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The AAPG/Datapages Combined Publications Database

AAPG Special Volumes

Abstract


AAPG Memoir 69: Reservoir Quality Prediction in Sandstones and Carbonates, 1997
Edited by J. A. Kupecz, J. Gluyas, and S. Bloch
Pages 19-27

Prediction of Porosity in Compacted Sands

Jon Gluyas,1, Christopher A. Cade

ABSTRACT

We present a new porosity-depth relationship for clean, rigid grain (quartz, feldspar) sands under hydrostatic burial. This allows the prediction of porosity in uncemented sandstones to an accuracy of ±2.5 porosity units at 95% confidence levels. The relationship was derived using experimental data from laboratory compaction experiments and field data for buried uncemented sandstones from around the world. The equation is:

0019_u1.jpg (3,479 bytes)

where porosity (phi.gif (848 bytes)) is in percentages and depth (z) is in meters.

By scaling this relationship in terms of effective stress rather than depth, it can be used to provide an equally accurate prediction of porosity for uncemented sands in overpressured settings. This is done using the following equation:

0019_u2.jpg (3,128 bytes)

where zprime1.gif (824 bytes) = effective burial depth (in meters); z = burial depth (in meters); rho.gif (842 bytes)r = density of rock (in Kgm-3 [kilograms per cubic meter]) = typically 2650; rho.gif (842 bytes)w = density of water (Kgm-3) = typically 1050; g = gravity (in ms-2 [meters per second squared]) = 9.8; phi.gif (848 bytes)sigma.gif (843 bytes) = average porosity of overburden = typically 0.2; and u = overpressure (in MPa [megapascals]).

We propose that there is considerable value in a "compaction only" porosity-depth relationship. A compaction-only trend allows the accurate prediction of porosity in uncemented sandstones, and gives a maximum porosity baseline to which cement volumes, and resultant cemented sandstone porosities, can be compared. If both cemented and uncemented sandstone data are included to produce a "porosity loss-depth" relationship, the resultant scatter (typically ±5 porosity units for a given depth) in the relationship limits its usefulness.

1 Present affiliation: Monument Oil and Gas, London, United Kingdom

End_Page 19-------------------------

Prior to drilling, the new relationships may be used either to predict the porosity of sands that are known to be uncemented or to place an upper limit on the porosity estimated for sandstones either known or suspected to be cemented.


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