Principal Stress Orientations Research Articles

Response of granular material under combined principal stress value and orientation change in 3D space

Response of granular material under combined principal stress value and orientation change in 3D space

Roof Fractures of Near-Vertical and Extremely Thick Coal Seams in Horizontally Grouped Top-Coal Drawing Method Based on the Theory of a Thin Plate

During the mining process of the near-vertical seam, there will be movement and collapse of the “roof side” rock layer and the “overlying coal seam,” as well as the emergence of the “floor side” rock layer roof which is more complicated than the inclined and gently inclined coal seams, which causes problems with slippage or overturning damage. With the increase of the inclination of the coal seam, the impact of the destruction of the immediate roof on the stope and roadway gradually becomes prominent, while the impact of the destruction of the basic roof on the stope and the roadway gradually weakens. The destruction of the immediate roof of the near-vertical coal seam will cause a large area of coal and rock mass to suddenly rush to the working face and the two lanes, resulting in rapid deformation of the roadway, overturning of equipment, overturning of personnel, and even severe rock pressure disaster accidents, all of which pose a serious threat to coal mine safety and production. It is necessary to carry out research on the mechanical response mechanism of the immediate roof of near-upright coal seams, to analyse the weighting process of steeply inclined thick coal seam sub-level mining. A four fixed support plate model and top three clamped edges simply supported plate model for roof stress distribution are established before the first weighting of the roof during the upper and lower level mining process. The bottom three clamped edges simply supported plate model and two adjacent edges clamped on the edge of a simply supported plate model are established for roof stress distribution before periodic weighting of the roof during the upper and lower level mining process. The Galerkin method is used to make an approximate solution of deflection equation under the effect of sheet normal stress, and then roof failure criterion is established based on the maximum tensile stress strength criterion and generalized Hooke law. This paper utilizes FLAC3D finite element numerical simulation software, considering the characteristics of steeply inclined thick coal seam sub-level mining. It undertakes orthogonal numerical simulation experiment in three levels with different depths, coal seam angles, lateral pressure coefficient, and orientation of maximum horizontal principal stress, and translates roof stress of corresponding 9 simulation experiment into steeply inclined roof normal stress. We conclude that the distribution law of normal stress along dip and dip direction of a roof under the circumstance of different advancing distances and different sub-levels. The caving pace of first weight and periodical weight were counted under the effect of the roof uniform normal stress. It can better predict the weighting situation of the working face and ensure the safe, efficient, and sustainable mining of coal mines.

Mechanism for seismic supershear dynamic rupture based on in-situ stress: a case study of the Palu earthquake in 2018

The Palu earthquake, with a magnitude of 7.5, severely devastated the region in Central Sulawesi, Indonesia, but the mechanism of dynamic rupture considering in-situ conditions is not well understood. The in-situ stress field and fault geometry play important roles in the evolution of the supershear rupture. This study investigated the in-situ stress field, critical factors including the inversion of principal stress orientation by the focal mechanism solution, and constraint magnitude using the improved lateral pressure coefficient polygon. Furthermore, the dynamic rupture process is simulated using the finite element method (FEM) considering the strike and in-situ stress magnitudes of the seismogenic fault. The result shows that the principal stress orientation rotates counterclockwise about 15° from north to south, and the northern part of the fault zone is less straight than the fault from Palu Bay to the south, which are two decisive factors for supershear in the north. The damage zones considering the peak acceleration and co-seismic displacement are enlarged in Palu City, which is attributed to the rupture mode. It provides plausible explanations for this supershear event and sheds light on different dynamic rupture processes and seismic hazards that can be predicted by fully understanding the regional in-situ stress field.

