Simulation of Land Subsidence Due to Gas Production at Ravenna Coastline

A large number of deep gas reservoirs are scattered both inland and offshore in the Upper Adriatic Sea sedimentary basin. Among these gas fields, the Angela Angelina reservoir is of major concern for the CENAS project because it is directly overlain by the shoreline extending from Lido Adriano to Lido di Dante in the Ravenna area. Angela Angelina is a very complex field consisting of 47 variously shaped pools located at a burial depth ranging between 3000 and 4000 m. The 31 largest pools will be depleted by 2014 with an ultimate pressure decline that is expected to exceed 300 kg/cm2 in some pools. A numerical study of the reservoir compaction and land settlement expected over Angela Angelina is performed by a new three-dimensional nonlinear finite element model. The constitutive relationship of the most important parameter controlling the event, i.e. the vertical soil compressibility vs the effective intergranular stress, is derived from a number of oedometric tests carried out on samples cored from deep boreholes scattered throughout the Upper Adriatic Sea basin. The results predict a final maximum land subsidence due to reservoir depletion equal to 20 cm over the gas field central area and a settlement of 12÷14 cm on the coastal area directly overlying the reservoir. An approximate evaluation of the influence of the active waterdrive surrounding the gas field is also given. Field measurements of land subsidence occurred from 1977 to 1992 are shown to be sufficiently in agreement with the simulated results.