Low Vertical Stress Research Articles

Study on the influence of vertical stress difference coefficient on fracture characteristics of shale under high stress

AbstractTo study the effect of vertical stress difference coefficient on fracture characteristics of shale fracturing, high stress true triaxial hydraulic fracturing test was carried out. By analyzing the profile after fracturing, it was found that the area of hydraulic fracture increased with the increase of vertical stress difference coefficient, and the probability of shear fracture will increase when the stress difference coefficient was high. A high vertical stress differential coefficient exerts a strong control over the direction of crack propagation, while a low vertical stress difference coefficient is beneficial to improve the roughness of hydraulic fracture surface and promote the formation of complex fracture network. By analyzing the pump pressure curves of different tests, it was found that with the increase of vertical stress difference coefficient, the formation and expansion of hydraulic fractures were more difficult. The surface characteristics of hydraulic fractures were quantified based on three‐dimensional topography scanning technology, combined with fractal dimension and fracture area calculation method, the results showed that with the increase of vertical stress difference coefficient, the fractal dimension and fracture area decreased. Since shale is stratified, and the transformation of reservoir is mainly reflected in the enhancement of fracture complexity through tensile failure, Xsite discrete grid method was used to study the influence of fracture propagation behavior with different bedding strengths. The results showed that when the bedding tensile strength was high, hydraulic fractures were easy to pass through the bedding, and when the bedding tensile strength was low, hydraulic fractures were easy to be captured by natural fractures. In addition, tensile cracks were easy to form when the tensile strength of bedding was low, shear cracks were easy to form when the strength of bedding was high, and the fracture volume was larger when the strength of bedding was low. This study provides a theoretical basis for hydraulic fracturing in engineering.

Evaluation of lightweight aggregate to reduce vertical stresses and improve load transfer in pile-supported embankments

Lightweight aggregate has unique characteristics of light self-weight, high friction angle, and good interaction with geosynthetics, and may be used as a fill material in load transfer platforms (LTP) over piles to support embankments on soft soils. This study included five plane-strain trapdoor tests to evaluate the use of lightweight aggregate to improve the performance of load transfer platforms over piled beams to support embankments on soft soils. These model tests consisted of trapdoor downward movement (settlement) and surface loading to investigate the mobilization and degradation of soil arching above the trapdoor. The surface loading was applied locally using a hydraulic jack through a footing. The vertical stresses on the trapdoor and the stationary supports were measured using earth pressure cells and the settlement of the footing was measured using a displacement transducer. Test results showed that soil arching mobilized with the settlement of the trapdoor and degraded with the footing loading. The geosynthetic reinforcement hindered the degradation of soil arching and increased the bearing capacity of the embankment under footing loading. Overall, the lightweight aggregate had favorable soil arching behavior with low vertical stresses on the trapdoor and the stationary supports as compared with the river dand. Lightweight aggregate is proven to be a good alternative material for load transfer platforms in pile-supported embankments.

Swelling behaviors of heterogeneous red-bed mudstone subjected to different vertical stresses

In this study, the axial swelling strain of red-bed mudstone under different vertical stresses are measured by swell-under-load method, and the microstructure of mudstone after hygroscopic swelling is studied by mercury intrusion porosimetry (MIP). The weakening coefficient and Weibull distribution function are introduced into the coupling model of mudstone moisture diffusion–swelling deformation–fracture based on finite–discrete element method (FDEM). The weakening effect of moisture on mudstone's mechanical parameters, as well as the heterogeneity of swelling deformation and stress distribution, is considered. The microcrack behavior and energy evolution of mudstone during hygroscopic swelling deformation under different vertical stresses are studied. The results show that the axial swelling strain of mudstone decreases with increase of the vertical stress. At low vertical stresses, moisture absorption in mudstone leads to formation of cracks caused by hydration-induced expansion. Under high vertical stresses, a muddy sealing zone forms on the mudstone surface, preventing further water infiltration. The simulation results of mudstone swelling deformation also demonstrate that it involves both swelling of the mudstone matrix and swelling caused by crack expansion. Notably, crack expansion plays a dominant role in mudstone swelling. With increasing vertical stress, the cracks in mudstone change from tensile cracks to shear cracks, resulting in a significant reduction in the total number of cracks. While the evolution of mudstone kinetic energy shows similarities under different vertical stresses, the evolution of strain energy varies significantly due to the presence of different types of cracks in the mudstone. The findings provide a theoretical basis for understanding the hygroscopic swelling deformation mechanism of red-bed mudstone at various depths.

