Core Disking Research Articles

Effect of height-diameter ratio on the mechanical characteristics of shale with different bedding orientations

Geological exploration cores obtained from shale gas wells several kilometers deep often show different height-diameter ratios (H/D) because of complex geological conditions (core disking or developed fractures), which makes further standard specimen preparation for mechanical evaluation of reservoirs difficult. In multi-cluster hydraulic fracturing, shale reservoirs between planes of hydraulic fractures with different lengths could be simplified to have different H/D ratios. Discovering the effect of H/D on the mechanical characteristics of shale specimens with different bedding orientations will support mechanical evaluation tests of reservoirs based on disked geological cores and help to optimize multi-cluster fracturing programs. In this study, we performed uniaxial compression tests and acoustic emission (AE) monitoring on cylindrical Longmaxi shale specimens under five bedding orientations and four H/D ratios. The experimental results showed that both the H/D-dependent mechanical properties and AE parameters demonstrated significant anisotropy. Increasing H/D did not change the uniaxial compressive strength (UCS) evolution versus bedding orientation, demonstrating a V-shaped relationship, but enhanced the curve shape. The stress level of crack damage for the specimens significantly increased with increasing H/D, excluding the specimens with a bedding orientation of 0°. With increasing H/D, the cumulative AE counts of the specimens with each bedding orientation tended to exhibit a stepped jump against the loading time. The proportion of low-average-frequency AE signals (below 100 kHz) in specimens with bedding orientations of 45° and 60° increased to over 70% by increasing H/D, but it only increased to 40% in specimens with bedding orientations of 0°, 30°, and 90°. Finally, an empirical model that can reveal the effect of H/D on anisotropic UCS of shale reservoir was proposed, the anisotropic proportion of tensile and shear failure cracks in specimens under four H/D ratios was classified based on the AE data, and the effect of H/D on the anisotropic crack growth of specimens was discussed.

FRACTAL CHARACTERISTICS OF CORE DISKING FRACTURE SURFACES

The morphological characteristics of core disking can reflect the in-situ stress field characteristics to a certain extent, but a quantitative description method for disking-induced fracture surfaces is needed. The fractal geometry was introduced to refine the three-dimensional characteristics of the core disking fracture surfaces, and the disking mechanism was explored through morphological characteristics. The innovative concentric-different scale method was proposed to divide the circular surfaces into square surfaces of different sizes. Furthermore, the applicability of four cubic covering methods for single fractal calculation was studied. Finally, the relative differential cubic covering method and multifractal method were used to calculate the three-dimensional data of core disking fracture surfaces in the Jinping area. The results showed that the concentric-different scale method fits the fractal dimension changing law in the exploration of different surface sizes. Using the differential cubic covering method and the relative differential cubic covering method can obtain good results for smoother surfaces. The disking was caused by tensile stress due to high crustal stress unloading, and the cracks originated from the inside of the core. Fractal theory is conducive to analyzing the morphology of rock fracture surfaces and provides guidance for exploring the failure mechanism and influencing factors.

Assessment of the Propensity of Rocks of the Irtysh Deposit to Rock Impacts by Core Disking and by the Brittleness Coefficient

An assessment of the stress-strain state at deep (below horizon 14) of the Irtysh mine has been carried out. The propensity of ores and rocks of the Irtysh deposit to rock burst hazard is estimated: by disking of core and fragility of rocks. Studies of physical and mechanical properties were carried out at all stages of the development of the Irtysh deposit. In order to include in the diagram the levels of the impact potential, a sample was made for more than 136 definitions of the compressive and tensile strength of ores and rocks of the deposit

Stress State in the Source Region of Mw2.2 Earthquake in a Deep Gold Mine in South Africa Determined from Borehole Cores

