In situ experiments, which were accessible for direct observations by mine back, have been conducted to determine the effect that material property interfaces and in situ stress differences have on hydraulic, fracture propagation and the resultant overall fracture geometry. Fractures were observed to terminate only in regions of high minimum principal in situ stress. Fracture growth into a higher (by a factor of 5 to 15) elastic modulus region was preferred to propagation into a region of higher (by a factor of 2) stress. Determination of the in situ stress in the reservoir rock and adjacent layers ca be used to assess the economical success of hydraulic fracture treatments a priori by predicting whether containment of the fracture within the reservoir or out-of-zone propagation and failure of the treatment will occur.
On a procede à des experiences in situ, suivies du creusement d'une galerie d'accès permettant des observations directes afin de determiner les proprietes caracteristiques des interfaces des materiaux et des differences de contraintes in situ sur la propagation des fractures hydrauliques et la geometrie d'ensemble qui en resulte. On a observe que toutes les fractures aboutissaient à des regions où la contrainte principale minimale in situ se trouvait être elevee. La propagation des fractures se faisait vers des règions à module plus eleve (de 5 à15 fois plus eleves) plutôt que vers des regions à plus forte contrainte (2 fois plus forte). Il est donc essentiel de determiner l'importance des contraintes in situ dans la roche reservoir et dans les couches adjacentes lorsqu'on evalue des traitements susceptibles de stimuler les fractures hydrauliques, car on peut ainsi predire si la fracture restera confinee au reservoir ou. au contraire, se propagera hors de la zone reservoir, causant ainsi l'echec du traitement.
Es wurden in situ Experimente durchgefuehrt, die direkter Beobachtung vor Ort (mineback) zuganglich waren, um den Einfluß verschiedener.Materialgrenzflachen bzw. Änderungen des in situ Spannungsfeldes auf das Fortschreiten von hydraulischer Spaltung und auf die daraus entstehende geometrische Anordnung zu bestimmen. Es wurde beobachtet, daß Risse nur in einer Region mit hohem in situ σ3 aufhörten. Fortschreiten der Spalten in eine Region mit höherem (5–15 fach) E-Modul wurde dem Fortschreiten in eine Region mit bis zu zweifach höheren Spannungen vorgezogen. Daraus ergibt sich, daß die Ermittlung der in situ Spannungsgrößen im Speichergestein und dessen Umgebung fuer die Beurteilung etwaiger hydraulischer Spaltanregungsverfahren wichtig ist, um damit voraussagen zu können, ob die Spaltung ausschließlich innerhalb der Lagerstatte stattfindet oder auch außerhalb der Zone, was ein Versagen des Verfahrens bedeuten wuerde.
Massive hydraulic fracturing is the most promising technique for stimulation of low-permeability gas reservoirs at the present time. This technique is at least an order of magnitude scale-up from conventional hydraulic fracture technology and it is designed to create long penetrating fractures which contact large areas of the reservoir. However, the results to date have often been disappointing and the general applicability of these treatments for unconventional gas resources is uncertain. Although there are several possible causes for the lack of success, one of the most likely reasons is the failure of the fracture to contact a sufficiently large area of the reservoir due to unfavorable vertical propagation out of the reservoir into formations lying above and below the producing zone. When a treatment is designed, the height of the fracture is the parameter about which the least is known a priori, yet this influences all aspects of the design (Perkins and Kern, 1961). Therefore, it is extremely important to recognize and understand the mechanism which may influence the height of a fracture by restricting vertical fracture propagation (containment). Several parameters have been suggested as being important for hydraulic fracture containment. A difference in elastic modulus between the reservoir rock and the barrier rock is often singled out as a primary mechanism controlling containment. In their work on composite, materials, Cookand Erdogan (1972) calculated the stress intensity factor for a two-dimensional crack approaching an interface between two materials with different elastic moduli. Simonson et al (1978) applied these results to hydraulic fracturing and observed that since the stress intensity factor, K, at the tip approaches zero as a fracture in a lower modulus material propagates toward a higher modulus material, the fracture will tend to be arrested. Daneshy (1978) conducted laboratory experiments and found that differences in rock properties were insufficient to stop fracture growth at an interface. He suggested that barriers may need to be defined as formations that reduce vertical fracture growth rather than prevent it. Daneshy further suggested that fracture containment may be more a result of the nature of the interface itself rather than any difference in material properties, but he thought that this would be most often the case at shallow depths where the bonding is likely to be weaker.
