Residual land subsidence over depleted gas fields in the northern adriaticbasin

An original nonlinear three-dimensional finite element model is developed t o 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 resid ual gas repressurized by the ground water which floods the field. The resul ting pore pressure distribution is used as input data in a pore-elastic str uctural model of land subsidence. The modeling approach is nonlinear becaus e of both the dynamic coupling between the flux from the aquifer and the re servoir gas pressure response, and the dependence of the porous medium elas tic properties on the effective intergranular stress and the loading/unload ing conditions. The model is applied to the 3,000 m deep gas reservoir of D osso degli Angeli, one of the major fields in the Northern Adriatic sedimen tary basin, made of three major gas pools. Representative basin-scale mecha nical 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/ cm(2). The largest land settlement from the modeling simulation turns out t o be 31 cm, in good agreement with the available leveling records. Numerica l 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 su bsidence experienced by those areas during the field development. Gas press ure recovery in 2042 ranges from 50 to 130 km/cm(2) according to gas pool, and with the cone of depression still expanding toward the far outer bounda ry of the adjacent aquifer.