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The AAPG/Datapages Combined Publications Database
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Abstract
Prediction of Porosity in Compacted Sands
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:
where porosity () 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:
where z = effective burial depth (in meters); z = burial depth (in meters); r = density of rock (in Kgm-3 [kilograms per cubic meter]) = typically 2650; w = density of water (Kgm-3) = typically 1050; g = gravity (in ms-2 [meters per second squared]) = 9.8; = 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
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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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