Principal Stress Difference Research Articles

Numerical Simulation of Hydraulic Fractures Breaking through Barriers in Shale Gas Reservoir in Well YS108-H3 in the Zhaotong Shale Gas Demonstration Area

Estimating the effectiveness of hydraulic fracturing in the context of the incrfease in the shale gas demand is of great significance for enhancing shale gas production, which aims to substantially reduce fossil energy consumption and CO2 emissions. The Zhaotong national shale gas demonstration zone has complex stress structures and well-developed fracture zones, and thus it is challenging to achieve targeted reservoir segment transformation. In this paper, we construct and optimize the geometry of hydraulic fractures at different pressures considering the upper and lower barriers in hydraulic fracturing simulation experiments and numerical modeling. The numerical simulation results show that the pore pressure exhibits a stepped pattern around the fracture and an elliptical pattern near the fracture tip. During the first time of injection, the pore pressure rapidly increases to 76 MPa, dropping sharply afterward, indicating that the fracture initiation pressure is 76 MPa. During the fracture propagation, the fracture length is much greater than the fracture height and width. The fracture width is larger in the middle than on the two sides, whereas the fracture height gradually decreases at the fracture tip in the longitudinal direction until it closes and is smaller near the wellbore than at the far end. The results revealed that the fracture width at the injection point reached the maximum value of 9.05 mm, and then it gradually decreased until the fracture width at the injection point dropped to 6.33 mm at the final simulation time. The fracture broke through the upper and lower barriers due to the dominance of the effect of the interlayer principal stress difference on the fracture propagation shape, causing the hydraulic fracture to break through the upper and lower barriers. The results of the physical simulation experiment revealed that after hydraulic fracturing, multiple primary fractures were generated on the side surface of the specimen. The primary fractures extended, inducing the generation of secondary fractures. After hydraulic fracturing, the width of the primary fractures on the surface of the specimen was 0.382–0.802 mm, with maximum fracture widths of 0.802 mm and 0.239 mm, representing a decrease of 70.19% in the maximum fracture width. This work yielded an important finding, i.e., the urgent need for hydraulic fracturing adaptation promotes the three-dimensional development of a gas shale play.

Mechanism of rock burst in deep gob-side entry based on dynamic and static stress: a case study

Deep gob-side entry (DGSE) shows rock burst risk under the condition of kilometer depth and hard roof. Based on the established conditions of the Tengdong coal mine, theoretical analysis, microseismic (MS) monitoring, and numerical modeling were used to study the rock burst mechanism of DGSE. Results show that MS events mainly occurred on the solid side of DGSE and the intense dynamic load was mainly caused by the breaking of low hard roof strata, which can release 2.64 × 105 J elastic energy per meter. The surrounding stress of DGSE was asymmetrical due to the coal pillar yielding and hanging roof’s weight, and the load of coal pillar is negatively correlated with that of solid. Simulation shows the vertical stress evolution of coal pillar and solid shows significant diversity. Coal pillar’s vertical stress first drops sharply, and then increases gradually, finally stable at 10.6 MPa with the DGSE’s excavation. Contrarily, solid’s vertical stress gradually rises and was finally stable at 40.9 MPa. Under roof dynamic loading, the vibration velocity of the entry’s top was higher than that of the floor which was caused by the increase of the propagation distance and the reflection and diffraction effect of waves. The vibration velocity of the coal pillar was significantly higher than that of the solid which is because higher stress can lead to faster attenuation of vibration velocity. After dynamic loading, the coal pillar’s principal stress and principal stress difference decreased while that of the solid can be divided into two drop areas and one rising area. Periodic pressure relief that was carried out in the rising area can reduce the rock burst risk on the solid side of DGSE.

The non-plane initiation and propagation mechanism of multiple hydraulic fractures in tight reservoirs considering stress shadow effects

