Breakout Formation Research Articles

Experimental investigation of borehole breakout formation in Gosford sandstone

This study investigates the in-situ borehole breakout formation using true-triaxial laboratory experiments on cubic Gosford sandstone. Four series of tests have been conducted under varying stress conditions following two approaches: PD method (loading onto specimens with pre-drilled boreholes) and PS method (loading onto intact specimens and then conducting drilling). Borehole camera observations show that a large amount of rock debris remains within the breakout zone under the PD conditions, while clear V-shaped notches are observed in the PS tests. The breakout geometries are extracted based on point cloud data obtained from 3D scanning. Both breakout width and normalised depth under both testing approaches exhibit positive correlations with the maximum horizontal stress while decreasing with minimum horizontal stress. The PS tests tend to yield wider and deeper breakouts compared to the PD tests. The difference in breakout width is generally consistent across various stress conditions at approximately 10° – 15°, whereas the variation in the normalised breakout depth increases with the severity of the borehole failure. The discrepancies are believed to be attributed to the excavation unloading induced strength-weakening and removal of rock debris by circulating fluid and drilling vibration. The outcomes of this study demonstrate that the stress path has critical impacts on the rock failure behaviours around openings, and the PS tests are suggested for investigating the borehole breakout phenomenon.

Numerical investigation of borehole breakout and rock spalling based on strain energy criteria

Borehole breakout is a natural phenomenon resulting from in-situ stress redistribution around a borehole, and its orientation and geometries are critical indicators for in-situ stress direction and magnitudes. The aim of the study is to better understand the borehole breakout formation mechanism and reproduce breakout geometries using numerical simulations. A 2D bonded-particle model (BPM) is developed based on Tenino sandstone using particle flow code (PFC), and simulations are conducted by applying different maximum horizontal stress values with the constant minimum horizontal stresses to a synthetic rock sample. V-shaped failure patterns are observed, and the obtained breakout geometries are compared with the published true-triaxial experimental results on the same rock properties and loading conditions. The modelled breakout widths are generally larger than the experimental results due to the lack of vertical stress in two-dimensional BPM simulation. For breakout depth, it is found that some broken particles remain within the breakout zone, constraining the damage zone development and thus withstanding more internal loads. Consequently, the breakout propagation process is suppressed, and the simulated breakout depths are significantly lower than the experimental values. To overcome the limitations of the breakout simulation, a particle removal algorithm using the strain energy release criteria is developed and implemented into the model. With the embedding of the algorithm, the breakout propagation process in the vicinity of the borehole shows a good agreement with the experimental breakout test results, indicating effective removal of failed rock is critical for accurately simulating borehole breakout. Model results from this study can provide new insights into the formation mechanism of V-shaped borehole breakout, as well as strain energy evolution during breakout initiation and propagation.

Borehole deformation based in situ stress estimation using televiewer data

The knowledge of in situ stress is critical in safe and optimised extraction of minerals and energy resources. In situ stresses are either measured directly (e.g. overcoring) or estimated indirectly (e.g. borehole breakouts). Borehole breakout analysis for in situ stress estimation is considered as a relatively simple and cost-efficient technique. This technique, however, poses certain limitations such as progressive formation of breakouts and requires inputs such as rock failure parameters that are not often available. As a result, significant effort has been made to develop new indirect methods for in situ stress estimation. Borehole deformation analysis using four-arm caliper has been recently proposed for in situ stress estimation and has shown promising results. In this study, we demonstrate a new methodology that analyses the borehole televiewer data with the technique of borehole deformation analysis to estimate the in situ stresses through a field case study. The advantages and challenges of using borehole televiewer data for stress estimation based on borehole deformation analysis are discussed and the best practice to obtain the reliable results is explained. The limitations of using four-arm caliper and borehole deformation analysis for in situ stress estimation are also discussed and it is shown how televiewer data can overcome such limitations. Finally, the in situ stress results obtained from televiewer data and borehole deformation analysis are compared with independent in situ stress measurements to show the robustness and reliability of the proposed methodology.

