Geomechanical response to seasonal gas storage in depleted
reservoirs: A case study in the Po River basin, Italy
P. Teatini, N. Castelletto, M. Ferronato, G. Gambolati, C. Janna
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
E. Cairo, D. Marzorati
Stogit S.p.A., San Donato Milanese, Italy
D. Colombo, A. Ferretti
Tele-Rilevamento Europa S.r.l., Milan, Italy
A. Bagliani, F. Bottazzi
Divisione E&P, Eni S.p.A., San Donato Milanese, Italy
Underground gas storage (UGS) in depleted hydrocarbon reservoirs is a strategic
practice to cope with the growing energy demand and occurs in many places in Europe
and North America. In response to summer gas injection and winter gas withdrawal
the reservoir expands and contracts essentially elastically as a major consequence of
the fluid (gas and water) pore pressure fluctuations. Depending on a number of factors,
including the reservoir burial depth, the difference between the largest and the smallest
gas pore pressure, and the geomechanical properties of the injected formation and the
overburden, the porous medium overlying the reservoir is subject to three-dimensional
deformation with the related cyclic motion of the land surface being both vertical
and horizontal. We present a methodology to evaluate the environmental impact of
underground gas storage and sequestration from the geomechanical perspective,
particularly in relation to the ground surface displacements. Long-term records of injected
and removed gas volume and fluid pore pressure in the "Lombardia" gas field,
northern Italy, are available together with multiyear detection of vertical and horizontal
west-east displacement of the land surface above the reservoir by an advanced permanent
scatterer interferometric synthetic aperture radar (PSInSAR) analysis. These data have
been used to calibrate a 3-D fluid-dynamic model and develop a 3-D transversally
isotropic geomechanical model. The latter has been successfully implemented and used
to reproduce the vertical and horizontal cyclic displacements, on the range of 8-10 mm
and 6-8 mm, respectively, measured between 2003 and 2007 above the reservoir
where a UGS program has been underway by Stogit-Eni S.p.A. since 1986 following
a 5 year field production life. Because of the great economical interest to increase the
working gas volume as much as possible, the model addresses two UGS scenarios
where the gas pore overpressure is pushed from the current 103%pi,
where pi is the gas
pore pressure prior to the field development, to 107%pi
and 120%pi. Results of both
scenarios show that there is a negligible impact on the ground surface, with deformation
gradients that remain well below the most restrictive admissible limits for the civil
structures and infrastructures.