Multi-cluster fracturing in horizontal wells is an important stimulation technology for unconventional tight reservoirs. The interference among multiple simultaneously growing hydraulic fractures results in a non-uniform fluid distribution into fractures and non-planar fracture geometries. In this paper, the competition mechanism between perforations under the stress shadow, the formation mechanism of complex non-plane fractures near the wellbore, and the initiation and propagation mechanism of multiple hydraulic fractures are investigated by using the 3D lattice method. Also, the effects of fracture geometry, fracture spacing and rock mechanics parameters on the stress interaction among fractures are analyzed by the dislocation theory. The results showed the complexity of fracture initiation near the wellbore and the low efficiency of the perforation initiation in the case of spiral perforation, due to a strong interaction between perforations. However, these initial micro-cracks will eventually coalesce to be a macroscopical fracture which perpendicular to the direction of minimum principal stress. The study indicated that stress interaction and fracture propagation is significantly affected by inclined well orientation. The hydraulic fracture is more likely to deflect and propagate non-uniformly with the increasing width of hydraulic fracture, the decreasing spacing between perforation clusters and the increasing rock modulus. When the horizontal principal stress difference is small, it is obvious to cause a large deflection angle and sometimes furcation of fractures. This study can provide a theoretical basis and technical guidance for hydraulic fracturing design in tight reservoirs.

Elliptically polarized light photoelasticity based on LCD.

We propose a three-wavelength elliptically polarized light photoelasticity method for high efficiency and low-cost stress measurement. By illuminating the sample with two different forms of elliptically polarized light for each wavelength sources, twelve images are acquired. From these images, phase delay and the principal internal stress difference are precisely computed using developed algorithms. Our proposed method based on an LCD panel has the unrivalled advantage that elliptically polarized light can be automatically adjusted, which reduces the mechanical rotation of the system, in contrast to the traditional six-step phase-shifting photoelasticity method, which requires manual rotation for circularly polarized light. In addition, the system has the potential to theoretically expand the area of illumination infinitely, thereby expanding the measurement area. The viability of the suggested methods is confirmed with numerical simulation and stress measurement.

Optimization of integrated geological-engineering design of volume fracturing with fan-shaped well pattern

According to the variable toe-to-heel well spacing, combined with the dislocation theory, discrete lattice method, and finite-element-method (FEM) based fluid-solid coupling, an integrated geological-engineering method of volume fracturing for fan-shaped well pattern is proposed considering the geomechanical modeling, induced stress calculation, hydraulic fracturing simulation, and post-frac productivity evaluation. Besides, we propose the differential fracturing design for the conventional productivity-area and the potential production area for fan-shaped horizontal wells. After the fracturing of the conventional production area for H1 fan-shaped well platform, the research shows that the maximum reduction of the horizontal principal stress difference in the potential productivity-area is 0.2 MPa, which cannot cause the stress reversal, but this reduction is still conducive to the lateral propagation of hydraulic fractures. According to the optimized fracturing design, in zone-I of the potential production area, only Well 2 is fractured, with a cluster spacing of 30 m and an injection rate of 12 m3/min per stage; in zone-II, Well 2 is fractured before Well 3, with a cluster spacing of 30 m and an injection rate of 12 m3/min per stage. The swept area of the pore pressure drop in the potential production area is small, showing that the reservoir is not well developed. The hydraulic fracturing in the toe area can be improved by, for example, properly densifying the fractures and adjusting the fracture distribution, in order to enhance the swept volume and increase the reservoir utilization.

Using the Magnetic Anisotropy Method to Determine Hydrogenated Sections of a Steel Pipeline

The paper deals with a non-destructive method of detecting hydrogenated sections of pipelines, which is based on variations of the level of mechanical stresses generated in the surface layers of the steel pipe material during its hydrogenation. The use of a magnetoanisotropic method based on the phenomenon of metal magnetoelastic anisotropy development, which consists in the variation of the magnetic properties of ferromagnetic materials in direction and magnitude under the influence of mechanical stresses, is proposed as a way to register that variation. Based on the results of tensile testing of carbon steel plates with measurement of the difference in principal mechanical stresses (DPMS) occurring in metal, as well as experiments on electrolytic hydrogenation of specimens with measurement of the DPMS signal, it was confirmed that when steel structures are saturated with hydrogen, tensile stresses are generated in the surface layers, the magnitude of which increases as the concentration of hydrogen increases in the metal. In this case, it is assumed that the indicated dependence between the hydrogen concentration in the metal and the stresses arising as a result of hydrogenation is linear. For the example of lamellar specimens made of pipe low-carbon steel, the possibility of using the magnetoanisotropic method for registering sections of underground pipelines with a high content of hydrogen is substantiated, which can become the basis for a method of diagnosing sections of pipelines with broken insulation for the possibility of their further operation. The scientific novelty of this article is the establishment of a relationship between the hydrogen content in the metal, the stresses that arise in this case, and the change in the magnetic properties of ferromagnetic materials, characterized by the magnitude of the DPMS signal. This study contributes to the understanding of the process of hydrogenation of metals, and may be useful in detecting and preventing damage to gas and oil pipelines caused by hydrogen embrittlement as a cause of stress corrosion.

