4 - Radial Flow to Wells

Question 1

Why are we interested in radial flow/basics

Answer 1

• Important for water extraction or injection
o Fracking
o Geothermal
o Disposal of waste
• Generally, we interact with groundwater through wells
• When a well is pumped a cone of depression is developed
• If the system is homogenous and isotropic and the aquifer is sufficiently large we can expect flow to be axisymmetric
• It is said that flow to production wells is radially convergent
• Flow to injection wells is radially divergent

Question 2

Radius of influence and Thiem influence assumptions?

Answer 2

1. Homogenous (no variation of H and K in space)
2. Isotropic (no variation of K in direction)
3. One dimensional radial flow (head gradient only in radial direction)
4. Infinite aquifer
5. Steady state conditions (no variation with time)
6. Darcy law applies (head gradient is linearly proportional to flow rate)

Question 3

Pumping near geological boundaries

Answer 3

• Hydrogeological boundaries in aquifers are physical boundaries to a groundwater flow system such as streams (an equipotential boundary) or impermeable barriers (e.g. some faults or an impermeable hardrock formation).
• The well flow equations given so far have been developed for aquifers of infinite extent and physical boundaries represent finite constraints on such flow systems.
• However, due to the assumed linearity of the system, the boundaries can be replaced or represented by an imaginary, equivalent flow system that will allow the infinite aquifer equations to be used.
• In other words, an image well (or wells) can be used to simulate an aquifer boundary, thereby transforming a semi-finite aquifer flow system into an infinite one in which the radial flow equations can be used.
• This is known as the method of images.

Question 4

Pumping near a river in a confined aquifer assumptions

Answer 4

• Stream stage is constant, so that the interface with the aquifer is a constant head/equipotential boundary.
• Both well and stream fully penetrate the aquifer. Although, in practice, the stream rarely fully penetrates the aquifer, the consequential errors in the analysis turn out to be relatively small.
• There is no sealing layer of fine sediment on the streambed (i.e. full hydraulic connection).
• Pseudo-steady state conditions.

Question 5

What is a drawdown test?

Answer 5

Drawdown test – pumping at a constantly increasing rate for a certain period of time

Question 6

Jacobs equation breakdown

Answer 6

X-axis = Qw (m3/min)
Y-axis = Sw/Qw (min/m^3) - drawdown/production
A = intercept
B = Slope