Environmental Engineering Reference
In-Depth Information
Oil Extraction
Oil extraction differs from groundwater extraction mainly because much greater depths
are involved, and therefore much greater pressures. Oil (or gas) extraction results in a
reduction of pore-fluid pressures, which permits a transfer of overburden pressures to the
intergranular skeleton of the strata.
In the Wilmington oil field, Long Beach, California, Allen (1973) cites “compaction as
taking place primarily by sand grain arrangement, plastic flow of soft materials such as
micas and clays, and the breaking and sharding of grains at stressed points.” Overall,
about two thirds of the total compaction at the Wilmington field is attributed to the reser-
voir sands and about one third to the interbedded shales (Allen and Mayuga, 1969).
During a period of maximum subsidence in 1951-1952, faulting apparently occurred at
depths of 1500-1750 ft (450-520 m), shearing or damaging hundreds of oil wells.
10.2.2
Surface Effects
Regional Subsidence
General
Surface subsidence from fluid extraction is a common phenomenon and probably occurs to
some degree in any location where large quantities of water, oil, or gas are removed. Short-
term detection is difficult because surface movements are usually small, are distributed
over large areas in the shape of a dish, and increase gradually over a span of many years.
Monitoring Surface Deflections
Traditionally, subsidence has been measured periodically using normal surveying meth-
ods. When large areas are involved the procedure is time-consuming, costly, and often
incomplete. Since 1992, some major cities have been monitoring subsidence with InSAR
(
Interferometric Synthetic Aperture Radar
) (
Section 2.2.3).
SAR images are presently obtained
by the European Space Agency (ESA) satellites. Cities mapped include Houston, Phoenix,
and Las Vegas in the United States, and Mexico City. The new InSAR maps provide new
and significant aspects of the spatial pattern of subsidence not evident on conventional
mapping (Bell et al., 2002),
Some Geographic Locations
Savannah, Georgia
: The city experienced as much as 4 in of subsidence over the 29 year
period between 1933 and 1962 because of water being pumped from the Ocala limestone,
apparently without detrimental effects (Davis et al., 1962).
Houston, Texas
: A decline in the water table of almost 300 ft since 1890 has caused as much
as 5 ft of subsidence with serious surface effects (see
Section 10.2.4)
.
Las Vegas, Nevada, Tucson and Elroy, Arizona and the San Joaquin and Santa Clara valleys,
California
: They have insignificant amounts of subsidence from groundwater withdrawal.
Mexico City
: In the hundred years or so, between the mid-1800s and 1955, the city experi-
enced as much as 6 m (20 ft) of subsidence from compression of the underlying soft soils
because of groundwater extraction. By 1949, the rate was 35 cm/yr (14 in.). Surface effects
have been serious (see Section 10.2.4).
London, England
: A drop in the water table by as much as 200 ft has resulted in a little more
than 1in. of subsidence, apparently without any detrimental effects because of the stiffness
of the clays. A geologic section is given in
Figure 7.56.
Long Beach, California
: It has suffered as much as 30 ft of subsidence from oil extraction
between 1928 and 1970 with serious effects (see Section 10.2.4).