Fracability evaluation of shale reservoirs considering rock brittleness, fracture toughness, and hydraulic fracturing-induced effects

Fracability is a vital evaluation index for shale reservoir exploration and development and horizontal well fracturing. In this paper, we define the fracability of a shale reservoir as the ability of hydraulic fracturing to generate a complicated large-scale fracture network and large stimulated volume under the same stimulation parameters. Considering factors such as the shale sedimentary environment and confining pressure, we establish a brittleness index calculation method by coupling logging data and in situ rock mechanics test results to obtain parameters such as the reservoir Young's modulus and Poisson's ratio. According to the fractal method, an in situ rock bifurcation/irregular fracture toughness calculation method is established. The hydraulic fracture-induced index is introduced to reflect the influence of stimulation-induced displacement, reservoir seepage, fluid viscosity, horizontal principal stress difference and other parameters on fracability. A fracability evaluation method coupling the reservoir rock brittleness, fracture toughness and hydraulic fracture-induced index is formed. The results of the field application show that the established method can effectively characterize the ability of a reservoir to form complex fracture networks and promote the stimulation effect. This study provides more credible guidance for geological and engineering practices for developing shale reservoirs.

Numerical Simulation on Radial Well Deflagration Fracturing Based on Phase Field Method

A radial well has a unique wellbore configuration. Fracture propagation in radial well deflagration fracturing is studied rarely. The mechanism of interaction between deflagration fractures, natural fractures, and micro-fractures is still unknown. Based on continuum mechanics, damage mechanics, and variational principles, a numerical model of fracture propagation in deflagration fracturing is established with the Hamilton principle and phase-field fracture theory. The effects of horizontal principal stress difference, natural fracture distribution, and micro-fractures around the wellbore on fracture propagation in deflagration fracturing are studied. First, when no natural fractures are developed around the radial well, fractures are initiated at both ends of the radial well. Second, when there are three natural fractures around the radial well, the created fractures have the morphology of shorter fractures in the middle and longer fractures on both sides under stress interference mechanisms. Third, a larger density of natural fractures causes obvious stress superposition, changes the initiation points of radial wells and fracture morphology, and increases fracture width and reservoir stimulation volume. Fourth, as the micro-fractures increase, their interference and induction effects on deflagration fractures are enhanced gradually, and the deflection angle of fractures increases by 38.7%. The study provides a reference for optimizing deflagration fracturing in a radial well.

A criterion for a hydraulic fracture crossing a natural fracture in toughness dominant regime and viscosity dominant regime

Hydraulic fracturing is an efficient stimulation technology in the process of unconventional reservoir development, and the hydraulic fracture morphology directly affected oil and gas recovery. In this paper, we proposed a new criterion based on linear elastic fracture mechanics (LEFM) to predict the intersection behavior of hydraulic fracture (HF) and natural fracture (NF) with non-orthogonal angle. The new criterion considered the coupling effects of the fluid flow, stress shadow, and poroelastic response, and was validated with published analytical criteria and laboratory tests. Sensitivity analysis results indicated that an HF tends to cross NF under conditions of short HF half-length, high friction coefficient, high fracturing fluid viscosity and high fracture toughness. In addition, high principal stress difference inhibited the crossing behavior when the intersection angle was less than the transition angle, the HF would be arrested or divert along NF.

Numerical simulation of three-dimensional fracturing fracture propagation in radial wells

A fracture propagation model of radial well fracturing is established based on the finite element-meshless method. The model considers the coupling effect of fracturing fluid flow and rock matrix deformation. The fracture geometries of radial well fracturing are simulated, the induction effect of radial well on the fracture is quantitatively characterized, and the influences of azimuth, horizontal principle stress difference, and reservoir matrix permeability on the fracture geometries are revealed. The radial wells can induce the fractures to extend parallel to their axes when two radial wells in the same layer are fractured. When the radial wells are symmetrically distributed along the direction of the minimum horizontal principle stress with the azimuth greater than 15°, the extrusion effect reduces the fracture length of radial wells. When the radial wells are symmetrically distributed along the direction of the maximum horizontal principal stress, the extrusion increases the fracture length of the radial wells. The fracture geometries are controlled by the rectification of radial borehole, the extrusion between radial wells in the same layer, and the deflection of the maximum horizontal principal stress. When the radial wells are distributed along the minimum horizontal principal stress symmetrically, the fracture length induced by the radial well decreases with the increase of azimuth; in contrast, when the radial wells are distributed along the maximum horizontal principal stress symmetrically, the fracture length induced by the radial well first decreases and then increases with the increase of azimuth. The fracture length induced by the radial well decreases with the increase of horizontal principal stress difference. The increase of rock matrix permeability and pore pressure of the matrix around radial wells makes the inducing effect of the radial well on fractures increase.

