ENVIRONMENTAL ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 242

compressibility c m is a measure of the deformability of the sediment when

subjected to a change in the applied stress, such as the change in pore fluid pressure
induced by fluid extraction. c m is therefore a measure of stiffness, relating a stress
increment to the resulting increment of deformation.
Reservoir model
Development of the reservoir model generally comprises two stages: (1) a static
study (i.e. development of the geological model), and (2) a dynamic study (i.e.
development of the flow model). Development of the geological model requires an
interpretation of the main structural features (e.g. faults), the geometry of all the
porous levels forming the reservoir and their petrophysical characterization. For the
dynamic study (i.e. flow model) two different models are accepted as best practice
internationally: the material balance model and a 3D model.
A material balance or 'single cell' model was the first approach developed for
simulating the production of hydrocarbon reservoirs. Material balance models are
considered to be a well-established and sound approach and have been used
worldwide in reservoir engineering studies for over 70 years. In these models it is
assumed that the pressure is spatially constant throughout the reservoir or 'cell',
changing only as a function of production, i.e. with time. A 3D model is currently
considered the most advanced technology for studying and simulating reservoir
behaviour, and therefore for evaluating the spatial and temporal distribution of
fluid pressure and saturations inside a reservoir body. By discretising the model
into multiple small elements for the purpose of the analysis, a more realistic picture
of the distribution of the pressure change at different locations in the reservoir can
be developed.
The predictive accuracy of a reservoir model is related to the quality and
quantity of the available data. In studies for new small-medium sized gas field
development projects the amount of geological/geophysical information available
is inevitably limited and no past history data is available for any calibration. In this
case the use of a simple reservoir model, such as a material balance or 'single cell'
model, to simulate the future behaviour of the reservoir is considered to be a sound
technical approach, fully adequate in terms of the reliability of the results obtained.
Geomechanical Model
Internationally, the following three different approaches to the development of a
geomechanical model are accepted as good practice.
- Geertsma cylinder-shaped model. Geertsma's analytical solution for the
evaluation of the surface subsidence induced by the depressurisation of a
reservoir level was published in 1973. The reservoir geometry Geertsma
considered is a cylinder-shaped layer, with petrophysical and mechanical
properties assumed to be homogeneous and isotropic, and behaving linear-
elastically when stressed. In his analysis, the formations surrounding the
reservoir level (over-, side-, and under-burden) is considered to be homogeneous
and isotropic, with mechanical properties equal to those of the reservoir rock.
- Geertsma semi-analytical model. The second approach is known as the Geertsma
semi-analytical model and is a natural evolution of the Geertsma cylinder-
shaped model. This version of Geertsma is usually applied when a 3D reservoir
model is available. The model is based upon the same basic assumptions as the