Modeling geomechanical effects of the well case on radioactive
marker measurements in the Northern Adriatic, Italy
N. Castelletto, M. Ferronato, G. Gambolati, C. Janna, P. Teatini
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
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 cM , 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.
