very good and really informative demonstration dear Nicolas but the initiation of hydraulic fractures brought up a question that i encountered while watching your videos you mentioned in your advanced course that by depletion, the total horizontal stresses decrease as well but there was not a clear explanation answering why it is like that I saw this point in some papers as well, but none of them came up with a physical logic to explain this phenomenon either, what does decreasing pore pressure has anything to do with a decrease in total horizontal stress, they were just reports of recorded pore pressures and Sh min in those papers that confirmed the decrease in pore pressure and Sh min respectively but there was not a physical intuition and explanation for this can you please explain this for me?
Opening a hydraulic fracture requires overcoming *total* stress, it requires pushing *everything* the solid matrix and pushing the fluid too. This is easier to picture mentally if you think in a tight rock, where fracture pressure can be maintained with small leak-off. Since "total stress = pore pressure + effective stress", this latter function of the solid skeleton strain, lowering pore pressure will lower the total stress, and therefore will change the pressure required to open a hydraulic fracture. During depletion, the pore pressure decreases and the *effective* horizontal stress increases, but effective horizontal stress increases less than what pore pressure decreases. The overall result is a decrease of total horizontal stress and a lower pressure required to push the rock laterally with a vertical hydraulic fracture. More details here: ua-cam.com/video/RmhKqr7-UwA/v-deo.html
Hi Dr. Espinoza - I think you can suppress leakages from the needle’s contact with the gelatin by placing the needle into liquid gelatin and clumping it in place during curing. Very nice video!
Hi Andreas, the idea here was to design an experiment as simple as possible to be easily repeated by students/viewers. Your suggestion is great but adds one step in complexity to the preparation of the experiment. I'm thinking on doing another more realistic experiment where we will follow your recommendation!
take a taller glass and longer needle, so that your overburden becomes higher and we can see true vertical fractures and compare to the one you showed. But, this is already very nice visualisation, thanks!
It's not that easy, the Poisson ratio of gelatin is PR~0.5, hence sigma_h = PR/(1-PR ) * sigma_v becomes sigma_h ~ sigma_v. I have a solution in mind though. I will prepare another video soon.
In this YT video, they injected first at a shallow point (i.e. with low overburden) and got a fracture propagating transversely to the needle’s axis (horizontal fracture). Then went deeper (higher overburden, probably much higher than the horizontal confining stresses) and got the induced fracture to propagate longitudinally to the needle (vertical fracture). From the specimen’s color, it looks that it has a lower concentration than Dr. Espinoza’s specimen here. ua-cam.com/video/2UHTj9mn7h4/v-deo.html
Thanks for the demonstration!
What ratio of jello to water are you using?
Smart Teaching Experiment!
very good and really informative demonstration dear Nicolas
but the initiation of hydraulic fractures brought up a question that i encountered while watching your videos
you mentioned in your advanced course that by depletion, the total horizontal stresses decrease as well but there was not a clear explanation answering why it is like that
I saw this point in some papers as well, but none of them came up with a physical logic to explain this phenomenon either, what does decreasing pore pressure has anything to do with a decrease in total horizontal stress, they were just reports of recorded pore pressures and Sh min in those papers that confirmed the decrease in pore pressure and Sh min respectively but there was not a physical intuition and explanation for this
can you please explain this for me?
Opening a hydraulic fracture requires overcoming *total* stress, it requires pushing *everything* the solid matrix and pushing the fluid too. This is easier to picture mentally if you think in a tight rock, where fracture pressure can be maintained with small leak-off. Since "total stress = pore pressure + effective stress", this latter function of the solid skeleton strain, lowering pore pressure will lower the total stress, and therefore will change the pressure required to open a hydraulic fracture.
During depletion, the pore pressure decreases and the *effective* horizontal stress increases, but effective horizontal stress increases less than what pore pressure decreases. The overall result is a decrease of total horizontal stress and a lower pressure required to push the rock laterally with a vertical hydraulic fracture.
More details here: ua-cam.com/video/RmhKqr7-UwA/v-deo.html
Hi Dr. Espinoza - I think you can suppress leakages from the needle’s contact with the gelatin by placing the needle into liquid gelatin and clumping it in place during curing. Very nice video!
Hi Andreas, the idea here was to design an experiment as simple as possible to be easily repeated by students/viewers. Your suggestion is great but adds one step in complexity to the preparation of the experiment. I'm thinking on doing another more realistic experiment where we will follow your recommendation!
take a taller glass and longer needle, so that your overburden becomes higher and we can see true vertical fractures and compare to the one you showed. But, this is already very nice visualisation, thanks!
It's not that easy, the Poisson ratio of gelatin is PR~0.5, hence sigma_h = PR/(1-PR ) * sigma_v becomes sigma_h ~ sigma_v. I have a solution in mind though. I will prepare another video soon.
@@dnicolasespinoza5258 cool
In this YT video, they injected first at a shallow point (i.e. with low overburden) and got a fracture propagating transversely to the needle’s axis (horizontal fracture). Then went deeper (higher overburden, probably much higher than the horizontal confining stresses) and got the induced fracture to propagate longitudinally to the needle (vertical fracture). From the specimen’s color, it looks that it has a lower concentration than Dr. Espinoza’s specimen here.
ua-cam.com/video/2UHTj9mn7h4/v-deo.html