Core Discing Research Articles

Formation mechanism of core discing during drilling under deep in-situ stress environment: Numerical simulation and laboratory testing

Formation mechanism of core discing during drilling under deep in-situ stress environment: Numerical simulation and laboratory testing

The proneness assessment of a mineral deposit to tectonic rockburst: A case-study of Yuzhnoe deposit

The procedure of the proneness assessment to tectonic rockburst (with fault-slip mechanism) is insufficiently described in guidelines and regulatory documents, thus, additional investigations are required in this regard. The tectonic rockburst proneness criteria at Yuzhnoe deposit, Primorye are considered to be the location of large faults in the modern stress field and the critical stress state of rock mass. It is found that the maximum (compression) principal stress, reconstructed using the methods of tectonophysics and the data on rock mass faulting, fits the orientation of the maximum compression in the stress field, determined by instrumental measurements. In this stress field, in the plane nearby the plane of the effective maximum shear stresses, a large fault exists and is traced underground and on ground surface for more than 10 km. The analysis of geodynamics at the deposit, deformation of boundaries of underground excavations, core discing in wells far from the underground excavations, as well as rock bursts both at the study deposit and at the similar Nikolaevskoe deposit nearby shows that the level of the natural stresses at Yuzhnoe deposit is rather high and the rock mass is everywhere in the critical stress state. Yuzhnoe deposit is assumed to be to tectonic rockburst prone as both criteria are fulfilled: the rock massif is in the critical stress state and there is large fault which lies comfortably for mutual movement of rock blocks.The studies were carried out using facilities of the Center for Shared Use of Scientific Equipment at the Center for Processing and Storage of Scientific Data of the Far East Branch of the Russian Academy of Sciences, supported by the Ministry of Science and Higher Education of the Russian Federation, Project No. 075-15-2021-663.

Discrete element analysis on the influence of drilling fluid pressure on core discing in pressure-preserved coring

In the process of coring in a high in-situ stress environment, the rock core is damaged due to stress release, and core discing is a prominent manifestation of this damage. Pressure-preserved coring has no direct effect on in-situ stress, and its influence on stress release is related to drilling fluid pressure. Yet fluid pressure in the coring process was not considered in most former numerical simulation studies on core discing. In this paper, the discrete element method software PFC3D is used to conduct a numerical simulation of the coring process. According to the two conditions without drilling fluid pressure and with it, and different horizontal in-situ stresses of 5-160 MPa, fracture and discing of the core with drilling are observed. In addition, the law of fracture and discing in the process of coring was studied, and the function of pressure-preserved coring to prevent fracture and discing is explored. Simulation results show nonlinear trends in distribution of discs both in time and in space.

Preliminary Study on the Phenomenon and Mechanism of Granite Core Discing in Laboratory Drilling Test

It is generally believed that core discing is a rock failure phenomenon under high in-situ stress, and high stress is closely related to the characteristics of core discing. The core drilling tests of intact granite were conducted on the laboratory servo-controlled drilling test platform. It was observed that the granite core showed discing phenomenon at a high drilling rate (> 15 mm/min) under no confining pressure (0 MPA). The fluctuation of the drilling parameter was consistent with the core discing section along the core. The failure mode of the core discs was determined by morphology and surface fractal dimension characteristics. Numerical simulation models considering the interaction between the drill bit and the rock were established using ANSYS software. Based on core drilling tests, failure morphology analysis of the core discs, and numerical simulation, the mechanism of granite core discing under low confining pressure is as follows: Tensile failure is caused by the stress concentration of the bit at the core root. The drilling rate is the critical factor affecting core discing.

Laboratory Drilling Test of Core Discing Phenomenon under Low Stress

The phenomenon of core discing has always been considered as a unique sign of high geostress area. With the in-depth study of the discing mechanism, there are various indications that the high geostress is not the main controlling factor of the core discing. Based on the geostress classification standard and engineering cases data, the stress level of complete granite prone to discing is determined in this paper. On this basis, laboratory drilling tests with different drilling speeds under high and low stress conditions are designed. The results show that high stress is not the key factor for core discing. When the drilling speed is fast, even if the confining stress is 0, core discing will occur. When the drilling speed is slow, the core can be taken completely even at high stress levels. Drilling parameters fluctuate with discing, and its peaks correspond to the core fracture location.

