Induced fractures and damage in diamond core

by Callen Banff, Manager Open Pit Geotechnical Engineering at AngloGold Ashanti

Introduction

Over USD 4 billion was spent globally on diamond core drilling services in 2023 (Spherical Insights, 2024) by industries such as mining, oil & gas, construction, infrastructure, and geothermal energy. Within these industries, many professionals rely on the recovered core to understand subsurface conditions and anticipate what lies ahead. For example:

Geoscientists (geologists and hydrogeologists) characterize lithology, stratigraphy, structure, alteration, mineralization, and groundwater systems to interpret geological history and develop predictive models of subsurface conditions.
Metallurgists assess geochemistry and recovery characteristics.
Geotechnical engineers (mining and civil) evaluate rock mass and soil behavior to assess excavation performance, ground stability, and foundation conditions.
Geothermal engineers evaluate lithology, fracture networks, and thermal properties to assess reservoir potential and system performance.

All of these disciplines depend on representative, high-quality samples to make sound decisions regarding project viability and safety. Given the importance of this dataset, it is concerning that core quality is often given limited consideration and that logging and handling are frequently undertaken by relatively inexperienced personnel. Drill-induced damage is thus commonly overlooked or misinterpreted as natural fracturing, with potentially significant downstream consequences. For example, heavily fractured zones induced by drilling have the potential to be misclassified as fault zones which confuses geological interpretations, adds project complexity, and can increase costs.

Because many controls lie within drilling practices, it is appropriate to reconsider traditional drilling key performance indicators (KPIs). Drill contracts are still commonly structured around cost and recovery targets (e.g. >95%), which can incentivize speed over sample quality. Modern drill rigs increasingly provide telemetry and measure-while-drilling (MWD) data that are readily auditable. It is therefore reasonable to question why parameter-based conformance (e.g. acceptable ranges of torque, weight on bit, vibration) has not been more widely adopted, particularly when such approaches are well established in the oil and gas industry.

This article is intended as a practical reference describing common damage types, their likely causes, and potential remediation actions. The objective is to support both early-career and experienced practitioners in recognizing damaged core and initiating informed discussions when sample quality appears compromised.

Damage controlled by drilling practices

Spiral (pinecone) fractures

Spiral fractures result from torque being transferred to the core. They are readily identified by a spiraling fracture pattern along its length. In half-core specimens, these fractures often resemble stacked conical segments, hence the alternative term ‘pinecone fractures’.

**Figure 1** – Spiral fracture example. Image courtesy of AGA.

In principle, the inner barrel should rotate independently of the drill string and torque should never be applied to the rock. However, a core break must be induced to allow retrieval of the inner tube using the overshot assembly. This should occur by lifting the drill string, which locks the inner barrel and creates a break at the bit interface. If the rod string rotates during this process, torque can be transferred into the core, inducing spiral fractures. Although this practice is sometimes used deliberately to facilitate breaking, it should be strongly discouraged as it can significantly degrade core orientation reliability.

A recent discussion (Cowan, 2026) illustrated the development of anti-clockwise beta smear on stereonets as a result of torque effects. Identifying such deformation is critical for structural geology and geotechnical interpretation, as it can compromise orientation data and reduce confidence in structural measurements used for design.

If spiral fractures are identified during logging, or systematic anti-clockwise skew is observed in stereonets, this should trigger a discussion with the driller to review recovery procedures.

Core-spin

Core-spin is among the most recognizable forms of drill-induced damage. These breaks occur at the drilling interface and are typically perpendicular to the core axis. The core effectively rotates against itself until sufficient friction allows the inner barrel to re-engage. In competent rock, the resulting surface is often polished with concentric circular markings; in weaker rock, circular ridges may be visible.

Core-spin breaks mark a loss of orientation confidence, as the downhole section of core may have rotated to an unknown position. Orientation can only be tentatively inferred where strong, consistent fabric (e.g. bedding) is present; however, this approach is unreliable when the fabric is orthogonal to the core axis and can itself contribute to the development of core-spin.

Geological factors such as bedding, foliation, or other anisotropy oriented perpendicular to the core axis increase the likelihood of core-spin by introducing preferential weakness planes along which rotation can initiate.

Although occasional occurrences of it are common, frequent core-spin typically indicates an external influence temporarily increasing applied force. Contributing factors may include pump fluctuations, excessive rig vibration, or high rod friction. In practice, adjustments to drilling fluid properties or additional lubrication can often mitigate the issue.

A useful operational indicator is alternating fast and slow rod rotation, which can suggest the rods are sticking. This behavior is analogous to elastic energy storage and release (similar to a twisted rubber band), where accumulated torsion is suddenly released, contributing to core breakage. Differential polishing of the core, such as shown in Figure 3, is an indicator that extra attention is needed when considering if perpendicular breaks across the core are natural or induced.

