Surface flow boundary conditions in modelling land subsidence due to
fluid withdrawal
D. Bau'
Dept. Geological and Mining Engineering and Sciences, Michigan Technological
University, Houghton, Michigan, USA
M. Ferronato, G. Gambolati, P. Teatini
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
ABSTRACT
Land subsidence due to subsurface fluid (water, gas, oil) withdrawal is often
predicted by either finite element or finite difference numerical models based
on coupled poroelastic theory, where the soil is represented as a semi-infinite
medium bounded by the traction-free (ground) surface. One of the variables
playing a most important role on the final outcome is the flow condition used
on the traction-free boundary, which may be assumed as either perme-able or
impermeable. Although occasionally justified, the assumption of no-flow
surface seems to be in general rather unrealistic. A permeable boundary
where the fluid pressure is fixed to the external atmospheric pressure
appears to be more appropriate. This paper addresses the response, in terms
of land subsidence, obtained with a coupled poro-elastic finite element model
that simulates a distributed pumping from a horizontal aquifer confined
between two relatively impervious layers, and takes either a permeable
boundary surface, i.e., constant hydraulic potential, or an impermeable
boundary, i.e., a zero Neumann flow condition. The analysis reveals that
land subsidence is rather sensitive to the flow condition implemented on
the traction-free boundary. In general, the no-flow condition leads to an
overestimate of the predicted ground surface settlement, which could even be
1 order of magnitude larger than that obtained with the permeable boundary.