Optimizing borehole location for stress state assessment by geomechanical method of core discing

The article addresses the reliability of the stress–strain analysis and rockburst hazard assessment in rock mass using the geomechanical method of core discing. The theoretical experimentation used CAE Fidesys system. This system performs the strength and other-type computations using the finite element method—a numerical method of problem solution in applied physics. The experiments have two stages. At the first stage, the model of rock mass with core drilling for the stress state assessment by core discing is constructed; the computation aims to determine vertical stresses such that the tensile stresses along the core axis exceed the ultimate tension strength (discing condition). At the second stage, the rock mass with the destress boreholes is modeled; the aim of the modeling is to determine the influence zones of the destress boreholes. The applied experimentation was carried out in underground openings of Taimyrsky Mine. The research has found the influence exerted by some destress boreholes on the stress state assessment by core discing. The theoretical experiments and practical testing results show a good agreement, which proves their reliability. The final conclusions are: inside the protected zones formed by destress drilling, the stresses effectively reduce down to safe values; to avoid errors inrockburst hazard assessment by core discing, core drilling should be performed at a distance from the destressing surface not less than 5 diameters of the destress boreholes; interpretation of the stress state assessment results should take into account the nonuniform stress distribution in the plane of destressing. The authors appreciate participation of D. Kh. Gilyazev, Head of the Rockburst Prediction and Prevention Department, Taimyrsky Mine (till 2021) in this study.

Exploration on Formation Mechanism of Core Discing Based on Energy Analysis

The basic premise of deep in situ fluidized mining is to study the in situ stress state of deep rocks, and coring is one of the basic physical means. However, core discing often occurs in the process of coring. And core discing is one of the symbols of high in situ stress in deep engineering area, and revealing the mechanical response of core stub under different conditions is an important premise to explore the formation mechanism of core discing. Therefore, the effects of in situ stress combination condition, core diameter, and drilling depth on the maximum tensile stress after core stub unloading are discussed by PFC2D discrete element numerical simulation method, and the formation mechanism of core discing is preliminarily explored based on strain energy and dissipated energy. Research shows that in situ stress combination is the most dominant factor of core discing. When the hydrostatic stress increases from 40 MPa to 80 MPa, the maximum tensile stress at the core stub increases by 98.42%. When the horizontal stress is 60 MPa and the axial stress increases from 40 MPa to 100 MPa, the maximum tensile stress at the core stub increases by 53.33%. However, when the axial stress is 60 MPa and the horizontal stress increases from 40 MPa to 80 MPa, the maximum tensile stress at the core stub first increases and then decreases, and there is an inflection point when the horizontal stress is 75 MPa. When the core diameter increases from 8.00 mm to 13.00 mm, the maximum tensile stress at the core stub increases by 1.58 times. When the drilling depth increases from 3.00 cm to 4.00 cm, the maximum tensile stress at the core stub increases by 4.01 times; that is, small diameter cores and cores with large drilling depth are more prone to core discing. The total energy of the system increases with the increase of in situ stress. When the hydrostatic pressure is 80 MPa, the dissipated energy of the system is 2.80 times greater than that when the hydrostatic pressure is 40 MPa. More energy must be dissipated when the core discing occurs under higher in situ stress conditions. It is preliminarily proved that the formation mechanism of core discing is that the tensile cracks extend and penetrate at the core stub, and the core is pulled off to form a core disc, which is reciprocating to produce core discing. The research results are expected to provide a certain reference for understanding the formation mechanism of core discing and provide useful ideas for the related research of discing phenomenon.

