4.2 Earth Resources and Engineering Flashcards
What does a hydrogeologist have to do in order to ensure a reliable supply of water?
- asses geological factors affecting siting of reservoirs and dams
- identify underground supplies of water in aquifers
When water reaches the surface as rain, what can happen to it?
- run off over surface
- evaporate into atmosphere
- percolate downwards as groundwater
What factors does the passage of water into rocks depend on?
- porosity
* permeability
When is a rock porous?
If its grains have spaces between them that can hold water, or if there are other features in which water can be held
What type of rock is usually more porous?
Sedimentary
When is a rock permeable?
If water can travel freely from its upper surface to its lower surface
What type of rock is usually more permeable?
Sediments with larger, not so compacted grains
Texture of conglomerate rock?
- individual grains - clastic
- poorly sorted
- rounded, coarse grains
Texture of granite?
- crystalline
* coarse grained
Texture of desert sandstone?
- clastic - medium grained
- well sorted
- well rounded clasts
Texture of oolitic limestone?
- crystalline and clastic (individual oolites)
* medium sized oolites - well sorted
Practical to investigate porosity?
- weigh rock
- rock put into container and water poured over
- rock removed from water and weighed again - difference is amount of water it has absorbed
- rock put into 200cm3 water - record difference for volume
How to calculate percentage porosity?
Percentage porosity = (difference in weight / volume of water rise) x 100
Which rocks are more porous; clastic or crystalline?
Clastic
Factors for a good dam location?
- high rainfall
- flow of water into reservoir
- steep sided valley
- low construction cost
- impermeable rock to prevent water loss downwards
- little/no faulting to cause weaknesses
- stable slopes to prevent landslides
- no earthquake activity
What is the easiest way of storing water?
Building a dam and letting streams and rivers fill up the area behind the dam to form a reservoir
Is shale permeable?
No, it is impermeable
What is the water table?
The boundary between the saturated (where porous rocks have become totally saturated with water) and unsaturated rocks
What does the water surface come to the surface of the earth in the form of?
Rivers, springs and lakes
What is an aquifer?
A layer of porous rock saturated with water and underlain by an impermeable layer
What is a cuesta?
A hill with one steep side and one gentle side
What happens if too much water is taken from an aquifer?
The wells may dry up, or the water level in them might fall significantly
How is a cone of depression formed?
From taking too much water from a well
What is an artesian basin?
Where water is obtained under pressure from the porous rock lying between the impermeable rock - water rises automatically to the surface due to the pressure it is under from the overlying rocks
What does petroleum include?
- crude oil
- natural gas
- tar
What does petroleum form from?
- decomposition of tiny plankton
- die and sink to sea bed and lack oxygen - decomposed to form sapropel
- increased pressure and converts organic remains to hydrocarbons
What is sapropel?
Black, organic mud
What is the rock called in which the petroleum forms?
Source rock
What type of rock does petroleum usually form in?
Black mudstone
What happens when the source rock of petroleum is compacted?
The petroleum is squeezed out and migrates upwards through the pores and joints in the overlying rocks
What are reservoir rocks?
Rocks that oil or gas may collect in
What are the most common reservoir rocks?
Sandstone and limestone
What must be true for petroleum to remain in the reservoir rock?
It must be sealed by an overlying rock - through which the petroleum cannot pass
What is the overlying rock that often overlies reservoir rocks?
Shale or hailte
What are traps?
When reservoir rocks are part of a structure that can contain the petroleum
Types of petroleum trap?
- anticlinal trap
- fault trap
- unconformity trap
- salt dome trap
What might be the cause of leakages in petroleum traps?
Up faults or along unconformities
How are petroleum reserves found?
- study of surface geology to see if there are structures that might contain petroleum
- geophysical methods - to find deeply buried structures e.g. seismic survey
How can seismic surveys find petroleum reserves?
- seismic waves are generated by detonation of explosives in shallow holes or by vibroseis trucks
- waves bounce off underlying rock
- when they return to surface, they are received by geophones - record similar to seismograph obtained5
What must be drilled where a possible petroleum trap been found?
An exploration well
What type of oil rigs are used at sea, and why?
Semi-submersible and jack-up rigs - as they can move around easily
What is a production platform?
If petroleum is discovered by drilling and more wells are drilled
What is secondary recovery?
- after oil in trap is pushed up through borehole under its own pressure, pressure will drop
- so gas or water may need to be pumped into trap to recover more petroleum
What is drilling mud used for when wells for oil are drilled?
To lubricate it
How is fracking carried out?
- shale gas companies drill boreholes down into gas-bearing shales
- borehole lined with steel and concrete casing
- perforating gun lowered into borehole to make small holes in concrete casing
- mix of water, chemicals and sand pumped at high pressure along borehole and through perforations which fractures shale
- water opens up cracks in rock, and sand grains lodge into spaces and keep them open - so gas flows out of rocks and travel back up borehole casing
Benefits of fracking?
- ↑ oil and gas production
- opens new oil resources
- ↓ price of oil and gas
- ↑ life of existing oil fields
- ↑ energy security
Drawbacks of fracking?
- use large amount of water
- pollution of groundwater by chemicals
- makes existing oil fields unprofitable
- ↑ consumption of hydrocarbons
- ↑ drilling activity on land
What is carbon capture also known as?
Geo-sequestration
What is carbon capture?
When CO2 is injected into underground geological formations
Where can CO2 be injected to increase oil recovery?
Declining oil fields
Why is injecting CO2 into declining oil fields an attractive option of carbon capture?
The geology of hydrocarbon reservoirs is generally well understood and any additional oil can be sold
What are disadvantages of injecting CO2 into declining oil fields as a form of carbon capture?
Their location and their limited capacity - as well as CO2 emissions from burning the additional oil
Where can carbon be stored?
- old hydrocarbon fields (highly impermeable caprock and permeable reservoir rock to trap the CO2)
- declining oil fields to increase recovery
- unmineable coal seams
- salt dome formations - contain highly mineralised brines
Advantages and disadvantages of storing carbon in salt dome formations?
- advantages - large potential storage volume
* disadvantages - relatively little known about them
Is chalk porous?
Yes
Can evaporites hold a lot of CO2?
Yes
What are the reserves of a resource?
The calculated amount that is economic to extract
What are resources?
Naturally occurring useful substances
What are the two major problems with the productions of oil from rigs?
The safety of the rig and the transport of the petroleum to where it is needed
What are the techniques for prospecting oil reserves?
- geological mapping
- borehole correlation
- geophysical (seismic, magnetic and ground penetrating radar)
- geochemical (soil and river sediment analysis)
Which factors affect the extraction of underground water from aquifers?
- height of water table
- porosity/permeability of the aquifer
- presence of natural springs
- distribution of wells
