Direction Of Fracture Propagation Research Articles

Meso-mechanical behavior of crystalline rocks: An image-based discrete element method

This research examines the meso-mechanical behavior of rock materials under various loading conditions, with a focus on the influence of mesoscale heterogeneity in crystalline rock specimens. Traditional approaches often rely on a limited set of statistical parameters for constructing numerical models, failing to capture the complex mesoscale heterogeneities integral to rock mechanics. To overcome this limitation, the study utilizes an advanced image-processing technique to accurately identify mineralogical features within rock microstructures. The extracted mineralogical data were then integrated into a discrete element method (DEM) model to explore the mesoscale mechanical responses of crystalline rocks. The results of the simulations indicated that mesoscale heterogeneities play a critical role in stress concentration localization, which in turn dictates the direction of fracture propagation and leads to a sophisticated interaction among various fracture pathways.

Data-Driven Dynamic Inversion Method for Complex Fractures in Unconventional Reservoirs

Abstract Hydraulic fracturing is a crucial technology for enhancing the recovery of oil and gas from unconventional reservoirs. Accurately describing fracture morphology is essential for accurately predicting production dynamics. This article proposes a new fracture inversion model based on dynamic data-driven methods, which is different from the conventional linear elastic fracture mechanics model. This method eliminates the need to consider complex mechanical mechanisms, resulting in faster simulation speeds. In the model, the fracture morphology is constrained by combining microseismic data and fracturing construction data, and the fracture tip propagation domain is introduced to characterize the multi-directionality of fracture propagation. The simulated fracture exhibits a multi-branch fracture network morphology, aligning more closely with geological understanding. In addition, the influence of microseismic signal intensity on the direction of fracture propagation is considered in this study. The general stochastic approximation (GSA) algorithm is employed to optimize the direction of fracture propagation. The proposed method is applied to both the single-stage fracturing model and the whole well fracturing model. The research findings indicate that in the single-stage fracturing model, the inverted fracture morphology aligns closely with the microseismic data, with a fitting rate of the fracturing construction curve exceeding 95%, and a microseismic data fitting rate exceeding 93%. In the whole well fracturing model, a total of 18 sections were inverted. The fitting rate between the overall fracture morphology and the microseismic data reached 90%. The simulation only took 5 minutes, demonstrating high computational efficiency and meeting the needs of large-scale engineering fracture simulation. This method can effectively support geological modeling and production dynamic prediction.

Study of stress field induced by natural fracture and its influence on hydraulic fracture propagation

The change of fracture propagation direction caused by stress interference between fractures is one of the main reasons that affect shale gas productivity. Natural fractures will be damaged by the induced stress, and the induced stress field produced by natural fracture damage will in turn affect the propagation of hydraulic fractures. Previous studies usually ignored the impact of stress field variation caused by natural fracture damage, leading to inaccurate fracture propagation simulation results. A new model for simulating hydraulic fracturing-induced stress field is established with consideration of the influence of natural fracture damage. Then, the natural fracture-induced stress is analyzed in open and closed fracture states. Through superposition of stress fields of natural fractures and hydraulic fractures, the interaction among open and closed natural fractures, reservoir and hydraulic fracture, and the corresponding fracture property evolution are studied. The results demonstrate that induced stress is not affected by elastic modulus and Poisson's ratio of the rock. The induced stress difference at the fracture tip is proportional to the net pressure and the length of hydraulic fractures. The results provide guidance for the optimal design of fracturing. When the deflection angle of hydraulic fracture is less than 90°, the maximum induced stress difference at the fracture tip decreases with the reduction in the deflection angle. When the deflection angle of hydraulic fracture is smaller than 90° or the deflection angle of natural fracture is smaller than 45°, the steering of hydraulic fracture is less hindered, which is beneficial to the formation of complex fracture network.

