Basin-scale compressibility of the Northern Adriatic by the
radioactive marker technique
D. Bau'
Dept. Geological and Mining Engineering and Sciences, Michigan Technological
University, Houghton, Michigan, USA
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
ABSTRACT
The paper presents a constitutive model for the one-dimensional vertical
compressibility mv of the Northern Adriatic sedimentary basin
based on in-situ deformation measurements by the radioactive marker technique.
Gas reservoir compaction and expansion occurred during the field
production life and after its abandonment have been measured in three deep
boreholes of the Northern Adriatic by FSMT (Formation Subsidence
Monitoring Tool) and CMI (Compacting Monitoring Instrument) developed
by Schlumberger and Western Atlas,
respectively. The compressibility values, as obtained by relating the
shortening of the distance between several pairs of adjacent markers
to the pore pressure decline experienced by the productive levels during
the monitored time interval, are
statistically processed to reduce the influence of operational and
instrumental errors. The method of the moving weighted averages is
used over clustered depth intervals. Compressibility thus derived is
related to the in-situ vertical effective stress to provide basin-scale
constitutive relationships for the expected mv in both virgin
loading and unloading-reloading conditions, together with the 68% and 95% confidence
intervals characterizing the law uncertainties. The results indicate that
for virgin loading mv ranges between 7x10-4
MPa-1 and
5x10-5 MPa-1 over the effective vertical stress
interval 10 < p' < 80 MPa, i.e. for a depth between 900 m and 7000 m in
undisturbed conditions, and decreases
by 2-3 times in unloading-reloading conditions.