Numerical simulation and theoretical analysis of coal-rock meso-mechanics on core discing

Numerical simulation and theoretical analysis of coal-rock meso-mechanics on core discing

Characteristics and Mechanical Mechanism of In Situ Unloading Damage and Core Discing in Deep Rock Mass of Metal Mine

Based on the phenomena of core damage and core discing in deep strata of metal mines in Laizhou area, the physical and mechanical mechanism behind these phenomena and characteristics is discussed. The analysis shows that the damage effect of deep high stress rock is obvious after in-situ unloading, the core discing occurs when the damage reaches a certain degree, and the time effect of damage increases with the increase of buried depth. The characteristics of core discing are closely related to the stress environment and the mesostructure and mineral composition of the rock. Monzogranite with a large particle size and high content of potassium feldspar is easier to occur core discing than granodiorite in this area. The mechanism of core discing in Sanshandao gold mine is as follows: tensile failure occurs in the middle of the core, and composite failure of shear and tension occurs at the edge. The crack extends from the middle of the core to the outer edge, and finally, the tensile crack and shear crack intersect and connect to form a cake crack. Considering the stress condition and strength conditions of core discing, the energy release rate of rock mass in the core discing area is large, which has the conditions for strong rock burst. In addition, the ratio of maximum stress to tensile strength at the starting position of core discing is about 6.5 in this area, which is consistent with the previous analysis, so as to provide a strong basis for calculating in-situ stress based on the core discing phenomenon.

Evaluation of the Diametrical Core Deformation and Discing Analyses for In-Situ Stress Estimation and Application to the 4.9\xa0km Deep Rock Core from the Basel Geothermal Borehole, Switzerland

The in situ state of rock mass stresses is a key design parameter, e.g., for deep engineered geothermal systems. However, knowledge of the stress state at great depths is sparse mostly because of the lack of possible in situ tests in deep boreholes. Among different options, core-based in situ stress estimation may provide valuable stress information though core-based techniques have not yet become a standard. In this study we focus on the Diametrical Core Deformation Analysis (DCDA) technique using monzogranitic to monzonitic rock drill cores from 4.9 km depth of the Basel-1 borehole in Switzerland. With DCDA the maximum and minimum horizontal stress (SHmax and Shmin) directions, and the horizontal differential stress magnitudes (∆S) can be estimated from rock cores extracted from vertical boreholes. Our study has three goals: first, to assess photogrammetric core scanning to conduct DCDA; second, to compare DCDA results with borehole breakout and stress-induced core discing fracture (CDF) data sets; and third, to investigate the impact of rock elastic anisotropy on ∆S. Our study reveals that photogrammetric scanning can be used to extract reliable core diametrical data and CDF traces. Locally aligned core pieces showed similar SHmax orientations, conform to borehole breakout results. However, the variability of core diametrical differences was large for the Basel-1 core pieces, which leads to a large spread of ∆S. Finally, we demonstrate that core elastic anisotropy must be considered, requiring robust estimates of rock elastic moduli, to receive valuable stress information from DCDA analyses.

Core discing characteristics and mitigation approach by a novel developed drill bit in deep rocks

Core discing often occurs in deep rocks under high-stress conditions and has been identified as an important characteristic for deep rock engineering. This paper presents the formation mechanism of core discing firstly. Then, the interaction between diamond drill bits and rock was analyzed based on numerical modeling. A novel drill bit with an inner conical crown for the mitigation of core discing was designed and verified by simulation experiments. The mitigation method was applied in the cavern B1 of CJPL-II and satisfactory results had been achieved. The percentage of core discing had been obviously decreased from 67.8% when drilling with a rectangular crown drill bit, to 26.5% when an inner conical crown drill bit had been adopted. This paper gives full insight into core discing characteristics and provides a new method for core discing mitigation; it will potentially contribute to stress measurement in deep rock engineering.

Numerical analyses of mesh size effects on core discing

Although the nonlinear numerical methods to study core discing mechanisms have their merits in terms of linear elastic models, the effects of mechanical parameters and mesh size in numerical simulations still have limits on the accuracy of the results. To address this problem, a parametric sensitivity analysis was conducted to understand the influence of shear and tensile mechanical parameters and mesh size on the numerical results of core discing. Numerical results show that the shear and tensile mechanical parameters have a “cliff effect” on the core discing process. The mechanical responses of the core, the borehole sidewall, and the bottom of the kerf were also contributing to this effect. Next, the simulations using different mesh sizes were conducted to achieve a best fit between the stress-strain curves and the failure modes. Finally, a method was introduced to eliminate the mesh size effects on rock discing in our numerical simulations.

