Subsidence associated with oil extraction, measured from time-series analysis of Sentinel-1 data : case study of the Patos-Marinza oil field, Albania

The Patos-Marinza oil field in Central Albania (40.71°N,19.61°E), operated since 1939, is one of the largest onshore fields in Europe. More than 7 millions oil barrels are extracted per year from the Messinian sandstone formations of the Durres Basin in the Albanian Peri-Adriatic Depression by the Bankers company operating the field since 2004. In the region, the background seismicity culminated in December 2016, when a shallow seismic swarm developed in the oil field triggering the opening of a public inquiry. However, because of the lack of a dense local seismic array and incompleteness of historical catalogues for such moderate events, whether this seismicity could be induced by the extraction/injection activities remains unclear. In this study, we take advantage of the new Sentinel-1 radar images acquired every 6 to 12 days over Albania to measure the surface displacement in the Myzeqeja plain and in the Patos-Marinza oil field in particular. Images from two ascending and descending tracks covering the area are processed through a radar interferometry (InSAR) time-series analysis over the 2014 to 2018 time-span, providing consistent average Line-Of-Sight velocity maps and displacement time-series. The regional deformation field exhibits a slow subsidence of the entire sedimentary basin relative to the highlands that we interpret as a combination of natural and man-induced compaction. This broad picture is complicated by a strong local subsidence signal with rates as high as 15 mm/yr that spatially correlates with the oil field and is maximal in the zone holding most of the operating horizontal wells, where EOR techniques are used. The striking spatial correlation between the maximum subsidence area and the active wells argues in favor of an oil-extraction induced surface deformation. The observed surface deformation is well reproduced by elastic models mimicking the basin and reservoir compaction using planar crack dislocations. Such modeling provides a first-order estimation of the volumetric deflation rate in the oil reservoir. This strong subsidence signal, together with the increase of the background seismicity since the oil field reactivation, are evidences of significant man-induced stress changes in the basin that should be further monitored and taken into account for seismic hazard assessment.