2 - Darcy’s Law and Permeability

Question 1

Aquifer Terms

Answer 1

Aquifer – A saturated permeable geological unit that stores groundwater and allows it o flow under normal conditions

Aquiclude – A saturated geological unit that is incapable of transmitting significant quantities of water
• Possible that it can hold water in its pore spaces, but they won’t be connected

Aquitard – A ‘less permeable’ bed in a stratigraphic sequence (may be significant in a regional groundwater balance but insignificant for a local production well)

Question 2

Porosity types:

Answer 2

Total porosity of rock: void space/total volume of rock
Effective porosity of rock: volume of pores accepting water/total volume of rock
Drainable porosity: Volume of voids drained by gravity/total volume of rock
Kinematic porosity: Volume of flowing water/total volume of rock

Question 3

Why total porosity is not the true porosity

Answer 3

• Some voids are too small to accommodate water molecules
•  Effective porosity
• Some water is held against gravity by surface tension
•  Drainable porosity
• Some pores are bypassed by or not connected to the flowing network
•  Kinematic porosity

Question 4

Fluid potential:

Answer 4

Potential: “A physical quantity capable of measurement at every point in a flow system whose properties are such that flow always occurs from regions in which the quantity has higher values to those in which it has lower regardless of the direction in space.”

Fluid Potential for flow through porous media = mechanical energy per unit mass of fluid.

Work done in moving a mass of fluid, m from a reference state, A, to a new point, B:

Question 5

Measuring the hydraulic head:

Answer 5

Observation Well:
Essentially an open borehole. Water level in such a well represents the average hydraulic head over the length of the open section. In unconfined aquifers, the water level in an observation well is often assumed to be the level of the prevailing water table

Piezometer:
A device for measuring hydraulic head at a point (or averaged over a relatively small depth) in the ground. The water level in an open piezometer well is not necessarily the same as the water table level in unconfined conditions.