Download Porosity (2/7/05)

Aquifer
• Properties: Porosity,
specific yield, specific
retention.
• Potential: Transmissivity,
storativity.
• Types: confined,
unconfined.
• Hydraulic conductivity,
Physical Laws controlling
water transport.
Porosity
• Porosity is percent of rock or soil that is
void of material.
• n = 100 Vv/V
• n = porosity (percentage)
• Vv = volume of void space (L3, cm3, m3)
• V = unit volume of material including voids
and solids.
How to measure porosity
Density of a fluid
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ρ = m/v (M/L3)
m = mass (M)
v = volume (L3)
Total Porosity
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n = 100 [1 – (ρb/ρd)]
n = porosity (percentage).
ρb = bulk density of aquifer material.
ρd = particle density of aquifer material.
Factors affecting porosity
• Packing
• Grain-size distribution - sorting
Grain packing, grain size, and
porosity
• Cubic packing: n = 47.65%
• Rhombohedral packing: n = 25.95%
• Grain size mixture => porosity lowered.
Processes affecting grain sorting
• Well-sorted sediments
=> wind, running water, wave action …
=> higher porosity.
• Poor sorting
=> glacial action, landslides …
=> lower porosity.
• Grain shape
=> sphere, rod, disk, book …
Sediment Classification
• Sediments are classified on basis of size of
individual grains
• Grain size distribution curve
• Uniformity coefficient Cu = d60/d10
• d60 = grain size that is 60% finer by weight.
• d10 = grain size that is 10% finer by weight.
• Cu = 4 => well sorted; Cu > 6 => poorly
sorted.
d60
d60
d10
d10
d60
d10
d60d10
Porosity of sedimentary rocks
• Diagenesis => compaction, removal or
addition of material, transformation of
minerals by replacement or phase change.
Porosity of sedimentary rocks
• Diagenesis => compaction, removal or
addition of material, transformation of
minerals by replacement or phase change.
• Primary porosity – pores between grains.
• Secondary porosity – fractures in rock.
• Clastic rocks – n = 3% - 30%.
• Limestones and dolomites – n = 1% - 30%.
Porosity of plutonic and
metamorphic rocks
• Weathering and fracturing – increase overall
porosity.
• Tectonic stresses - fracturing, faulting,
folding.
• Fracturing increases porosity by 2% - 5%.
• Weathered plutonic rocks can have
porosities of 30% - 60%.
Depth 100 (meters)
Porosity of volcanic rocks
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Degassing products –> holes in the rock.
Shrinkage cracks -> joints
Basalt – 1%-12%.
Pumice – glassy rock formed from a magma
with high gas -> can be as high as 87%.
• Pyroclastic deposits – Tuff – 14% - 40%
• Ash – 50%.