Direction Of Maximum Horizontal Stress Research Articles

Geomechanical risk assessment of injection test for a carbon storage site appraisal, offshore Northern Territory

A geomechanical study has been conducted to assess the risk of formation fracturing and sanding during a water injection test. Considering the temperature difference between the storage formation and the injection fluid, the cooling effects could lead to a reduction of almost 15% in fracture initiation pressure (FIP). Notably, the FIP for perforations aligned with the maximum horizontal stress direction are lower than the minimum horizontal stress. While injection pressures surpassing FIP might trigger tensile fracturing in specific perforations, fracture propagation from the wellbore remains unlikely if the injection pressure remains below the field’s minimum horizontal stress. The pressure requirements for current envisaged matrix injection rates fall below the estimated FIPs. Consequently, the risk of formation fracturing during the injection test is deemed low. The propensity of sanding during flowback, considering a range of rock weakening and thermal effects of water injection, is also found to be low.

Mechanisms for Microseismicity Occurrence Due to CO2 Injection at Decatur, Illinois: A Coupled Multiphase Flow and Geomechanics Perspective

ABSTRACT We numerically investigate the mechanisms that resulted in induced seismicity occurrence associated with CO2 injection at the Illinois Basin–Decatur Project (IBDP). We build a geologically consistent model that honors key stratigraphic horizons and 3D fault surfaces interpreted using surface seismic data and microseismicity locations. We populate our model with reservoir and geomechanical properties estimated using well-log and core data. We then performed coupled multiphase flow and geomechanics modeling to investigate the impact of CO2 injection on fault stability using the Coulomb failure criteria. We calibrate our flow model using measured reservoir pressure during the CO2 injection phase. Our model results show that pore-pressure diffusion along faults connecting the injection interval to the basement is essential to explain the destabilization of the regions where microseismicity occurred, and that poroelastic stresses alone would result in stabilization of those regions. Slip tendency analysis indicates that, due to their orientations with respect to the maximum horizontal stress direction, the faults where the microseismicity occurred were very close to failure prior to injection. These model results highlight the importance of accurate subsurface fault characterization for CO2 sequestration operations.

Seismicity and state of stress in northeast Arabia

Abstract Eastern Arabia exhibits low seismic activity although damaging earthquakes are historically known. Despite the proximity to convergent plate boundaries in the Arabia-Eurasia collision zone, the state of stress in northern Oman with its persistent topography of the Oman Mountains (OM) remains enigmatic. We revised the earthquake catalogue of northern Oman and confirm detection and location accuracy of the permanent network by comparison with a temporarily densified network across the OM. For the first time, we infer focal mechanisms (FM) for earthquakes in Oman from P-wave polarities. Seismic activity is high in the Northern OM but diminishes rapidly south of 24.5°N. In the Central and Eastern OM, low-magnitude earthquakes occur along topography bounding faults with mostly transtensional FM. Offshore, seismicity follows NE-trending lines in extension of the Semail Gap and the Masirah Fault Zones, up to the Makran trench. Except for an isolated patch of repeated small-magnitude earthquakes, the western Makran trench is seismically quiet. Inversion of FM confirms a NE-SW direction of maximum horizontal stress that aligns with Arabia-Eurasia convergence. In the Central and Eastern OM, maximum horizontal and vertical stresses are balanced. The topography of the OM appears, therefore, not to be generally sustained by compressional forces.

The shrinkage behavior of boreholes drilled through deepwater creeping salt formations in West Africa

Introduction: Salt formations are complex and pose significant risks during oil and gas drilling. Creep behavior in salt formations under geostress can jeopardize drilling safety.Methods: This study analyzes the shrinkage behavior of boreholes drilled through salt formations in West Africa’s Block B, with emphasis on the differential creep rates in two horizontal principal stress directions and the evolution of wellbore shape over time. The impact of drilling fluid density on shrinkage rates is also investigated.Results: After drilling through salt formations, the creep rates differ between the two horizontal principal stress directions. Shrinkage is faster in the direction of minimum horizontal principal stress and slower in the direction of maximum horizontal principal stress. Over time, shrinkage rates converge, resulting in a transition from elliptical to circular wellbore shape. Higher drilling fluid density leads to reduced shrinkage rates.Discussion: These findings contribute to the theoretical guidance for drilling fluid density selection in salt formations.