Experimental investigation of hydraulic fracture propagation behavior in layered continental shale

In this study, numerous triaxial hydraulic fracturing experiments are performed in continental shale outcrop samples and artificial samples with similar properties to investigate the HF propagation behavior under single-cluster and multi-cluster fracturing treatments. The single-cluster fracturing experimental results show that the interlayer interface (IF) has a significant effect on the HF height containment, which tends to shear slip under conditions of low IF bonded strength, low vertical stress difference, low wellbore traversing layer strength and low q⋅μ value (the product of injection rate and fracturing fluid viscosity), resulting in the arrest of HF by the IF, and the vertical propagation of HF is hindered. Seven types of HF geometries were observed under the interference of multiple IFs, which could be classified into three categories in the vertical plane based on degree of connection with IFs, namely, simple fracture, small-scale fishbone-like fracture, and large-scale fishbone-like fracture. The multi-cluster fracturing experimental results indicate that the vertical propagation law of HF under multi-cluster fracturing is similar to single-cluster fracturing. Under the conditions of small cluster spacing and large number of perforation clusters, it is difficult for each perforation cluster to initiate HF evenly, and the middle perforation cluster is more difficult than outers. In addition, injection-pressure curves in theory can provide some useful information for HF geometries identification. But it is difficult to judge whether the HFs initiate evenly and develop uniformly at multiple perforation clusters by observing the injection fracturing curves because the evolution law of the injection-pressure curves under the simultaneous propagation of multiple HFs and the propagation of a single HF are similar.

Reinforcing effects of herbaceous plant root systems under low vertical stresses simulated at the extreme surface layer of a slope with vegetation work

Reinforcing effects of herbaceous plant root systems under low vertical stresses simulated at the extreme surface layer of a slope with vegetation work

Discrete element modeling of the effect of particle size distribution on direct shear behavior of hydrate-bearing sediments

Natural gas hydrates are a new energy resource with a great potential for commercial exploitation. The exploitation of natural gas hydrate can cause a degradation of the mechanical properties of hydrate-bearing sediments (HBS) and may trigger geohazards. Natural gas hydrates exist widely in marine sediments which have different types of sediment matrix with different particle size distributions (PSD). The effect of the PSD of hydrate-bearing sediments has not been investigated clearly. In this paper, a direct shear test model of HBS is built using the discrete element method. A series of numerical simulations with different PSD of HBS under different hydrate saturations and vertical stresses are conducted. The results show that the shear strength, residual strength, and dilatation of HBS increase with increasing hydrate saturation at the same PSD. It is found that the mechanical properties of HBS are not sensitive to the PSD at the same median particle size at low vertical stress. However, the shear strength can be enhanced with the increase of median particle size. The results obtained in this study can facilitate a deep understanding of the mechanical properties of HBS.

