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When hydraulic fractures do not expand in the direction required by a project, it is difficult to enhance coal seam permeability effectively. Slotting–directional hydraulic fracturing (SDHF) has been proposed as a possible alternative, but there is not enough theoretical mechanism to guide the construction. Based on preliminary study of the directional mechanism of single slotted hole, we established a new slotting–directional hydraulic fracturing (NSDHF) mechanical model by using the complex function theory of elasticity, and the influence of stress interference between adjacent slotted holes and non-uniform pore water pressure was considered. We carried out true triaxial double slotted holes SDHF experiments and used large-scale nondestructive computer tomography to scan the fractured samples to ensure accurate measurement of directional distance. The measured directional distance was used to verify the NSDHF model; the maximum deviation was 5.1%. Taking the experimental data in this paper as example, the stress interference between adjacent slotted holes decreased the fracture directional distance by 20.3%, and the non-uniform pore water pressure increased the fracture directional distance by 47.6%. NSDHF mechanical model realized the quantitative description of the influence of non-uniform pore water pressure on directional distance. The contribution of non-uniform pore water pressure to directional distance accounted for more than 25% of the total directional distance; the effect of non-uniform pore water pressure on fracture direction distance was almost twice the stress interference between adjacent slotted holes. The verified NSDHF model was used to study further the influence of horizontal stress difference, azimuth of slotted hole, slotting size and fluid injection pressure on directional distance with different slotting spacing. The work discussed in this paper will contribute to promoting and apply SDHF on a large scale in low-permeability coal mines.

Estimation of horizontal stress magnitudes from borehole breakouts has been an attractive topic in the petroleum and mining industries, although there are critical research gaps that remain unfilled. In this paper, numerical simulation is conducted on Gosford sandstone to investigate the borehole breakout and its associated borehole size effect, including temperature influence. The discrete element method (DEM) model shows that the borehole breakout angular span is constant after the initial formation, whereas its depth propagates along the minimum horizontal stress direction. This indicates that the breakout angular span is a reliable parameter for horizontal stress estimation. The borehole size effect simulations illustrated the importance of borehole size on breakout geometries in which smaller borehole size leads to higher breakout initiation stress as well as the stress re-distribution from borehole wall outwards through micro-cracking. This implies that the stress may be averaged over a distance around the borehole and breakout initiation occurs at the borehole wall rather than some distance into the rock. In addition, the numerical simulation incorporated the thermal effect which is widely encountered in deep geothermal wells. Based on the results, the higher temperature led to lower breakout initiation stress with same borehole size, and more proportion of shear cracks was generated under higher temperature. This indicates that the temperature might contribute to the micro-fracturing mode and hence influences the horizontal stress estimation results from borehole breakout geometries. Numerical simulation showed that breakout shape and dimensions changed considerably under high stress and high temperature conditions, suggesting that the temperature may need to be considered for breakout stress analysis in deep locations.

Electro-osmotic consolidation has been applied in several geotechnical engineering applications that contain a series of complex processes, including electrochemical processes, temperature changes, and mechanical evolution. To explore the combination of electrochemical–temperature–mechanical processes in marine clay, electro-osmotic consolidation experiments were conducted using a self-made electro-osmotic consolidation system under various durations and voltages. The following findings was obtained: (1) the change in the pH value increased during electro-osmotic consolidation and as the voltage rise; (2) the temperature increased with a rise in voltage in the initial stage of the experiments, which was induced by Joule heating; (3) the temperature rise promoted the electro-osmotic consolidation process, which included a rise in the coefficient of consolidation and a reduction in water content; (4) horizontal shrinkage occurred when the horizontal stress increment was greater than the critical stress condition. In addition, the volume difference reached a constant value, and was proportional to the voltage rise. After the discussion, a coupling analysis was conducted, which can help to better understand the mechanism of electro-osmotic consolidation and can provide reference for engineering applications.

