# Hydro-mechanical Calculation **URL:** \ **Category:** 3DEC\ **Created:** [June 12, 2023, 3:38pm UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053 "2023-06-12T15:38:53Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![DariushJ](https://avatars.discourse-cdn.com/v4/letter/d/ba8739/32.png) [@DariushJ](https://forum.itascainternational.com/u/DariushJ)\ **Post date:** [June 12, 2023, 3:38pm UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053/1 "2023-06-12T15:38:53Z")
Hi, I try to understand how 3DEC performs hydro-mechanical calculations. From manual of 3DEC 5.20 I understood a little, but I will be grateful if someone can simply explain the computation loop from start point to the end point. For example, if a constant injection into a flow-know is prescribed, how 3DEC deals with this problem to obtain new values of hydraulic aperture, fluid pressure, flow rate and so on along the flow plane in the next time-steps? ---
**Author:** ![DariushJ](https://avatars.discourse-cdn.com/v4/letter/d/ba8739/32.png) [@DariushJ](https://forum.itascainternational.com/u/DariushJ)\ **Post date:** [June 15, 2023, 12:39pm UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053/2 "2023-06-15T12:39:43Z")
It is probably better to ask the question step by step. If a constant flow rate of Q is prescribed to a flow-knot, an extra fluid pressure, in consequence, will be generated which will change the flow-knot volume as well depending on the bulk modulus of the fluid as \deltaP=\K\_f\*\deltaV/\V\_0. \deltaV\ at this problem will be Q (assuming that the time-step is 1 second). Then new P will be old P plus \K\_f\*Q/\V\_0. If it is so, then what is the third term in right hand side of equation (4.18) at page 4-12 of “Fluid in 3DEC” version 5.20 manual (or equation 18 on webpage: [Joint Fluid Flow — 3DEC 7.0 documentation](https://docs.itascacg.com/3dec700/3dec/docproject/source/theory/fluid/fluid_joint.html?node2385#id1))? and why the denominator of the second term of RHS is V instead of V\_0? Any help will be highly appreciated. ---
**Author:** ![jhazzard](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@jhazzard](https://forum.itascainternational.com/u/jhazzard)\ **Post date:** [June 15, 2023, 4:04pm UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053/3 "2023-06-15T16:04:54Z")
The third term is change in pressure caused by a change in volume (a decrease in volume increases the pressure, and vice versa). This term is calculated first, then the volume is updated, and the second term is calculated using the updated volume. ---
**Author:** ![DariushJ](https://avatars.discourse-cdn.com/v4/letter/d/ba8739/32.png) [@DariushJ](https://forum.itascainternational.com/u/DariushJ)\ **Post date:** [June 15, 2023, 8:33pm UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053/4 "2023-06-15T20:33:13Z")
Thanks @jhazzard If the third term is change in pressure caused by change in volume, then what is the second term? The reason behind my question is that the second term itself is change in pressure caused by a change in volume because Q\*deltat is in fact change in volume. And why the denominator or third term is average volume instead of initial volume (based on the definition of bulk modulus)? ---
**Author:** ![jhazzard](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@jhazzard](https://forum.itascainternational.com/u/jhazzard)\ **Post date:** [June 15, 2023, 11:00pm UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053/5 "2023-06-15T23:00:48Z")
The second term is the change in pressure due to the change in _fluid_ volume. The third term is change in pressure due to change in _mechanical_ volume. ---
**Author:** ![DariushJ](https://avatars.discourse-cdn.com/v4/letter/d/ba8739/32.png) [@DariushJ](https://forum.itascainternational.com/u/DariushJ)\ **Post date:** [June 16, 2023, 9:04am UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053/6 "2023-06-16T09:04:04Z")
Understood! Many thanks @jhazzard. One last question, what is the reason behind use of average flow-knot volume in denominator of the third term instead of initial flow-knot volume? ---