Experimental visualization of the wear and scuffing evolution of a flake graphite cast iron cylinder liner

Scuffing is a wear mechanism that can lead to catastrophic failure of various technical applications. Even though the mechanism has been researched in countless publications for decades, it is not fully understood yet. An experimental approach was used in this paper to investigate the origin and evolution of scuffing in a ring-on-liner contact. For this purpose, test runs in an application-oriented test rig were stopped at different points in time. A linear tribometer was used to transfer a reciprocating motion of a real piston ring segment to a specimen made from a real cylinder liner. Based on a comprehensive analysis, a four-stage scuffing hypothesis for a ring-on-liner contact was developed. In the first stage, smearing of the honing structure and the formation of surface cracks lead to the development of a scaly structure on the liner surface. Crack growth induced by adhesive shear forces leads to the formation of breakouts in the liner surface in the second stage. Both stages are stable and only the specific load increase of the used test strategy causes the transition to stage three. Here, large crack growth induced breakouts result in the local destruction of tribofilms and, subsequently in local metallic contact between the specimens. When this local destruction becomes rampant and expands to a macro scale, adhesive bonds lead to macroscopic scuffing. It can be deduced that the first occurring damage stage is a result of a surface fatigue process.

Discrete Element Method Simulation of Borehole Breakout Based on the Strain Energy Concept

This study focused on studying the V-shaped rock failure around borehole walls and reproducing borehole breakout using numerical simulations. Discrete element method (DEM) is adopted, and a two-dimensional particle flow code (PFC2D) model is built based on Tenino sandstone. The simulations are conducted by applying different maximum horizontal stress values under the same minimum horizontal stress condition, and the results are compared with the true-triaxial breakout experiments on the same rock properties and loading conditions. Due to the absence of vertical stress in 2D simulations, the obtained angular spans are generally larger than the experimental values. On the other hand, the breakout depths are found to be significantly smaller than the experimental results since the failed particles within the breakout zone remained to withstand internal loads and constrain the damage zone. In order to better simulate the rock detachment process during breakout formation, a particle removal algorithm using the strain energy release concept is developed and embedded into the model. With the implementation of the algorithm, the breakout propagation process in the vicinity of the borehole shows a good agreement with the experimental observations, suggesting that the effective removal of failed particles is essential for accurately simulating borehole breakout. The results from this study can provide new insights into the V-shaped breakout formation mechanism and strain energy changes during borehole breakout initiation and propagation.

Tool damage state condition monitoring in milling processes based on the mechanistic model goodness-of-fit metrics

Monitoring of the end milling tool wear in real-time is very crucial for the quality control of a surface finish. In the model-based approaches, usually the tool wear is monitored by tracking the force model coefficients during the milling operation. In this work, we explore a monitoring approach based on two metrics derived from the goodness-of-fit of the milling mechanistic model. The cumulative sum control charts are constructed to monitor changes occurring to the tool condition during a milling operation. Compared to previously suggested methods based solely on the values of force model coefficients, our approach facilitates a preciser and less conservative identification of the point of transition from homogeneous wear to onset of breakout formation at the cutting edges. The proposed strategy is an alternative approach for real time monitoring of tool condition based on mechanistic models.

Analysis of fracture nucleation around wellbores

Crack nucleation has been the subject of important contributions in the two last decades. Starting from the double criterion of Leguillon (Eur J Mech A 21:61–72, 2002), the present paper suggests that an energy criterion is sufficient for the prediction of the loading level that triggers nucleation as well as its geometrical characteristics. In view of application to saturated porous materials, the role of the pore fluid during nucleation is considered. In case of low fluid compressibility, the presence of the fluid delays the brittle failure usually associated with nucleation, as long as the fluid pressure remains above the saturation vapour pressure. The energy nucleation criterion is then applied to discuss the formation of breakouts around wellbores subjected to remote compressive stresses.

