Waterdrive dynamics and enhanced land subsidence over productive gas fields: Application to Dosso degli Angeli reservoir, Ravenna, Italy

Abstract

An active waterdrive (or confining aquifer) generally plays a favorable role in sustaining the pore-pressure in productive gas reservoirs. However, it may contribute significantly to spread the pressure decline and enlarge the land settlement bowl around the field. Moreover, it may continue to compact, and hence produce additional subsidence, for 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 program. 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-thirds 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.