An unequal fracturing stage spacing optimization model for hydraulic fracturing that considers cementing interface integrity

Determining reasonable fracturing stage spacing is the key to horizontal well fracturing. Different from traditional stage spacing optimization methods based on the principle of maximum stimulated reservoir volume, in this paper, by considering the integrity of the wellbore interface, a fracture propagation model was established based on displacement discontinuity method and the competition mechanism of multifracture joint expansion, leading to the proposal of an unequal stage spacing optimization model. The results show that in the first stage, the interfacial fractures spread symmetrically along the axis of the central point during that stage, while in the second and subsequent stages, the interfacial fractures of each cluster extend asymmetrically along the left and right sides. There are two kinds of interface connectivity behaviour: in one, the existing fractures first extend and connect within the stage, and in the other, the fractures first extend in the direction close to the previous stage, with the specific behaviour depending on the combined effect of stress shadow and flow competition during hydraulic fracture expansion. The stage spacing is positively correlated with the number of fractures and Young’s modulus of the cement and formation and is negatively correlated with the cluster spacing and horizontal principal stress difference. The sensitivity is the strongest when the Young’s modulus of the cement sheath is 10–20 GPa, and the sensitivity of the horizontal principal stress difference is the weakest.

Genetic Mechanism of Lower-Order Faults in Shale Formations in Rift Basins

The distribution of lower-order faults affects the development of oil and gas and the distribution of remaining oil, which is also the key to the development of fault block reservoirs in eastern China. The lower-order faults are characterized by short extension, small fault displacement, and difficult identification by traditional seismic interpretation methods. Therefore, it is necessary to predict the development law of faults in combination with the paleostress field during the fault formation period to improve the accuracy of fault seismic interpretation. In this paper, on the basis of reservoir structural analysis and rock mechanics experiments, the geological and mechanical model of the Sanduo period is established to predict the simulation of the paleostress field during the development of lower-order faults. The development of lower-order faults in rift basins is mainly controlled by the minimum principal stress and stress difference, and the shear stress in the profile controls the tendency of faults. Controlled by the paleostress field, the occurrence of lower-order faults in the Weizhuang area, Gaoyou Sag, and Subei Basin is very complex, and the strike of the fault is distributed as a tension shear broom on the plane. The prediction results can provide a reference for the excavation of remaining oil and the development of oil and gas.

Stress tensor similarity index based on Euclidean distance for numerical back analysis of in situ stress fields

Evaluating the similarity between measured and predicted stress states is critical for the numerical back analysis of three-dimensional in situ stress fields. However, conventional nontensor approaches process stress components or the magnitude and orientation of principal stresses separately, which may yield contradictory results among multiple indices. To address this issue, we propose the Euclidean distance similarity (EDS), which is a dimensionless single index based on spatial distance. As a case study, EDS was applied to a site with complex geological conditions for verification. EDS can be derived in the form of stress component or principal stress. EDS exhibited a positive correlation with the initial difference and magnitude error of principal stresses and changed periodically with the Euler angle of rotation around the principal stress. Moreover, three magnitude errors and three Euler angles had different effects on EDS. Finally, EDS can be classified into five levels (i.e., extremely high, high, normal, low, and very low) according to the magnitude errors of principal stresses and Euler angles, where the “normal” level is recommended as a general error standard. The thresholds of EDS differ according to the initial stress level. These results are a valuable contribution to the similarity evaluation of stress tensors.

