Utilisation des effets poroelastiques et des contrastes en permeabilite comme moyen de reorientation des fractures hydrauliques

Gebrauch der porositatselastischen Effekten und Verschiedenheit in Durchdringbarkeit zur Kontrolle der Bruch Orientierung

Abstract:

This paper demonstrates how poroelastic effects, acting in combination with anisotropic permeabilities (i.e. a vertical permeability that is less than the horizontal permeability), will typically favour reorientation of a propagating fracture from vertical to horizontal. The paper includes examples of fracture reorientation using a specially adapted finite element code that can model curved fracture propagation.

Resume: Cet article demontre comment les effects de poroelasticite peuvent se combiner avec l'anisotropie des permeabilites de milieu et changer l'orientation d'une fracture de la verticole a l'horizontale. Un logiciel a base d'elements finis a etre modifie dans ce travail a fin de representrer les effects de courbure durant croissance d'une fracture.

Zusammenfassung: Diese Abhandlung demonstriert in welche Weise Porositatselastischen Effekten, wenn betaticht zusammen mit Verschiedenheit in Durchdringbarkeit, normaler Weise eine Orientierung der Bruch Fortpflanzung vom senkrecht nach horizontal begunsticht. Die Abhandlung enthalt Bruch Fortpflanzungsbeispiele, und gebraucht dafur ein besonders modifiziertes Computer Modell das auch gebogene Bruche modellieren kann.

Introduction

It is commonly assumed that the initial in-situ stress state is the dominant factor controlling the orientation of hydraulic fractures. However, it is known that changes in reservoir pressure alter the stress state and that this can in some cases alter the orientation of an induced fracture. This effect generally requires the reservoir pressure to be altered over a large volume and it is independent of formation permeability. It is also known that both thermoelastic and poroelastic effects can alter fracture pressures [Cleary].

In this paper it will be shown that for reservoirs where there is a permeability contrast, there is a concomitant contrast in the poroelastic confining stresses or backstresses that can potentially develop during hydraulic fracturing. The backstress is induced by expansion of the rock mass in the vicinity of the fracture due to increases in the pore pressure. Figure 1 plots the pressure contours surrounding two fractures, oriented vertically and horizontally, in a formation where the horizontal permeability is ten times the vertical permeability. The pressure has increased significantly in a much larger volume of rock for the case of a vertical fracture. This implies that the backstress in this case will be much greater than for a horizontal fracture.

This potential stress contrast depends on the orientation of the fracture relative to the principal permeability axes. It is referred to herein as a potential stress contrast since the stress only develops in the presence of a fracture and is not present initially as are the in-situ stresses. Furthermore, the magnitude of the potential stress contrast is dependent on the poroelastic time scaling of the fracture and the magnitude of the permeability contrast. Most importantly, it will be shown that the potential confining stresses acts in a manner similar to an initial stress contrast. Specifically, it can be expected that, in the absence a more dominant significant stress contrast, a propagating fracture will deviate to an orientation perpendicular to the minimum principal permeability regardless of the initial fracture orientation. This effect can occur at what would typically be considered low injection rates when injecting fluids, such as water, that readily leak-off into the formation. It typically will not occur at high injection rates with viscous fluids like those used in conventional fracturing treatments.

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