[Geoqus] borehole injection boundary conditions

[Manooch Salehabadi]

Hi Shengli,
 
as you know on the surface of bore hole shear stress is zero, about your
first  boundary condition "total radial stress sigma_rr = 0, shear
stress sigma_rz = 0, and flux rate q = q0" , please  impose a surface
load for example with Dload on the surface of borehole which normally
comes from the pressure of drilling mud or  injection fluid so the
Sigmar=0  then asign surface flux rate or node flux rate depends on your
problem,
 and when it comes to your second boundary condition "total radial
stress segma_rr = pore pressure p (which indicates zero Terzaghi
effective stress), shear stress sigma_rz = 0, and flux rate q = q0"
again here
impose appropriate flux rate depends on the type of your problem, for
considering effective stress zero on the borehole wall please assing a
surface load through Dload as equal as pore pressure on the surface of
the borehole then the effective pressure on the surface of borehole is
zero.
 
I hope that it helps
 
Regards
 
Manooch 

 
________________________________

From:
geoqus-bounces+manooch.salehabadi=pet.hw.ac.uk at lists.ruhr-uni-bochum.de
[mailto:geoqus-bounces+manooch.salehabadi=pet.hw.ac.uk at lists.ruhr-uni-bo
chum.de] On Behalf Of shengli chen
Sent: 12 January 2009 21:41
To: geoqus at lists.ruhr-uni-bochum.de
Cc: shengli.chen at yahoo.com
Subject: [Geoqus] borehole injection boundary conditions


Dear Geoqus members,
 
I am working on the borehole prolem but recently got stuck with the
specification of the boundary conditions. Consider a borehole subjected
at its surface r =  R (R is the borehole radius) to a uniform fluid
injection rate q0, What should be the boundary condition at the borehole
wall r = R looks like? (1) total radial stress sigma_rr = 0, shear
stress sigma_rz = 0, and flux rate q = q0; or (2) total radial stress
segma_rr = pore pressure p (which indicates zero Terzaghi effective
stress), shear stress sigma_rz = 0, and flux rate q = q0???
 
If (1) is correct then we can easily impose q = q0 in the Abaqus and
just do nothing with the sigma_rr = sigma_rz = 0. However, I think the
boundary condtions (2) is more reasonable as the injection of fluid will
be accompanied by an induced pore pressure thus the total radial stress
segma_rr should be equal to the pore pressure p. If so, how can we
assign at the borehole surface sigma_rr = p (both sigma_rr and p are
unknown) or Terzaghi effective stress equal to zero in additon to q =
q0??? In my understanding, the applied surface stress in ABAQUS should
have a meaning of total stress for porous medium.
 
Thanks in advance for any advice!!!
 
Shengli

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