In December 2007, an Mw2.2 earthquake occurred in a gabbroic dike at 3.3 km depth in a deep gold mine in South Africa. The fore- and aftershock activity was analyzed in an effort to understand the preparation and generation processes of earthquakes; these findings have already been published. The present paper focuses on the stress state in the source region of the mainshock. A 90-m-long borehole across the mainshock fault was drilled ~ 1.5 years after the mainshock and logged by an optical televiewer. The fault can be identified by severe damage to the borehole wall at the point where the borehole intersected the aftershock cluster. Except for a 10-m section in the hanging wall of the fault, borehole cores were fully recovered. Borehole breakout (BO) and core disking (CD) were found to occur. Two stress measurement techniques [Deformation Rate Analysis (DRA) and Diametrical Core Deformation Analysis (DCDA)] were applied to the borehole cores. By combining their results with occurrence criteria for BO and CD, the principal stress state in the source region of the mainshock was determined. The principal directions in the hanging wall of the fault were nearly identical to the virgin stress state, while it was significantly disturbed in the footwall. The vertical stresses were 106 MPa and 40 MPa in the hanging wall and footwall, respectively. The significant difference in the vertical stress between the two sides of the fault can be explained by the stress redistribution associated with a nonuniform slip on a nonplanar fault.

The rock mass as the governing factor for successfully developing deep geothermal systems in Southern Germany

AbstractIn Southern Germany, geothermal wells target Upper Jurassic carbonates, formerly referred to as Malm, below the Northern Alpine Foreland Basin (NAFB). In the northern, shallower carbonates (3000 m depth), e.g., in the Munich area, hydrothermal systems prevail, and wells yield proper flow rates. Deeper carbonates (< 4000 m) towards the south indicate a petrothermal system due to a decline in porosity and permeability. Inexplicably, fractures only contribute poorly to larger flow rates. This sets the goal for the project Dolomitkluft to analyse the rock mass for options countering low flow rates in two geothermal wells at Geretsried, Bavaria. Therefore, 20 m drill cores from 4600 to 4715 m total vertical depth (TVD) depict the initial structure of the rock mass: the rock parameters, the discontinuity network, and the in situ stresses. Alternating homogeneous, micritic, strong, to inhomogeneous, porous, weak limestones and massive, crystalline dolostones define the stratigraphy. Five joint sets from the cores can be distinguished into the bedding, two paleo anti‐ and synthetic normal faults and two recent strike‐slip faults. The latter, in combination with core disking, allows exclusively a strike‐slip (SS) stress regime and delimits the ratio to at least SH : Sv : Sh = 1.7 : 1.0 : 0.7. According to the results, including dilatation‐ and slip‐tendencies, drill trajectories of prosperous geothermal wells should head ESE/WNW, perpendicular to SS‐faults, and avoid the presence of normal faults.

New criterion for the spalling failure of deep rock engineering based on energy release

Ground pressure hazards in deep rock engineering, such as rock spalling and rock burst, are big challenges and threats to personnel and equipment safety. In order to investigate the mechanism of ground pressure disasters in deep rock engineering, in-situ investigations were carried out in a deep gold mine in China. The survey results show that with the increase of the mining depth, the number and severity of ground pressure disaster events increase significantly. The investigation results of ground pressure disaster in the gold mine are analyzed by three methods. The effects of the maximum tangential stress, strain rebound and strain energy release on the failure of surrounding rock under different burial depths are obtained. The failure behaviors of surrounding rock are well explained by these methods at different mining depths. The new method considering strain energy release can well explain the phenomenon of core disking and it can accurately calculate the theoretical thickness of core disk. In addition, a theoretical calculation basis for the support design of energy absorbing bolt is provided in the paper and the support design figure is put forward by considering energy absorbing bolt. • According to in-site investigations, ground pressure hazard events increased significantly with depth. • Strain energy released by surrounding rock increases rapidly with a function of second power of mining depth. • Phenomenon of core disking is well explained, and calculation formula of rock disk thickness is provided. • The energy-absorbing materials such as energy-absorbing bolts can well absorb the released strain energy.

Analysis Of Methods And Means Of Bump Hazard Prediction

The article analyzes the methods and tools for predicting the impact hazard in the conditions of underground mining of gold deposits. To assess the stress state of a rock mass, the core disking method is proposed as a basic method. The degree and categories of impact hazard of sections of the rock mass are estimated. Due to the impossibility of solving many problems by geomechanical only field studies. The reliable efficiency of the use of the finite element method and the boundary element method in predicting the impact hazard of the field sites a priori is shown.