**Figure 3** – Differential polishing example. Image courtesy of AGA.

Mushroom core

Mushroom core represents an extreme form of core-spin in weak rock. Excessive water flow at the face can weaken the material to the point where it is abraded rather than fractured, leading to significant material loss and formation of clay-like coatings on subsequent runs.

**Figure 4** – ‘Mushroom’ example. Images courtesy of AGA.

Although the specific bit type is often unknown in retrospective examples, field experience suggests that open-channel bits exacerbate this issue in weak formations. Face-discharge bits, which direct flow toward the cutting interface and away from the recovered core, are likely to reduce this risk and should be considered where weak lithologies are anticipated.

Standard open channel bit and face-discharge bit — **Figure 5** – Standard open-channel bit (left) and face-discharge bit (right) (Boart Longyear, 2024)

Elephant (alligator) skin

The rough, scaly appearance observed on core surfaces is commonly attributed to vibration-induced damage during drilling. The root causes can be varied and may include excessive weight on bit, excessive torque, or loss of circulation.

Beyond aesthetic concerns, such damage often renders core unsuitable for geotechnical testing, as surface disturbance can significantly reduce apparent strength. Where this condition affects substantial intervals, additional drilling may be required to obtain representative specimens.

In most cases, pronounced vibration indicates that the rig is being operated beyond optimal conditions. Excessive vibration is typically perceptible to experienced drillers and should prompt immediate investigation before drilling continues.

**Figure 6** – Alligator or elephant skin caused by chattering. Images courtesy of AGA.

Friction burns

Friction burns are of particular operational importance as they commonly indicate overheating of the drill bit, often due to insufficient cooling. Causes may include pump failure, reduced flow, loss of circulation into cavities, or blocked channels.

Friction burn on fresh core and drill bit with all cutting matrix work away — **Figure 7** – Example of friction burn on fresh core (left) and example of a drill bit with all cutting matrix work away (right). Images courtesy of AGA.

In severe cases, overheating can lead to catastrophic bit failure. The presence of metallic filings within core can also indicate abnormal wear and should prompt inspection of the bit and string.

**Figure 8** – Steel flecks and fingernail-like fragments shown sitting on core surface. Image courtesy of AGA.

Boart Longyear (2022) provided a comprehensive diagnostic guide for drill bit and string wear. The document is a practical and accessible reference for operational troubleshooting and a good resource to have in the toolkit.

Variable diameter core

Core diameter variability can result from several mechanisms. Gradual increases in diameter commonly reflect bit wear, as the internal face typically wears faster than the external. While this effect is commonly minor, it is included here for completeness.

Of greater concern are abrupt diameter changes that introduce stress concentrations and premature breakage in laboratory specimens. One cause is excessive weight on bit, which can induce a bowing effect at the drill face and lead to uneven loading. Examples from oil and gas demonstrate that extreme cases can produce corkscrew-like core geometries. Such effects are generally controllable through appropriate management of thrust and operating parameters.

**Figure 9** – Core bulge and divot. Images courtesy of AGA.

**Figure 10** – Examples of variable diameter core (Lorenz & Cooper, 2018)

Longer-wavelength undulations in core are more consistent with drilling using a bent barrel. In such cases, rods should be withdrawn and the faulty component replaced promptly, as continued drilling increases the risk of downhole equipment loss.

**Figure 11** – ‘Wobbly’ core example. Image courtesy of AGA.

Oval or ‘football-shaped’ core fragments are commonly produced when pieces fall from the inner barrel and are subsequently over-cored. While occasional occurrences are difficult to avoid, persistent repetition may indicate that lifters require replacement or that fingers should be installed in the inner assembly. Repeated mid-run loss of core should prompt a direct review with the driller.

**Figure 12** – Football-shaped core caused by over-coring. Images courtesy of AGA.

**Figure 13** – Partially over-cored example. Image courtesy of Brett Davis.

River rocks / tumbled core

Tumbled core refers to rounded fragments resulting from repeated impact and abrasion within the barrel. This is widely recognized and generally easy to identify.

**Figure 14** – Broken and tumbled core. Image courtesy of AGA.

While often minor, this condition can also reflect suboptimal drilling practices. For example, drilling beyond tube length in an attempt to wedge the core may worsen fragmentation rather than improve recovery. Such practices should be discouraged, as they compromise sample quality.

**Figure 15** – Induced axial break. Image courtesy of AGA.

Stress-induced damage mechanisms

Some drill-induced structures appear to be influenced by the in-situ stress field. These features may provide valuable insight for structural geology, seismology, and geotechnical engineering. This area remains under-researched, but emerging studies suggest that such observations warrant greater attention.

The intent of this section is not to present definitive conclusions, but to highlight phenomena that may offer future value if systematically observed and documented.

