Land subsidence spreading factor in the Northern Adriatic gas fields, Italy

The land subsidence spreading factor provides a useful straightforward indication on how much of a gas/oil reservoir compaction induced by field development migrates to ground surface with a possible adverse impact on the stability of low-lying coastal areas. This factor depends primarily on the ratio between the depth of burial and a representative horizontal dimension of the reservoir. However, an important influence is also exerted by the active bottom/lateral aquifer hydraulically connected to the field (called “waterdrive” in reservoir engineering) that may undergo an extensive depressurization also after the field abandonment, thus contributing to enhance the overall land settlement. In the Northern Adriatic basin, Italy, is evaluated using a nonlinear finite element model of three representative gas reservoirs (Chioggia Mare, Dorotea, and Dosso degli Angeli) surrounded by important bottom/lateral aquifers in the interval depth between 1000 and 3300 m. Results show that may easily approach and even exceed one for the deepest field as well, contrary to the prediction of land subsidence based on the compaction of the gas-bearing formations alone, that can thus be largely underestimated.