Lithological Constraints on Borehole Wall Failure; a Study on the Pennine Coal Measures of the United Kingdom

Stress-related borehole deformation features have been documented across the UK, most commonly as borehole breakouts and drilling induced tensile fractures. Recent studies using borehole imaging have allowed more detailed investigation of these features and the processes that control their formation. Within the Pennsylvanian Pennine Coal Measures Group (PCM) of the UK borehole imaging has highlighted a disproportionately high number of breakouts occurring within palaeosols located immediately below coal seams. To understand the processes controlling breakout formation, a 10.5 m section of core from the Melbourne 1 borehole, incorporating a typical coal seam / palaeosol sequence, was analysed using multiple techniques including: scanning electron and optical microscopy, X-Ray Fluorescence (XRF), X-ray radiography, Point Load testing, wireline petrophysics and track-based core scanning for physical properties. Strength measurements highlight that breakouts form preferentially in poorly consolidated sediments, with low tensile strength, cross-cut by listric surfaces. The formation and termination of breakouts also corresponds to zones of diagenetic iron mineral growth with a lower propensity to fail. These coincide with greater preservation of sedimentary structures and an increase in the rock’s tensile strength; this intra-unit variation in tensile strength constrains breakout length. This demonstrates that secondary diagenetic processes, including the growth of iron minerals impose, lithological controls on the formation and length of borehole breakouts within the UK PCM.

The origin and evolution of breakouts in a cooling-limited rhyolite lava flow

Understanding lava flow processes is important for interpreting existing lavas and for hazard assessments. Although substantial progress has been made for basaltic lavas our understanding of silicic lava flows has seen limited recent advance. In particular, the formation of lava flow breakouts, which represent a characteristic process in cooling limited basaltic lavas, but has not been described in established models of rhyolite emplacement. Using data from the 2011–2012 rhyolite eruption of Puyehue-Cordon Caulle, Chile, we develop the first conceptual framework to classify breakout types in silicic lavas, and to describe the processes involved in their progressive growth, inflation, and morphological change. By integrating multiscale satellite, field, and textural data from Cordon Caulle, we interpret breakout formation to be driven by a combination of pressure increase (from local vesiculation in the lava flow core, as well as from continued supply via extended thermally preferential pathways) and a weakening of the surface crust through lateral spreading and fracturing. Small breakouts, potentially resulting more from local vesiculation than from continued magma supply, show a domed morphology, developing into petaloid as inflation increasingly fractures the surface crust. Continued growth and fracturing results in a rubbly morphology, with the most inflated breakouts developing into a cleft-split morphology, reminiscent of tumulus inflation structures seen in basalts. These distinct morphological classes result from the evolving relative contributions of continued breakout advance and inflation. The extended nature of some breakouts highlights the role of lava supply under a stationary crust, a process ubiquitous in inflating basalt lava flows that reflects the presence of thermally preferential pathways. Textural analyses of the Cordon Caulle breakouts also emphasize the importance of late-stage volatile exsolution and vesiculation within the lava flow. Although breakouts occur across the compositional spectrum of lava flows, the greater magma viscosity is likely to make late-stage vesiculation much more important for breakout development in silicic lavas than in basalts. Such late-stage vesiculation has direct implications for hazards previously recognized from silicic lava flows, enhancing the likelihood of flow front collapse, and explosive decompression of the lava core.

Emplacing a Cooling-Limited Rhyolite Lava Flow: Similarities with Basaltic Lava Flows

Accurate forecasts of lava flow length rely on estimates of eruption and magma properties and, potentially more challengingly, an understanding of the relative influence of characteristics such as the apparent viscosity, the yield strength of the flow core, or the strength of the surface crust. Consequently, even the most straightforward models of lava advance involve sufficient parameters that constraints can be relatively easily fitted within the uncertainties involved, at the expense of gaining insight. Here, for the first time, we incorporate morphological observations from during and after flow field evolution to improve model constraints and reduce uncertainties. After demonstrating the approach on a basaltic lava flow (Mt. Etna, 2001), we apply it to the 2011-12 Cordon Caulle rhyolite flow, where unprecedented observations and syn-emplacement satellite imagery of an advancing silica-rich lava flow have indicated an important crustal influence on flow emplacement. Our results show that an initial phase of viscosity-controlled advance at Cordon Caulle was followed by later crustal control, accompanied by formation of flow surface folds and large-scale crustal fractures. Where the lava was unconstrained by topography, the cooled crust ultimately halted advance of the main flow and led to the formation of breakouts from the flow front and margins, influencing the footprint of the lava, its advance rate, and the duration of flow advance. Highly similar behaviour occurred in the 2001 Etna basaltic lava flow. The processes controlling the advance of crystal-poor rhyolite and basaltic lava flow therefore appear similar, indicating common controlling mechanisms that transcend profound rheological and compositional differences.

