Drilling‐induced core fractures and in situ stress

Abstract

The relationship between the shapes of drilling-induced core fractures and the in situ state of stress is developed. The stress concentrations at the well bore bottom are first determined using a complete three-dimensional finite element analysis. Existing in situ compressional stresses generate large tensions in the immediate vicinity of the bottom hole which are sufficient to rupture the rock. Tensile fracture trajectories within these concentrated stress fields are predicted using a simple model of fracture propagation. These modeled fracture trajectories resemble well the observed shapes of drilling-induced core disking, petal, and petal-centerline fractures. Further, this agreement suggests that both the shape of the drilling-induced fracture and the location at which it initiates depends on the in situ stress state existing in the rock mass prior to drilling; the core fractures contain substantial information on in situ stress conditions. In all faulting regimes the coring-induced fractures initiate near the bit cut except for most cases under thrust faulting regime where the fracture initiates on the well bore axis. Further, under thrust faulting conditions only disk fractures appear possible. Both petal and disking fractures can be produced in strike-slip and' normal faulting regimes depending upon the relative magnitudes between the least compressive horizontal principal stress and the vertical overburden stress. The predicted fracture shapes are in good qualitative agreement with observations of drilling-induced fractures described in the literature from laboratory experiments and field programs in which in situ stresses are measured by other means. The relationship of the morphology of coring induced fractures and in situ stresses suggests that the fractures can be used as independent complementary indicators in identifying stress regimes.

Data report: tectonic and induced structures in the JFAST core

“…Characteristic petal-centerline fractures dip between 30°a nd 75° in a downcore direction near the core edge and curve to dip vertically near the center of the core (Kulander et al, 1990). Petal-centerline fractures form in response to an induced principal tension that rotates downward in a vertical plane from an inclined orientation to horizontal (Kulander et al, 1979), and the strike of the fracture surface is aligned with the direction of the greatest horizontal stress (Li and Schmitt, 1998). We observe no clear petal-centerline fractures in the JFAST cores, similar to other studies in thrust faulting regimes (Li and Schmitt, 1998).…”

“…We observe no clear petal-centerline fractures in the JFAST cores, similar to other studies in thrust faulting regimes (Li and Schmitt, 1998). However, in drill cores of well-indurated rock, petal-centerline fractures are commonly used to estimate in situ principal stress orientations (e.g., Li and Schmitt, 1998;Davatzes and Hickman, 2010).…”

“…We observe no clear petal-centerline fractures in the JFAST cores, similar to other studies in thrust faulting regimes (Li and Schmitt, 1998). However, in drill cores of well-indurated rock, petal-centerline fractures are commonly used to estimate in situ principal stress orientations (e.g., Li and Schmitt, 1998;Davatzes and Hickman, 2010).The second type of core axis-parallel induced fractures is typically smaller in scale and interpreted as related to both the coring and handling processes. Induced core axis-parallel fractures, sometimes several centimeters in length and open several millimeters, are best observed in Section 20R-2 (Fig.…”

“…Petal-centerline fractures form in response to an induced principal tension that rotates downward in a vertical plane from an inclined orientation to horizontal (Kulander et al, 1979), and the strike of the fracture surface is aligned with the direction of the greatest horizontal stress (Li and Schmitt, 1998). We observe no clear petal-centerline fractures in the JFAST cores, similar to other studies in thrust faulting regimes (Li and Schmitt, 1998). However, in drill cores of well-indurated rock, petal-centerline fractures are commonly used to estimate in situ principal stress orientations (e.g., Li and Schmitt, 1998;Davatzes and Hickman, 2010).…”

“…Additionally, two types of vertical induced fractures are recognized that vary in scale and orientation. The first are petalcenterline fractures, described in detail by Kulander et al (1979Kulander et al ( , 1990 and Li and Schmitt (1998). Characteristic petal-centerline fractures dip between 30°a nd 75° in a downcore direction near the core edge and curve to dip vertically near the center of the core (Kulander et al, 1990).…”