What Does Core Discing Mean?
Core discing (or core disking) is an indicator of the in-situ stress state of rocks. When drilling a core, if the ratio of the induced stress to the strength of the rock is high enough, there is a change in stress. This stress change induced by excavation can lead to crack initiation, propagation, and coalescence causing rock failure such as core discing, damage, and spalling. Core discing is considered an indicator of high in-situ stress.
In the core discing phenomenon, there is a discing of the core with uniform spacing and shape due to the transient change in stress and the release of stress during drilling. Core discing can be solid core discing which occurs during normal coring, and ring core discing that occurs during overcoring. There are three major failure mechanisms of core discing namely:
- Tensile failure.
- Shear failure.
- Extension strain failure.
Core discing can be used to establish principal stress directions and to indicate far-field stress magnitudes in combination with other stress indicators and stress measurements. The shape of the disc gives a good estimate of the directions of the principal stress and is a good indicator of in-situ stress in regions where the horizontal in-situ stress is high.
In vertical boreholes, core discing is a result of high horizontal stresses. As maximum horizontal stress increases, the average disc thickness consistently declines. In vertical boreholes, discing can determine the direction and approximate ratio of the horizontal principal stresses. Any asymmetry in the disc can give a measure of the inclination of the principal stress away from the vertical.
Trenchlesspedia Explains Core Discing
Core discing is considered an indicator of high in-situ stress in the earth’s crust. It has been found that core discing in vertical boreholes is due to high horizontal stresses.
Core Discing Phenomenon
Core discing occurs at the coring bit-rock interface due to the tensile stress induced from drilling. During drilling, the stress concentrations across the core result in fractures. If these stresses are high enough on a brittle rock, the core discs into thin chips. If the discs are saddle-shaped, it is observed that the convex axis is aligned with the direction of minimum horizontal stress.
The saddle shape and the lineation markings on the surface of the core discs observed under a scanning electron microscope (SEM) help indicate the direction of maximum horizontal stress. Therefore, core discing is used to determine horizontal stress orientation. This method uses a naturally occurring event during coring and does not incur an additional cost. However, this technique cannot consistently provide reliable estimation hence it is only used as a stress indicator. Moreover, core discing occurs only in certain conditions when the stress concentrations overcome the tensile strength.
Failure Mechanism of Core Discing
Research by various scientists suggests three major failure mechanisms: tensile failure, shear failure, and extension strain failure, and their combinations.
Tensile failure mechanism: Core discing occurs when applied lateral stress reaches nearly double the tensile stress of the rock
Shear failure mechanism: Core discing begins at the point of maximum shear stress whose magnitude is much higher than the rock’s shear strength.
Extension strain failure mechanism: When the rock’s total extension strain exceeds a critical value, the rock begins to fracture. These fractures form in the plane normal to the direction of extension strain, corresponding to the direction of minimum principal stress.Combination failure mechanism:
Tensile stress plays a major role in initiating fractures that cause core discing as shear stress magnitudes by themselves are insufficient to cause fractures.