[Geoqus] borehole injection boundary conditions

[shengli chen]

Manooch,
 
Thank you very much for your suggestion!!! I will try to use the User-subroutines in ABAQUS to fix this problem. 
 
Best regards.
Shengli Chen
 
PhD student, PoroMechanics Institute
The University of Oklahom,
Norman, US

--- On Thu, 1/15/09, Manooch Salehabadi <Manooch.Salehabadi at pet.hw.ac.uk> wrote:

From: Manooch Salehabadi <Manooch.Salehabadi at pet.hw.ac.uk>
Subject: RE: [Geoqus] borehole injection boundary conditions
To: shengli_chen at yahoo.com, "Geoscientific Abaqus User Group" <geoqus at lists.ruhr-uni-bochum.de>
Cc: shengli.chen at yahoo.com
Date: Thursday, January 15, 2009, 4:40 PM

Hi Shengli,

Ok, It seems that the problem regarding the first boundary condition is sorted
out.
about the second boundary condition,

yes, you are right, pore pressure will be increased by fluid injection
(fracturing and so on), in this case i recommend you to use two availabe
User-subroutines in ABAQUS, DlLOAD and USDFLD.

in USDFLD you can get pore pressure from the previous increament in nodes
adjacent to wellbore face through GETVAM subroutine called inside of USDFLD then
by the aim of common blocks in fortran send this pore pressure to DLOAD
subroutine. In DLOAD adjust the magnitude of your load so that surface load on
the wellbore face is equal to pore pressure which means zero effective stress on
the wellbore face.
based on my knowledge, ABAQUS calls USDFLD before than DLOAD so there should
not be any problem, but you need to take care of time increment in this case.

I hope that it help

Regards
Manooch 

PhD Student, Petroleum Geomechanics
Institute of Petroleum Engineering
Heriot-Watt University
Edinburgh, UK

 

-----Original Message-----
From: shengli chen [mailto:shengli_chen at yahoo.com]
Sent: Thu 15/01/2009 19:05
To: Geoscientific Abaqus User Group; Manooch Salehabadi
Cc: shengli.chen at yahoo.com
Subject: RE: [Geoqus] borehole injection boundary conditions
 
Dear Manooch,
 
Thank you very much for your help!!!
 
I agree with you that for the first boundary conditions, the total radial
stress should be equal to the pressure from the drilling mud. This can be easily
done in the ABAQUS by impose a surface load equal to the mud weight pressure.

As for the second boundary conditions (which actually I think should be
correct for the fluid injection or borehole fracturing problem), you suggest to
"impose appropriate flux rate depends on the type of your prolem, 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".
The problem is, however, how could we assign a surface load equal to the pore
pressure on the borehole surface? Because the pore pressure is induced by the
fluid injection and itself is unknown to be determined. This means that the pore
pressure and the total stress are coupled with each other.
 
What do you think about this point? Thanks again.
 
Best regards.
Shengli Chen
 

--- On Thu, 1/15/09, Manooch Salehabadi <Manooch.Salehabadi at pet.hw.ac.uk>
wrote:

From: Manooch Salehabadi <Manooch.Salehabadi at pet.hw.ac.uk>
Subject: RE: [Geoqus] borehole injection boundary conditions
To: shengli_chen at yahoo.com, "Geoscientific Abaqus User Group"
<geoqus at lists.ruhr-uni-bochum.de>
Cc: shengli.chen at yahoo.com
Date: Thursday, January 15, 2009, 3:57 AM



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-bochum.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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