Geomechanical effects of gas storage in depleted gas fields
N. Castelletto, M. Ferronato, G. Gambolati, C. Janna, I. Salce, P. Teatini
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
ABSTRACT
Cyclic gas storage in and withdrawal from subsurface gas fields generate a
sequence of loading-unloading cycles in both the sandy permeable formation
and the confining clayey caprock. A basic requirement for the safety of the
underground gas disposal is that the related stress variation does not
jeopardize the sealing capacity of the caprock, especially if the pore fluid
pressure should exceed the original in situ value in order to increase the
volume of the injected gas. Moreover, as a side effect the reservoir
deformation can induce a non negligible cyclic motion of the ground surface,
with possible undesired consequences for the existing civil structures and
infrastructures. The present communication aims at investigating numerically
the expected geomechanical effects of gas storage in a depleted gas reservoir
located in Italy. Computer simulations are performed using quite realistic
information on the geometrical and geomechanical properties of the field.
Two disposal scenarios are assumed where the maximum pore pressure in the
injection stage may increase from 120% up to 150% of its original value.
The results show that no significant stress changes are caused in the caprock
in the most unfavourable scenario as well, thus ensuring the safety of the
operation. However, a shear failure condition can be locally attained in the
injected formation. Such an occurrence should be carefully considered and
possibly monitored, especially if the failure is predicted close to the
injecting wells with potential large displacements which could damage the hole
casing. Finally, the reservoir deformation can induce a cyclic vertical ground
motion up to a few centimetres according to the generated pore pressure
fluctuation, the burial depth and the actual rock geo-mechanical properties of
the field and the overburden. Such a motion, however, is expected to occur
mainly in the elastic range and is almost fully recovered over a complete
injection-withdrawal sequence, with little concern for the safety of the
existing civil structures.
