Subsidence risk in Venice and nearby areas, Italy, owing to offshore gas fields: A stochastic analysis

Subsidence due to fluid withdrawal from the subsurface is being experienced in many regions. Water extraction for agricultural and civil needs is the most common cause of this phenomenon, However, hydrocarbon production from deep formations sometimes has been blamed for substantial rates of subsiden ce (e,g,, Wilmington, California; Ekofisk, the North Sea). Hydrocarbon rese rvoirs in sensitive areas (lowlands), such as the Netherlands and the easte rn Po River Plain in Italy, which includes the city of Venice, must be main tained under control to avoid actual or alleged subsidence, Because compact ion in deep sediments is a relatively new research held, applicable data ar e sparse, and much work remains to be done. In particular, a strong discrep ancy exists between rock compressibility values measured in the laboratory and in the field, these latter ones being probably more representative. Lab oratory compressibility values are at least one order of magnitude larger t han in situ values. Given this discrepancy, and in order to accommodate als o for the variability of natural materials, a stochastic risk analysis is t he most reliable tool for predicting subsidence due to hydrocarbon extracti on. This approach is here applied to a planned gas production project from offshore fields in the northern Adriatic Sea, in order to assess the risk o f subsidence on the coast and the nearby city of Venice, No stochastic theo ry of subsidence is available to date; consequently, a Monte Carlo techniqu e is applied with respect to soil compressibility, which affects both the r eservoir and subsidence simulations. A semi-analytical three-dimensional mo del based on the theory of Geertsma is used as a subsidence model. The resu lts show that no actual risk of subsidence exists on the coast. Against int uition, the worst case scenarios coincide with the lowest compressibilities , since these values contribute to higher pressure declines in the reservoi rs and the adjacent aquifers, and to a faster propagation of pressure chang e towards the shoreline. The results for the worst-case scenarios are confi rmed by more sophisticated non linear, three-dimensional, finite element si mulations.