Groundwater and Aquifers Flashcards
Including porosity, permeability and the flow of fluids in rocks
Define porosity
Porosity is the amount (volume) of pore space in a rock or sediment
Expressed as a percentage of total rock volume
What is the formula for porosity?
% porosity = total volume of pore space / x100
total volume of rock or sediment
The total volume of pore space can be calculated by weighing a rock dry, then weighing it once it has been submerged underwater, this is as 1cm³ of water weighs 1g
Define permeability
Permeability is the rate at which a fluid (usually water, oil or gas) flows through a rock
What is the formula for permeability?
Permeability = distance fluid has travelled /
time taken
Primary porosity
Is the pore spaces at the time the sediments were deposited
Secondary porosity
Develops after in the rock, and includes cavities or fractures that may have formed
Effective porosity
Total porosity minus any cement and this contributes to fluid flow through the rock.
Clean sands have an effective porosity that is almost equal to the total porosity
Clays though having high porosity (near 50%) have an effective porosity of nearly 0.
How does sorting affect porosity?
Well sorted = high porosity
Poorly sorted = lower porosity (as finer grains fill in the spaces between the coarser grains)
How does diagenesis affect porosity?
Is there a trend with depth?
Loose unconsolidated rock has higher porosity than a rock that has undergone compaction and cementation (due to diagenesis)
Because of this there is a trend with porosity decreasing with depth
How does grain shape affect porosity?
Rocks containing rounded grains have a higher porosity than rocks containing angular grains (as long as these grains fit together)
How does the packing of grains affect porosity?
Cubic packing = increased porosity
Rhombic packing = decreased porosity
Depth is also important here, as compaction can occur due to overburden pressure which causes particles to pack closer together
How does grain size affect porosity?
It doesn’t
Coarse-grained rocks tend to have larger pore spaces, but fewer of them
Whereas fine-grained rocks have many small pore spaces (of which many are not connected to one another)
What other factors can affect porosity?
The presence of geological structures such as joints and faults
Through the dissolution or alteration of mineral grains and/or cements to create caves or voids
Both of these are especially important in limestones
How does porosity affect permeability?
A rock that has a high effective porosity with good interconnections between poor spaces will also have a high permeability
How grain size affect permeability?
Coarse-grained rocks = higher permeability
This is as there is less resistance to flow around the coarse grains than around fine grains
How does temperature affect permeability?
It can affect the viscosity of a fluid (such as oil), usually, viscosity decreases with increasing temperature
What is secondary permeability? How does it affect permeability?
Develops from the presence of geological structures such as joints, faults, voids and caves
(similar to secondary porosity)
What is capillary pressure?
How does it work? (Think oil and water)
The force that exists across the interface which separates two immiscible fluids (like oil and water).
In a rock there is a balance between the attractive force (surface tension) and adhesive (wettability) force between each fluid and the rock
Before a non-wetting fluid can displace a wetting fluid the pressure must exceed this threshold
THINK: As hydrocarbons begin to migrate into a rock they displace the pore water (as oil is less dense than water).
They first enter pores with the largest entrance, leaving connate water in the pores with smaller entrances / those with smaller nooks and crannies
As the hydrocarbon level increases, the capillary pressure increases, this forces hydrocarbons to enter smaller pore spaces and displace water in those pores
What is connate water?
Water trapped in the pores of a rock as the rock formed
It includes water trapped in the original sediment and water released during diagenesis