Modeling of cryogenic cracking behavior of tight sandstone under confinement

Stimulation is a must for commercial development of tight sandstone hydrocarbon reservoirs. Cryogenic fracturing using liquid nitrogen (LN) is a promising clean technique for efficiently stimulating reservoir rocks given its waterless nature. We developed a fully coupled thermo-mechanical (TM) model that incorporates strain-based damage theory for simulating LN cryogenic cracking in tight sandstone. Particularly, the compressive and tensile strengths, Young’s modulus, and thermal expansion coefficients and conductivity are designated as dynamic functions of the damage variable during the TM coupling process. Then the initiation, propagation, and cessation of multiple cracks from a borehole within a 2D heterogeneous tight sandstone plate were systematically scrutinized. Cryogenic cracks are found to emerge in short and long forms. In accordance with experiments, multiple long cracks emanate radially from the borehole along the maximum horizontal stress direction. In the investigated parameter ranges, the fracture morphology, including numbers, lengths, and coverage, is susceptible to changes in in-situ stress, Young's modulus, and thermal conductivity, but relatively insensitive to the variations of heat transfer coefficient and sandstone density. These results deepen our understanding of cryogenic shock on tight sandstone and provide a theoretical reference for designing cryogenic treatment operations.

Imaging faults and fractures with the difference of fast and slow shear-wave splitting reflectivity, Δγ(S): 3D/9C survey in Midland Basin, West Texas, and 3D/3C survey in Washakie Basin, Wyoming

One of the most important applications of shear-wave (S-wave) seismic exploration has been in reservoir fracture characterization. While many advancements have been made over the past 30 years to compute and correct for the long-wavelength kinematics of S-wave splitting (SWS) (fast S-wave polarization directions and slow S-wave time delays), practically no progress has been made in imaging the short-wavelength reflectivity of fractures directly with Δγ(S). This property is the contrast in the SWS anisotropy parameter, γ(S), and represents the reflection amplitude at vertical incidence for changes in fracture density and orientation across an interface. In this article, we examine the Lupin nine-component survey in Midland Basin, Texas, for the technical reliability of imaging fractures in depth with converted P to S waves (PS waves) guided by pure-mode horizontal shear waves and vertical shear waves. Final Δγ(S) amplitude maps for each mode show sensitivity to fractures, faults, and the maximum horizontal stress direction. These maps are computed from the difference between fast and slow S-wave stacks (after SWS analysis and correction) and P-wave amplitude variation with offset gradient stacks. The S-wave difference maps identify an east–west lineament, possibly a strike-slip fault or fracture corridor, that is not observed by P-wave depth slices. Pure-mode S waves and PS waves are orders of magnitude more sensitive to Δγ(S) than P waves. We also review the development of Δγ(S) and find that it has been relatively unexploited by the exploration industry. In addition, we demonstrate that Δγ(S) can be obtained directly from the objective function of the transverse energy to correct for SWS, and show a four-component Alford rotation example from a previous PS-wave survey in the Washakie Basin, Wyoming.

