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A nonlinear poroelastic finite element model to predict land subsidence due to gas withdrawal is developed and applied to simulate land settlement caused by gas production from the Angela Angelina field, one of the major gas reservoir located close to Ravenna.

The gas field is quite complex being formed by 47 variously shaped mineralized pools located in the pre-Quaternary basement at a burial depth ranging from 3000 down to 4000 m.

As the gas extraction produces a large decrease of the pore pressure within the productive pools, while the pore pressure variation in the remainder of the rock matrix is negligibly small, a new 3-D modeling approach is developed: a 1-D nonlinear vertical compaction model of the gas-bearing strata is coupled with a 3-D linear poro-elastic finite element model of the remaining porous medium. Tetrahedral elements for the discretization of the non-homogeneous porous medium are used.

Horizontal and vertical views of the tetrahedral
finite element mesh used to discretize one of the
Angela Angelina pools and the surrounding porous medium

Land subsidence is simulated from the inception of the production in 1973 to the end of the field production life in 2014.

Land subsidence over the Angela Angelina gas field
due to reservoir compaction in 1995

Land subsidence over the Angela Angelina gas field
due to reservoir compaction in 2014

Maximum land subsidence and subsidence at the Fiumi Uniti
river mouth versus time

Depressurization of the gas-bearing rocks induces a pressure decrease in the aquifer (waterdrive) surrounding the field with a further land settlement contribution. Following the results from the land subsidence simulation over the Dosso degli Angeli gas field (another reservoir located in the Ravenna area and with a structure quite similar to that of Angela Angelina) where computation indicates a maximum land settlement due to waterdrive depletion of the same order of magnitude as that caused by reservoir depletion, we can reasonably assume that the final maximum subsidence over Angela Angelina will be about 40 cm, with 25 cm at the coastline overlying the reservoir.

The modeling results are compared with the available topographic leveling data, consisting of the leveling surveys carried out in 1977 and 1992 using the `linea 11' benchmarks of the Ravenna Municipality leveling network located close to the Fiumi Uniti river. Measured land subsidence are decreased by the natural basin sinking, which is estimated in about 2 mm/y.

Measured and simulated land subsidence in the period
1977-1992 along the `linea 11' Ravenna Municipality
leveling network

Comparison of measured and simulated subsidence provides the following partial indications:

Presentation author: Andrea Pellizzon