Abstract

Seafloor subsidence is a matter of concern in areas with offshore hydrocarbon exploitations relatively close to the coast, like the central and northern Adriatic Sea. Traditional bathymetry methods provide sensitivities to seafloor deformations of the order of tenths of centimetres, which is often insufficient to determine subsidence patterns and rates.

We propose a new, patented method for regional subsidence monitoring with an absolute accuracy of 2 to 5 millimetres, based on a technology developed for offshore reservoir monitoring. The method uses water pressure measurements as a starting point and reaches accuracies much better than pressure sensor specifications.

The new method will allow distinguishing natural from anthropogenic subsidence in the Adriatic, and monitoring the extension of the anthropogenic subsidence bowls. The new measurements can cover the gap between the fine grid of land measurements and the sparse measurements obtained by GPS on offshore hydrocarbon platforms. In particular, the new information will allow to determine how close to the coastline the effects of hydrocarbon production extend. The method can also be used to enable accurate real-time subsidence monitoring in the area by using pressure sensors on the seabed.

Regional subsidence monitoring can provide value to hydrocarbon exploitation itself. A good understanding of compaction processes is required for an optimal management of oil and gas reservoirs. Compaction depends on key properties of the reservoir like lateral compartmentalization and pore compressibility. In documented field cases, for instance, the analysis of subsidence patterns above producing fields have allowed to identify non-depleted compartments and have been used to identify target locations for infill wells.

Introduction

The context of the main Adriatic fields offshore is represented by young unconsolidated terrigenous sediments, where hydrocarbon production is likely to cause seafloor subsidence. The area features shallow water depths and a naturally subsiding sedimentary basin (Teatini et al., 2005). The coast presents large areas with elevations below 2 m above mean sea level. The Italian Adriatic coast features important natural sites and cities, including Venice and Ravenna.

Italian authorities require that offshore development projects include a subsidence monitoring plan. ENI, the major operator in the area, has implemented a monitoring network since the beginning of the 90's (Dacome et al., 2015). The network includes onshore measurements with several technologies, and a network of 48 continuous GPS measurement stations placed on offshore platforms.

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