Bob Seymour
(Dr Robert H Seymour, Seismic Petrophysics)

Thesis – The effects of stress on the seismic velocities of reservoir rocks. (pdf, 5 Mb)

See illustrated abstract, below.

For Appendix (flow diagram for model algorithm) email bob@bobseymour.net.

For spreadsheet implementation of model (not free!) give affiliation and expected usage.

Most recent publications:

Prediction of shear wave velocities in multi-porosity carbonate reservoirs using effective medium theory, R. H. Seymour, R. Lubbe and J. Yan, Research Workshop W2, SEG Annual Meeting, San Antonio, Texas, September 2007.

A Crack Closure Explanation for the Exponential Variation of Velocity with Pressure in Reservoir Rocks, R. H. Seymour and R. W. Zimmerman, PETEX 2004, London, November 2004.

Multiple porosity Differential Effective Medium models: improvements and approximations. R.H. Seymour, R.W. Zimmerman and R.E. White, EAGE Conference, Stavanger, June 2003.

(Full publication list: press here)

Biography

Now retired.

Seismic petrophysicist (consulting), 2005-2008

PhD (Supervisor Dr Robert W Zimmerman), Imperial College, 2002-2005

Time-lapse seismic ("4D") specialist, 1988-2001

MSc (Petroleum Engineering), Imperial College 1986-1987

Exploration & research geophysicist, 1963-1986

Thesis abstract, May 2005 (illustrated)

An improved petrophysical model has been developed to predict seismic velocities, primarily for consolidated sandstones. The new model includes the effects of stresses on velocity, as well as the effects of porosity, lithology and saturation.

Such a model is needed to help in the interpretation of well logs and seismic surveys, especially time-lapse repeated seismic surveys.

For the model, a new theory was developed to link the commonly-observed form of pressure-dependent seismic velocity variations, with hypothetical microcrack population characteristics. The derived forms of the microcrack aspect ratio spectra (exponentially declining crack density) were confirmed by the results of inversion work using published velocity data.

The specific forms of the implied aspect ratio spectra, together with the elastic properties of cracks, were combined to demonstrate the existence of a "characteristic" or "effective" crack aspect ratio for each rock sample, which remained almost constant under variations of pressure. This gave a rock mechanics foundation to a finding by other workers, namely that excess compliances were often proportional to excess soft porosities.

It was found that Gassmann’s theory was accurate for saturated clay and sand porosity components, but that microcracks were better modelled as being isolated. In the new model, the contributions from isolated and equilibrating theories may be varied continuously in different classes of pores, as required.

The model also benefited from a number of other improvements. One was the introduction of near-symmetry among the various classes of porosity, by adding pore increments of each type in rotation.

Parameterized microcrack aspect spectra were estimated using the theory, with published velocity data, for each of about two hundred core samples. The parameters correlated only weakly with the median grain sizes and grain volume fractions in the rocks.

The modelled velocity predictions compared very well with published laboratory dry and saturated rock velocity measurements.

[ref: Seymour, R. H., 1992, Interpretation of 3-D Seismic Reservoir Monitoring Results, SEG 62nd Conference, New Orleans, Paper W-2.4.]

[illustrations in this document are greatly reduced versions of those in the full thesis.]