**Author:** ![Theophile](https://yyz1.discourse-cdn.com/flex031/user_avatar/forum.itascainternational.com/theophile/32/299_2.png) [@Theophile](https://forum.itascainternational.com/u/Theophile)\ **Post date:** [June 20, 2023, 12:14pm UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053/7 "2023-06-20T12:14:33Z")
Hi @DariushJ ! My guess is that this is due to the rightmost term refering to a _relative change_ in pore space that requires a _reference_ volume. There is no physical reason for having `V0` or `V` as the reference and a complete solving would require non-linear equations, hence drastically dampening computation times. To keep things simple, you then have to select a reference volume that remains consistant with the physics at stake. You _could_ choose `V0` or `V` as the reference but since we are dealing with linearized equations, the mean volume between the two timesteps seems to be a reasonable and less impacting choice. Just my thoughts! Théophile ---
**Author:** ![DariushJ](https://avatars.discourse-cdn.com/v4/letter/d/ba8739/32.png) [@DariushJ](https://forum.itascainternational.com/u/DariushJ)\ **Post date:** [June 22, 2023, 8:50am UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053/8 "2023-06-22T08:50:30Z")
Thanks @Theophile for sharing your thoughts. ---
**Author:** ![LorenZafrilla](https://avatars.discourse-cdn.com/v4/letter/l/8edcca/32.png) [@LorenZafrilla](https://forum.itascainternational.com/u/LorenZafrilla)\ **Post date:** [March 21, 2025, 8:31pm UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053/9 "2025-03-21T20:31:49Z")
Hi sorry for bump the post but I don’t know why I can create a new post. I’m doing a fully coupled hydromechanical analysis to calibrate the model with my permeability tests in 3DEC 7. I begin with the Itasca exemple for 1 joint fully coupled exemple and then I do the modifications to adapt to my situation. I see that we have to change the fluid bulk to 2e9 to 0 to have the initial state, but I see that in the line before the command: model mechanical substep 10 we have : block fluid property bulk **2e8** Why the fluid bulk is 2e8 and not 2e9? Is this an error? Is there a reason why in an order less? @jhazzard Thanks ---
**Author:** ![jhazzard](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@jhazzard](https://forum.itascainternational.com/u/jhazzard)\ **Post date:** [March 22, 2025, 12:51pm UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053/10 "2025-03-22T12:51:13Z")
@LorenZafrilla you need to enter a license number when you set up your account in order to create a new post. If you have a license, you can send me the number and I can activate your account. ---
**Author:** ![golu007](https://avatars.discourse-cdn.com/v4/letter/g/13edae/32.png) [@golu007](https://forum.itascainternational.com/u/golu007)\ **Post date:** [August 27, 2026, 9:34am UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053/11 "2026-08-27T09:34:22Z")
@jhazzard Sir, how can I incorporate strain softening and water weakening together in rock? In hydro-mechanical coupling. ---
**Author:** ![jhazzard](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@jhazzard](https://forum.itascainternational.com/u/jhazzard)\ **Post date:** [August 27, 2026, 1:18pm UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053/12 "2026-08-27T13:18:36Z")
I’m not sure what you mean by “water weakening”. The water weakens the rock? By some chemical reaction? We don’t have any logic like that in the software. You would have to implement something with FISH or Python. ---
**Author:** ![golu007](https://avatars.discourse-cdn.com/v4/letter/g/13edae/32.png) [@golu007](https://forum.itascainternational.com/u/golu007)\ **Post date:** [August 31, 2026, 2:02pm UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053/13 "2026-08-31T14:02:55Z")
@jhazzard Yes, sir, both strain softening and water saturation (water pressure) weaken the rock. I want to use a coupled hydro-mechanical model in 3DEC via FISH. ---
**Author:** ![jhazzard](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@jhazzard](https://forum.itascainternational.com/u/jhazzard)\ **Post date:** [September 1, 2026, 3:35pm UTC](https://forum.itascainternational.com/t/hydro-mechanical-calculation/1053/14 "2026-09-01T15:35:43Z")
OK - I don’t think there is any magic here. You can take any one of the fluid flow examples in the manual and just use a strain softening material instead of an elastic material. It should work.