ABSTRACT:

This paper presents the use of two-dimensional models to analyze the behaviour of a pre-existing fracture intersecting a pressurized wellbore. The fracture has an arbitrary location and inclination with respect to the wellbore and the far-field state of stress and it is assumed that the wellbore fluid penetrates the fracture only in the section where the fracture is open. The analysis shows that the wellbore pressure and width responses, as a function of the length of the open section, are extremely sensitive to the location of the fracture with respect to the wellbore. In particular, situations exist where the near wellbore state of stress dominates the pressure response. This result has strong consequences for stress measurement and interpretation. This work has application not only to stress measurement, but also to wellbore stability and hydraulic fracturing.

1 INTRODUCTION

Downhole imaging techniques reveal that numerous discontinuities intersect a wellbore (Zemanek et al 1970, Shanks et al 1976, Suau and Gardner 1980, Paillet 1981, Heliot et al 1989, Plumb and Luthi 1989). Most of them are natural discontinuities such as fractures, faults, or bedding planes which have been penetrated during the drilling process. Others have been induced by the drilling process (Zemanek et al 1970, Laubach et al 1988), or even created on purpose for stress measurement when fracturing techniques (Scheideger 1962, Fairhurst 1964) are used. Induced or natural fractures can be reopened in extension if the wellbore pressure is high enough. This process is quite similar to the hydraulic fracturing process, where the wellbore pressure is increased up to the creation of a tensile crack. In hydraulic fracturing, it is expected that the fracture propagates normal to the direction of the far field minimum stress (assuming that the wellbore axis is a far field principal stress direction). As a result, the fracture propagates in a pure opening mode. This is not the case if the fracture pre-exists because it can have an arbitrary direction with respect to the principal stress directions. The opening of the pre-existing fracture will therefore generate some shear along the fracture plane. The pressure and width response during the initiation and propagation of a hydraulic fracture has been intensively studied, especially when the plane of the fracture contains the wellbore axis (Abou-Sayed et al 1978, Rummel 1987, Atkinson and Thiercelin 1993, Carbonell and Detournay 1995). Little is known, however, about the wellbore pressure and fracture behaviour during the injection of a fluid when the fracture has an arbitrary location with respect to the wellbore and the farfield state of stress. However, under- standing the pressure response and fracture behaviour in this situation is quite important as it has applications in stress measurement, wellbore stability analysis and stimulation: - In stress measurement, the pressurization of a natural fracture is called the H.T.P.F. (Hydraulic Tests on Pre-Existing Fracture) method (Cornet and Valette 1984). With this method, the closure stress is determined on pre-existing fractures by performing a series of fluid injections and shut-ins.

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