Effects Of Bedding Planes Research Articles

Studies on rock failure of layered rock in underground mining-face and control techniques

The failure of layered rock in underground mining face may be influenced by the slipping among the layers. The shear failure around the excavation circular roadway is the main failure type in layered rock. To Study the rock failure in layered rock, the condition of bedding slipping in rock is analyzed, and the stresses distribution around circular roadway is calculated. Based on the analysis results, the Flac 3D software is used to analyze the failure of layered rock in roadway floor. Discontinuous interfaces are created to study the effect of bedding planes. The failure of floor is described at two phases: roadway stability phase and mining-face recovery influencing phase. The results indicate that a shear failure surface occurs at the side of coal pillar during the first phase; a shear arched plane occurs at the side of mining-face during the second phase. The results of numerical model are in good agreement with monitoring data. Finally, bolts reinforcement design is proposed to eliminate the development of failure zone in layered rock.

Cracking processes affected by bedding planes in Opalinus shale with flaw pairs

Understanding the cracking processes of rock is important regarding the fundamental material behavior and in engineering applications. Characterizing and predicting the cracking processes in shale is necessary when assessing unconventional oil and gas reservoirs and designing nuclear waste repositories. Of particular interest is the effect of bedding planes on the cracking processes. For this reason we tested one such shale, Opalinus shale, in this study. A series of unconfined compression tests were conducted on prismatic specimens with two pre-existing flaws and various bedding plane orientations. High speed- and high resolution imagery were used to capture crack initiation, -propagation and -coalescence between the flaw pairs and study the effects of bedding plane orientation and flaw pair geometry. It was found that in specimens with bedding planes aligned perpendicularly to the maximum applied compressive stress, the effects of the flaw angle and flaw pair bridging angle on the cracking processes were similar to those observed in previously tested rocks without bedding planes. However, as the bedding planes became more aligned with the direction of the maximum compressive stress the cracks initiating at the flaw tips propagated more frequently along the bedding planes. This visual observation is unique as previous studies did not characterize, in real-time, cracking along bedding planes (weak surfaces). Hence, the results from this study contribute to the understanding of cracking in anisotropic rocks.

Effect of bedding planes on wave velocity and AE characteristics of the Longmaxi shale in China

The experimental work described in this paper was carried out in order to discover more about the effects of bedding planes on wave velocity and acoustic emission (AE) characteristics of shale. Two groups of specimens, which were collected from the Longmaxi shale outcrop in Chongqing, China and cored perpendicular and parallel to the bedding planes, were tested under uniaxial compression, and the wave velocity and AE were monitored. There were obvious differences in the acoustic characteristics of shale with different bedding plane orientations. The experimental results show that (1) the average increasing rates of P- and S-wave velocities were 39.86 and 54.41%, respectively, for the specimen with a load perpendicular to the bedding planes (Y-0). The P-wave velocity and axial strain of specimen show a marked logarithmic relationship. However, the average increasing rates of P- and S-wave velocities were 5.44 and 10.54%, respectively, for the specimen with a load parallel to the bedding planes (Y-90). The good linear relationship between P-wave velocity and axial strain before failure of specimen has been built. Generally, S-wave velocity was more sensitive to axial strain than P-wave velocity. (2) AE characteristics for Y-0 showed that a few signals → quiet period → stable increase → steep increase; for Y-90: quiet period → stable increase → sudden increase → sharp increase. The AE energy for two groups of specimens was concentrated on low and middle of amplitudes (45-80 dB), but the proportion of amplitudes (80–100 dB) and the total counts of AE for Y-0 was 1.95, 2.2 times as much as that for Y-90, respectively. The results preliminarily revealed the effect of bedding orientation on the wave velocity and AE properties of shale and may provide guidance for the improvement of acoustic logging and microseismic monitoring in the field.