In-site core disking phenomenon and break mechanism of hard marble: Investigation in 2400 m deep-buried underground laboratory

The core-disk phenomenon has been observed generally in the drilling process under high-stress conditions. This paper presents the in-situ experimental study of the coring-disking failure mechanism of marble in an underground cavens with 2400 m depth. Based on the disk samples in several boreholes with different diameters, both macro- and micro-morphological characteristics of core-disks’ break surface were analysis, using 3D optical scanning and electron microscope scanning. Moreover, the numerical back analysis was also used to simulate the drilling process for demonstrating the development of core disking. The in-situ experiment results showed that the failure types of core disking consisted of tensile break and shear break, i.e., the shear break usually appears in the edge part of break surface, and tensile break appears in the central part. What’s more, the ration of core-disks thickness to borehole diameter is in a relatively stable range. Numerical back analysis indicated this micro asynchronous break of hard marble is induced by high geostress and unloading drill.

Characterisation of rock fracturing ahead of the preconditioned mining faces in a hard rock mining

A study on the characterisation of rock fracturing ahead of the preconditioned mining faces in a hard rock mining is described. The study involves the use of borehole periscope, ground-penetrating radar (GPR), the concept of core disking and numerical modelling. The results of the study have shown that extension fractures are firstly developed as groups, separated with solid zones, and as time progresses, the grouping of the fractures disappeared. In supporting the above findings, extension fractures were noted to be developed in a manner of micro-fractures, close fractures to open fractures as time progress. However, open fractures were observed to be developed quicker closer to the borehole core as compared with several metres within the tunnel front. In the meantime, GPR results correspond very well with the borehole periscope observations, by revealing the maturity of fracture as time progresses. In supporting the periscope observation and GPR, the recovered cores have shown a variety of core disking with the decrease in core thickness as the depth increases. The numerical simulation revealed a gradual closure at the vicinity of the borehole with extensive closure taking place along the sidewall of the borehole. The study concluded that the initiation of extension fractures was not limited to a certain depth; it was observed and validated to depend on the rate at which the rock mass deteriorates, which is mostly controlled by the rock mass properties.

Three-dimensional discrete element method simulation of core disking

The phenomenon of core disking is commonly seen in deep drilling of highly stressed regions in the Earth’s crust. Given its close relationship with the in situ stress state, the presence and features of core disking can be used to interpret the stresses when traditional in situ stress measuring techniques are not available. The core disking process was simulated in this paper using the three-dimensional discrete element method software PFC3D (particle flow code). In particular, PFC3D is used to examine the evolution of fracture initiation, propagation and coalescence associated with core disking under various stress states. In this paper, four unresolved problems concerning core disking are investigated with a series of numerical simulations. These simulations also provide some verification of existing results by other researchers: (1) Core disking occurs when the maximum principal stress is about 6.5 times the tensile strength. (2) For most stress situations, core disking occurs from the outer surface, except for the thrust faulting stress regime, where the fractures were found to initiate from the inner part. (3) The anisotropy of the two horizontal principal stresses has an effect on the core disking morphology. (4) The thickness of core disk has a positive relationship with radial stress and a negative relationship with axial stresses.

Theoretical explanation of rock splitting based on the micromechanical method

In this paper, in order to explain the splitting of cylindrical rock specimen under uniaxial loading, cracks in cylindrical rock specimen are divided into two kinds, the longitudinal crack and the slanting crack. Mechanical behavior of the rock is described by elastic-brittle-plastic model and splitting is assumed to suddenly occur when the uniaxial compressive strength is reached. Expression of the stresses induced by the longitudinal crack in direction perpendicular to the major axis of the crack is deduced by using the Maxwell model. Results show that the induced stress is tensile and can be greater than the tensile strength even before the uniaxial compressive strength is reached. By using the Inglis\'s formula and simplifying the cracks as slender ellipse, the above conclusions that drawn by using the Maxwell model are confirmed. Compared to shearing fracture, energy consumption of splitting seems to be less, and splitting is most likely to occur when the uniaxial loading is great and quick. Besides, explaining the rock core disking occurred under the fast axial unloading by using the Maxwell model may be helpful for understanding that rock core disking is fundamentally a tensile failure phenomenon.