Numerical Simulation of the Simultaneous Development of Multiple Fractures in Horizontal Wells Based on the Extended Finite Element Method

Simultaneous multiple-fracture treatments in horizontal wellbores have become an essential technology for the economic development of shale gas reservoirs. During hydraulic fracturing, fracture initiation and propagation always induce additional stresses on the surrounding rock. When multiple fractures develop simultaneously, the development of some fractures is limited due to the stress-shadow effect. An in-depth understanding of the multiple-fracture propagation mechanism as reflected by fracture morphology and flow rate distribution can help to set reasonable operation parameters for improving the uniformity of multiple fractures and forming a complex fracture network according to the different in situ stress conditions in a reservoir to increase the shale gas recovery and reduce the cost. In this study, a two-dimensional (2D) fracture propagation model was developed based on the extended finite element method (XFEM). Then, the influences of various factors, including geological and operational factors, on the development of multiple fractures were studied. The results of numerical simulations showed that increasing the cluster spacing or injecting fracturing fluid with lower viscosity can help reduce the stress-shadow effect. In the case of smaller in situ stress differences, the deflection of the fractures was larger due to the stress-shadow effect. As the stress difference increased, the direction of the propagation of the fracture was gradually biased towards the direction of maximum horizontal stress. In addition, the injection rate had some effects on the fracture morphology and flow rate distribution. However, as the injection rate increased, the dominant fracture developed rapidly, and the fracture length significantly increased.

Interpretation of low-frequency distributed acoustic sensing data based on geomechanical models

Distributed acoustic sensing (DAS) is a technology that enables continuous, real-time measurements along the entire length of a fiber-optic cable. The low-frequency band of DAS can be used to analyze hydraulic fracture geometry and growth. In this study, the low-frequency strain waterfall plots with their corresponding pumping curves were analyzed to obtain information on fracture azimuth, propagation speed, number of fractures created in each stage, and restimulation of preexisting fractures. We also use a simple geomechanical model to predict fracture growth rates while accounting for changes in treatment parameters. As expected, the hydraulic fractures principally propagate perpendicular to the treated well, that is, parallel to the direction of maximum horizontal stress. During many stages, multiple frac hits are visible, indicating that multiple parallel fractures are created and/or reopened. Secondary fractures deviate toward the heel of the well, likely due to the cumulative stress shadow caused by previous and current stages. The presence of heart-shaped tips reveals that some stress and/or material barrier is overcome by the hydraulic fracture. The lobes of the heart are best explained by the shear stresses at 45° angles from the fracture tip instead of the tensile stresses directly ahead of the tip. Antennas ahead of the fracture hits indicate the reopening of preexisting fractures. Tails in the waterfall plots provide information on the continued opening, closing, and interaction of the hydraulic fractures within the fracture domain and stage domain corridors. The analysis of the low-frequency DAS plots thus provides in-depth insights into the rock deformation and rock-fluid interaction processes occurring close to the observation well.

Seismic anisotropy and stress-field variations along the Dead Sea Fault zone in northern Israel

Geological observations based mainly on stress (strain)-indicating meso-scale markers suggest that the local maximum horizontal compressive stress (SHmax) direction along the Dead Sea Fault (DSF) system in northern Israel changes considerably over short distances from a ∼ N-S, fault-parallel orientation in the south to an ∼E-W, fault-perpendicular orientation in the north. However, the association of the meso-scale markers to the present-day stress field is not well constrained as many of them could have formed at any stage during the DSF evolution. In this study, we use a large dataset of Shear-Wave Splitting (SWS) measurements, taken from local DSF-related earthquakes, to characterize the azimuthal seismic anisotropy of the upper-crust at this critical turning point of the DSF system, near the Lebanese Restraining Bend. The SWS directions commonly align parallel to the local SHmax as revealed by the meso-scale markers, supporting their association with the current stress field. In several stations, SWS fast directions indicate azimuthal anisotropy at high angles to the trace of the master fault, indicating that stress-induced anisotropy represents a more plausible mechanism for SWS than structure-controlled anisotropy. The SWS measurements suggest that a northward counterclockwise rotation of the crustal SHmax is present along this segment of the DSF, supporting previous notation on the transition from divergent to convergent strike-slip faulting in this area. Plain langauge summaryGeological observations based mainly on stress (strain)-indicating meso-scale markers suggest that the local maximum horizontal compressive stress direction along the plate-boundary Dead Sea Fault (DSF) system in northern Israel changes considerably over short distances. However, the association of the meso-scale markers to the present-day local stress field is not well constrained since many of them could have formed at any stage of the DSF evolution. In this study, we use a common method of measuring local seismic anisotropy called “shear wave splitting” (SWS) to characterize the directional-dependent seismic wave speeds in the upper crust. The SWS measurements are obtained from local seismic and micro-seismic DSF-related events, and characterize the upper-crust seismic anisotropy at this critical turning point of the DSF system. The SWS directions commonly align parallel to the maximum compression as revealed by the meso-scale markers, supporting their association with the current stress field. The SWS measurements suggest that a northward counterclockwise rotation of the crustal maximum compression is present along this segment of the DSF.