The influence of water-based drilling fluid on mechanical property of shale and the wellbore stability

Abstract Because of high cost and pollution of oil-based drilling fluid, the water-based drilling fluid is increasingly used now. However, bedding planes and micro-cracks are rich in shale formation. When water-based drilling fluid contacts formation rock, it causes the propagation of crack and invasion of drilling fluid, which decrease shale strength and cause wellbore instability. In this paper, we analyzed influence of water-based drilling fluid on shale strength and failure mode by mechanics experiment. Based on those experimental results, considering the effect of bedding plane and drilling time, we established modeling of wellbore stability for shale formation. The result from this model indicates that in certain azimuth of horizontal well, collapsing pressure increases dramatically due to shale failure along with bedding plane. In drilling operation, those azimuths are supposed to be avoided. This model is applicable for predication of collapsing pressure in shale formation and offers reference for choosing suitable mud weight.

Effect of bedding planes, their orientation and clay depositions on effective re-injection of produced brine into clay rich deep sandstone formations: Implications for deep earth energy extraction

The rapidly increasing world energy demand requires the development of deep earth energy extraction methods such as unconventional oil and gas production, including coal bed methane recovery (CBM) and utilisation of hot pore fluid in deep geothermal reservoirs. However, all these deep earth energy production methods are associated with huge wastewater (brine) production. Disposal of this brine has become a major challenge in the energy industry, as direct disposal is highly undesirable due to its organic and inorganic pollutant content and its high salinity. Re-injection of brine into deep aquifers is therefore a feasible option to overcome this issue and the flowability through the sedimentary formation or its permeability plays a major role in this process. However, most of previous works mainly focus on the homogenous formations and therefore the brine permeability of deep heterogeneous sandstone aquifers is still not well understood. This study intends to obtain a comprehensive knowledge of this process by conducting a series of core flooding experiments for brine injection into two types of clay rich sandstone, WWS and WGS, obtained from Marburg formation, Queensland, Australia along different bedding orientations and under different reservoir conditions. A series of core flooding tests were conducted on 38mm diameter and 150mm long cylindrical core samples and the pressure developments along the samples were recorded over time for a range of confining pressures (10–25MPa), temperatures (30–60°C), and injection over-pressures (4–20MPa). According to the results, brine permeability in clay rich sandstone increases with increasing injection over-pressure and reduces with increasing depth and aquifer temperature. Furthermore, the flow characteristics of sandstone aquifers are largely dependent on their heterogeneous nature, and the presence of authigenic quartz, clay depositions, low permeable beddings and their orientation have significant influences on reservoir permeability. A stepped pattern was observed in the pore pressure development along the tested sandstone cores which depicts a possible pore structure variation during the fluid injection. A rapid pressure development occurs throughout the sample at the initial stage of injection may be due to the flux tends to pass through the preferable flow paths in the rock mass and then over the time this pressure development diminishes, possibly due to the gradual building of flow barriers by grain particles extracted from the initial rapid flux. These flow barriers obstruct the expected flow towards the downstream and cease the flow for some time until the pressure builds up steadily up to a certain level as it can break the flow barrier by re-arranging the pore structure. The existence of beddings further delays the demolishment of this barrier and beddings perpendicular to the flow direction creates a greater permeability reduction in the aquifer than parallel beddings, because the influence of these low permeable zones on permeability depends on the extent to which they limit the flowability across the rock mass. The experimental results reveal that the brine permeability in sandstone can be reduced significantly up to 7 and 40 times respectively for parallel and perpendicular beddings in this study compared to no-bedding types.

Effect of bedding plane on prediction blast-induced ground vibration in open pit coal mines

Effect of bedding plane on prediction blast-induced ground vibration in open pit coal mines

Effects of coring directions on the mechanical properties of Chinese shale

This paper presents an experimental study of the effects of bedding planes on the mechanical properties of shale samples. Two specimens, sourced from Sichuan Basin in China, cored perpendicular and parallel to the beddings were tested using a uniaxial compressive strength (UCS) machine, and the corresponding crack propagation patterns were analysed using acoustic emission (AE) data. In addition, a digital camera system called ARAMIS was used to acquire the lateral and axial strains during the loading. It was observed that there was significant influence on the peak strength of shale when tested for various bedding planes. As expected, the sample with perpendicular load had nearly twice as high maximum axial strength than the sample with parallel load, and the peak cumulative energy release of the parallel load on the specimen was 1.5 times higher than that of the perpendicular load. The axial strain was higher than the lateral strain in perpendicular load, while under parallel load, the lateral strain was higher than the axial strain. When the axial stress was small, for the shale sample with perpendicular load, the axial strain increased gradually with the increase of stress. In contrast, the lateral strain remained near to zero during the loading period for the sample with parallel load, and both the axial strain and lateral strain were not affected by the increasing load and remained zero. The volumetric strain of the two samples indicated that shale with a perpendicular load shows a compaction behaviour, while parallel load leads to a dilatancy characteristic. The AE data showed that the compression period consists of three stages on both of the two tests and energy release occurs mainly in the third stage. From the ARAMIS results, no perpendicular cracking was observed in the normal stress test, while under parallel loading, the cracks were mostly parallel to the load direction.

