ENVIRONMENTAL ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 241

Here, superposition m refers to the number of aquitards (type 2 and 3, see 14.13)
per vertical position r . The Hantush well-function, commonly used in
geohydrology, is not suitable for land subsidence. The well-function (14.13d) and
the corresponding subsidence (14.14) can be used in complex situations (multiple
well systems), since the principle of superposition holds. It has been successfully
applied for the simulation of land subsidence at the Bolivar coast in Venezuela (Fig
14.11), reaching at the coast up to 5 metre subsidence in 2010.

Peat oxidation
Peatlands cover an estimated area of 3% of the Earth's land surface. Most of it is
in the northern hemisphere, covering large areas in North America, Russia and
Europe. Tropical peatlands occur in mainland East Asia, Southeast Asia, the
Caribbean and Central America, South America and southern Africa where the
current estimate of undisturbed peatland is 10-12% of the global peatland resource
(International Peat Society). Agriculture (drainage for agricultural practices
enhances the loss of mass due to oxidation of organic matter), forestry and peat
extraction for fuel and horticultural use are the major causes of peatland
disappearance. As these land-use changes require alteration of peatland hydrology,
peat oxidation results leading to land subsidence, which could reach 2-3 cm per
year. This subsidence causes increased pumping and drainage costs.
An empirical formula to quantify the effect of water table depth on subsidence
due to peat oxidation per year is (Stephens)

S = 0.01( a + bh ) 2 ( T- 5)/10 [m/yr] (14.15)

with a 1.5 are empirical constants, h is actual groundwater depth in

metre, and T the temperature in

0.1 and b

C at 0.1 m depth.
Peatland restoration is receiving attention in Europe and North America. It is
also recognised in tropical peatland areas. While peatland restoration is primarily
designed for global biodiversity protection, it can also play an important role in
reducing CO 2 emissions.

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Oil and gas recovery
The standard process, applied internationally by the gas and oil mining industry
for subsidence prediction, may be considered as being divided broadly into two
steps
- Reservoir modelling. This step is used to simulate the flow processes of
hydrocarbons and water in the porous layers and to compute the pressure and
saturation changes that take place during hydrocarbon extraction.
- Geomechanical modelling. This step is used to compute the deformation
(including the compaction and resulting subsidence) of the porous layers and to
compute the change in stresses and porosity due to fluid pressure changes, e.g.
as a consequence of hydrocarbon extraction.
There are many factors that influence the magnitude and aerial extent of subsidence
associated with gas production. The key factors are compressibility of reservoir
formation, thickness, depth and radius of reservoir, influence of adjacent water
aquifers, and volume of gas production and related pressure drop. The sediment