Chapter 9 - Economic and Engineering geology Flashcards
Define porosity
Volume of pore space
as a %
volume of pore space/ volume of rock or sediment
x100
Define permeability
rate at which a fluid (usually water, oil or gas) flows through a rock
Distance fluid has travelled/ time taken
Name and describe factors affecting porosity
- degree of sorting: well sorted = high porosity
- structure of the rock: crystalline = no porosity
- amount of diagenesis: loose unconsolidated rock = much higher porosity than a rock that has undergone compaction and cementation.
- grain shape: rounded grains = high porosity than angular
- packing of the grains: the way they fit together
Name and describe factors affecting Permeability
- Porosity - high porosity = high permeability
- Temperature - permeability affects viscosity of a fluid.
(ie oil, viscosity decreases with increasing temperature) - Secondary permeability: results from the presence of fractures like joints, faults and voids.
- Capillary pressure
Define capillary pressure
the pressure between two immiscible fluids in a narrow pore space, resulting from interactions of forces between the fluids and solid grains.
What is Connate water?
water trapped n pores of a rock as the rock formed.
includes water trapped in the original sediment and water released in diagenesis.
what is groundwater?
Water occupying pores and other spaces in rocks and sediments which is derived mostly from
rainfall percolating into the underlying rock
Below watertable
What is the watertable?
the surface separating unsaturated rock above from the saturated rock below
What is Hydrostatic pressure?
pressure at a point in a body of water due to the mass of the overlying column of water
What is hydraulic gradient?
difference in hydrostatic pressure between two points divided by the distance between them
(gradientA - gradientB)/ (A-B)
What is an aquifer?
a body of porous and permeable rock capable of storing and yielding significant amounts of water
What is an unconfined aquifer? 4 points
open to the atmosphere
under atmospheric pressure
recharged by rain water from directly above.
Water will need to be pumped to the surface from a well or borehole sunk into an unconfined aquifer.
What is a confined aquifer?
overlain by impermeable rocks
the GW held within it is under hydrostatic pressure
GW can only be replenished if it has recharge zones that are open to the atmosphere.
What is a recharge zone?
the area of an aquifer open to the atmosphere, allowing replenishment of water
Perched aquifer
an aquifer that sits above the regional WT and is underlain by a lens of impermeable rock which prevents the water from percolating further downwards.
