Discriminating between Drilling Induced and Natural Fractures in Wellbores: A Reappraisal of Criteria and Methodologies

Pedram Zarian
Geoscience, Houston, TX

The orientation, concentration and interconnectivity of natural open fractures are often the most significant issues affecting production in many gas fields in the Rocky Mountain region. Identification and characterization of highly fractured intervals using borehole imaging and other advanced wireline techniques can greatly benefit the field development. However, a fundamental challenge in wellbore fracture characterization is to distinguish between natural open fractures and drilling-induced fractures, especially in situations where they are parallel to each other. The discrimination becomes more difficult if the principal stress field is not coaxial with the borehole (deviated wellbores or non-vertical overburden stress). This study reappraises degrees of confidence in applying the discrimination criteria offered by many previous authors. Criteria such as: the geometry of fracture termination, spalling, en-echelon morphology and vertical offsets between the fracture pairs. The main ambiguity is in understanding the relative reliability of the different criteria and the potential effects on reservoir producibility.

Detailed analysis of fracture morphology on the borehole images from several wells in Wyoming provided the basis for this work. Wherever available core data were used to calibrate and integrate with borehole images. Further, the outcome of the fracture interpretations from borehole images and core analysis were integrated with cross-dipole acoustic anisotropy and Stoneley wave reflectivity data. The results of this study helped in establishing a set of confidence-weighted-criteria, the application of which can improve the prediction of performance of fractured reservoirs by reducing the uncertainty associated with the characterization of natural fracture parameters such as orientation, spacing, aperture and size.