Drilling induced segmentation

The term ‘Drilling Induced Segmentation’ (DIS) is used here to distinguish this phenomenon from classical discing. This is currently a working descriptor rather than a formally established classification. In this context, DIS refers to incipient disc-like features observed at shallow depth or under relatively low confinement. Laboratory work by Tang et al. (2023) demonstrated that disc-like features consistent with DIS could be induced in granite blocks without confining stress through increased weight on bit, and that these effects correlated with MWD parameters such as torque.

In the oil and gas industry, the concept of critical weight on bit (typically ~50% of expected rock strength) has been applied to minimize coring-induced damage (Hettema et al., 2002). While this specific threshold may not be directly transferable to other industries, it raises the possibility that alternative operational thresholds could improve recovery and reduce damage. Such approaches would require validation and adjustment for the drilling application but represent a potentially valuable avenue for future research.

Another variation of low-stress discing can be attributed to vibration. In the example below, lost circulation caused the rig to vibrate and the core to break as it was drilled. Due to the loss of drill fluids, it could also be argued the weight on bit could have been a factor if the pull back pressure wasn’t adequate to deal with the loss of buoyancy provided by a full drill column.

**Figure 17** – Vibration-induced DIS example. Image courtesy of AGA.

Petal and petal-centerline fractures

Petal and petal-centerline fractures, well documented in oil and gas literature, are typically induced in the direction of minimum horizontal stress. These fractures therefore have potential value as indicators of stress orientation.

They are characterized by short, concave, parabola-like fractures that converge toward the core centerline. They may occur on one or both sides of the core, but do not fully transect the diameter.

**Figure 18** – Petal-type fractures (Yatsenko, 2008)

Their formation appears to be influenced by both stress conditions and applied weight on bit. Although not widely applied outside of the oil and gas industry, their recognition could provide useful supplementary information on stress directions where present.

**Figure 19** – Example of petal fractures (Lorenz & Cooper, 2018)

Saddle fractures

Saddle fractures resemble petal-type features but occur under different stress regimes. They are more likely where horizontal stresses exceed vertical stress (analogous to thrust faulting when drilling from surface).

**Figure 20** – Saddle terminology as described by Ziegler & Valley, 2021

They may also be more prevalent when drilling from underground, where the drilling direction alters the relationship between applied load and in-situ stress. Where identified, the orientation of the saddle trough may indicate the direction of maximum stress.

**Figure 21** – Example of saddle-type fractures. Image courtesy of Brett Davis.

Cupping

Cupping is often grouped with discing, but field examples indicate that cup-and-bowl shaped fractures can also form outside highly stressed environments. These breaks appear tensile in nature and lack clear indicators of rotational movement associated with core-spin.

**Figure 22** – Cup mechanism, hypothesized to be created by shock-loading the core. Image courtesy of AGA.

The mechanism remains under investigation. Hypotheses include shock-related Ortlepp shear processes or lower-frequency percussive loading ahead of the bit. Until laboratory work confirms the dominant mechanism, appropriate controls can only be inferred, but consistent drilling parameters, control of vibration, and careful management of weight on bit are likely to reduce the likelihood of such damage.

Discing and plumose (feathering) structures

Discing and plumose (feathering) structures appear to share related fracture mechanisms. In fine-grained, weaker lithologies such as shale, feather patterns may be clearly visible and can indicate the direction of maximum horizontal stress. In stronger rocks where classical discing occurs, such features may not be apparent at the macro-scale but have been identified using scanning electron microscopy (Xiang et al., 2022). The growth pattern in plumose or traditional discing occurs in the direction of the greatest horizontal stress.

These observations suggest potential for estimating both stress orientation and magnitude where disc thickness can be reliably measured (Lim & Martin, 2010). Application of these concepts in routine industry practice remains limited and should be approached with appropriate caution.

**Figure 23** – Annotated example of a plumose structure (Kulander et al., n.d.)

Some concluding remarks

Despite the substantial investment in drilling, significant uncertainty remains in how data quality is controlled and optimized. There remains a clear need for geologists and engineers to maintain direct engagement with drilling operations and develop familiarity with common failure modes.

Oil and gas industries have long benefited from systematic use of MWD data. With improved connectivity now widely available, there is no technical barrier to closer integration between field and office. Parameter-based drilling ‘playbooks’ tailored to lithology could support real-time decision-making and improve overall sample quality.

KPI frameworks may benefit from evolving beyond recovery-based metrics toward conformance with defined operational envelopes. As demonstrated throughout this article, 100% recovery does not guarantee that core is fit for geotechnical or structural purposes if disturbance is significant.

Even relatively simple measures, such as mandatory camera systems at drill pads, could provide benefits for quality assurance, safety culture, and root-cause investigation.

Acknowledgements

The author thanks AngloGold Ashanti for supporting the sharing of these learnings and acknowledges the contributions of colleagues who have assisted in collecting examples and imagery to support this visual guide.

References

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Xiang, P., Ji, H., Geng, J., & Zhao, Y. (2022). Characteristics and mechanical mechansims of in situ unloading damage and core discing in deep rock mass of metal mine. Shock and Vibration.
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