Coring-induced fractures have recently become a reliable source of information on the orientation of the in-situ stress field. There are two important aspects to the use of coring-induced fractures for stress information. First, the coring-induced fracture must be distinguished from natural fractures, and second, a clear understanding of the fracture, its source, and its relation to drilling parameters must be made. The criteria for the distinction between artificial and natural fractures in core are not always obvious, and the possible mechanisms for the formation of many types of artificial fractures are poorly understood. Kulander and others and Pendexter and Rohn listed recognition characteristics that differentiate induced from natural fractures in core, and a new and comprehensive volume on the identification and logging of core fractures has been published. Kulander and others determined that induced fractures are extensional in origin and describe several types of induced fractures in core. Only petal/petal- centerline and scribeline fractures are appropriate here.

The significance of coring- and drilling- induced fractures lies primarily in the relationship of their orientation to the in-situ horizontal stresses. If induced fractures are found in oriented core, the true orientation of the in-situ stresses can usually be determined. If induced fractures are found in unoriented core but in conjunction with natural fractures, at least the relative angle between the natural fracture trend and a hydraulic stimulation fracture can be determined. If borehole image log data is subsequently acquired after coring, it may be possible to quantitatively resolve frac azimuths.

The most fundamental principle for discriminating between natural fractures and all types of induced fractures, tensile or shear: Induced fractures are geometrically related to the wellbore but natural fractures are not. This relationship causes the traces of natural fractures that do not cross the entire wellbore to appear at different azimuthal positions in image logs even if the fractures have similar orientations. However, the traces of induced fractures that do not cross the entire wellbore tend to stack in depth.

Petal, centerline, and petal-centerline fractures form ahead of the bit during both coring and normal drilling operations. They normally extend beyond the final borehole diameter so that they can usually be correlated between core and image logs. The direction of fracture propagation is easily determined in core and is always downhole. Petal fractures form just ahead of the bit and are due to excessive bit weight. Centerline fractures propagate ahead of the bit but probably within approximately 1/2 meter of the bottom of the hole. Centerline fractures are driven by a combination of mud pressure and bit-induced stresses.

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Aspect of petal-centerline fractures			Petal fracture & natural fracture