ABSTRACT:

The possible influence of the well case on the compaction measurements made in producing gas/oil reservoirs by the Radioactive Marker Technique (RMT) is investigated. The issue is a very important one as RMT data are currently used to derive the vertical rock compressibility M c, i.e. an essential parameter controlling anthropogenic land subsidence due to gas/oil field development. A geomechanical Finite Element (FE) model is implemented to compute the disturbance exerted by the combined rigidity of the steel casing and the surrounding cement on the amount of compaction around the borehole as detected by RMT. The FE model is integrated by a class of Interface Elements (IE) accounting for the possible sliding between the different materials, i.e. along the contact surfaces between the steel case and the cement, and the cement and the producing formation. A set of simulations are performed using a real casing geometry and geomechanical information from the Northern Adriatic basin, Italy, where a few boreholes have been instrumented with RMT since the early 1990s. Although sliding is not likely to occur along the actual contact surfaces, the results show that the effects of the different rigidity between the casing and the formations under depletion are practically negligible on the RMT measurements, i.e. are below the nominal RMT accuracy, for the overall depth range spanned by RMT in the Northern Adriatic. Hence, it may be concluded that for the Northern Adriatic gas reservoirs RMT as is currently implemented represents a quite reliable tool for assessing the actual geomechanical properties of the porous medium

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