Effect of cyclic shearing on contact surfaces of geo-materials under constant normal force

A large amount of research has been done on conforming surfaces in rock joints as well as on the contact between individual grains; however, not much exist in nonconforming contact surfaces subjected to friction, such as flat contacts between ballast particles, stone columns, or riprap; applications that involve the use of coarse gravel subjected to low vertical stresses. Therefore, this article aims to study changes in contact properties between nonconforming flat contacts between large geo-material particles that have been subjected to cyclic shearing under a constant low vertical force, using a direct shear apparatus. Two different silica carbide sandpapers that do not loose particles were used, to simulate different morphologies, a nominally fine and a coarse surface texture. The results show a passivation of the shear strength where a constant value of friction coefficient is reached after around 15 to 17 cycles for all tests, except the tests at the lowest vertical force. For the tests at the highest vertical force, similar friction coefficients were determined for the coarse and fine surfaces. The mass broken during the 10th and 20th cycles was collected at the end of the tests and seemed to show a linear relationship with the vertical force used in the test. Particle analysis, determined via microscopy, shows that the grading is dependent on the initial topography of the surfaces. Despite being subjected to 10 and 20 cycles of shearing, the broken particles look similar in shape with sharp, jagged edges and having different shapes and roundness values with a large variation, indicating that the breakage was not enough to fill in the space between the particles.

Experiments and modelling of horizontal loading tests on small-scale foundations in sand

The accurate prediction of settlements of shallow foundations under operational conditions is still an open issue for both scientists and engineers, in particular in the case of complex loading conditions (e.g. combined vertical and horizontal loads). The complexity of the soil mechanical response and the difficulty in managing a multiscale problem (ranging from the local scale of the soil representative elementary volume to the macro-scale of the structure) makes a rigorous modelling of such problems particularly demanding for standard numerical tools such as three-dimensional finite-element codes. In this perspective, the well-known macroelement approach may represent a valid simplified approach, capable of overcoming these limitations and allowing a comprehensive description of both serviceability limit state and ultimate limit state of such systems, particularly useful even for practitioners. In the paper, based on a small-scale 1g experimental campaign on a strip foundation, the effect of embedment is investigated over horizontal loading paths at a constant vertical load. In particular, relatively low vertical stress values are considered for the foundations, as it often happens in real structures under usual working conditions. A simple macroelement plastic model, based on a strain hardening law with non-associated flow rule is then proposed and validated over the available experimental data.

Compaction and sealing properties of bentonite/claystone mixture: Impacts of bentonite fraction, water content and dry density

Compacted block composed of 70% crushed Callovo-Oxfordian claystone and 30% MX80 bentonite has been proposed as a possible material for sealing a potential repository of radioactive waste. In this study, the compaction property of the bentonite/claystone mixtures with various bentonite fractions and water contents were first investigated to define an appropriate protocol for the fabrication of blocks. Results show that the increase of bentonite fraction decreased the overall compressibility and the achievable dry density at a given stress. As the water content increased, the compressibility at a relatively low vertical stress increased due to the increasing deformability of aggregates and the decreasing inter-aggregate friction; contrarily, at a high vertical stress, a lower compressibility was observed for the samples with a higher water content because a larger stress was required to squeeze water within aggregates. During demoulding, the required demoulding pressure increased with the increase of bentonite fraction and decreased with the increasing water content. Then, to assess the suitability of the material, the swelling pressures and hydraulic conductivities of the compacted blocks were determined. It was found that the swelling pressure increased and the hydraulic conductivity decreased with the increases of bentonite fraction and dry density. By contrast, as the water content increased, more water was intercalated in the interlayer spaces and the inter-aggregate pore volume decreased, resulting in reductions of the swelling pressure and hydraulic conductivity. • Compressibility, rebound property, demoulding pressure, swelling pressure, and hydraulic conductivity were investigated; • Effects of bentonite fraction, dry density and water content were identified; • The effect of water content on the compressibility depended on vertical stress; • The increase of water content reduced the swelling pressure and the hydraulic conductivity.