Determination of initial breaking pattern in the bending failure of a semi-infinite ice sheet

There is a growing interest in the development of real-time ice-ship or ice-structure interaction simulators based on theoretical ice sheet bending failure models. The initial ice-structure interaction process is commonly idealized as a bending failure of semi-infinite ice sheets. Existing theoretical models either assume a breaking pattern of radial-before-circumferential cracking, or that of a circumferential-before-radial cracking. However, field observations indicate that the breaking pattern depends on various factors. For generic applications of real-time simulators, the determination of breaking patterns is thus an essential first step towards reasonable ice load predictions. This paper addresses the current research gap by considering a normalized formulation of the ice sheet bending failure. The locations and orientations of the maximum principal stresses at the upper and bottom surfaces of a semi-infinite ice sheet are obtained, from which the criterion for breaking pattern determinations is determined. It is found that factors such as large structure waterline curvature radii, small ice thicknesses, large ice flexural strengths, and small ice compressive strengths induce a breaking pattern of circumferential-before-radial cracking. Conversely, small structure waterline curvature radii, large ice thicknesses, small ice flexural strengths, and large ice compressive strengths result in a breaking pattern of radial-before-circumferential cracking.

Application of Electron Backscatter Diffraction to Calcite-Twinning Paleopiezometry

Electron backscatter diffraction (EBSD) was used to determine the orientation of mechanically twinned grains in Carrara marble experimentally deformed to a small strain (≤4%) at room temperature and at a moderate confining pressure (225 MPa). The thicknesses of deformation twins were mostly too small to permit determination of their orientation by EBSD but it proved possible to measure their orientations by calculating possible twin orientations from host grain orientation, then comparing calculated traces to the observed twin traces. The validity of the Turner & Weiss method for principal stress orientations was confirmed, particularly when based on calculation of resolved shear stress. Methods of paleopiezometry based on twinned volume fraction were rejected but a practical approach is explored based on twin density. However, although twin density correlates positively with resolved shear stress, there is intrinsic variability due to unconstrained variables such as non-uniform availability of twin nucleation sites around grain boundaries that imposes a limit on the achievable accuracy of this approach.

Anisotropic change in apparent resistivity before earthquakes of M S ⩾ 7.0 in China mainland

Since 1967 when DC apparent resistivity observation was carried out in China, there have been 16 times or groups of earthquake of M S ≥ 7.0 occurred within about 400 km from the monitoring stations. The apparent resistivity showed medium-short term continuous changes before 13 times or groups of these earthquakes. The medium-term changes usually lasted for several months to about two years. Apparent resistivity of monitoring arrays placed in different directions at the same station showed anisotropic change related to the maximum principal compressive stress orientation (i.e. P axis). The larger the angle between a monitoring array and the P axis is, the greater the magnitude of change in apparent resistivity is. According to the results from rock physics experiments, resistivity model of cracked rock-soil medium, and fault virtual dislocation model, the changes in apparent resistivity before earthquakes may be related to the seismogenic process through a way of resistivity change caused by medium deformation.

Experimental investigations of the effectiveness of simultaneous topology/orientation optimization via SOMP and principal stress directions

This article presents an analysis of simultaneous topology and orientation optimization based on a principal stress method within the Solid Orthotropic Material with Penalization (SOMP) framework. Numerical case studies were done to assess the expected benefits of including orientation in the design process for minimum compliance structures under single load cases printed with orthotropic materials. In particular, comparisons were made between structures designed with and without consideration of orthotropic material orientations during the topology optimization (TO) process. Direct-Ink Writing (DIW) additive manufacturing (AM) was used to print experimental specimens, which were tested in bending to validate numerical results. The novelty of the work centers on the experimental evaluation of the performance of optimized topologies designed with SOMP and manufactured via DIW. Additionally, new knowledge is presented in the form of corresponding numerical and physical experiments showing that with a principal-stress based material orientation updating scheme, equivalent performance can be achieved with respect to compliance by 1) optimizing topology and orientation concurrently with orthotropic material orientations controlled during TO using maximum in-plane principal stress directions; or 2) optimizing topology assuming equivalent isotropic properties, and substituting the original orthotropic properties along principal stress orientations post-optimization. This experimentally validated finding confirms that including orientation in the compliance-based design framework for AM of structures with orthotropic materials is not advantageous. For single load cases, using the standard Solid Isotropic Material with Penalization (SIMP) method for design purposes and then aligning material orientations with principal stress orientations after the design is produced is sufficient for obtaining equivalent performance benefits.

