ENVIRONMENTAL ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 246

contaminated streamlines end up in the well. However years later the groundwater
downstream beyond the well shows contamination. Why?

application 14.2
If the groundwater table in a peat land is at 0.35 m depth, what is the expected
land subsidence per year, if the ground temperature at 10 cm depth is 15 = C?

application 14.3
A proper Environmental Impact Assessment (EIA) related to oil and gas
recovery involves the evaluation/prediction of deep effects (pressure decline in
adjacent aquifers), surface effects (land subsidence, vibrations, and water
management), and a monitoring plan. The following example is worked out for a
small reservoir that will be exploited for 25 years.
The compaction coefficient is obtained from general data of sandstone: c m =
6x10 -9 Pa -1 . The reservoir has been preloaded in the glacial periods (ice-cap) and a
factor 5 is adopted to account for the re-loading: c mr = 1.2x10 -9 Pa -1 . The maximum
pressure drop in the reservoir is p = 20 MPa. The reservoir height involves the net

height: h net = 50 m, gross height: h = 100 m (including shales, etc.). The maximum
(ultimate) reservoir compaction becomes

20 10 6 =
2.4 m. The choice of h and not h net is appropriate, if the intermittent shales also
compact finally.
For the maximum subsidence S max at the surface the rebound of the underburden
is considered for 20%, yielding and effective reservoir compaction of

p = 1.2x10 -9

h = c mr h E 100 E

h eff =

0.8 2.4 = 1.92 m. The arching: overburden is incorporated in formula (14.15) S max

= 2(1

E

(1+ C 2 )]

h , with C = D/R . The reservoir depth is D = 2000 m, the
reservoir apparent radius is R = 4000 m, and Poisson's ratio is

)[1 C /

= 0.3. Hence, S max

0.5/(1.25) 0.5 ] 1.92 = 1.79 m. The subsidence bowl will have a radius
of about R + 2 D = 8000 m. The compaction of the intermittent shales
(consolidation) gives another retardation effect. The direct subsidence can be found
by using

= 2(1 0.3)[1

2.4 = 1.2 m, which leads to S max = 0.895 m.
Moreover, this subsidence will become manifest at a later stage due to the
retardation time (viscous effects at the start), which could be 2 years.
Monitoring and a mitigation plan are therefore important. They should include
measuring the zero state (before exploitation), and regularly thereafter by levelling
and/or InSAR / GPS.

h dir = h net /h h = 0.5 E