Residual land subsidence over depleted gas fields in the Northern Adriatic basin

摘要

An original nonlinear three-dimensional finiteelement model is developed to predict the residual landsubsidence adjacent to depleted gas fields, as a delayed responsefrom an active aquifer which my keep on compacting for a longtime after the field abandonment. The pore pressure recoverywithin the reservoir and the depletion of the lateral/bottomaquifer are simulated by a subsurface flow model coupled withthe equation of state of the residual gas repressurized by theground water which floods the field. The resulting pore pressuredistribution is used as input data in a poro-elastic structuralmodel of land subsidence. The modeling approach is nonlinearbecause of both the dynamic coupling between the flux from theaquifer and the reservoir gas pressure response, and thedependence of the porous medium elastic properties on the ef-fective intergranular stress and the loading/unloading conditions.The model is applied to the 3,000 m deep gas reservoir of Dossodegli Angeli, one of the major fields in the Northern Adriaticsedimentary basin, made of three major gas pools. Representativebasin-scale mechanical parameters have been obtained fromlaboratory triaxial and oedometric tests, density logs, and recentmeasurements of in situ compaction by the use of radioactivemarkers. In 1992, at the end of 21 year production life themaximum pore pressure drawdown in the depleted poolsapproached 300 kglcm~2. The largest land settlement from themodeling simulation turns out to be 31 cm, in good agreementwith the available leveling records. Numerical predictionssuggest that a residual land sinking of about 10 cm is yet to beexpected in 2042, i.e., 50 years after the field abandonment, closeto the areas of Porto Garibaldi and Casal Borsetti a fewkilometers south and north of the field, respectively, namelybetween two and three times the subsidence experienced by thoseareas during the field development. Gas pressure recovery in2042 ranges from 50 to 130 km/cm~2 according to gas pool, andwith the cone of depression still expanding toward the far outerboundary of the adjacent aquifer.