A Theoretical Explanation for Rock Core Disking in Triaxial Unloading Test by Considering Local Tensile Stress

Rock is a typical inhomogeneous material with a large number of flaws in different scales; the stress field of the rock in its elastic state consists of two parts: the elastic stress, which distributes uniformly in the entire region; and an additional stress, which only exists around the flaws. Theoretical expressions of the additional stress and local stress are derived based on the Maxwell model. Core disking which takes place under the condition that the axial stress is rapidly reduced while the confining pressure is kept unchanged is explained with a new method. Unloading duration’s effect on core disking is analyzed. A new criterion for core disking is presented based on attributing the core disking to the result of the exceedance of local tensile stress over the tensile strength. Based on our theoretical analysis and the conclusions from published resources, core disking is most likely to occur if the maximum principal stress is more than five to six times the tensile strength.

Estimation of in situ stress along deep tunnels buried in complex geological conditions

The headrace tunnels of the Jinping II Hydropower Station are 16.7km in length and have a maximum depth of 2525m. Many difficulties have been encountered in estimation of virgin rock stresses along the tunnels. One method was presented including the macroscopic analysis of the region geological structures and topography and the integrated analysis of in situ stresses based on the multi-source information in local tunnel sections. The characteristics of geological structures and tectonic movement were studied to determine the orientations of the tectonic stresses within the Jinping II tunnel project region. The influences of the topography on the distribution of in situ stress were analyzed using numerical methods. The virgin stress regimes were preliminarily estimated based on the geological and topographical analysis and the World Stress Map. Under the guidance of these results, multi-source information was collected regarding rock brittle failures, EDZs, core disking and large-scale deformation of soft rock. Several techniques were adopted, including multivariate regression, numerical simulation, and stereographic projection. An iterative method for the orientations and magnitudes of in situ stresses is presented. The in situ stress regimes, orientations, and magnitudes in tunnel sections of different depths were obtained and verified. They can provide the basic conditions for analysis of the stability of surrounding rock mass during excavation.

Core disking and its relationship with stress magnitude for Lac du Bonnet granite

Core disking is an indicator of elevated stress magnitudes. Disked cores from boreholes drilled from underground excavations in massive unfractured granite, where the stress magnitudes are known with confidence, were used to establish a relationship between core disk thickness and the stress magnitude. Relationships were established for three disk thickness categories; (1) thin ( t / D < 0.2 ) , (2) medium ( 0.2 < t / D < 0.4 ) and (3) thick ( 0.4 < t / D < 2.2 ) and partial disking. The data suggests that core disking initiates when the maximum principal stress normalized to the tensile strength is 6.5.

Estimate of stress state in rock masses adjacent to open pit walls

It is proposed to study the action exerted by the tectonic stresses on stability of open pit walls by using the method of core disking in exploratory holes and the tectono-physical method based on the analysis of tectonic structure of a rock mass.

Evaluation of core disking rock stress and tensile strength via the compact conical-ended borehole overcoring technique

To evaluate core disking rock stress and tensile strength via the compact conical-ended borehole overcoring (CCBO) technique, the mechanism of core disking by means of the CCBO technique is discussed. It is simulated that overcoring of the CCBO under three-dimensional initial stress state coincides with borehole axis and is one of principal directions. The characteristics of induced stress and initiated failure due to tensile stress on the borehole bottom surface during overcoring are discussed. They are based on the results calculated by a semi-analytical boundary element process for axisymmetric elasticity with arbitrary boundary element conditions. From the results, an initiation criterion of core disking is suggested. It is shown that the failure is initiated from outside surface of core and that the failure initiation criterion on the longitudinal tensile stress outside of core can define the criterion of core disking. Secondly, since tensile strength of rock is closely related to core disking, an estimation procedure of it is suggested using strains during compact overcoring. It is shown that representative rock volume for tensile strength estimated by the filed measurement data is smaller than that by Brazilian test, and that the fracture initiation criterion may estimate rock stress using tensile strength obtained by Brazilian test with small volume. Finally, an estimation procedure of rock stress is suggested by means of core disking phenomenon on the CCBO technique.