A Study on Fracture Propagation Law of CO2 Composite Fracturing in Shale Reservoir

Background: In recent years, CO2 composite fracturing technology has been widely used in unconventional reservoirs. Compared to conventional hydraulic fracturing, CO2 fracturing can create complex fractures, replenish formation energy and reduce oil flow resistance. For shale oil reservoirs with natural fractures, CO2 composite fracturing can not only give full play to the advantages of complex fracture networks created by CO2 but also make use of water-based fracturing fluid to create long fractures with high conductivity. Methods: Based on fracture fluid flow, stress interference, natural fracture description, and CO2 phase change equation, a CO2 composite fracture propagation model was established in this paper to simulate the effects of fracturing fluid type, CO2 proportion, construction scale, natural fracture development, fracturing fluid injection rate and other factors on the propagation morphology of CO2 injection fracture network in shale oil reservoirs. Results: The results show that the water-based fracturing fluid is beneficial to the formation of long main fractures, but the overall complexity of the fracture network and the effective stimulated volume of the fracture network are significantly lower than that of CO2 fracturing. The application of the appropriate proportion of CO2 composite fracturing fluid can give full play to the comprehensive advantages of CO2 and water-based fracturing fluid and realize the full stimulation of the reservoir. CO2 fracturing in shale oil reservoirs with low principal stress difference and high natural fracture development extent can communicate natural fractures in a large range and form a complex fracture network. For shale oil reservoirs with natural fractures, a high fracturing fluid injection rate can significantly improve the complexity of the fracture network. Conclusion: The CO2 composite fracturing technology is applied to horizontal wells in X shale reservoir, and the production after fracturing is significantly higher than that of offset wells, which can be applied in the same type of reservoir and has broad application prospects.

A DEM–CFD coupling method for modelling two-hole synchronous hydraulic fracturing

This work introduced a novel numerical approach for modelling hydraulic fracturing processes. The Edinburgh bonded particle model was employed to establish the discrete element model of the rock samples, and it was further coupled with computational fluid dynamics to simulate the hydraulic fracturing. The fracture propagation law was taken into account under various principal stress differences to confirm the viability of the model. In addition, the model was used to investigate the fracture propagation mechanism of two-hole synchronous hydraulic fracturing. The results indicate that the arrangement and spacing of two holes will affect the complexity of hydraulic fractures. The principal stress difference becomes more prominent in the direction of hydraulic fracture propagation with the increase in the principal stress difference.

Study on the Law of Hydraulic Fracture Propagation in Low Permeability Thin Interbed Reservoir

The third member of Weixinnanliu in the west of the South China Sea develops thin interbeds, and the vertical extension of fracturing fractures is excessive. Once the fractures extend vertically to the upper and lower aquifers, it is easy to cause water flooding of oil wells, and the effect after fracturing is not obvious. The present work aims to explore the longitudinal extension law of fractures in Low Permeability Thin Interbed Reservoir based on the finite element calculation platform. A three-dimensional expansion model of hydraulic fractures in the target reservoir was established, and the displacement, fracturing fluid viscosity, minimum horizontal principal stress difference, vertical stress, interlayer thickness, perforation point separation were studied. The interlayer distance and other factors affect the crack propagation law. The research results show that the thin interbed fractures have three forms: T-shaped fractures, through-layer fractures, and I-shaped fractures; for the target layer, the overlying stress is relatively large, and the minimum principal stress is along the horizontal direction. Vertical cracks; the farther the perforation point is or the greater the stress difference, the smaller the thickness of the interlayer required to control the fracture height; the stress difference is 3 MPa, and the distance between the perforation points exceeds 10 m, the thickness of the interlayer is required to be ≥4 m; In order to ensure that the width of the fracture in the middle spacer does not affect the placement of the proppant, it is recommended that the displacement be controlled within 3 m3/min and the viscosity of the fracturing fluid is 150 mPa·s; in addition, the thickness of the spacer required to control the fracture height is different due to different geological parameters. Different, different wells need targeted analysis.

Method to quantify the dynamic near-fault full-field stress evolution associated with rough fault shear deformation based on 3D printed models and photoelastic measurements

Quantifying the dynamic near-fault stress evolution associated with fault shear deformation is crucial for revealing the earthquake nucleation mechanisms of rough faults. However, it is still challenging via traditional methods because of the difficulties in resolving continuously evolved stresses and high stress heterogeneity. In this study, we present a novel method to quantify the dynamic full-field stress evolution using three-dimensional printed models and photoelastic techniques. The dynamic full-field stresses in a circular disc determined by this method and an analytical model are compared to verify the accuracy of the proposed method. A new method to count the fringe orders near the rough fault is proposed to achieve the stress quantification. The full-field principal stress difference in a rough fault model under shear forces is derived. Our findings indicate that the proposed method can significantly improve the efficiency of stress calculation and effectively quantify the dynamic full-field stress near rough faults.