Stress background and rock fractures revealed by ultrasonic borehole television in the Fankou Lead-Zinc Mine

The stress background and rock fractures are essential factors affecting the stability of mines. In order to better understand the in situ stress background and rock fractures in the Guangdong Fankou Mine, we use ultrasonic borehole television scanning to measure rock fractures. The results indicate that rock fractures are intensively distributed at depths of −360 m to −450 m below the surface, suggesting the effect of intensive mining activities. The present maximum horizontal principal stress direction is NWW, which is consistent with the regional tectonic stress field direction. Systematic measurement of rock fractures is fundamental for further three-dimensional geological modeling and is significant for mining engineering.

Crustal stress pattern at Mt. Etna volcano

Stress fields may exhibit variegated patterns, especially in volcanic areas where several processes superimpose their effects in space and time. The comprehension of such patterns may not be straightforward to investigate. This work investigates the pattern of the crustal stress in the area of Mt. Etna Volcano (Sicily, Italy). This has been possible through a collection of more than 800 stress indicators derived from seismological and volcanological/geological information. In particular, the type of collected data allows to consider, for the first time in this area, two different temporal steps in the evolution of Etna volcano: the present-day and the previous volcanic phase at 15 ka. Results indicate a transition between a background shallow NW-SE tensional regime and a deep SW-NE compressional one that occurs between 6 and 16 km depth and which well fits with the present-day geodynamic framework of the area. The occurrence of small-scale lateral variations is interpreted as the second-order effect of the structures of the active front buried beneath the volcano, to the volcano loading, and to the feeding system. The temporal variations in the area surrounding the volcano suggest a major rearrangement of the background stress field evidenced by the swap between minimum and maximum horizontal stress directions. Conversely, during the same period, the stress pattern in the exact correspondence of the volcanic edifice showed to be stable and with a radial arrangement. Such coherence would support the literature which suggests a long-term inflation process started at least 15 kyr ago.

Predicting The Azimuth of Stress from Wellbore Breakouts in (X) Oil Field

The current minimum and maximum horizontal stresses orientations are influential parameters in the development of any Geomechanical model; these models aim to reduce wellbore instability and non-productive time during drilling by selecting the best wellbore direction when drilling inclined or horizontal wells. Minimum horizontal stress orientations can be established by examining wellbore breakouts. Borehole breakouts result from a natural failure compression process in which the maximum hoop stress surrounding the hole exceeds the rock strength, which may be detected using an oriented caliper log tool. There is no data in the World Stress Map (WSM) in the research area. As a result, data from 10 wells in the (X) oilfield's Oriented caliper well logs were analyzed for this study. The breakouts azimuth is consistent with depth, and the minimum horizontal stress azimuth from the north is 140o - 150o and 320o - 330o, which is compatible with the regional maximum horizontal stress direction (Shmax). As a result, the recommended drilling direction of deviated, and horizontal wells in the (X) oil field area should parallel the maximum horizontal stress.