Mechanism of core discing in the relaxation zone around an underground opening under high in situ stresses

Core discing is a brittle rock failure during borehole drilling in intact rock subjected to high in situ stresses. This failure mode has been recognized as an important index for assessing underground projects under high in situ stresses. With increasing depth of underground space exploitation, core discing occurs not only in highly-stressed intact rock, but also in the relaxation zone of the surrounding rock masses. In the latter case, the disc thickness is even smaller, and the mechanism is significantly different from that of core discing in highly-stressed intact rock. In this study, the correlation between the regional geological structures and the disc thickness in the relaxation zone is analyzed based on the deep auxiliary tunnel of the Jinping hydropower project. A “structural arch” model, applicable for analyzing the mechanical behaviors of fractures and rock blocks in the relaxation zone, is proposed. The model is then verified by laboratory similarity tests and numerical simulations. Finally, it is revealed that the “discontinuous” stress field in the relaxation zone is the main cause of core discing in the relaxation zone around underground openings subjected to high in situ stresses.

Evaluation of the Integrity of Deep Rock Masses Using Results of Digital Borehole Televiewers

Rock mass integrity is regarded as an important index to evaluate rock mass quality. Core drilling technology is one of the effective methods used for this. To overcome the problem of core discing and core breakage from the drilling process, a new evaluation method was proposed: a rock mass integrity index (RMIBT) based on high-definition digital borehole televiewer data. The RMIBT values were obtained by measuring the mass proportions of the length of the rock mass without macroscopic fractures on the borehole wall. Their scoring criteria were determined based on rock quality designation. It was applied in multiple deep rock excavations and therefore proved to be useful, especially where core discing occurs, in specific spatial distributions of structural planes, as well as logging errors from core breakages induced by drilling. In addition, the RMIBT can be used to assess dynamically the integrity of macroscopic rock masses and the evolution of fractures in the excavation damaged zone, thus providing a basis for the evaluation of rock masses in deep rock excavations.

Mechanism and Stress Analysis of Rock Core Discing

Mechanism and Stress Analysis of Rock Core Discing

Drilling disturbance and constraints on the onset of the Paleocene–Eocene boundary carbon isotope excursion in New Jersey

Abstract. The onset of the Paleocene–Eocene thermal maximum (PETM) and associated carbon isotope excursion (CIE; approx. 56 Mya) was geologically abrupt, but it is debated whether it took thousands of years or was effectively instantaneous. Wright and Schaller (2013) published a significant new record of the onset of the CIE, and claimed that it could be resolved across 13 annual layers in a drill core through the Marlboro clay at Millville, New Jersey (Ocean Drilling Program (ODP) Leg 174X). Supporting evidence for similar layering was reported from another New Jersey drill site, Wilson Lake B, and a photograph of the Marlboro clay in outcrop (Wright and Schaller, 2014). Such a short duration would imply an instantaneous perturbation of the atmosphere and surface ocean and the impact of a comet or asteroid as the likely cause. However, Pearson and Nicholas (2014) suggested, based on the published core photographs, that the layers in the Marlboro clay cores could be artifacts of drilling disturbance, so-called biscuiting, wherein the formation is fractured into layers or biscuits and drilling mud is injected in between the layers. (We now prefer the term core discing following Kidd, 1978.) Here we report new observations on the cores which support that interpretation, including concentric grooves on the surfaces of the core discs caused by spinning in the bit, micro-fracturing at their edges, and injected drilling mud. We re-interpret the limited outcrop evidence as showing joints rather than sedimentary layers. We argue that foraminifer concentrations in the sediments are far too high for the layers to have been annually deposited in turbid waters at depths of 40–70 m, indicating that the onset of the CIE in the Marlboro clay likely took on the order of millennia, not years (Zeebe et al., 2014). Re-coring of Millville aimed at minimizing drilling disturbance to allow a higher-resolution study of the carbon isotope excursion is highly desirable.

