Residual land subsidence over depleted gas fields in the Northern Adriatic basin
D. Bau', G. Gambolati, P. Teatini
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
ABSTRACT
An original nonlinear three dimensional finite element model
is developed to predict the residual land subsidence
adjacent to depleted gas fields, as a delayed response from an active aquifer
which may keep on compacting for a long time after the field abandonment.
The pore pressure recovery within the reservoir and the depletion
of the lateral/bottom aquifer
are simulated by a subsurface flow model coupled with the
equation of state of the residual gas repressurized by the groundwater
which floods the field. The resulting pore pressure distribution
is used as input data in a poro-elastic structural model of land subsidence.
The modeling approach is nonlinear because of both the dynamic coupling between
the flux from the aquifer and the reservoir gas pressure response and the
dependence of the porous medium elastic properties on the effective
intergranular stress and the loading/unloading conditions.
The model is applied to the 3000 m deep gas reservoir of Dosso degli Angeli, one of the
major field in the Northern Adriatic sedimentary basin, made of three major gas pools.
Representative basin-scale mechanical parameters have been obtained from
laboratory triaxial and oedometric tests, density logs, and recent measurements
of in situ compaction by the use of radioactive markers.
In 1992 at the end of 21 year
production life the maximum pore pressure drawdown in the depleted pools approached
300 kg/cm2. The largest land settlement from the modeling simulation
turns out to be 31 cm,
in good agreement with the available leveling records.
Numerical predictions suggest that a residual land sinking of about 10 cm is
yet to be expected in 2042, i.e. 50 years after the field abandonment, close to the areas of Porto
Garibaldi and Casal Borsetti a few kilometers south and north of the field,
respectively, namely between two and three times the subsidence experienced
by those areas during the field development. Gas pressure recovery in 2042
ranges from 50 to 130 km/cm2 according to gas pool, and with the cone of
depression still expanding toward the far outer boundary of the adjacent aquifer.