Study of fractography of ferritic ductile iron at different temperatures and loading conditions

This study characterizes the fractography of ferritic ductile iron under various loads, including tensile, fatigue and bending, and impact conditions. The results indicate that ductile fracture is the primary mechanism observed during tensile testing at room temperature. The fractography resulting from fatigue testing exhibits characteristics similar to cleavage fracture, and explains the formation of fatigue striations caused by the joint effects of dislocation slip and oxidation under crack tip stress. Under impact testing, the main fracture mechanism transitions from ductile to brittle with decreasing temperature. At high temperatures, fractography is mainly characterized by elliptical dimples with graphite nodules at the center that deform along the stress direction. In the ductile-brittle transition temperature range, a mixed fracture mechanism involving both dimples and cleavage patterns is observed. At low temperatures, the fracture mechanism is cleavage fracture, cleavage fracture is mainly caused by the deformation twin, inducing crack nucleation. These findings further validate D.O.Frenandino’s quantitative analysis method for determining the main crack propagation direction of ductile fracture and brittle fracture. By employing larger statistical datasets, it is shown that this method yields high accuracy in determining the main crack propagation direction of ferritic ductile iron, thereby promoting its application as a general method for impact fractography analysis.

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

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

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

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

Orthogonal decomposition of anisotropic constitutive models for the phase field approach to fracture

We propose a decomposition of constitutive relations into crack-driving and persistent portions, specifically designed for materials with anisotropic/orthotropic behavior in the phase field approach to fracture to account for the tension–compression asymmetry. This decomposition follows a variational framework, satisfying the orthogonality condition for anisotropic materials. This implies that the present model can be applied to arbitrary anisotropic elastic behavior in a three-dimensional setting. On this basis, we generalize two existing models for tension–compression asymmetry in isotropic materials, namely the ‘volumetric–deviatoric’ model (Amor et al., 2009) and the ‘no-tension’ model (Freddi and Royer-Carfagni, 2010), towards materials with anisotropic nature. Two benchmark problems, single notched tensile and shear tests, are used to study the performance of the present model. The results can retain the anisotropic constitutive behavior and the tension–compression asymmetry in the crack response, and are qualitatively in accordance with the expected behavior for orthotropic materials. Furthermore, to study the direction of maximum energy dissipation, we modify the surface integral based energy release computation, Gθ, to account only for the crack-driving energy. The computed energies with our proposed modifications predict the fracture propagation direction correctly compared with the standard Gθ method.

Dual bilinear cohesive zone model-based fluid-driven propagation of multiscale tensile and shear fractures in tight reservoir

PurposeHydrofracturing technology has been widely used in tight oil and gas reservoir exploitation, and the fracture network formed by fracturing is crucial to determining the resources recovery rate. Due to the complexity of fracture network induced by the random morphology and type of fluid-driven fractures, controlling and optimising its mechanisms is challenging. This paper aims to study the types of multiscale mode I/II fractures, the fluid-driven propagation of multiscale tensile and shear fractures need to be studied.Design/methodology/approachA dual bilinear cohesive zone model (CZM) based on energy evolution was introduced to detect the initiation and propagation of fluid-driven tensile and shear fractures. The model overcomes the limitations of classical linear fracture mechanics, such as the stress singularity at the fracture tip, and considers the important role of fracture surface behaviour in the shear activation. The bilinear cohesive criterion based on the energy evolution criterion can reflect the formation mechanism of complex fracture networks objectively and accurately. Considering the hydro-mechanical (HM) coupling and leak-off effects, the combined finite element-discrete element-finite volume approach was introduced and implemented successfully, and the results showed that the models considering HM coupling and leak-off effects could form a more complex fracture network. The multiscale (laboratory- and engineering-scale) Mode I/II fractures can be simulated in hydrofracturing process.FindingsBased on the proposed method, the accuracy and applicability of the algorithm were verified by comparing the analytical solution of KGD and PKN models. The effects of different in situ stresses and flow rates on the dynamic propagation of hydraulic fractures at laboratory and engineering scales were investigated. when the ratio of in situ stress is small, the fracture propagation direction is not affected, and the fracture morphology is a cross-type fracture. When the ratio of in situ stress is relatively large, the propagation direction of the fracture is affected by the maximum in situ stress, and it is more inclined to propagate along the direction of the maximum in situ stress, forming double wing-type fractures. Hydrofracturing tensile and shear fractures were identified, and the distribution and number of each type were obtained. There are fewer hydraulic shear fractures than tensile fractures, and shear fractures appear in the initial stage of fracture propagation and then propagate and distribute around the perforation.Originality/valueThe proposed dual bilinear CZM is effective for simulating the types of Mode I/II fractures and seizing the fluid-driven propagation of multiscale tensile and shear fractures. Practical fracturing process involves the multi-type and multiscale fluid-driven fracture propagation. This study introduces general fluid-driven fracture propagation, which can be extended to the fracture propagation analysis of potential fluid fracturing, such as other liquids or supercritical gases.