Image log analysis of in situ stress orientation, breakout growth, and natural geologic structures to 2.5 km depth in central Scandinavian Caledonides: Results from the COSC‐1 borehole

AbstractStress‐induced borehole deformation analysis in the Collisional Orogeny in the Scandinavian Caledonide deep scientific borehole establishes in situ stress orientation in a poorly characterized region in central Sweden. Two acoustic televiewer logging campaigns, with more than 1 year between campaigns, provide detailed images along the full length of the 2.5 km deep borehole for breakout, drilling‐induced tensile fracture (DITF), and natural occurring structural analysis. Borehole breakouts occur in 13 distinct zones along total length of 22 m, indicating an average maximum horizontal stress, SHmax, orientation of 127° ± 12°. Infrequent DITFs are constrained within one zone from 786 to 787 m depth (SHmax orientation: 121° ± 07°). These SHmax orientations are in agreement with the general trend in Scandinavia and are in accordance with many mechanisms that generate crustal stress (e.g., ridge push, topographic loading, and mantel driven stresses). The unique acquisition of image logs in two successions allows for analysis of time‐dependent borehole deformation, indicating that six breakout zones have crept, both along the borehole axis and radially around the borehole. Strong dynamic moduli measured on core samples and an inferred weak in situ stress anisotropy inhibit the formation of breakouts and DITFs. Natural fracture orientation below 800 m is congruent to extensional or hybrid brittle shear failure along the same trend as the current SHmax. Analysis of foliation in the image logs reinforces the interpretation that the discontinuous seismic reflectors with fluctuating dip observed in seismic profiles are due to recumbent folding and boudinage.

Influence of Bedding Angle on Borehole Stability: A Laboratory Investigation of Transverse Isotropic Oil Shale

The stability of wells drilled into bedded formations, e.g., shales, depends on the orientation between the bedding and the borehole axis. If the borehole is drilled sub-parallel to bedding, the risk of borehole instabilities increases significantly. In this study, we examined the formation of stress-induced borehole breakouts in Posidonia shale by performing a series of thick-walled hollow cylinder experiments with varying orientations of the bedding plane with respect to the borehole axis. The thick-walled hollow cylinders (40 mm in diameter and 80 mm in length containing an 8 mm diameter borehole) were loaded isostatically until formation of breakouts. The onset of borehole breakout development was determined by means of acoustic emission activity, strain measurements, ultrasonic velocities and amplitudes. The critical pressure for breakout initiation decreased from 151 MPa by approximately 65 % as the bedding plane inclination changed from normal to parallel to the borehole axis. The finely bedded structure in the shale resulted in an anisotropy in elasticity and strength from which the variation in strength dominated the integrity of the thick-walled hollow cylinders.

Time-dependent brittle creep as a mechanism for time-delayed wellbore failure

By drilling a wellbore cavity, high stresses arise at the wellbore wall, leading to the formation of breakouts which enlarge the hole to an elongated shape oriented along the direction of the minimum principal stress. If formation of breakouts is delayed, rock debris falls on the drill bit which may lead to stuck pipe problems or even abandonment of the drill string. Reasons for such time-delayed failure of the wellbore may be due to chemical fluid–rock interaction, especially in swelling clays. However, such delayed instabilities have also been observed e.g. in gneiss formations at the KTB borehole (Germany) that are not known to exhibit a swelling behavior. We propose to explain observations of delayed wellbore failure by time-dependent brittle creep, which has been observed for many types of rocks. Following this approach, rock fails under loads less than their short-time strength but after a long enough time span. This time is in exponential relation to the load applied to the rock. We implement a model developed for the creation of shear bands on the basis of time-dependent brittle creep by Amitrano and Helmstetter (2006). Here, progressive damage of the formation is captured by a damage parameter D and the time-to-failure TTF. Young׳s modulus E is decreased by a factor every time TTF is expired, i.e. when failure is reached. Subsequently, stresses are redistributed according to the new distribution of E in the formation. Using this approach, we obtain closure of the well with primary and secondary creep phases. Wellbore breakouts are formed progressively with deepening and widening of the initially damaged zone. After a certain time the formation of breakouts comes to an end with an Omori-like decay of failure approaching a steady-state.