Hydraulic fracturing is one of the common methods to increase the efficiency and production of oil and gas reservoirs. Low-permeable rocks are good candidates for creating a hydraulic fracture since the fracture creates a permeable path that connects the reservoir and the well. To perform hydraulic fracture operations, it is necessary to conduct geomechanical studies, with their most important outputs being formation fracture pressure and direction of fracture. As the most important input data in this study, well logging was used to build a mechanical model of the earth for the rock and then estimate the fracture pressure, accordingly. In the studied well, the tensile fracture pressure in this rock is in the range of 17.87 ppg–23.74 ppg in all directions. In this case, by increasing the angle of the well, the pressure required for tensile fracture is increased such that the fracture pressure of the well wall was 23.74 ppg and 17.87 ppg, respectively, in horizontal and vertical wells. The studied well was vertical in which the pressure required to create a hydraulic fracture was about 17.9 ppg. Also, in this well, the angle of fracture was 135 in the direction of Azimuth. Meanwhile, with increasing the angle of the well and becoming more horizontal, the tendency of the well to break in the 45-degree azimuth direction (i.e., the maximum horizontal stress) is increased. Factors such as azimuth, minimum horizontal stress, Biot number, maximum horizontal stress, formation pressure, and Poisson’s ratio can also have a significant effect on fracture pressure to create a fracture.

Based on literature data and original constructions of the focal mechanisms of earthquakes within the Romashkinskaya seismically active zone of the Tatar Arch for 1992–1995, additional information was obtained on the nature of the stress–strain state in the central Russian Platform, which is a “white spot” on the well-known world stress maps (WSM). Predominant subhorizontal uniaxial compression in the southeast–northwest direction is established, which is agrees well with the observed regional maximum horizontal stress field SHmax in Western Europe. As one of the possible reasons for this orientation of this compression, along with a plate tectonics interpretation, a system of planetary tectonic fracturing of rock material is proposed. This fracturing, in light of the ideas obtained here on the nature of the stress–strain state of the crust, can be understood as numerous planes of thrust and shear dislocations that formed during deformation of crustal material under horizontal compression in the indicated direction.

Geological characteristics, geomechanical behavior and hydraulic fracture propagation mechanism in the Marcellus shale gas play are analyzed and compared with China’s Fuling shale play. Successful experiences in hydraulic fracturing and shale gas development in the Marcellus shale gas play are summarized, which might be applicable in other shale plays. The main factors contributing to the successful development of the Marcellus shale gas play include adoption of advanced drilling and completion technologies, increases of hydraulic fracturing stages, proppant concentration and fluid injection volume. The geological and geomechanical mechanisms related to those technologies are analyzed, particularly the in-situ stress impacts on hydraulic fracturing. The minimum horizontal stress controls where the fractures are initiated, and the maximum horizontal stress dominates the direction of the hydraulic fracture propagation. Hydraulic fracturing performed in the shale reservoir normally has no stress barriers in most cases because the shale has a high minimum horizontal stress, inducing hydraulic fractures propagating beyond the reservoir zone, resulting in inefficient stimulation. This is a common problem in shale plays, and its mechanism is studied in the paper. It is also found that the on-azimuth well has a higher productivity than the off-azimuth well, because shear fractures are created in the off-azimuth well, causing main fractures to kink and increasing fracture tortuosity and friction. The Fuling shale gas play has a markedly higher minimum horizontal stress and much smaller horizontal stress difference. The high minimum horizontal stress causes a much higher formation breakdown pressure; therefore, hydraulic fracturing in the Fuling shale gas play needs a higher treatment pressure, which implies higher difficulty in fracture propagation. The small difference in the two horizontal stresses in the Fuling shale gas play generates shorter and more complex hydraulic fractures, because hydraulic fractures in this case are prone to curve to preexisting fractures. To overcome these difficulties, we recommend reducing well spacing and increasing proppant concentration to increase gas productivity for the Fuling shale gas development.

In fully mechanized top‐coal caving (FMTC), the effects of coal caving are closely related to the degree of damage to the top‐coal which is determined by the stress state therein. Therefore, research into the evolution of mining‐induced stress field is key to understanding failure mechanisms of top‐coal. By taking the 12 309 FMTC face in Wangjialing Coal Mine (China) as the engineering background and using field measurement and numerical simulation, we studied the evolution of the principal stress field in FMTC under high horizontal stress. The research results demonstrated that in situ stress in Wangjialing Coal Mine was a high horizontal stress and lateral pressure coefficients in the north‐south and east‐west directions were 1.52 and 0.45, respectively. Under the influence of mining, principal stresses were concentrated in front of the coal wall, with concentration factors of 2.26, 2.24, and 2.85, respectively. The minimum principal stress in unconstrained sides of the top‐coal in the roof‐control zone was tensile. The directions of intermediate and minimum principal stresses in the top‐coal were more sensitive to the influence of mining compared to the maximum principal stress. During rotation of the principal stress direction in top‐coal, the location (19 m in front of the coal wall) where the maximum and intermediate principal stresses were equal and the location of the coal wall are key positions.

