ENVIRONMENTAL ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 238

shrinkage noticed as surface subsidence comes from the aquitards, which are
relatively more compressible. In the aquitards, in fact, a process of consolidation is
triggered by the water withdrawal. Oil/gas withdrawal takes place from relatively
deep reservoirs (sandstone, shales), which are confined by impermeable rock layers
(salt, claystone). Sometimes there is a connected adjacent aquifer. The pressure
drop here is significantly higher than for shallow water withdrawal, causing the
reservoir itself to compact, which results in land subsidence. So, for water
withdrawal the storage effect is in the adjacent layers, while for oil/gas withdrawal
it is in the reservoir itself.

Q 1 Q 1 Q 1 Q 1

Q 2 Q 2 Q 2 Q 2

w w w w

aquifer aquifer top layer top layer

aquitard aquitard aquitard aquitard

aquifer aquifer

aquitard aquitard aquitard aquitard

aquifer aquifer aquifer aquifer

aquiclude aquiclude aquiclude aquiclude

Figure 14.10 Geological system and a production well

Water withdrawal
The geohydrological system from which groundwater is recovered consists of
aquifers (permeable reservoir) and aquitards (semi-pervious confining layer) or
aquicludes (impervious confining layer), see Fig 14.10. The drawdown (pore
pressure head lowering in the aquifer) s at time t at distance r from a production
well with constant discharge Q is formulated by

Q

s W ( r , t ) (14.9)

4 kH

Here, kH represents the aquifer transmissivity (permeability x height) and W is a
well-function. For a confined aquifer the well-function is (Theis) 54

54

Horizontal strains are disregarded.