State of stress and stress rotations: Quantifying the role of surface topography and subsurface density contrasts in magmatic rift zones (Eastern Rift, Africa)

In rift settings, the crustal stress field is dominated by extension, which leads to rift-parallel topography and basin alignments. However in some continental rift systems, some observables of the orientation of principal stresses show substantial deviations from these patterns. Such stress field rotations are currently poorly understood and could reflect the critical role of rift magmatism in the creation of topography, the plate state-of-stress, and volcanic and tectonic processes. Yet the role of magma intrusions, crustal thinning, and rift basin and flank topography on rift zone stress field rotations remain poorly quantified. The seismically- and volcanically-active Magadi-Natron-Manyara region of the East African Rift shows a 60∘ local stress field rotation with respect to regional extension. Here, we test the hypothesis that such rotation is due to the cumulative effects of surface and subsurface loads (lateral subsurface density contrasts). We use analytical and calibrated numerical models of magmatic rift zones to simulate lithospheric deformation in the presence of magma bodies, crustal thinning, and topography to quantify their effect on intrusions and fault kinematics in a rift setting. Our 3D static models of a weakly extended rift suggest that surface topography influences shallow stress localization, whereas subsurface density contrasts play a larger role in lower crustal stress localization. Both patterns suggest a preferred region for melt storage beneath the rift valley. We show that the interaction between topography, crustal thinning, extension, and a pressurized magma reservoir could generate principal stress orientations consistent with the local stress rotation observed from earthquake focal mechanisms. Our results demonstrate how rift topography and the geometry of crustal thinning can guide magmatism and strain localization, highlighting the need for a three-dimensional treatment of rift kinematics.

Effects of parent well spacing on the poroelastic behaviors in the infill zone in shale oil reservoirs: A case study in Jimsar Shale Oil, China

AbstractShale oil resources are usually developed by tightly spaced horizontal wells that are hydraulically fractured. Infill wells are often used to develop the zones not thoroughly produced by previously fractured parent wells, and determining the parent well spacing has become a key issue in the field as it affects the overall fracturing and production efficiency. This study employs a coupled flow and geomechanics model to investigate the poroelastic changes induced by parent well depletion in the infill zone. Based on poromechanics, the temporal and spatial changes in pore pressure, minimum horizontal principal stress, breakdown pressure, and maximum horizontal principal stress orientation are quantified. Pore pressure is used to estimate the remaining hydrocarbon exploitation potential in the infill zone, while minimum horizontal principal stress, breakdown pressure, and maximum horizontal principal stress orientation are used to estimate the difficulties of establishing hydraulic fracture networks in the infill zone. Results show that increasing parent well spacing helps to maintain the pore pressure in the infill zone, while the difficulties of initiating hydraulic fracture networks in the infill zone are slightly increased. Tight well spacing intensifies the infill zone depletion and lowers the infill well development potential, while the breakdown pressure is decreased which can facilitate future infill well fracturing. The poroelastic effect of parent well spacings on the infill zone is not monotonic.

Geology and Structural Studies of Precambrian Basement Rocks Around Mahuta, Zuru Schist Belt, Northwestern Nigeria

The study of geology and structures in part of Zuru schist belt northwestern Nigeria was aimed at understanding the rock units, mineralogy, structural configuration, distribution and deformation. This was achieved through lineament analysis, petrographic study and structural analysis (geometric, kinematic and dynamic). Lineament analysis revealed the distribution, relative abundance and orientations of linear-curvilinear features. Three (3) main lithologic units were revealed; medium-grained granite, quartz-mica schist, and quartzite while minor ones are quartz vein, quartzofeldspathic pegmatite and aplitic dyke; this was arrived at because of minerals assemblages like quartz, feldspars, biotite, and muscovite. Structural analysis revealed the presence of normal fault, strike-slip fault and systematic joint (orthogonal). The joints trend is dominantly NE-SW, N-S, E-W. The normal faults reveal maximum principal stress (σ1) orientation in 087, the intermediate stress (σ2) was oriented in 222 while the minimum stress (σ3) was oriented in 312 directions. The kinematic axes revealed from the fault data are P-axis (Compressional) is 87/087; B-axis (Tensional) is 02/222 while T-axis (Transpressional) is 02/312. The component of deformation of the normal fault is elongated by 10-30 cm, while the vertical deformation effect is neutral. The area experienced multiple phases of deformation. The study will aid in mineral exploration.