Compaction by vertically or horizontal oscillating probes increases not only the soil stiffness but also the horizontal effective stress. An important, but often neglected, consequence is that compaction also causes preloading of the soil. The change in horizontal stress following vibratory compaction can be measured independently by two in situ methods – a cone penetration test (CPT) and a flat dilatometer test (DMT). Values of sleeve resistance (CPT) and horizontal stress index (DMT) can be determined prior and after compaction. In this work, five case histories reported in the literature where different vibratory compaction methods were used (dynamic compaction, vibroflotation, VibroWing, TriStar and resonance compaction) were re-analysed. For each test site, the change in CPT cone resistance and sleeve resistance was determined and compared with the increase in horizontal stress index from DMTs. The preloading effect due to vibratory compaction was estimated using empirical correlations.

Pre-existing borehole cracks result in hydraulic fracture deflection, which causes sand screen-out in advance and even fracturing failure. Based on a simplified three-dimensional displacement discontinuity method, a model to simulate the initiation and propagation of hydraulic fractures from a borehole with pre-existing cracks was established. It is found that the effects of injection rate and fracturing fluid viscosity on fracture propagation are similar. When the effects are smaller, hydraulic fractures quickly turn perpendicular to the minimum horizontal stress. With injection rate or viscosity increase, the injection pressure at the well bottom increases, and the turning degree and width of hydraulic fractures increase. The length of pre-existing cracks dramatically affects the breakdown pressure. With increasing pre-existing crack length, the breakdown pressure decreases. With increasing horizontal stress contrast, the hydraulic fracture path clearly changes, and the required extension pressure also increases when the injection rate and fracturing fluid viscosity both remain constant.

Geomechanical analysis is performed at Gundih Field to obtain detail stress condition and mechanical properties from interval of interest. Ngrayong Formation is targeted as CO2 injection interval of Gundih Field. The detail interval 8 is determined using multi-mineral modelling and calibrated using XRD and petrographic data. Overpressure is detected from this well at Ngrayong Formation. Stress direction from adjacent well showed NE – SW direction of maximum horizontal stress. Stress magnitudes and mechanical properties are calculated using available log data through well-established empirical equation resulted thrust faulting regime acting on this area. Injection capability of formation is examined from Mohr – Coulomb diagram with input from calculated stress.Keywords: Geomechanics; Gundih Field; Ngrayong Formation; Reservoir; overpressure.

A complex fracture network is composed of many similar structures. The migration law of proppant at each structure is the core and basic content of the migration law of proppant in complex fracture network, and there is little research. In this study, the EulerianEulerian method (TEM) is used to analyze the migration and distribution characteristics of solid–liquid two phases at the fracture corner according to different corner types of the fracture network. The results show that the migration characteristics of proppant in the corner area can be divided into the corner anomaly area, buffer area, and stability area; the influence of the turning angle on proppant migration is mainly concentrated at the corner and in the range of 4 times the fracture width after turning. The probability of sand plugging at the corner of the “Y → T” fracture is lower than that of “L → l”, higher than that of the “X → +” wing branch fracture, and lower than that of the main fracture. At the corner of the fracture network, after the solid flow turns, the proppant will form a high sand area on the side of the impact fracture surface, then rebound back to the fracture, form a sand-free area on the other side, and form a high-velocity core in the refraction interval. At the corner of the “L → l” fracture, there are one high sand area, one non-sand area, two low-velocity areas, and one high-velocity area; there are three low-velocity areas, two sand-free areas, and one high sand area at the corner of the “Y → T” fracture; at the corner of the “X → +” fracture, there is a high sand area and no sand-free area, and the flow velocity of the main fracture is much greater than that of the wing branch fracture.