Physical and Mechanical Characteristics of Shallow Expansive Soil due to Freeze-Thaw Effect with Water Supplement

Shear strength of shallow expansive soil varies along with the depth under the freeze-thaw effect. This work investigates shear strength characteristics of shallow expansive soil by simulating the actual freeze boundary conditions of seasonal frozen areas with water supplement. An integrated approach incorporating the freeze-thaw test and direct shear test was adopted. Firstly, unidirectional freezing tests for expansive soil columns under three different freezing temperature gradients were carried out. Secondly, direct shear tests under low vertical stress were performed on the standard samples, which were prepared by using cutting rings cut the thawed expansive soil columns into nine segments along with the depth. Temperature, water content, and dry density at different depths were also investigated after the freeze-thaw process. The test results showed that, after the freeze-thaw process, the shear strength of expansive soil columns showed significant differences along with the depth and highly correlated with water content, specifically the higher water content and the lower shear strength. The minimum shear strength in the expansive soil columns occurred at the soil layer below the frozen and unfrozen zones interface. The expansive soil column’s shear strength changed most under the moderate freezing temperature gradient corresponding to the most considerable shear strength reduction. Moreover, the significant decrease in cohesion was the main reason for the shear strength reduction of expansive soil after the freeze-thaw process. These results indicate significant depth variability in shear strength of expansive soil under the freeze-thaw effect.

Numerical Investigation of Particle Shape Effect on Sand Shear Strength

The particle shape contributes significantly to sand mechanical properties but is difficult to study in laboratory experiments. In this study, microscopy experiments were used to classify sand particle morphologies as spherical, non-spherical and ball-shaped. Particles with these three morphologies were modeled using the discrete element method to analyze the effect of the particle shape on the shear strength in a direct shear test simulation. The simulation results showed that the non-spherical particles and balls exhibited the highest and lowest peak shear strengths, respectively, under a low vertical stress. The highest and lowest contact force numbers were observed for the non-spherical particles and balls, respectively. The sand particles exhibited a higher resistance to motion with increasing contact force numbers, resulting in a higher shear strength. For particles with low sphericity (S) and roundness (R) values, a high number of contacts and interlocking structures can form, increasing resistance to external forces. As the S decreased, the anisotropy in the contact force increased during the test, and the contact number distribution of the spherical particles tended to remain isotropic during the entire process.

Effect of fracture voids on the swelling behaviour of Callovo-Oxfordian claystone

The Callovo-Oxfordian (COx) claystone has been considered as a possible host rock for deep geological repository of high-level radioactive waste in France. When constructing the repository, it is unavoidable to generate a fractured zone in the vicinity of excavated repository walls. In the assessment of the repository performance, it is important to investigate the role of the fractures on the swelling capacity of COx claystone and to further characterize the fractured zone evolution. In this study, the effect of fracture voids on the swelling behaviour of COx claystone was investigated by simulating the fracture voids with a space created by a limited amount of grains sandwiched between claystone disks, and by performing a series of swelling tests in oedometer under a low vertical stress (0.05 MPa) and swelling pressure tests under constant-volume condition. Results showed that, with water infiltrating into the sample with fracture voids, the grains collapsed before the swelling of claystone disks. The whole sample was composed of swelling zones corresponding to the claystone disks with higher dry density than the expected final value, and of compression zones corresponding to the initial fractures with lower dry density. Moreover, the axial strain and swelling pressure decreased as the fracture void increased. The maximum fracture voids that might be fully filled by the swelling of COx claystone (sealing) were found to be 5.14%–7.65% without considering the long-term creep contribution in the field condition.

An Experimental Study on the Shear Characteristics of Typical Colluvial Soil with Gravel and the Stability of Slopes in Wenzhou, China

Slope disasters are an important area of concern in the prevention and mitigation of national land disasters. To study the stability of the slope in Wenzhou, China, the basic physical and mechanical properties of undisturbed soil in this area were obtained from laboratory experiments of typical colluvial soil. From experimental results, it was determined that the soil was low-liquid limit clay containing gravel. The shear characteristics of soil for different water content, gravel ratios, and burial depth were also obtained from large-scale indoor direct shear tests. The test results show that: (1) for colluvial soil without gravel, increasing water content increases cohesion, but decreases internal friction angle; (2) for colluvial soil containing gravel, the shear stress drops sharply when the shear displacement is close to approximately 15 mm; (3) for the same gravel ratio, the contribution of the gravel to the shear strength of the shear surface is not significant at lower vertical stresses, but increases at higher vertical stresses. A finite element strength reduction method has been used to numerically simulate the typical binary slope in Wenzhou. The simulation results show that the factor of safety of the binary slope is dependent on the strength of the overlying soil. The variations of the safety factor with water content and the gravel ratio are consistent with the variations of shear strength with the water content and the gravel ratio obtained from the tests. Simultaneously, the soil–rock interface strength needs to be addressed in the analysis of binary slopes.