Prediction of Fracture Opening Pressure in a Reservoir Based on Finite Element Numerical Simulation: A Case Study of the Second Member of the Lower Triassic Jialingjiang Formation in Puguang Area, Sichuan Basin, China

All stages of oil and gas exploration and development involve the study of in-situ stress. Since the traditional two-dimensional and three-dimensional homogeneous models can no longer fulfil the requirements of research and production, numerical simulation of the stress field has become an effective study method. In this study, we took the Jia 2 member in Puguang area as a case to establish a geological model and a mechanical model based on the tectonic framework and the distribution characteristics of the rock mechanical parameters, respectively, and loaded the model with the present-day in-situ stress state calculated from the logging data as the boundary conditions. The simulation results show that 1) the orientation of the maximum horizontal principal stress in the study area is near E-W, and the in-situ stress orientation is locally deflected due to the influence of faults; and 2) the magnitude of in-situ stress is predominantly affected by the burial depth and lithology, and the minimum horizontal principal stress, maximum horizontal principal stress, and differential stress are mainly concentrated in the ranges of 30–60, 50–80, and 10–40 MPa, respectively. We also analysed the opening sequence of the multiple fracture systems during development, using the present-day stress field model. The analysis revealed that the E-W fractures will open first, and the continuously increasing operating pressure will lead to formation breakdown, producing a fracture network.

4D-stress evolution of tight sandstone reservoir during horizontal wells injection and production: A case study of Yuan 284 block, Ordos Basin, NW China

To investigate the 4D stress change during injection and production in tight sandstone reservoirs, a multi-physical fields modeling method is proposed considering the reservoir heterogeneity, hydraulic fracture and complex injection-production system. The 4D stress evolution of tight sandstone reservoir in Yuan 284 block of Huaqing oilfield, Ordos Basin, during injection-production in horizontal well network is investigated by modeling coupled flow and geomechanics. Results show: (1) Induced by injection and production, the 3D stress increases near the injectors but decreases near the producers, and the horizontal stresses are distributed in obvious strips along their respective stress directions. (2) The horizontal stress difference is the highest at the horizontal wellbore beside injectors during injection and production, while it is the lowest in undeveloped zone between the injectors, and the orientation of maximum horizontal principal stress changes the most near the injectors, which is distributed radially. (3) The hydraulic fracture in re-fracturing well was observed to be asymmetrical in geometry and deflected as the stress changed. The results provide theoretical guidance for horizantal well network modification and re-fracturing optimization design in tight sandstone reservoir.

Investigation on the influence of multiple fracture interference on hydraulic fracture propagation in tight reservoirs

The combination of horizontal well drilling and staged fracturing technology is one of the most effective ways to increase unconventional oil and gas production. The orientation of the horizontal wellbore is mostly considered to be consistent with the minimum horizontal principal stress in the numerical simulation. However, horizontal wellbores are usually at an angle to the minimum principal stress in field operations. In this case, the dynamic growth of the fractures will be limited during subsequent perforation and fracturing operations. Therefore, the objective of this paper is to build an effective numerical model to investigate the propagation of hydraulic fractures in the case of a horizontal wellbore that is not co-linear with the minimum horizontal principal stress. The extended finite element method is applied for multi-stage fracturing flow-solid coupling in horizontal wells. In addition, the effect of sequential and alternate fracturing on the propagation of fractures in horizontal wells was compared. The simulation results show that fracture growth on one side of the horizontal wellbore is severely restricted as the pinch angle increases. It is most evident in sequential fracturing mode while increasing the fracture spacing is one of the effective ways to reduce inter-fracture interference. Meanwhile, compared to sequential fracturing, alternate fracturing is more effective in reducing inter-fracture interference, allowing for effective fracturing at closer fracture spacing. Finally, the density of the fracturing section has been increased, and a higher reservoir reconstruction area can be obtained. This work, for the first time, evaluates the dynamic propagation of fractures when the horizontal wellbore is not collinear with the minimum horizontal principal stress orientation and helps operators to optimize the fracturing process.