For reservoirs with abnormally high pressure and high geostress, formation resistivity can be greatly affected. This increase of resistivity resulting from high stress causes errors in the identification of reservoir fluids. In order to investigate the effect of stress on resistivity, resistivity measurement was conducted simultaneously with triaxial testing to obtain rock resistivity under high temperature and high pressure. The changes of resistivity and resistivity increasing coefficient with horizontal differential stress and minimum horizontal stress were revealed from experiments. Besides, field data were analyzed to show the main influencing factors of formation resistivity under reservoir conditions. In addition, a new resistivity correction model for high geostress formation was derived in this work. The interpretation results are in good agreement with well testing data in the Keshen area of the Tarim oilfield, China.

Hydraulic fracturing is an important process to increase the productivity of wells. In shallow reservoirs, tectonically active reservoirs, and abnormally high-pressure reservoirs, fractures caused by hydraulic stimulation are likely to be in the horizontal direction. To date, the study of transient pressure behavior arising from horizontal fractures has been limited to that of a single horizontal fracture, without consideration of the interference from multiple fractures. In this work, we develop an analytical method to model, and analyze the transient pressure behaviors from multiple horizontal fractures. The results show that the flow regimes of multiple horizontal fractures can be divided into linear flow, transition flow, first pseudo radial flow, bi-radial flow, and second pseudo radial flow. Compared to the case of a single horizontal fracture, the first pseudo radial flow and bi-radial flow represent new flow regimes inherent in the multiple horizontal fracture system. Sensitivity analysis indicates that changes in horizontal fracture length affect the linear flow, transition flow, and first pseudo radial flow. In addition, the first pseudo radial flow and bi-radial flow are closely related to the fracture spacing and fracture number. The transition flow is dominated by the fracture's vertical position and formation thickness.

Rock burst represents a very dangerous phenomenon in deep underground mining, as well as for underground structures in unfavourable conditions (great depth, high horizontal stress, proximity of major tectonic structures, etc.). The rock burst problem relates to the natural and mining conditions of the rock mass. The evaluation of rock burst is becoming increasingly important as mining activities reach greater depths. In the literature, rock burst assessment challenge was tackled by many researchers using various methods. However, no study providing review and comparison of different rock burst assessment methods is available. In this paper, rock burst classification is briefly summarized. This includes a classification based on rock burst type, and another classification based on rock burst severity. As an important method for rock burst prevention, some novel energy-absorbing bolts were developed. These bolts demonstrate constant resistance under both static and shock loads and large elongation ability enabling them to withstand large deformations of rock masses under rock burst-prone conditions. Among the novel energy absorbing bolts, Modified Cone Bolt (MCB) and Constant Resistance at Large Deformation (CRLD) Bolt are selected to be presented in this paper.

The coefficient of lateral pressure at rest (k0) is a key parameter required to determine the horizontal stress distribution on subsoils. In this paper, k0 was measured from consolidation test for ...

Hydraulic fracturing is an effective treatment technology to improve reservoir permeability. In this paper, the dual effective stress was introduced into the theoretical analysis of water pressure during crack initiation, and the theoretical analysis results are about 37% more accurate than Hubbert and Willis theory based on the numerical results. Moreover, the artificial-macrocrack effects on fracturing crack propagation and direction changes have been analyzed and compared to hydraulic fracturing without the artificial-macrocrack. The water pressure required for crack initiation with the artificial-macrocracks is less than that without the artificial-macrocracks. When α (α is defined as the angle between the artificial-macrocrack and the horizontal direction) reached or exceeded π/4, crack growth began at the tip of the macrocrack with a higher box counting dimension than that at a lower values of α. Furthermore, a series of simulations have been conducted to obtain the point of fracturing crack direction change when α = 0. The fracturing crack starts to propagate at the artificial-macrocrack tip when λ = 0.95 (λ is defined as the ratio of horizontal stress to vertical stress), and the crack propagation direction becomes vertical when λ = 0.9625. By extracting the crack morphology from the simulation results, it was determined that the box-counting dimension, the crack perimeter, and the crack area proportion reach maximal values at λ = 0.95, λ = 0.9625 and λ = 0.975, respectively. Using image segmentation technology, the impact area proportion reached a maximum at λ = 0.9625 with a more effective treatment than at other values of λ. Finally, beddings and cracks in natural strata were taken into account for fracturing crack propagation, and beddings and cracks could guide the fracturing cracks. More microcracks could be generated along the beddings with the artificial-macrocrack effect at λ = 0.9. Case study indicated that the gas extraction achieved high production in the first 40 days, and the field data with fracturing treatment show that the average pure gas flux is about 4 times greater than that without fracturing. This research also provides a new idea to mine intact rock of construction aggregate.