Effect of anisotropic borehole wall failures when estimating in situ stresses: A case study in the Nankai accretionary wedge

Breakouts observed in a vertical borehole (C0002A) drilled through two major tectonic sedimentary formations consisting of forearc basin (upper) and accretionary prism (lower) sediments in the Nankai accretionary wedge, Japan, exhibit distinctive geometric features in respective formations. Breakouts in the lower accretionary prism sediments are markedly wider than those in the forearc basin sediments, and breakout azimuths in the two units are horizontally rotated relative to one another. Breakout azimuths are widely used as a proxy for the determination of principal stress directions. However, strength anisotropies related to the presence of bedding planes may affect both breakout azimuths and widths, which can result in misleading in situ stress interpretations. While thinly bedded mudstones are the dominant lithology in both the forearc basin and accretionary prism sediments, bedding planes in the accretionary prism sediments are relatively steeper than those in the forearc basin sediments, with possible implications for breakout geometry and interpretations of principal stress directions. To investigate the effects of bedding planes on breakout geometry (azimuth and width), we conducted borehole wall failure analyses using a weak-plane failure model that incorporates material strength anisotropies. The model results show that breakout widths and orientations are strongly affected by steeply dipping (>40°) bedding planes in cases where dip directions are unaligned with the principal stress orientation. Our theoretical results suggest that variations in breakout azimuths in the C0002A site may be associated with bedding plane related strength anisotropy, and not associated with the rotation of stress field. That is, stress orientation is consistent throughout the borehole (down to the bottom-hole depth of 1495 m below sea floor). In addition, disregarding the effects of bedding planes tends to yield an overestimation of in situ stress magnitude.

Boundary element method of analyzing the interaction between roof strata and roof bolts

Based on the principle of fictitious displacement, equations were formulated by the boundary element method for determining the stress fields in an elastic material with a centrally located opening. The opening which can be of various geometry, is reinforced by roof bolting (either point-anchored or fully grouted bolt) of different systems. Also the effects of bedding planes in the roof strata are included. A computer program was written for the whole process and two example problems are illustrated. Comparisons with closed-form analytical and finite element methods demonstrate the accuracy of the method developed.

Fracture growth around openings in thick-walled cylinders of rock subjected to hydrostatic compression

Cylindrical specimens of rock into which holes of circular, elliptical and rectangular cross section had been cut, were tested in hydrostatic compression, in order to study the growth of fractures around the holes. The rock types used were an isotropic, homogenous sandstone and an anisotropic, argillaceous quartzite and the relative sizes of specimens and holes were such as to eliminate boundary effects for at least one of the specimen geometries. Failure around the circular holes initiated by shearing at diametrically opposite points and spalling continued at these loci until the opening was sufficiently large for ultimate collapse of the specimen to occur. In the elliptical and rectangular specimens spalling along similar shear planes occurred at the ends of the long axes of the holes. However, if the applied hydrostatic pressures were sufficiently large and were held for a sufficiently long time, a second set of conjugate shear fractures appeared, along which movement was such as to cause material to move into the opening. In addition, extension cracks parallel to the long axes of the holes tended to form in the region above the opening and these cracks often coalesced with shear cracks radiating out from the ends of the holes to form a series of domical cracks. There is a striking similarity between the observed fracture patterns and that predicted theoretically around an elliptical hole with an 8:1 axial ratio in an isotropic elastic medium although the effect of bedding planes in the quartzites is to modify the orientation of the fractures. There is also good agreement between the observed fractures and those observed in front of tabular excavations in mines.