Hydraulic fracture network propagation in a naturally fractured shale reservoir based on the “well factory” model

In different geological and engineering conditions, it is important to study the stress interference of multiple fractures and interaction of natural fractures in “well factory” fracturing to improve the complexity of fracture networks. For this purpose, a 2D, coupled stress-seepage-damage field cohesive model with random propagation of hydraulic fracture was developed, and a random function was used to simulate the natural fractures distribution in a real reservoir. Orthogonal test was designed to investigate the geological and engineering parameters impacting the evolution of zipper fracturing and modified zipper fracture networks. The results show that in naturally fractured reservoirs, hydraulic fracture can open natural fractures and promote the formation of complex fracture networks by combined, induced manner. Higher stress differences lead to simpler fracture networks and straighter hydraulic fracture propagation paths; When pre-existing natural fracture is longer, the propagation direction of hydraulic fracture is more likely to be paralleled with natural fracture; However, the stress shadow effect of multiple fractures may potentially lead to failure of some hydraulic fracture initiation. In addition, the fracture initiation sequence of multiple fractures has a significant effect on the evolution of the fracture networks. The results of the study have important significance for parameter optimization of well factory fracturing in naturally fractured reservoirs.

Coupled multiphysics 3-D generalized finite element method simulations of hydraulic fracture propagation experiments

Intentional creation and propagation of fractures driven by fluid under high pressure have several applications in oil and gas exploration, geological mining, disposal of toxic waste, flow of molten magma within crevices/fissures, etc. Due to the need to design hydraulic fractures, especially in the oil and gas industry, and the limitation of their full scale/field testing, the use of laboratory experiments and/or numerical simulations remain viable alternatives. While several computational methods for this class of problems exist in the literature, their reliability and robustness hinge on their ability to replicate experimental results with acceptable confidence. This paper presents the application of a coupled multiphysics 3-D generalized finite element method (GFEM) for the simulation of hydraulic fracture experiments based on a regularized Irwin criterion and adaptive mesh refinement. Built with an automatic time step search, the algorithm computes the time step size that ensures the satisfaction of the propagation criterion. Modes I, II, and III stress intensity factors are computed using the displacement correlation method, while the fracture propagation direction is determined using the Schöllmann’s criterion, which is a generalization of the maximum tangential stress criterion for 3-D stress state. The GFEM is validated against a hydraulic fracture experiment on PMMA specimen, on one hand, and against four different cases of experimental fractures within concrete (a granite-like) material which serves as the pioneer application of the algorithm to verify hydraulic fracture simulation in non-PMMA materials. The influence of the initial shear stresses along the fracture plane (assuming unnotched condition) on the resulting propagation paths in the latter benchmarked problem is also assessed and found to have good agreement with experimental observations.