Mapping of stress and structure controlled upper crustal anisotropy in Kumaon-Gharwal Himalaya

Shear wave splitting analysis of local earthquakes provides valuable insights into the structure and stress controlled upper crustal anisotropy signatures. We examine these signatures at the Kumaon-Garhwal Himalaya and provides an excellent example of anisotropic crustal structure in the ongoing continent-continent collision settings between Indian and the Eurasian plates. A total of 256 local earthquakes were selected for the analysis (1.0 ≤ M ≤ 5.4) between January 2017 and February 2021, recorded at a dense network of 51 broadband stations. The result indicates the observed crustal anisotropy is parallel to stress-aligned micro-cracks far from the major Himalayan fault zones and structure parallel near the fault zones. The dominant fast polarization directions (FPD) in the Inner Lesser Himalaya (ILH) and Higher Himalaya, are consistent with the maximum compressive horizontal stress directions (SHmax), essentially influenced by the local stress field. Whereas in the Outer Lesser Himalaya (OLH) and Sub Himalaya, fast directions are sub-parallel to the structural trends, suggesting that the anisotropy is associated with the shear fabric from recent deformation episodes related to structural induced anisotropy. The computed normalized delay times show a mean value of 1 ms/km, within the OLH and Sub-Himalaya, while in ILH and Higher Himalaya, this value increases up to 5.3 ms/km. The associated crack densities are 0.0038 and 0.0207, with shear wave velocity anisotropy of 0.38% and 2.07% respectively. Significant scatter within the depth range of 10–15 km is observed in normalized delay times, suggesting the source of anisotropy within the upper crust.

Stress Constraints From Shear‐Wave Analysis in Shallow Sediments at an Actively Seeping Pockmark on the W‐Svalbard Margin

AbstractMechanisms related to sub‐seabed fluid flow processes are complex and inadequately understood. Petrophysical properties, availability of gases, topography, stress directions, and various geological parameters determine the location and intensity of leakage which change over time. From tens of seafloor pockmarks mapped along Vestnesa Ridge on the west‐Svalbard margin, only six show persistent present‐day seepage activity in sonar data. To investigate the causes of such restricted gas seepage, we conducted a study of anisotropy within the conduit feeding one of these active pockmarks (i.e., Lunde Pockmark). Lunde is ∼400–500 m in diameter, and atop a ∼300–400 m wide seismic chimney structure. We study seismic anisotropy using converted S‐wave data from 22 ocean‐bottom seismometers (OBSs) located in and around the pockmark. We investigate differences in symmetry plane directions in anisotropic media using null energy symmetries in transverse components. Subsurface stress distribution affects fault/fracture orientations and seismic anisotropy, and we use S‐wave and high‐resolution 3D seismic data to infer stress regimes in and around the active seep site and study the effect of stresses on seepage. We observe the occurrence of changes in dominant fault/fracture and horizontal stress orientations in and around Lunde Pockmark and conclude minimum (NE‐SW) and maximum (SE‐NW) horizontal stress directions. Our analysis indicates a potential correlation between hydrofractures and horizontal stresses, with up to a ∼32% higher probability of alignment of hydrofractures and faults perpendicular to the inferred minimum horizontal stress direction beneath the Lunde Pockmark area.

Fracture geometry and breakdown pressure of radial borehole fracturing in multiple layers

Radial borehole fracturing that combines radial boreholes with hydraulic fracturing is anticipated to improve the output of tight oil and gas reservoirs. This paper aims to investigate fracture propagation and pressure characteristics of radial borehole fracturing in multiple layers. A series of laboratory experiments with artificial rock samples (395 mm × 395 mm × 395 mm) was conducted using a true triaxial fracturing device. Three crucial factors corresponding to the vertical distance of adjacent radial borehole layers (vertical distance), the azimuth and diameter of the radial borehole are examined. Experimental results show that radial borehole fracturing in multiple layers generates diverse fracture geometries. Four types of fractures are identified based on the connectivity between hydraulic fractures and radial boreholes. The vertical distance significantly influences fracture propagation perpendicular to the radial borehole axis. An increase in the vertical distance impedes fracture connection across multiple radial borehole layers and reduces the fracture propagation distance along the radial borehole axis. The azimuth also influences fracture propagation along the radial borehole axis. Increasing the azimuth reduces the guiding ability of radial boreholes, which makes the fracture quickly curve to the maximum horizontal stress direction. The breakdown pressure correlates with diverse fracture geometries observed. When the fractures connect multi-layer radial boreholes, increasing the vertical distance decreases the breakdown pressure. Decreasing the azimuth and increasing the diameter also decrease the breakdown pressure. The extrusion force exists between the adjacent fractures generated in radial boreholes in multiple rows, which plays a crucial role in enhancing the guiding ability of radial boreholes and results in higher breakdown pressure. The research provides valuable theoretical insights for the field application of radial borehole fracturing technology in tight oil and gas reservoirs.

