A series of numerical tests of hydraulic fracturing in heterogeneous rocks were performed to investigate the influence of pore pressure magnitude and gradient on initiation and propagation of tensile fractures. The numerical tests were run with a Rock Failure Process Analysis code, F-RFPA2D, incorporated with a Flow-Stress-Damage coupling model (FSD). To investigate the influence of pore pressure magnitude on fracture behaviour, "pinch-off" breaking tests, originally employed by Bridgeman to investigate the effective stress law for tensile fracture in a uniform pore pressure field, were numerically modelled. In another numerical test, a double-notched sample, with fluid pressure in one notch while keeping the other one open to the atmosphere, is numerically extended to investigate how the fluid flow direction or the pore pressure gradient will influence the fracture behaviour. The simulation results show that fracture is strongly influenced by both pore pressure magnitude and pore pressure gradient.
Um den Einfluss der Wasserdruckgroesse der Oeffnung und der Porisitaet auf den Bruchentstanden und Verbreiten der Tension von Gebirgen zu beurteilen, wurde viele nummerische Pruefungen auf den hydraulischen Bruch in dem Gebirgen aufgenommen.
Diese Pruefungen wurden mittels der Analyse- Software F-RFPA2D und Flow-stress-Damage Coupling model (FSD) durchgefuehrt. Um den Einfluess der Wasserdruckgroesse der Oeffnung auf den Bruch zu beurteilen, wurden auch Bruchpruefung und nummerische Analogexperimenten gemacht, und dadurch wurde die ganze Phase der Bruchentstanden und Verbreiten wiedergezeigt, und die Ergebnissen ergeben sich, dass die Wasserdruckgroesse der Oeffnung und der Porisitaet sehr viel auf den Bruchentstanden und Verbreiten der Tension von Gebirgen beeinfluessen.
Une serie de simulations numeriques a ete menee pour analyser l'influence de la grandeur de la pression de pore et du gradient sur la naissance et la propagation des fractures de tensions par le biais de la simulation numerique des fractures hydroliques dans des rochers heterogènes. Le logiciel F-RFPA (Rock Failure Process Analysis code) integrant FSD (Flow-Stress-Damage coupling model) y a ete employe. Les resultats des simultations montrent tout le processus de la naissance et de la propagation des fractures sous la pression de pore. Ces resultats nous ont permis de constater que la grandeur de la pression de pore et le gradient exercent une influence importante sur la naissance des fractures dans les rochers et sur leur mode de propagation.
As pointed out by Bruno and Nakagawa (1991), field evidence for pore pressure effects on fracture initiation and extension is sometimes ambiguous. It is found that pore pressure has two effects depending on its magnitude and gradient. While a local increase in pore pressure around the crack tip may enhance fracture extension, a global increase in pore pressure may inhibit fracture by increasing the compressive in situ stresses for the field.
The purpose of the present work is to numerically investigate the influence of pore pressure fields in terms of magnitude and gradient on hydraulic fracture initiation and propagation direction. A brief introduction on F-RFPA2D, incorporated with a Flow-Stress-Damage coupling model (FSD) (Tang et al. 2002), is first presented. The validity of effective stress laws for tensile fracture in porous rock is numerically demonstrated using a numerical sample modelling the "pinch-off" breaking test originally employed by Bridgeman (1912). Next, the influence of pore pressure gradients on hydraulic fracturing is studied by using double-notched square samples under pore pressure gradients. Our results verified that fracture is influenced by both pore pressure magnitude on a local scale around the crack tip and by the orientation.