Influence of borehole diameter on the formation of borehole breakouts in black shale

We performed a series of borehole breakout tests on Posidonia shale to study the influence of borehole diameter on borehole stability in unconventional black shale. Thick-walled cylindrical samples (20–80mm long, 10–50mm in diameter) with varying borehole diameters of 1–19mm drilled perpendicular to bedding were loaded under increasing hydrostatic pressures until formation of borehole breakouts. The critical hydrostatic pressure for breakout formation decreased significantly from about 275MPa for samples with 1mm borehole diameter, to 89MPa for those containing a 19mm borehole, approaching the uniaxial compressive strength of the material. The observed scale effect was fitted to various failure criteria predicting fracture initiation at a depth of about 0.15mm around the borehole wall in the fine-grained shale.

Study on mechanisms of burr formation and edge breakout near the exit of orthogonal cutting

The scanning electron microscope (SEM) technique had been used as an instrument to study chip formation mechanism in metal cutting. In this paper the mechanisms of burr formation and edge breakout near the exit of orthogonal cutting are analyzed, based on the direct observation using a machining substage inside the SEM. It is found that a negative deformation plane begins to form when the steady-state chip formation stops as the tool approaches the end of the cut. Plastic bending and shearing of the negative deformation plane are the dominant mechanisms in burr formation whereas crack propagation along the plane causes the edge breakout. Two modes of breakout formation were observed. Fracture mechanisms involved during the cut were identified by examining the fracture surfaces obtained from the impact machining tests. These observations are of crucial importance to the proper mathematical formulation and development of a burr/breakout model in orthogonal cutting.

Effect of stress fields on magma chamber stability and the formation of collapse calderas

The summits of many of the Earth’s and other planets’ larger volcanoes are occupied by calderas that formed by collapse into an evacuating, underlying magma chamber. These collapse calderas are typically several tens of square kilometers in area and are commonly elliptical in shape. We show that the long axes of late Quaternary collapse calderas in the Kenya rift valley, the western Basin and Range province, the Snake River‐Yellowstone Plateau, and the Iceland rift zone are parallel to the upper crustal minimum horizontal stress direction (Sh) as determined by independent criteria. We suggest that circular magma chambers beneath these volcanoes became elliptical by stress‐induced spalling of their chamber walls, by a mechanism that is analogous to the formation of breakouts in boreholes and tunnels. In breakouts, the hole becomes elongate parallel to the far‐field minimum stress. In the Kenya rift, Late Pleistocene caldera collapse was accompanied by a 45° rotation of Sh and an increase in the magnitude of the maximum horizontal stress (SH). The breakout model predicts increasingly unstable caldera walls under these conditions, a possible explanation for the sudden appearance of so many collapse events in a volcanic setting that had never experienced them before. This mechanism of stress change‐induced collapse may have played a role in other caldera settings.

Borehole instability in high-porosity Berea sandstone and factors affecting dimensions and shape of fracture-like breakouts

