53rd U.S. Rock Mechanics/Geomechanics Symposium,
New York City, New York
2019. American Rock Mechanics Association
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ABSTRACT: Drilling-induced tensile fractures (DITFs) are located orthogonally from wellbore breakouts and are observed from image logs obtained during drilling operations. Fully analytical criteria for the orientation of DITFs initiating from vertical and horizontal wellbores in porous, permeable media are derived considering fluid infiltration using Biot’s theory of poroelasticity. Independent variables are discussed and expressions for the initiation pressures are presented for each well configuration. Dimensionless plots are used to indicate the region of the possible stress state at a given DITF orientation and the in-situ stress regime. Parameters that affect the size of this region and hence the magnitude range of components of the local stress tensor is discussed. Subsequently, the diagnostic power of DITFs is demonstrated under different scenarios, where the DITF orientation relative to the wellbore can be used to constrain the local in-situ stress tensor. For vertical and horizontal wells, the magnitude of SHmaxcan be constrained with closed-form analytical solutions for the corresponding upper and lower bounds of the SHmaxmagnitude. If the DITF initiation location is near the casing shoe, numerical modeling is necessary for determining the DITF orientation. While, similar work has been done considering linearly-elastic media, the novelty of this work lies in considering fluid infiltration.
1. INTRODUCTION During drilling, tensile fractures initiate from the wellbore, opening against the least principal stress at a location perpendicular to wellbore breakouts on the borehole wall. The initiation mechanism of these drilling-induced tensile fractures (DITFs) is the same as that for hydraulic fractures induced during completions (Aadnoy, 1990; Brudy and Zoback, 1999; Nelson et al., 2005; Cao et al., 2017; Michael et al., 2018) although DITFs are only shallow cracks. Also similarly to completion-induced hydraulic fractures, DITFs can be oriented longitudinally (axially), or transversely (laterally) relative to the wellbore as shown on the schematic of Fig. 1.
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