Study on H-shaped fracture propagation and optimization in shallow dull-type coal seams in the Hancheng block, China

The initiation and propagation of hydraulic fractures dominate the fracture morphology and conductivity, thereby determining the fracturing stimulation effect. This work took the shallow dull-type coal seam in the Hancheng block as the research object, and focused on the propagation and optimization of the H-shaped fracture. By using a combination of mechanical theoretical analysis, laboratory experiment, numerical simulation, field test, and optimization evaluation, an integrated study on the formation conditions, morphology evolution, propagation laws, main controlling factors, and optimal design of H-shaped fractures was conducted. The mechanical criterion for identifying H-shaped fracture propagation was derived through the analysis of the stress fields on the fracture surface. A 3D numerical model of H-shaped fractures was established with cohesive element method, and the propagation law and influencing factors of H-shaped fractures were investigated. Sensitivity analysis of the influencing factors on the H-shaped fractures was performed with grey incidence analysis method. The optimization of the H-shaped fracture parameters was performed by fracture propagation numerical simulation. The results show that H-shaped fractures dominated by horizontal fractures are generally formed in shallow dull-type coal seams. H-shaped fractures can improve the CBM development effect by parameter optimization of indirect fracturing technology. The judging criterion for H-shaped fracture formation is that the tensile strength of the upper and lower weakly cemented horizontal interfaces is less than that of the coal seam and the vertical principal stress is lower than the minimal horizontal principal stress. The incidence degree of fracturing fluid injection rate, viscosity, perforation location, minimum horizontal stress difference, and coal seam permeability on the H-shaped fracture propagation decreases in turn. The engineering factors (fracturing fluid injection rate and viscosity and perforation location) are the major impact factors. With the increase in coal seam permeability, the horizontal fracture develops from stubby-type to slender-type. The minimum horizontal principal stress difference mainly affects the horizontal fracture. As the fracturing fluid viscosity increases, the horizontal fracture length increases first and then decreases. The fracturing fluid injection rate has a great impact on the height-length profile of horizontal fractures. The optimal injection rate seems to be 6.5–7 m3/min, the optimal fracturing fluid viscosity is between 10 mPa·s and 15 mPa·s, and the optimal perforation location is selected near the formation interface. These research results could provide a reference for the design and optimization of efficient fracturing of coalbed methane reservoirs.

Numerical Simulations of Radial Well Assisted Deflagration Fracturing Based on the Smoothed Particle Hydrodynamics Method

The technology of radial-well-assisted hydraulic fracturing is applied in the stimulation of low-permeability hydrocarbon reservoirs where commercial production cannot be achieved by the conventional fracturing method. Here, a study on the reservoir stimulation effect and the fracture propagation pattern of radial-well-assisted deflagration fracturing was carried out. Based on smooth particle hydrodynamics (SPH), rock mechanics theory, and finite element theory, a numerical model of radial-well-assisted deflagration fracturing was established by integrating the JWL state equation. Research on the effects of the deflagration position, radial well azimuth and horizontal principal stress difference on the fracture propagation was carried out. The results show that the deflagration position, radial well azimuth and horizontal principal stress difference have significant effects on the fracture area in deflagration fracturing. The closer distance from the deflagration position is, the larger the radial well azimuth and the smaller the horizontal stress difference are, leading to a larger fracture area, which is conducive to reservoir stimulation. During fracturing, both shear fractures and tensile fractures are formed. The formation and conversion of shear fractures and tensile fractures are related to the deflagration position, radial well azimuth, horizontal principal stress difference, etc.

On various mechanisms of shear wave generation from underground chemical explosions in hard rocks

SUMMARY Significant shear waves observed during underground explosions can make them look similar to natural earthquakes. In the near field, they can be caused by the source geometry and emplacement conditions, heterogeneous and anisotropic properties of the rock mass, relaxation of a non-uniform in situ stress around the source after the departure of the shock wave as well as by activation of nearby faults. The stress relaxation mechanism is important at depth when the difference in main principal stresses becomes significant. Another mechanism of shear motion generation is due to explosive gas flow from the source cavity into the cracks initiated by the shock wave and joints connected to the cavity. Also, the shock wave from the explosion can cause weakening of the rock either due to permanent damage or shock-induced liquefaction at saturated joints. Numerical modelling of underground chemical explosions in hard rock at various confinement stresses has been conducted to compare various mechanisms of shear wave generation and their seismic characteristics with that of a simple fault rupture process observed during an earthquake.