Fluid In Rocks Flashcards
What is porosity?
The volume of pore space.
The amount of free space in a rock which allows water storage. Usually expressed as a percentage of total rock volume
What can a reduction in porosity result in?
Fluid escaping out of the rock
What is the formula for calculating percentage porosity?
% porosity = total volume of pore space / Total volume of rock or sediment
then x100
How do interlocking crystals affect porosity?
There will be no porosity at all
What is permeability?
Ability of rock or sediment to transmit fluids such as water, oil or gas.
Can be expressed as a rate of flow of a fluid through a rock or sediment.
Rate at which fluid flows through a rock.
What is the formula for permeability?
permeability = distance fluid travelled / time taken.
measured in units for velocity e.g. m/s, m/h, mm/s etc - interchangeable for whatever is suitable
What are the factors affecting porosity? (5)
Amount of diagenetic processes,
Interlocking crystals,
Degree of sorting,
Roundness,
Packing of grains
How does the amount of diagenetic processes affect porosity?
Compaction and pressure makes the porosity less depending on how much diagenesis.
Pressure dissolution can produce a liquid that fills pores, e.g. quartz cement
How does interlocking crystals affect porosity?
Most igneous and metamorphic rocks are not porous (excluding pumice). No pore spaces, no room for water
How does the degree of sorting affect porosity?
Well sorted rocks have higher porosity.
Poorly sorted rock pores are filled in with smaller grains (less porosity)
How does the roundness of grains affect the porosity?
Angular grains can fill in pore spaces. More rounded grains have more porosity
How does the packing of grains affect the porosity?
Caused by compaction.
Grains may have moved to fill in pore spaces.
What is secondary porosity?
Includes cavities or fractures that have formed. Geological formations such as joints and faults affect this.
Also be caused if grains dissolve or alter grains or cement or both
What are the factors affecting permeability? (3)
Porosity, grain size, temperature
How does porosity affect permeability?
Pores have to be connected.
If a rock is not porous, it cannot be permeable
How does grain size affect permeability?
Coarse grains have higher permeability than fine grains due to less resistance to flow
How does temperature affect permeability?
Rocks expand in heat, causing pores to get smaller.
However increase temps also can increase flow because it decreases the liquid’s viscosity
What is secondary permeability?
Results from the presence of fractures, faults, voids or caverns than connect
What is capillary action in rocks?
Pressure between two immiscible fluids in narrow pore spaces, resulting from interactions of forces between the fluids and solid grains.
Fluids can be attracted to grain surface, and also each other.
Different fluids have different levels of attraction to a surface or itself.
Therefore, one fluid can be displaced by another
What are some examples of rocks with high porosity?
Clay, pumice, sand, chalk
What are some examples of rocks with low porosity?
Granite, shale, basalt
What are some examples of rocks with high permeability?
Sand, chalk
What are some examples of rocks with low permeability?
Clay, shale, granite, pumice, limestone
What is infiltration?
Above ground to below ground water movement (soil)
What is percolation?
Movement underground through permeable rock
Where is the water table?
The layer between the saturated and unsaturated zones.
What is hydrostatic pressure?
The pressure at a point in a body of water due to the mass of the overlying column of water. e.g. pressure that builds as you go deeper into oceans (but can’t use this example)
What is hydrostatic head?
The height of the water level above. Is a measurement typically in metres
What is hydraulic gradient?
The difference in hydrostatic pressure or head between two points, divided by distance between them.
Difference in depth divided by horizontal distance
What is an aquifer?
A body of permeable rock which can contain or transmit groundwater
What is an unconfined aquifer?
Permeable rock above. Is open to the atmosphere pressure, and is recharged by rainwater from directly above. Water will be pumped to the surface from a well or borehole sunk into an unconfined aquifer
What is an aquitard?
Rock that has low permeability. It allows transmission of water but at low rates
What is a confined aquifer?
Overlain by impermeable rocks and the groundwater held withing it is under hydrostatic pressure. Groundwater can only be replenished in a confined aquifer if it has recharge zones that are open to the atmosphere
What is a perched aquifer?
Sits above the regional water table and is underlain by a lens of impermeable rock which prevents the water from percolating downwards
What is an aquiclude?
Impermeable rock that does not transmit water
What is a live aquifer?
An aquifer currently being recharged/replenished by rainwater via a recharge zone on the surface
What is a fossil aquifer?
No longer being replenished and represent a relic of a past wetter climate
What is an artesian basin?
E.g. london basin.