Using pressure changes in offset wells for interpreting fracture driven interactions (FDI)

Infill completions have been used in the field to enhance ultimate recovery of unconventional low permeability reservoirs. The stimulation of the infill wells can cause pressure increase in nearby wells. Most available studies have focused on the propagation of fractures from infill wells and pressure changes in treatment wells rather than offset wells which their pressure data may reveal important signatures of fracture-driven interactions (FDI). In this study, we looked at different signatures of FDI data collected in the field and then used a numerical approach to model the pressure fluctuations in offset wells induced by fracturing infill wells and provide insight into how field operators can use the pressure data from nearby wells to identify different forms of FDI, including frac/frac connections and fracture shadowing. The trajectory of a fracture propagating from an infill well is modeled using the extended finite element methods (XFEM). This method allows us to model change of fracture propagation direction due to the rotation of stress field caused by the production of the offset wells. Then, we calculate the pressure response after the fracture intersection using a multi-phase flow model. We investigate the effect of several parameters on the pressure behavior such as the fracture intersection location, the vertical well offset and the fluid compressibility. Typically, a sudden increase in the offset well pressure over a short period is interpreted as a fracture intersection. However, our numerical examples reveal that a gradual increase in the offset well pressure may also indicate a frac/frac connection, especially if the offset well is initially pressurized by injection of fluid before stimulating the infill well. Therefore, only looking at the magnitude of the pressure increase may not be very definitive to make a judgement. Thus, we propose to use diagnostic pressure plots to differentiate fracture connection and fracture shadowing. The proposed analytical model for frac/frac connection pressure threshold and the pressure diagnostic plots can be combined to quickly assess fracture's connections.

Staged multicluster hydraulic fracture propagation mechanism and its influencing factors in horizontal wells for CBM development

AbstractStaged multicluster fracturing (SMCF) is crucial for the low‐permeability coalbed methane (CBM) reservoirs development. To reveal the fracture propagation mechanism and characteristics in SMCF and select appropriate hydraulic fracturing parameters, this study considers CBM exploitation in the Shizhuang block, Qinshui Basin in China as the engineering background. Then, XSite hydraulic fracturing software based on the discrete lattice theory and synthetic rock mass method is used to construct a three‐dimensional model. The influence mechanisms of the cluster spacing (CS) and lateral pressure coefficient (LPC) on the fractures propagation characteristics were investigated. With an increase in the CS, the stress interference between fractures gradually weakens, and the fracture deflection also decreases. Both the stimulated reservoir area (SRA) and fracture aperture of a single cluster increase with an increase in CS, but the SRA and fracturing efficiency exhibit limited growth when the CS is greater than 15 m. The SRA decreases exponentially with an increase in LPC. When the LPC increases from 0.4 to 1.4, the SRA decreases by 74.2%. The LPC plays a decisive role in the fracture propagation direction. When the LPC reaches 1.4, the fractures propagate along the horizontal direction. With an increase in LPC, the fracturing fluid pressure increases linearly, and the fracturing efficiency decreases exponentially. The results of this study reveal the fracture propagation law of SMCF and can provide a reference for engineering applications. In the SMCF at the 3# coal seam of the Shizhuang block, the CS can be set to approximately 15 m, and the CS can be appropriately reduced when the LPC is large.

Simulation of Polymer Injection in Granular Media: Implications of Fluid-Driven Fractures, Water Quality, and Undissolved Polymers on Polymer Injectivity

Summary Polymer flooding is an enhanced oil recovery (EOR) method which improves the mobility ratio and sweep efficiency of a waterflood. In theory, the high viscosity of the polymer reduces its injectivity compared to water. However, field studies have documented much higher polymer injectivity than predicted by theoretical models. There are various reasons for high polymer injectivity. The objective of this work is to predict polymer injectivity in granular media accounting for fluid-induced fractures, water quality, polymer rheology, and undissolved polymers. We perform grain-scale, coupled fluid dynamics and granular mechanics modeling. Fluid-particle interactions are modeled by coupling computational fluid dynamics (CFD) and the discrete element method (DEM). Simulation results show that polymer injection can create fractures in the granular media along the direction perpendicular to the minimum principal stress, thereby reducing wellbore pressure buildup at a constant polymer injection rate. The polymer tends to flow in the direction of fracture propagation in granular media, so the direction of the fracture affects the swept area of the polymer. Polymer rheology, water quality, and undissolved polymer also affect the polymer injectivity. Suspended solid particles may plug pores and reduce the injectivity of polymer by ~25%. Mechanically trapped undissolved polymers can greatly reduce polymer injectivity in low-permeability granular media. This work shows for the first time initiation of polymer-driven fractures in a granular model and demonstrates its implications on polymer injectivity.

