Unloading-reloading uniaxial compressibility of deep reservoirs by marker
measurements
M. Ferronato, G. Gambolati, P. Teatini
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
ABSTRACT
The radioactive marker technique appears to be a promising tool for
the in situ measurement of the uniaxial vertical compressibility
cM of deep gas/oil reservoirs. A constitutive
model for cM,unload in the Northern Adriatic
sedimentary basin in unloading-reloading (II cycle) conditions
is derived. This relies on marker measurements obtained from an
instrumented borehole located in depleted gas reservoirs that
had experienced a pore pressure recovery over the period 1997-1999.
Record analysis indicates that cM,unload
is on the order of 5-6·10-5 MPa-1 over the effective
stress interval 35 &leσz&le 65 MPa,
i.e. for a burial depth
between 3200 and 5800 m in undisturbed conditions.
The ratio of virgin loading cM,load to
cM,unload varies between 1.5 and 3 with
an average value equal to 2.2. The cM,unload
constitutive law derived for rock expansions is implemented
into a coupled finite element consolidation model in an attempt
to reproduce, also quantitatively, the vertical deformation
recorded in the marker borehole after field abandonment.