Characteristics and mechanism of rockburst at five deep gold mines in Jiaodong Peninsula of China

Rockburst in deep rock engineering poses serious threat to personnel safety and engineering. In order to reveal the rockburst characteristics in deep engineering, the rockburst investigations and rockburst mechanisms based on in-situ stress were carried out at five deep gold mines in Jiaodong Peninsula, China. At the cover depth between 400 m and 600 m, the ground pressure hazards are mainly local spalling. At the cover depth between 600 m and 1000 m, the ground pressure hazards are mainly rockburst, accompanied by serious spalling. When the cover depth is more than 1000 m, in addition to rockburst, the large area collapse occurred in some tunnels. The maximum horizontal principal stress in the northwest of Jiaodong Peninsula is the largest, indicating that the regional in-situ stress field is dominated by horizontal tectonic stress. The average maximum horizontal principal stress direction is about 105°, which is approximately perpendicular to the strike of regional metallogenic fault zone. The increase of in-situ stress is the main factor leading to the serious ground pressure hazards. The tangential stress threshold for rockburst is 8 times the tensile strength of rock at the five gold mines, which is consistent with the stress threshold (8 times tensile strength) leading to brittle failure of rock in Griffith criterion. This indicates that the strain rockburst is caused by the concentration of tensile stress in the micro-cracks of surrounding rock.

Geophysical characteristics of a fault system in the northern Central Range of Taiwan and its applications for geothermal energy exploration

The northern Central Range of Taiwan is a high-potential geothermal region. Since the formations are mainly tight metasandstone and slate, permeable structures associated with faults are commonly considered as conduits of geothermal fluids. This study determines the characteristics and orientations of the permeable fault zones by analyzing the geophysical logs and microresistivity formation image log (FMI) of the JT-4 well in Jentse, an important geothermal area in the northern Central Range. Between 720 and 1480 m measured depth (MD), the effective porosity of the intact host rock is mostly below 3% calculated by the geophysical log. Zones with porosity greater than 5% are only clustered within a few thin intervals. The FMI interpretations show these porous zones are in the interior of the fractured and faulted intervals. These porous fault zones comprise fault damage zones with a high density of open fracture planes and fault cores with porous fault breccias. There is a highly brecciated fault core in 1334–1339 m MD, which would be the most permeable interval of the well. Additionally, some healed fault zones with sealed fractures are observed. The picked drilling-induced tensile fractures signify that the direction of the present-day maximum horizontal principal stress is N40–50°E, and most of the open fractures also strike parallel to the NE–SW direction. The study results show that the open fractures are concentrated in the four fault zones belonging to one major normal fault system. After integrating the orientations and locations of the fault zones, we propose that the permeable normal fault system is about 200 m wide, trends N50–70°E, and dips 70–80° to the NW. The development of the open fractures and the permeable fault system in the northern Central Range may be controlled by the current rifting of the Okinawa Trough offshore northeastern Taiwan. The study exhibits the characteristics of fractured fluid conduits of the regional geothermal system, which will benefit future geothermal exploration in northeastern Taiwan.