Effect of In-Situ Stress on Hydraulic Fracturing of Tight Sandstone Based on Discrete Element Method

The tight sandstone reservoir in the Qianfoya formation of well PL-3 of the Puguang gas field in Sichuan, China, obtained a high-yield gas flow after a volume fracturing treatment. However, the stimulated reservoir volume (SRV), fracture morphology, scale and formation law still remain unclear. Based on particle flow discrete-element theory in this paper, we carried out a few trials of the Brazilian splitting test, uniaxial compression and triaxial compression of rock mechanics. Meanwhile, the research also testified to the conversion relationship between macroparameters and microparameters, established the numerical simulation on hydraulic fracturing through PFC2D discrete element software, and finally analyzed the influence of difference coefficients on the fracturing effect, in terms of different in-situ stresses. The conclusions are as follows: firstly, the influence of in-situ stress is essential for the direction, shape and quantity of fracture propagation, and the fractures generated by hydraulic fracturing are mainly tension fractures, accounting for over 90% of the total longitudinal fractures. Secondly, it is indicated that when the difference coefficient is small in the in-situ stress, the fractures formed by hydraulic fracturing expand randomly around the wellbore. When the difference coefficient Kh of in-situ stress is above 0.6, the development of hydraulic fractures is mainly controlled by in-situ stress; as a result, the fractures tend to expand in the vertical direction of the minimum horizontal principal stress and the fracture shape is relatively singular. When the difference coefficient of in-situ stress was 0.3, in total, 3121 fractures were generated by fracturing, and the fractal dimension D value of the fracture network complexity was 1.60. In this case, this fracturing effect was the best and it is the easiest to achieve for the purpose of economical and effective development on large-scale volume fracturing.

Asymmetric propagation mechanism of hydraulic fracture networks in continental reservoirs

AbstractHydraulic fracturing technology is relatively mature in North America, but under complex geological conditions, such as those in China, the application of this technology still faces great challenges. At present, techniques for the numerical simulation of hydraulic fracture networks are mainly based on the prediction of the fracture half-height and half-length, which cannot capture the heterogeneity of continental low-permeability sandstone reservoirs in China and the distribution of the asymmetric hydraulic fracture network present in them. Therefore, determining the asymmetric propagation mechanism of hydraulic fracture networks is very important for improving the recovery rates of continental reservoirs. In this paper, taking the Ordos Basin in China as an example, the spatial distribution of the stress field of a heterogeneous continental reservoir is precisely predicted by reservoir mechanical heterogeneity modeling. By using a microseismic monitoring method, the 3-D morphology of the hydraulic fracture network is determined. Through the coupling of multisource data, the frequency distributions of the determined in situ stress magnitudes in different hydraulic fracturing stages are obtained. The propagation direction of the hydraulic fracture network changes under the control of the horizontal stress difference (Δσ) and the presence of natural fractures. The smaller Δσ is, the greater the deflection of the hydraulic fracture propagation direction. The asymmetric propagation of these fractures is related to the frequency distribution of Δσ. As the frequency of Δσ approaches a normal distribution, the two wings of the hydraulic fracture network become basically equal in length, and as Δσ deviates more from a normal distribution, the difference between the two wings of the hydraulic fracture network increases. These research results will provide new insight for modeling, exploring, and developing continental reservoirs.