Measurements of induced stress and strength in the near-field around a tunnel and associated estimation of the Mohr–Coulomb parameters for rock mass strength

Abstract The state of induced stress measured by the compact conical-ended borehole overcoring technique in the immediate roof of an approach tunnel excavated under high rock stress is described. During the measurements, core disking was observed. An X-ray Computed Tomography (CT) scanner was used to select strain data uninfluenced by the core disking; then the induced rock stress was estimated from selected strain data. From these results, it is shown that the non-destructive investigation using X-ray CT is effective for visualization of the fracturing within cores and the selection of strains measured during overcoring. Furthermore, the Mohr–Coulomb failure criterion parameters for the rock mass were estimated by comparing the measured stresses with the shear strength of in situ rock and the uniaxial compressive strength determined in laboratory tests.

凝灰岩類からなる軟岩地山におけるコアディスキングとトンネル変形

Core disking and tunnel deformation like rockburst have occurred at soft tuffaceous rock masses at a point 220 meters deep from the surface in Iwate Tunnel. The physical properties suggest that these rock masses have the possibility of swelling. The estimated earth pressure affecting a drilling hole is larger than five times the tensile strength of the rocks, which is considered to be a condition to cause core disking. Plate-shaped rock fragments continuously exfoliated like rockburst from the face of this tunnel, and deformation at the wall exhibited plasticity. The magnitude of this tunnel deformation is not influenced by either overburden or physical properties of rock masses, but mainly by geological structure.

円錐孔底ひずみ法におけるコアディスキング現象に関する基礎的研究

Core disking is sometimes observed in high stress region where rock stress measurements are performed by the Compact Conical-ended Borehole Overcoring (CCBO) Technique, which is one of the stress relief methods. When the tensile stress induced on the borehole bottom surface becomes larger than the tensile strength of the rock, a failure from the borehole bottom surface will be imitated. Therefore, if the stress changes on the borehole bottom surface can be analyzed during overcoring, the rock stress prior to boring may be estimated.In this paper, assuming that the tensile stress on a borehole bottom surface induced due to overcoring is one of factors of generating core disking, the mechanism of the core disking in the CCBO is discussed as well as the location of failure initiation and the type of failure. Firstly, the characteristics of core disking in the CCBO is shown. Secondly, simulating the overcoring of the CCBO under three dimensional initial stress state in which the axis of a principal stress coincides with that of a borehole, the failure initiated on the borehole bottom surface during the overcoring is discussed, based on the results calculated by a semi-analytical boundary element process for axisymmetric elasticity with arbitrary boundary element conditions. From the results, the criterion of core disking is suggested and the applicable limitation of the CCBO is made clear. Furthermore, the X-ray CT method is applied to the recovered core with core disking, and then the state of failure within the core is visualized. Finally, it is discussed that the rock stress prior to boring is estimated by the suggested criterion of the core disking, comparing the neighboring rock stress measurement and tensile strength obtained from cores.

Core Disking and "Rockburst" in Soft Tuffaceous Rock Masses at Iwate Tunnel

Core disking and tunnel deformation like rockburst occurred at soft tuffaceous rock masses at a point 220 meters below the surface in Iwate Tunnel. The physical properties suggest that these rock masses have the possibility of swelling. The estimated earth pressure affecting the drilling hole is larger than five times the tensile strength of the rocks, which is considered to be a cause of core disking. Plate-shaped rock fragments continuously raveled like rockburst from the face of this tunnel, and deformation at the wall exhibited plasticity. The magnitude of these tunnel deformation is not influenced by either overburden or physical properties of rock masses, but mainly by geological structure.