Numerical simulation analysis of vertical propagation of hydraulic fracture in bedding plane

Hydraulic fracturing is one of the principle technologies employed for achieving the efficient development of shale oil and gas reservoirs, and the bedding plane of the reservoir is a key factor affecting the geometrical distribution of hydraulic fractures in three dimensions. The present study adopts cohesive force unit theory to construct a multi-layer hydraulic fracture propagation model with coupling stress damage filtration, and the effects of changes in the reservoir stress field, angle of the bedding planes relative to vertically propagating hydraulic fractures, and tensile strength of the bedding planes on the vertical propagation of hydraulic fractures are analyzed. The simulation results show that the hydraulic fracturing process of penetrating the bedding planes can be divided into three stages. Here, the hydraulic fracture propagates in the rock matrix and intersects with the bedding planes with continuous injection of fracturing fluid in stages I and II, while the vertical stress, bedding plane angle, and tensile strength of the bedding planes determine the direction of hydraulic fracture propagation in the third stage. Reservoirs with low vertical stress differences and nearly horizontal bedding planes with low dip angles are found to be favorable for opening the bedding planes, and reservoirs with high vertical stress differences and bedding plane dip angles are favorable for the longitudinal expansion of hydraulic fractures. Simultaneously, the degree of opening of the bedding planes also increases as the tensile strength of the bedding planes decreases. This is mainly because the energy consumption of the bedding layer opening process decreases as the tensile strength of the bedding planes decreases.

Experimental Investigation on Hydraulic Fractures in the Layered Shale Formation

Multistage fracturing of horizontal wells to form a complex fracture network is an essential technology in the exploitation of shale gas. Different from the conventional reservoirs, the mechanical characteristics of shale rock have significant heterogeneity due to the existence of beddings, which makes it difficult to predict the fracture geometry in the shale reservoir. Based on the laboratory experiments, the factors that affect fracture propagation were analyzed. The experimental results revealed that the hydraulic fracture would cross the beddings under the high vertical stress difference, while it would propagate along with the bedding under the low vertical stress difference; besides, the low injection rate and viscosity of the fracturing fluid were beneficial to generate a complex fracture network. Under the high injection rate and viscosity, a planar fracture was created, while a nonplanar fracture was observed under the low injection rate and viscosity, and branch fracture was created. According to the acoustic emission events, the shear events were the main events that occurred during the hydraulic fracturing process, and the acoustic emission events could be adopted to describe the fracture network. Lastly, the supercritical carbon dioxide fracturing was more effective compared with the hydraulic fracturing because the fracture network was more complex.

Numerical investigation on the hydraulic fracture propagation based on combined finite-discrete element method

Hydraulic fracturing technology is widely used in the exploration of shale gas. Stratum interface is the key element which affect the form of fracture network. This paper proposed a numerical study based on finite-discrete element method to investigate the influence of bedding plane on fracture propagation. A hydraulic fracture would cross the beddings under the high vertical stress difference between a vertical stress and horizontal minimum stress, while a hydraulic fracture propagates along the beddings under a low vertical stress difference. The complex fracture network is more easily generated under the low net pressure, the fracturing fluid leaking into the beddings can act as a lubricant, thus reducing or eliminating the shear strength of the fractures, it is easier for hydraulic fractures to deviate than to cross the weakly cemented and large-aperture bedding plane. Based on the simulation, a mixed fracturing method was proposed to increase the stimulated reservoir volume which was benefit to the generation of fracture network.

