, W.H. Freeman and Co.
Connolly, P., 1999, Elastic Impedance, The Leading Edge, April Issue, 438-452.
Sena, A., 1997, Far Angle Stack Seismic Inversion: A Feasibility Study, AEPT Rearch Memorandum 97-0013.
VerWest, B., 1998, Elastic Impedance for the Inversion of Far Offt Seismic Sections, ARCO U.S. Patent Notes.
Gingrich, D., Hughes, D., Kerr, D., Savic, M., Hannon, R., Knock, D., and Sena, A., 2000, Alpine Oil Field: Geophysics from wildcat to development, SEG E. Abstr., Calgary, Canada.
VerWest, B., Masters, R., and Sena, A., 2000, Elastic Impedance Redefined, SEG E. Abst., Calgary, Canada
奶骑手法Acknowledgement
Authors would like to express their gratitude to ARCO Alaska Inc. and Anadarko Petroleum Corporation for permission to publish this project.