Visualization of hydraulic fracture interacting with pre-existing fracture

Hydraulic fracturing is considered the main stimulation method to develop shale gas reservoirs. Due to its strong heterogeneity, the shale gas formation is typically embedded with geological discontinuities such as bedding planes and natural fractures. Many researchers realized that the interaction between natural fractures and hydraulic fractures plays a crucial role in generating a complex fracture network. In this paper, true tri-axial hydraulic fracturing tests were performed on polymethyl methacrylate (PMMA), on which pre-existing fracture was pre-manufactured to simulate natural fracture. A cohesive model has been developed to verify the results of the experimental tests. The key findings demonstrate that the experimental results agreed well with the numerical simulation outcomes where three main interaction modes were observed: crossing; being arrested by opening the pre-existing fracture; being arrested without dilating the pre-existing fracture. Crossing behavior is more likely to occur with the approaching angle, horizontal stress difference, and injection rate increase. Furthermore, the higher flow rate might assist in reactivating the natural fractures where both sides of the pre-existing fractures were reactivated as the injection rate increased from 5 to 20 mL/min. Additionally, hydraulic fractures show a tendency to extend vertically rather than along the direction of maximum horizontal stress when they are first terminated at the interface. This research may contribute to the field application of hydraulic fracturing in shale gas formation.

Stress interference and interaction between two fractures during their propagation: insights from SCDA test and XFEM simulation

Understanding and predicting the propagation and disturbance of multiple fractures is the key to the successful implementation of hydraulic fracturing. In this study, the two-hole fracturing tests are carried out on granite samples by using soundless cracking demolition agent (SCDA). Combined with the extended finite element method (XFEM) simulation and the induced stress analysis, the interaction and stress interference during the propagation of two fractures are systematically investigated. The relative position of two holes determines whether attraction or repulsion effect occurs between adjacent fractures, the pattern of which can be classified into three modes: the fracture collinear, staggered propagation and parallel propagation. The fractures in collinear pattern are prone to coalescence, the staggered fractures are easy to attract each other and the parallel fractures tend to propagate independently, which ultimately lead to the formation of single, spiral and multiple main fractures, respectively. The stress near the fracture tip controls the possibility and direction of fracture propagation. The superposition of tensile stress is prone to promote fracture propagation, and the stress interference tends to make fractures deflection and mutual exclusion. The fracturing results are quantitatively evaluated by using fractal dimension and fracture surface characteristics. It is found that increasing perforation spacing makes the degree of internal rupture of the sample increase first and then decrease, and the fracturing scheme is optimal when the perforations are arranged perpendicular to the direction of maximum horizontal principal stress.

Early estimation method of rockburst and large deformation of surrounding rock based on the deep borehole test

In geotechnical engineering, several major catastrophic accidents occur frequently in areas of high in-situ stress. To study the influence of high in-situ stress on deep mining, the in-situ stress was tested using the hydraulic fracturing method in the mine. Based on the measured data of the initial stress, a comprehensive evaluation of the stress field of the deep surrounding rocks was carried out. Combining the physical and mechanical indexes of the surrounding rocks, field measurements, and theoretical analysis, the Russenes criterion and Turchaninov criterion were applied to evaluate the propensity of rockbursts of hard rocks in the mine area. In addition, the large deformation of the soft rock in the mine was predicted based on the large deformation classification criteria. The results demonstrate that the vertical stress is linearly related to the depth. The horizontal principal stress values tested in all holes except boreholes G and I are approximately linearly distributed with depth. The greater the depth, the greater the propensity for rockburst. For obvious deviation from the maximum horizontal main stress direction of the mining tunnel, the tendency of rockburst during construction is greater. Slight deformation occurs when the tunnel surrounding rock is shallow than 660 m; larger deformation occurs when the burial depth exceeds 660 m. Level-Ⅱ or level-Ⅲ deformations may occur near the bottom of holes F, G, and I due to the lower uniaxial compressive strength of the phyllites of these holes.