A low lying region where groundwater is cramped under pressure from surrounding layers of rock.
Kinda looks like a syncline, clay sandwich with permeable rock between
What is an artesian well?
A water well that doesn’t require a pump to bring water to the surface. Pressure/capillary action pushes water to the surface
What is a piezometric surface?
The layer at which water will rise in a well or piezometer that tap a confined aquifer. The level pressure pushes water too (above water table)
What is a spring?
Occurs when water table intersects land surface, and groundwater flows onto the surface. Often a junction between permeable and impermeable rock. We often see a spring line
Why may a spring flow intermittently?
The water table may rise and fall
Where are the three types of springs?
Lithological spring (2 of these)
At a fault
An uncomformity
What are lithological springs?
There’s two.
In a valley, where permeable rocks and water table reach permeable rock.
Where permeable rock meets a dyke (impermeable)
What causes springs at a fault?
Fault causes impermeable rock to rise, making permeable rock reach impermeable rock, causing a spring
What causes springs at an unconformity?
Where the permeable rock meets an unconformity, there’s a spring
What is a seep?
Sometimes rock has oil. The oil (hydrocarbons) migrate upwards, because it is displaced by water in the rock
Why is there a seep at Kimmeridge Bay, Dorset?
Rocks here composed of highly organic, black mudstones with slate-like bedding. Rocks called kimmeridge clay formations and formed in kimmeridge age (157-152 Ma). Rock acts as the source rock for much of UK’s oil reserves under the north sea.
Rocks are significantly folded and laminated layers of shale
What is connate water?
Water included in a sediment when it is deposited, trapped in the pores of the sediment
What can happen when connate water and its solutes during diagenesis?
Water is usually saline with Sodium and Calcium chlorides. Brines may be richer in silica and bicarbonate than sea water.
During diagenesis, connate waters are often expelled. Some solutes may be precipitated in the pores
How does connate water affect porosity and permeability?
After diagenesis processes.
Reduces porosity and permeability. Make the rocks less suitable as a reservoir rock or aquifer
What is abstraction?
Extracting water from aquifer
What are the three main problems caused by groundwater abstraction?
Over extraction, subsidence, salt water encroachment
How does over extraction cause problems in aquifers?
This lowers the water table, creating cones of depression. Results in loss of springs, meaning loss of surface water.
Also results in loss of habitats, particularly freshwater.
This causes problems with wells as existing access to water may not be deep enough
How is subsidence a problem caused by groundwater extraction?
Water extraction means pore spaces are empty. Then compaction and collapse, Vertical downward movement of soil and rocks above.
Once the pore spaces have compacted, it cannot be reversed so no new aquifer is possible.
This is the biggest issue around buildings and infrastructure.
This sinking is uneven, e.g. Tower of Pisa
How is salt water encroachment a problem caused by groundwater extraction?
Water is over extracted from aquifer but near the sea. Salt water can then fill the aquifer.
Once contaminated, it limits water usage for drinking, infrastructure or agriculture.
Residence time. Average aquifer is 10s thousands of year, meaning it will be contaminated for a long time
What are the three main threats to groundwater supplies?
Pollution, overpumping, soluble materials in the water
How is pollution causes a threat to groundwater supplies?
Pollution enters the groundwater and is difficult to clean. Unconfined aquifers are more at risk - more porous rocks at surface.
Nitrates, pesticides, agricultural runoff, sewage, toxic fluids from landfill sites, hydrocarbons & solvents from petrol + factories all pollute groundwater.
Acid mine draining has toxic metals, e.g. lead.
How are soluble materials in the water a threat to the groundwater supply?
Salts or ions. Hard water has Ca2+ and Mg2+ ions.
Some groundwater has dissolved fluoride. This reduces tooth decay, but too much can lead to dental fluorosis.
Some have high levels of arsenic, e.g. India. Which is unfit for human consumption.
What is Hydraulic Gradient?
The change in total head (vertical movement) divided the distance over which the change occurs (horizontal movement)
What is the Hydraulic Gradient equation?
(h2-h1) / length
Difference in height / difference in length.
What are the units for hydraulic gradient?
There are no units.
What is Darcy’s law?
A method of calculating the permeability of a rock
What is the equation for Darcy’s law?
Q = -KA((h2-h1)/L)
Q: Flow rate, m^3/sec
K: Coefficient of permeability, m/sec (these -ve cancel out)
A: The cross-sectional area, m^2
(h2-h1): the Hydraulic gradient
