Waterdrive dynamics and enhanced land subsidence over productive gas fields:
application to Dosso degli Angeli reservoir, Ravenna, Italy
D. Bau', G. Gambolati, P. Teatini
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
ABSTRACT
An active waterdrive (or confining aquifer) generally plays a favourable
role in sustaining the pore pressure in productive gas reservoirs.
However, it may significantly contribute to spread the pressure decline
and enlarge the land settlement bowl around the field. Moreover, it may
keep on compacting, and hence produce additional subsidence, for
quite a long time after the field abandonment. The dynamics of
waterdrive depletion, and related compaction, are primarily controlled
by the aquifer extent and its rock hydromechanical properties for any given
field pumping programme. After production has been completed water
progressively floods the reservoir with a moving water-gas
interface until a stable equilibrium is ultimately achieved.
The process of residual waterdrive depressurization and
concurrent reservoir repressurization is a nonlinear one, where
nonrecoverable sediment compaction exerts an important influence
on the overall occurrence. A nonlinear finite element model to predict
land subsidence over a depleted gas field and the associated
waterdrive is developed and applied to the gas reservoir of
Dosso degli Angeli, Ravenna, Italy. Numerical results show
that the waterdrive compaction accounts for two third of the
maximum land subsidence of more than 30 cm recorded in 1992 at the well
shutdown and suggest that a residual land settlement of almost 10 cm
is to be expected by the year 2042 over the two areas of Porto Garibaldi and
Casal Borsetti located a few kilometers south and north of the field,
respectively.