Stress-induced breakouts in vertical boreholes are failure zones caused by excessive compressive stress concentration at the borehole wall along the springline of the least horizontal far-field stress. Wellbores are sometimes drilled into aquifers or oil reservoirs that are weak, poorly consolidated, and highly porous sandstone formations, which are often conducive to breakout formation. Breakouts are an expression of borehole instability and a potential source of sand production. On the other hand, the breakout phenomenon can be used advantageously in obtaining an estimate of the in situ stress condition. The average orientation of breakouts, as identified by borehole geophysical logging, is a reliable indicator of in situ stress directions. It has also been suggested that breakout dimensions could potentially be used as indicators of in situ stress magnitudes. The research reported here has concentrated on the unique type of breakouts observed for the first time in high-porosity Berea sandstone. Drilling experiments in rock blocks subjected to critical far-field true triaxial stress regimes, simulating in situ conditions, induced breakouts that were unlike the ‘dog-ear’ ones previously observed in granites, limestones, and low-porosity sandstones. The newly observed breakouts were thin, tabular, and very long, resembling fractures that counterintuitively extended perpendicular to the maximum principal stress. We found that a narrow zone ahead of a fracture-like breakout tip underwent apparent localized grain debonding and compaction. In the field, such zones have been termed ‘compaction bands’, and are a source of concern because in oil fields and aquifers they constitute curtains of low permeability that can impede the normal flow of oil or water. In order to determine whether a correlation exists between fracture-like breakouts and in situ stress, we conducted several series of tests in which the minimum horizontal and vertical stresses were held constant and the maximum horizontal stress ( σ H) was increased from test to test. These tests showed strong dependence of the breakout length on far-field stress, signaling that potentially the ability to assess fracture-like breakout length in the field could be used to estimate in situ stress magnitudes in conjunction with other indicators. Another series of tests revealed that breakout length increased substantially when borehole diameter was enlarged. This result suggested that in the field, where wellbore size is considerably larger, fracture-like breakout could extend to sizable distances, creating a sand production hazard. Two series of tests, one to evaluate the effect of drill-bit penetration rate, and the other to verify the drilling-fluid flow rate effect on breakout formation and dimensions yielded inconsistent results and showed no unique trends. Remarkably, fracture-like breakouts maintained a consistent narrow width of about 5–10 grain diameters, irrespective of the test conditions. This characteristic supports the suggestion that fracture-like breakouts are emptied compaction bands.

Stress Concentration at the Bottom of a Borehole and its Effect on Borehole Breakout Formation

Stress concentration at the bottom of a borehole due to the corners with small radius of curvature in an axial section and its effect on the azimuth of breakout was studied. To this end, a 3-D finite element analysis was conducted and the stress around the borehole was examined for boreholes arbitrarily oriented to three principal axes of remote stress. Results show that, in the case of high strength rock, compressive failure resulting in spalling of a borehole may occur only at the bottom of the borehole. The spalling can occur continuously with drilling, and results in continuous spalling with depth, i.e., a breakout. This type of breakout tends to form on one side of the borehole and its orientation is approximately perpendicular to the orientation of standard breakouts, inferred from the stress concentration due to the cylindrical shape of the borehole.

Laboratory study of borehole breakouts in Cordova Cream: a case of shear failure mechanism

We subjected vertical boreholes drilled in cubical specimens of Cordova Cream to a 3-dimensional stress field, and increased one or both of the horizontal principal stresses until breakout failure occurred. Acoustic emission, borehole strain, and visual monitoring of the borehole wall during testing revealed four stages of breakout development: crack initiation, extension, breakout formation, and stabilization. Thin section analysis suggests that breakout formation in Cordova Cream is the result of shear failure mechanism. The relationships at breakout stabilization between the two horizontal principal stresses and between the span at the borehole wall and the least horizontal stress suggest a potentially simple method of estimating the far-field state of stress if ‘breakouted’ borehole geometry is accurately mapped.

Character and distribution of borehole breakouts and their relationship to in situ stresses in Deep Columbia River Basalts

The character and distribution of borehole breakouts in deeply buried basalts at the Hanford Site in south central Washington State are examined in light of stress indicator data and hydraulic‐fracturing stress data by means of acoustic televiewer and acoustic waveform logging systems. A series of boreholes penetrating the Grande Ronde Basalt of the Columbia River Basalt Group were logged to examine the extent of breakouts at depths near 1000 m. Breakouts occur discontinuously throughout the interiors of most flows. In some boreholes the distribution of borehole wall breakouts closely correlates with the incidence of core disking. Differences in the distribution of breakouts and disking are attributed to differences in failure mechanisms. A thin interval of breakout‐free basalt occurs near the upper and lower limits of flow interiors, with many intervals of breakouts terminating at the intersection of oblique fractures with the borehole. Hydraulic‐fracturing stress measurement results obtained from four deep boreholes indicate anisotropic horizontal principal stresses, with maximum principal stress along an approximate north trending axis, consistent with the east trending orientation of breakouts. Acoustic waveform logs indicate that there is no measurable difference between the seismic properties of breakout‐free flow tops and flow interiors. The highly coherent waveforms obtained in almost all flow interiors indicate that damage to the borehole wall associated with breakout formation remains confined to the thin annulus of stress concentration.