ENVIRONMENTAL ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 243

full Geertsma model, with similar rock properties for the reservoir and the
surrounding formations. However, as the reservoir is discretised into the same
3D cells as the reservoir model, the resulting prediction gives a more realistic
distribution of subsidence because it can more accurately reflect the real
geometry of the reservoir and actual depressurisation field.
- Finite Element Model. Use of Finite Elements is the most accurate (but also
most complex) tool for the assessment of subsidence. The reliability and
accuracy of a Finite Element model depend on the availability of a large amount
of data describing both the geological setting and the mechanical properties of
the reservoir rocks and of the surrounding formations.
The choice of any one particular model depends largely on the availability of data
and whether the assumptions inherent in the model can reasonably accurately
represent the physical situation. The proper use of simple models, such as those
developed by Geertsma, can often provide realistic estimates of the subsidence
magnitude. In practice, if the reservoir is small, of limited thickness and remote
from subsidence-sensitive objects, industrial practice considers the analytical
Geertsma-type model adequate for the assessment of subsidence.
Sediment compaction coefficient c m
The sediment compaction (vertical compressibility) coefficient c m can be
measured via laboratory experiments (e.g. by oedometer tests), or by in situ
monitoring (using markers). Geotechnical engineering practice favours in situ tests
as a source of compressibility data because these tests can be made with a
minimum of soil disturbance and, by definition, will take into account the
integrated effect of the full layer rather than relying on the extrapolation of
properties derived from a small element test. This preference has led to the
development of an industry of field-testing, with numerous methods and test tools.
For deep sediment compressibility, radio-active borehole marker measurements
have become standard practice internationally. However, in cases where marker
measurements are not available or limited, laboratory experiments based on
second-cycle compressibility derived from oedometer testing is considered to be
the only practical alternative. Experience suggests that the second-cycle
compressibility is generally quite reliable as an average value.
Geertsma's analytical solution
The internationally accepted approach used by industry to assess the
approximate order of magnitude of the expected subsidence is to apply the
following standard formula, which relates the approximate maximum subsidence to
the geometric properties of the reservoir (depth, radius), the thickness,
compressibility and expected pressure drop, according to

) (1 - C/ (1 +C 2 ) 0.5 ) h c m S max = 2(1 p (14.16)

Poisson ratio, C
reservoir factor C = D/R , D approximate depth of the reservoir, R approximate
radius of the reservoir, h average reservoir thickness, c m average compressibility of
the reservoir, and

With S max maximum settlement in the subsidence bowl,

p average pressure drop in the reservoir.