Arrest and aperture variation of hydrofractures in layered reservoirs

Abstract

AbstractHydrofractures are extension fractures generated by internal fluid overpressure (net or driving pressure): they include dykes, veins and many joints. The growth of a hydrofracture depends primarily on the mechanical properties of the host rock and the overpressure of the hydrofracture. Field observations show that in heterogeneous and anisotropic rocks, many hydrofractures change their apertures on passing through layers with different mechanical properties. Alternatively, hydrofractures may become arrested at contacts (and other discontinuities) between layers. We present boundary-element models on hydrofracture arrest and aperture variation that focus on the effects of abrupt changes in Young’s modulus in layered reservoirs. The results show that, for internal fluid pressure as the only loading, high tensile stresses concentrate in the stiff layers. When approaching a stiff layer, the hydrofracture tip becomes sharp and narrow, and would normally continue its propagation through that layer. By contrast, soft layers suppress the tensile stresses associated with the hydrofracture tip and blunt the tip itself. Without a nearby weak, subvertical discontinuity that could open up, the hydrofracture tends to become arrested on meeting with a soft layer. When fluid over-pressure is the only loading, the aperture of a hydrofracture is normally larger in soft layers than that in stiff layers, which may lead to flow channelling in a layered reservoir.