The Influence Mechanism of In Situ Stress State on the Stability of Deep-Buried-Curved Tunnel in Qinghai-Tibet Plateau and Its Adjacent Region

In China, rockburst disaster occurs mostly in construction of underground engineering in Qinghai-Tibet Plateau and its adjacent region. Previous research on deep-buried tunnels has indicated that tunnels stability is related to in situ stress state. To quantify these relationships, three-dimensional finite element modeling was done to analyze the influences that the angle φ between the maximum horizontal principal stress orientation and tunnel axis, and the lateral pressure coefficient KH, had on the tangential stress σ θ in a deep-buried-curved tunnel. Based on the in situ stress condition in Qinghai-Tibet Plateau and its adjacent region, 50 different simulation conditions were used to analyze the relationship that φ and KH had on σ θ for the rock mass surrounding the tunnel. With the simulation data produced, predictive equations were generated for σ θ as a function of φ and KH using multivariate regression analysis. These equations help estimate σ θ at various key positons along the tunnel boundary at Qinghai-Tibet plateau and its adjacent region. The equations were then proved by a set of typical tunnels to ensure validity. The results concluded that the change in φ has a significant impact on σ θ , and thus, the stability of the tunnel, when 30° < φ < 60°, with the most obvious influence being when φ is about 45°. With the equations, the rockburst potential at a certain location within a curved tunnel can be quickly estimated by calculating φ and KH on σ θ , without need of geo-stress background knowledge and heavy simulation, allowing for the practical value in engineering at design phase for the projects in Qinghai-Tibet Plateau and its adjacent region.

Fault Geometry and Mechanism of the Mw 5.7 Nakchu Earthquake in Tibet Inferred from InSAR Observations and Stress Measurements

Different types of focal mechanism solutions for the 19 March 2021 Mw 5.7 Nakchu earthquake, Tibet, limit our understanding of this earthquake’s seismogenic mechanism and geodynamic process. In this study, the coseismic deformation field was determined and the geometric parameters of the seismogenic fault were inverted via Interferometric Synthetic Aperture Radar (InSAR) processing of Sentinel-1 data. The inversion results show that the focal mechanism solutions of the Nakchu earthquake are 237°/69°/−70° (strike/dip/rake), indicating that the seismogenic fault is a NEE-trending, NW-dipping fault dominated by the normal faulting with minor sinistral strike-slip components. The regional tectonic stress field derived from the in-situ stress measurements shows that the orientation of maximum principal compressive stress around the epicenter of the Nakchu earthquake is NNE, subparallel to the fault strike, which controlled the dominant normal faulting. The occurrence of seven M ≥ 7.0 historical earthquakes since the M 7.0 Shenza earthquake in 1934 caused a stress increase of 1.16 × 105 Pa at the hypocenter, which significantly advanced the occurrence of the Nakchu earthquake. Based on a comprehensive analysis of stress fields and focal mechanisms of the Nakchu earthquake, we propose that the dominated normal faulting occurs to accommodate the NE-trending compression of the Indian Plate to the Eurasian Plate and the strong historical earthquakes hastened the process. These results provide a theoretical basis for understanding the geometry and mechanics of the seismogenic fault that produced the Nakchu earthquake.