Direct shear tests of shear strength of soils reinforced by geomats and plant roots

To investigate the shear strength of root–geomat reinforced soil (RMS), direct shear tests were conducted in laboratory on soil samples cultivated with three–dimensional geomats and Bermuda grass. The test results showed that the shear strength of RMS could be significantly improved by the combined reinforcement with grass roots and geomats, particularly at a low vertical stress level. The shear strength increment was increased exponentially with the total reinforcement content of roots and geomats. Concurrently, the soil cohesion was significantly increased, but the influence on the friction angle was generally negligible. With the increase in root or geomat content and decrease in water content, both the soil shear strength and cohesion were remarkably increased. Owing to the higher tensile strength of geomat than that of roots, the effect of geomat content on shear strength was larger than that of root content. Furthermore, the shear strength increment caused by root–geomat reinforcement was larger than the sum of the respective increments caused by root and geomat. Moreover, the soil shear strength and cohesion increments induced by the combined root–geomat mutual interlocking effect were increased exponentially with the total reinforcement content.

Thermal–mechanical effects on volume-change behavior of compacted GMZ bentonite during cyclic wetting–drying processes

For investigation of temperature and vertical stress influences on the swelling–shrinkage deformation of GMZ bentonite specimens, cyclic wetting–drying tests with suction ranging between 0 and 110 MPa were conducted under different constant vertical net stresses and temperatures using a suction-temperature controlled oedometer. Results show that temperature effects on the cumulative strains depend on the vertical stress. Under lower vertical stresses, rising temperature intensifies the tendency of the accumulation of expansive strains, while under higher stresses, rising temperature intensifies the tendency of accumulation of contractive ones. In addition, for a given constant temperature, the accumulative deformation recorded on the specimen tested was an expansion or a shrinkage strain depends on the vertical stress applied. Meanwhile, the corresponding vertical stress at transition was different at different temperatures. A logarithm correlation was developed between the final accumulative strain and vertical stress during wetting–drying cycles. According to the test results, the Barcelona expansive model was modified for simulation of the results of the cyclic wetting–drying tests conducted at temperatures. Simulation showed that the results calculated by the modified BExM agreed well to that measured in the present work.

Relation between the critical state friction angle of sands and low vertical stresses in the direct shear test

In this study, the critical state friction angle in a direct shear box was tested at vertical effective stresses between 4 kPa and 150 kPa for seven sands of different origins. The results show a stress dependency on the angle of friction at low vertical stresses, and that the critical state friction angle decreases with increase in the vertical effective stress. The largest drop occurs for vertical effective stresses between 4 kPa and 50 kPa and varies between 4° and 10° depending mostly on the shape of the sand particles and the difference in the extreme void ratios.

Laboratory investigation of shale rock to identify fracture propagation in vertical direction to bedding

Affected by beddings and natural fractures, fracture geometry in the vertical plane is complex in shale formation, which differs from a simple fracture in homogeneous sandstone reservoirs. However, the propagation mechanism of a hydraulic fracture in the vertical plane has not been well understood. In this paper, a true tri-axial pressure machine was deployed for shale horizontal well fracturing simulation experiments of shale outcrops. The effects of multiple factors on hydraulic fracture vertical propagation were studied. The results revealed that hydraulic fracture initiation and propagation displayed four basic patterns in the vertical plane of laminated shale formation. A hydraulic fracture would cross the beddings under the high vertical stress difference between a vertical stress and horizontal minimum stress of 12 MPa, while a hydraulic fracture propagates along the beddings under a low vertical stress difference of 3 MPa. Four kinds of fracture geometry, including a single main fracture, a nonplanar fracture, a complex fracture, and a complex fracture network, were observed due to the combined effects of flow rate and viscosity. Due to the influence of binding strength (or cementing strength) on the fracture communication effects between a hydraulic fracture and the beddings, the opening region of the beddings takes the shape of an ellipse.