[Geoqus] borehole injection boundary conditions

[Manooch Salehabadi]

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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