Glossary - P - permeability
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In geology, permeability is a measure of the ability of a material (typically, a rock or unconsolidated alluvial material) to transmit fluids through it. It is of great importance in determining the flow characteristics of hydrocarbons in oil and gas reservoirs, and of groundwater in aquifers. The usual unit for permeability is the darcy, or more commonly the milli-darcy or md (1 darcy = approx 10−12m²).
Permeability is part of the proportionality constant in Darcy's law which relates discharge (flow rate) and fluid physical properties (e.g. viscosity), to a pressure gradient applied to the porous media. The proportionallity constant specifically for the flow of water through a porous media is the hydraulic conductivity; permeability is a portion of this, and is a property of the porous media only, not the fluid. In naturally occurring materials, it ranges over many orders of magnitude (see table below for an example of this range).
| Relative Permeability | Pervious | Semi-Pervious | Impervious | ||||||||||
| Aquifer | Good | Poor | None | ||||||||||
| Unconsolidated Sand & Gravel | Well Sorted Gravel | Well Sorted Sand or Sand & Gravel | Very Fine Sand, Silt, Loess, Loam | ||||||||||
| Unconsolidated Clay & Organic | Peat | Layered Clay | Fat / Unweathered Clay | ||||||||||
| Consolidated Rocks | Highly Fractured Rocks | Oil Reservoir Rocks | Fresh Sandstone | Fresh Limestone, Dolomite | Fresh Granite | ||||||||
| κ (cm²) | 0.001 | 0.0001 | 10−5 | 10−6 | 10−7 | 10−8 | 10−9 | 10−10 | 10−11 | 10−12 | 10−13 | 10−14 | 10−15 |
| κ (milliDarcies) | 10+8 | 10+7 | 10+6 | 10+5 | 10,000 | 1,000 | 100 | 10 | 1 | 0.1 | 0.01 | 10-3 | 10-4 |
Source: modified from Bear, 1972
Permeability of Aquifer Materials and relation to drawdown cone
Pumping rates are important for efficient use of aquifers, and knowledge of aquifer material is important. This is the case for the Lockyer valley alluvial aquifer, and it is important to know what material the bore is screened in.
This is shown in the figure comparing three aquifer materials at the same pumping rate.
Coarse sand and gravels: highly transmissive and water is released easily - little drawdown, and cone will spread out; could sustain pumping at a higher rate.
Fine sand: moderately transmissive, steeper gradient and drawdown.
Sandy silt: quite low permeability, low transmissivity, needs steep gradient to release water, so long drawdown, but less spread of cone. Can easily reach maximum production rate.
example of different materials with constant pumping rate:
- slope of cone: by permeability and quantity of water removed
- depth of cone: by pumping rate
- radius of cone: by time since pumping began