Influence of rock failure and damage on in situ stress measurements in brittle rock

The modified Doorstopper stress measurement technique consists in recording the recovered strains caused by stress relief at the bottom of a borehole drilled in a rock mass and converting the relieved strains into stresses by using the deformability parameters at the measuring point. This conversion is usually based on the assumption of a linear elastic behaviour. Hence, when plastic strains are induced at or near the borehole bottom prior to or during stress relief, the stress tensor calculated by assuming an elastic behaviour can be wrong. Plastic strains develop at the borehole bottom or in its vicinity, causing local damage. Core discing is probably the most visible manifestation of core failure, however, even if no visible signs of damage are present, plastic strains can be induced at or near the borehole bottom, affecting the recovered strains and the calculated stresses. Using Flac3D, the stress relief process for different 3D in situ stress states in an elasto-plastic cohesion softening friction hardening model is simulated in order to determine if the non-elastic behaviour can be detected from the strain recovery curves. Errors on the far-field stress state obtained by assuming an elastic behaviour are calculated. Finally, means of minimizing core damage in the presence of high in situ stresses are briefly explored and discussed.

Evolution of fractures in the excavation damaged zone of a deeply buried tunnel during TBM construction

This paper presents results of in situ measurements of fracture evolution within the excavation damaged zone (EDZ) surrounding the tunnel boring machine (TBM) excavated headrace tunnel No. 3 of Jinping II hydropower station, China. Fractures were measured by borehole images obtained with a digital panoramic borehole camera throughout the processes of tunnel excavation and support installation. Boreholes for the digital camera were pre-drilled perpendicular to the tunnel axis from a pre-excavated branch tunnel, and a series of digital images of the borehole walls were made as the TBM passed. Digital processing revealed the lithology, pre-existing fractures and characters of rock core discing of the surrounding rock mass. Processes of fracture initiation, propagation, and closure were observed, and width, occurrence and zoning of EDZ were measured. The relationship between fracture evolution and TBM construction progress is described in detail. Based on true triaxial tests and theoretical analysis of unloading conditions of deep rock masses, the occurrence of tunnelling induced new fractures at the EDZ is discussed with respect to in situ measurement results. The results provide valuable data for EDZ analysis, future designs and the construction of deeply buried tunnels using TBM.

In Situ Stresses in Borehole Blanche-1/South Australia Derived from Breakouts, Core Discing and Hydraulic Fracturing to 2 km Depth

The development of Hot-Dry Rock (HDR) geothermal energy in Australia with drillings to some kilometres depth yields an impetus for deep stress logging. For the Olympic Dam HDR-project, borehole Blanche-1 was drilled to almost 2 km depth and provided the possibility to estimate the in situ stresses within the granitic borehole section by the analysis of borehole breakouts and core discing, as well as by hydraulic fracturing combined with acoustic borehole televiewer logging for fracture orientation determination. Although the stress magnitudes derived by the different methods deviate significantly, they clearly indicate for the depth range between 800 and 1,740 m a compressional stress regime of Sv ≤ Sh < SH and a consistent East–West orientation of maximum horizontal compression in agreement with existing stress data for Australia. The minor horizontal stress Sh derived from the hydraulic fracturing closure pressure values is about equal to the overburden stress and may be regarded as most reliable.

The Improved PSO Arithmetic for Identifying Ground Stress Based on Rock Core Discing Observation

Identifying ground stress based on rock core discing can save a lot of surveying cost, so studying “rock core disicing” has great meaning. Aiming at the limitation of conventional methods, the particle swarm optimization (PSO) arithmetic is introduced to be combined with numerical simulation, so as to identify the ground stress based on rock core disicing observation. The objective function of optimization is constructed by combing Mohr-Coulomb criterion and tension criterion. Adopts dynamically adjusting momentum term method and steric compression method so as to improve the PSO in convergence velocity and globally searching property. The proposed method is mentioned and used to identify the ground stress of Bakken formation in USA. The arithmetic has good convergence speed and the identified results are coincident with the former reported results.