This study was conducted to examine the influence of vugs on hydraulic fracture propagation, which is very significant for hydraulic fracturing design in fractured-vuggy carbonate reservoirs. In this paper, a series of hydraulic fracturing physical simulation experiments were performed under true triaxial stress states and acoustic emission devices were used to monitor real-time hydraulic fracture propagation. Moreover, the key influencing factors and the typical fracturing curve characteristics of vug-hydraulic fracture interaction were studied and analyzed. The experimental results demonstrated that vugs exert an important effect on hydraulic fracture propagation. Three main vug-hydraulic fracture interaction modes were observed in our experiments: a) the hydraulic fractures directly crossed the vugs, which usually happened for small vugs. b) the vugs arrested the hydraulic fractures for further propagation when the hydraulic fractures tip reached the vugs that are in large size. c) the hydraulic fractures bypassed the vugs by propagating around it, which usually happened under low horizontal stress difference states. Furthermore, the fracturing curve response was obvious when the interaction between vugs and hydraulic fractures occurs. Our research findings provide a reliable basis for optimizing hydraulic fracturing design in fractured-vuggy carbonate reservoirs.

A three-dimensional finite element model of layered porous media containing interlayers and reservoirs was established with ABAQUS software and the three-dimensional fluid-solid coupling solid elem...

A complex fracture network, which is the precondition of stimulated reservoir volume for shale formation, depends on the interaction between hydraulic fractures and discontinuities, such as bedding planes and natural fractures. However, the fracture propagation behavior in laminated shale formation still maintains ambiguous and the formation mechanism of complex fracture network has not been well understood. Therefore, hydraulic fracturing simulated experiments were conducted on shale outcrops to clarify above issues under the effects of multiple factors, such as horizontal stress difference, pump rate, and fluid viscosity. The results reveal that dilation and crossing are two main hydraulic fracture propagation behaviors because of discontinuities influencing, resulting in three categories of the ultimate fracture morphologies. Simple fracture is a single induced fracture. Complex fracture is consisted of two or more induced fractures which do not link up with each other. However, network fracture includes induced fractures which interweave with each other like a network. All the fracture geometries are generated under the mutual interaction of natural factors and engineering factors. A large horizontal stress difference and a high fracturing energy reduce the complexity of fracture geometry, whereas, a small horizontal stress difference and a low fracturing energy enhance the activated probability of discontinuities, resulting in a complex fracture network. Furthermore, weak cementing strength and large opening degree of discontinuities increase the complexity of the fracture. In addition, the fracturing curve reflects fracture morphology indirectly. Simple fracturing curve and steady extensive pressure indicate the simple fracture. The rising extensive pressure implies the complex fracture, whereas, the fluctuation of fracturing curve suggests the network fracture. In the meantime, each fracture morphology has its own dimensionless net pressure features, which could be applied to judge fracture morphology in shale formation.

Refracturing is an effective technology for reinstituting a percolation path and improving the fracture conductivity in coal measure strata. Hydraulic fracture (HF) propagation is complicated due to the presence of cleats and stress change caused by pore pressure changes. Many scholars have studied HF propagation in the initial fracturing of coal, but the refracturing in coal seams is rarely mentioned. In this study, laboratory refracturing experiments were conducted on large natural coal specimens under various triaxial stress states to investigate the propagation of HFs in coal seams. The mechanical properties of coal were tested before refracturing. The maximum and the minimum horizontal principal stresses are inverted to simulate the stress change caused by the production and pore pressure reduction of the stress condition after initial fracturing. Experimental results showed three different types of HF initiation and propagation during refracturing: (1) under low horizontal stress differences (0-2 MPa), HF propagated along the cleats, and no new HFs were formed on the walls of the initial HFs regardless of changes in the horizontal stress; (2) under high horizontal stress differences (4–8 MPa) with no stress inversion, a major HF was initiated parallel to the orientation of maximum horizontal stress during initial fracturing; new branches propagated along cleats in the orientation of the minimum horizontal stress during refracturing; and (3) under high horizontal stress differences (4–8 MPa) with maximum and minimum horizontal stress inversions, the main HF formed along the orientation of the maximum horizontal stress, and a new HF perpendicular to the initial HF was formed during refracturing. Multiple factors affect fracture morphology during refracturing. Cleats affect the HF growth path and the creation of new branches. The in situ stress determines the initiation and propagation of new HFs.