Kinematic analysis of fault-slip data in the Nanling Tectonic Belt and Cretaceous to Paleogene tectonic evolution of the Central South China Block

• Six tectonic events were inversed from fault-slip analysis in the Nanling Tectonic Belt. • Four Cretaceous tectonic events resulted from the Paleo-Pacific subduction . • Two Paleogene tectonic events were due to the far field effect of the India-Asia collision. Cretaceous tectonic events and Cenozoic geodynamics of the central South China Block are still under debate. A long-term extensional regime or two extension-contraction tectonic cycles were proposed for the Cretaceous tectonic evolution. While Cenozoic geodynamics of the central South China Block were ascribed to the Pacific subduction or the far field effect of the India-Asia collision. Kinematic analysis of fault-slip data were carried out for the Cretaceous to Paleogene Chaling-Yongxing Basin and the Lianshan-Yangshan Domain in the Nanling Tectonic Belt. Kinematic analysis of fault-slip data (409 groups of faults and striations) reveal four Cretaceous tectonic events and two Paleogene tectonic events. i) The early Early Cretaceous extensional regime is represented by a NW-SE horizontal minimum principal stress and a vertical maximum principal stress, which generated NE-striking boundary normal faults of Early Cretaceous basins. ii) The late Early Cretaceous strike-slip regime is characterized by a N-S horizontal maximum principal stress and a E-W horizontal minimum principal stress, which led to tectonic inversion of Early Cretaceous extensional basins and nearly N-S-striking folding of the Lower Cretaceous. iii) The early Late Cretaceous extensional regime is represented by a NW-SE horizontal minimum principal stress and a vertical maximum principal stress, which activated the NE-striking boundary normal faults of Late Cretaceous basins. And iv) the latest Late Cretaceous strike-slip regime is characterized by NW-SE horizontal maximum principal stress and a NE-SW horizontal minimum principal stress, which led to tectonic inversion of Late Cretaceous extensional basins and NNE-striking folding of the Upper Cretaceous. Orientations of maximum or minimum principal stresses imply that two extension-contraction tectonic cycles during the Cretaceous resulted from the Paleo-Pacific subduction. Two Paleogene tectonic events are v) the strike-slip regime characterized by NE-SW horizontal maximum principal stress and a NW-SE horizontal minimum principal stress, and vi) the subsequent extensional regime represented by a NE-SW minimum principal stress and a vertical maximum principal stress. The former generated NW-striking reverse faults and reactivated strike-slipping of NE- and NW-striking conjugate faults whereas the latter formed NW-striking normal faults in the Lower Paleogene clastic rocks. Orientations of maximum or minimum principal stresses indicate that two Paleogene tectonic events in the central South China Block were attributed to the far field effect of the India-Asia collision and subsequent stress relief.

Tectonic fracture prediction and its implication for hydrocarbon development with a case study in ultra-deep reservoirs of Kuqa Depression, Tarim Basin, NW China

Tectonic fractures can be important reserving space and the main contributor to permeability in ultra-deep reservoirs of Kuqa Depression. In this study, a workflow based on geomechanical method is presented. Firstly, a 3D heterogeneous mechanical field is constructed by core-logging-seismic data; then the finite element in-situ stress simulation is carried out, which is established using heterogeneous mechanical parameters instead of uniform parameters. Based on strain energy theory and energy conservation principle, the relationship between tectonic fracture parameters and in-situ stress field is established by a suitable rock fracture criterion for ultra-deep tight sandstone. Characteristics of tectonic fractures are predicted and revealed by the above stress field simulation and the established relationship between stress and tectonic fracture parameters. The results show that the maximum horizontal principal stress (SH) orientation is generally N-S-trending in Bozi-1 gas reservoir. The present-day in-situ stress magnitudes are generally high with the minimum horizontal principal stress (Sh) is about 110∼160MPa, and the stress difference is over 35MPa. The characteristics of present-day in-situ stress largely vary among different structural styles. The in-situ stress magnitude and burial depth generally indicates a linear relationship. The in-situ stress gradient is stratified with burial depth and is consistent with fold stress stratification. Controlled by the effect of fold bending and protection of salt layer, the development of tectonic fracture in pop-up structures is better with larger fracture aperture and higher permeability, and fracture strikes are generally consistent with the SH orientation. Tectonic fractures in imbricated structures indicate great heterogeneity because of the development of faults and folds, and the rebuilding effect (permeability reduction) under the present-day in-situ stress field. Optimization of favourable zone for in-situ stress and tectonic fracture is of practical effects during well drilling and completion works.