Most analytical approaches for the design of shallow plate and screw anchors in tension are based on the limit equilibrium of a rigid soil wedge for which a horizontal stress distribution acting on the failure plane is assumed. Finite element analysis for a wide range of soil properties was carried out to identify the shape of the failure mechanism and to study the stress distribution at failure. Results show that soil deformation modifies the stress field around the anchor and increases the uplift capacity. A semi-analytical approach is proposed to describe this stress distribution, based on peak friction angle.

Borehole breakout is a progressive failure process that results in major wellbore instability problems. Previous studies of progressive breakouts have been mainly for vertical wells, with fewer studies for deviated wells. Moreover, most existing breakout models cannot update the boundary conditions for the receding wellbore surfaces during progressive breakout propagation. This paper presents a finite-element-based method for simulating dynamic breakouts in deviated wellbores considering hydro-mechanical coupling. By incorporating an adaptive meshing technique, the method allows for timely updates of the boundary conditions on newly created wellbore surfaces due to breakout. Using the proposed method, the effects of well trajectory, in-situ stress anisotropy, fluid seepage, and formation properties on wellbore breakouts in a strike-fault stress regime are investigated. The results are analyzed in terms of the orientation and magnitude of wellbore breakouts. The analysis shows that breakout orientation on the cross-section plane normal to the axis of a deviated well is always aligned with the projection of the minimum horizontal stress direction on this plane. A smaller Young's modulus, a smaller cohesion, or a larger permeability increases the risk of borehole breakout.

Evaluating the interaction between backfilled stopes and the surrounding rock mass is a vital issue during secure backfill application and stope stability analysis. With the increase of rock mass complexity and mining depth, weak rock masses and high in situ stresses are increasingly encountered, emphasising the consideration of the creep behaviour of the rock mass (CBRM). In this paper, a modelling framework was proposed by considering the CBRM and the time-dependent characteristics of backfill (e.g., increasing stiffness and cohesion with time). A generic study was present to investigate the effect of the CBRM on the stress distribution in the backfilled stope. In the generic study, a reference case was investigated in detail followed by an extensive parametric study. Furthermore, the proposed modelling framework was applied to an engineering instance, namely, the Baixiangshan Iron Mine, to verify its robustness. The generic study shows that the horizontal stress was much larger than the vertical stress in the backfilled stope at day 21 and the stress was transferred from the rock mass to the backfill (‘squeeze-induced stress effect’). The horizontal displacement of rock mass was responsible for the long-term stress development in the backfilled stope. Backfill parameters and backfill delay had a strong influence on the stope stress development while the influence of backfill gap was mainly around the upper part of the backfilled stope. The engineering application in Baixiangshan Iron Mine indicates that the proposed modelling framework can be well adopted to analyse the continuous increase in stress and displacement during backfill operations.

Fractures are widely distributed within the Earth’s crust and have a substantial influence on wave propagation. Although many studies have been conducted on fracture characterization based on seismic anisotropic attributes, natural horizontal fractures and hydraulic fracture networks have yet to be characterized. Here, we investigate the influence of the fracture orientation and saturation on the Vp/Vs ratio via laboratory observations and theoretical modeling. Then, we discuss the feasibility of using the Vp/Vs ratio to predict natural horizontal fractures and evaluate hydraulic fracture networks. Our study demonstrates that an abnormally low Vp/Vs ratio, together with a lack of shear wave splitting, could be used to predict horizontal fractures. Fracture network development that results from hydraulic fracturing stimulation is associated with a much higher Vp/Vs ratio and no shear wave splitting; these two characteristics could be used to estimate fracture development and evaluate hydraulic fracturing stimulation.