Phase field modeling of multi-cluster hydraulic fracturing in horizontal wellbore with inconsistent direction to minimum principal stress

Zhongjiang gas field is a typical narrow channel tight sandstone gas reservoir. In order to obtain enough horizontal section length in high-quality reservoir, horizontal wellbore azimuth is basically consistent with direction of narrow channel sand body, resulting in a certain included angle θ between wellbore azimuth and in-situ minimum principal stress orientation. In this study, a seepage-stress-damage coupled dynamic fracture propagation model is established through phase field method to investigate the effect of angle θ on multi-cluster fracture simultaneous propagation based on geological engineering data of JS-X well. Propagation of hydraulic fracture is automatically tracked through phase field evolution without additional criteria or grid direction limitation. Simulation results show that: (1) Fracture tip of each cluster would approach to stress diversion zone created by adjacent clusters. Simultaneous propagating fractures of each cluster are prone to deflect, interconnect with each other and form one single fracture eventually. (2) Increasing cluster number or decreasing cluster spacing in the same fracturing stage would further promote diverting of each cluster fracture and accelerates fractures merging into one single fracture. We suggest that azimuth of horizontal wellbore in tight gas reservoir should be consistent with in-situ minimum principal stress orientation to reduce the effect of angle θ.

Variations in vertical stress in the Permian Basin region

Constraining the magnitude of vertical stress (Sv), or overburden pressure, is key in determining a region’s stress state and has implications for reservoir geomechanics and the potential for induced seismicity. Of the principal stress orientations (Sv, minimum horizontal stress [Shmin], and maximum horizontal stress [SHmax]), Sv is the most straightforward to constrain using wire-line log data. The magnitude of Sv varies because of lithology and burial history, potentially causing local perturbations in the in situ stress field. Previous studies on the state of stress in the Permian Basin use a constant Sv, relying on determination of SHmax and Shmin, and yield an interpretation that the faulting regime transitions from normal faulting in the west to normal to strike-slip faulting in the east. Here, we present an interpretation of the spatial and depth variability in Sv trends in the Permian Basin based on density log integration. Where density measurements are absent, values are calculated from compressional velocity logs using a transform that is fit to local data. Notable variations include higher Sv gradient on carbonate platforms and shelves, where high-density carbonates are thicker and are found at shallower depths than in the basins. Within the basins, the magnitude of Sv gradient is as low as 1.06 psi/ft at depth. This work shows the potential for regional interpretations of Sv to gain insight into the effect of variations in Sv on state of stress.

Characteristics of in situ stress field in the Huainan mining area, China and its control factors

Due to the high in situ stresses, dynamic disasters occurred frequently in the Huainan mining area, China. Our understanding of the in situ stresses in this area is still insufficient. In this study, the in situ stresses of 18 sections in two boreholes in the Xinji No. 1 coalfield were measured using the hydraulic fracturing method, and the distribution of in situ stresses in the Huainan mining area were investigated. The relationship between in situ stress and geological structure in the Huainan mining area was summarized and the limitation of fault friction strength on in situ stress was discussed. The result showed that the maximum horizontal principal stress (σH) at Xinji No. 1 mine was 13.95–25.23 MPa, the minimum horizontal principal stress (σh) was 12.16–21.17 MPa. The average azimuth of the maximum horizontal principal stress was N83.61°E. The statistical results showed that the in situ stresses in Huainan mining area were characterized by a strike-slip faulting regime. Both the horizontal and vertical principal stresses increased approximately linearly with the increase of burial depth. The orientation of the maximum principal stress in the study area is closely related to the tectonic movement and the ratio of maximum principal stress to minimum principal stress was primarily limited by the friction strength of the faults. The outcomes of this research can provide some reliable engineering parameters and benefit the roadway layout and support design in the Huainan mining area.