Integrating Petrophysical and Geomechanical Rock Properties for Determination of Fracability of the Iraqi Tight Oil Reservoir

Tight oil reservoirs have been a concerned of the oil industry due to their substantial influence on oil production. Due to their poor permeability, numerous problems are encountered while producing from tight reservoirs. Petrophysical and geomechanical rock properties are essential for understanding and assessing the fracability of reservoirs, especially tight reservoirs, to enhance permeability. In this study, Saadi B reservoir in Halfaya Iraqi oil field is considered as the main tight reservoir. Petrophysical and geomechanical properties have been estimated using full-set well logs for a vertical well that penetrates Saadi reservoir and validated with support of diagnostic fracture injection test data employing standard equations and correlations. Subsequently, breakdown pressures are computed, and two fracturing models have been developed. The petrophysical analysis infers that the reservoir has poor properties, while the findings of the geomechanical properties indicate that the reservoir is brittle with ductile rock strata. These ductile strata underlay and overlay more brittle formations than the reservoir. The results from diagnostic fracture injection test DFIT are quite consistent with well logs results. The breakdown pressure reflects that this reservoir could easily be fractured by inserting pressure equal to 6250 psi. However, the fracturing model design parameters manipulates the fracture height confinement within Saadi Formation and its propagation to Hartha and/or Tanuma Formations. Therefore, the employment of petrophysical and geomechanical properties of the rocks assists in understanding the fracability of the formation and demonstrating the orientation and the fracture propagation direction.

Parameter Sensitivity Analysis of the Hydraulic Fracture Growth Geometry in a Deep Shale Oil Formation: An Experimental Study

The depth of shale oil of Fengcheng Formation in Mahu of Junggar Basin, China, is 4500-5000 m. The horizontal principal stress difference of deep shale reservoir is high, which makes it difficult to form complex fractures during fracturing reconstruction. In order to fully understand the law of hydraulic fracture propagation in the formation during fracturing construction, the anisotropy characteristics and basic reservoir physical parameters (mineral composition and rock strength parameters) of rock were obtained through mineral composition test and indoor rock mechanics test (Brazil splitting test), and it was found that the heterogeneity was strong. The true triaxial fracturing simulation experimental system is used to carry out experimental research on full-diameter core rock samples, and the propagation patterns of hydraulic fractures under the influence of different geological factors (in situ stress difference and natural fractures) and engineering factors (pumping rate and fracturing fluid viscosity) are compared and analyzed. The results show that the in situ stress is the most important factor affecting fracture propagation, which determines the direction and shape of fracture propagation. The natural weak surface (lamina/bedding and natural fractures, etc.) in shale reservoir is an important reason for complex fractures. The nature of the weak plane, occurrence, and in situ stress jointly determine whether the fracture can extend through the weak plane. With the increase of pumping rate (18 mL/min to 30 mL/min), the ability of hydraulic fractures to penetrate layers is continuously enhanced. The horizontal principal stress difference of deep shale reservoir is high, and the low viscosity fracturing fluid (10 mPa·s) tends to activate the horizontal bedding, while the high viscosity fracturing fluid (80 mPa·s) tends to directly penetrate the bedding to form the vertical main fracture. Therefore, the fracturing technology of alternating injection of prehigh viscosity fracturing fluid and postlow viscosity fracturing fluid can be adopted to maximize the complexity of fracturing fractures in deep shale reservoirs. The research results are designed to provide theoretical guidance for prediction of hydraulic fracturing fracture propagation in shale reservoir and have certain reference significance for field construction.

Mechanical Properties of the Shear Failure of Rock Masses with Prefabricated Sinusoidal Fractures

A large number of sinusoidal fractures exist in the open-pit slopes of mines, and the mechanical properties of shear failure are of great significance to the stability of the open-pit slope. Sandstone specimens containing sinusoidal fractures with different undulated heights were prefabricated by the CNC sand-wire-electrode cutting technology to explore the effect of sinusoidal fractures on the mechanical properties of shear failure of rock masses. The TFD-20H/50J rock shear testing machine was used for shear tests on sinusoidal-fracture sandstone with different undulated heights. In the shear loading process, the prefabricated-fracture sandstone specimens with different undulated heights have the fracture-compacting stage, linear-elastic changing stage, bottom-up stage, and residual strength stage. Before peak strength, the sinusoidal prefabricated-fracture rock masses with different undulated heights have the precursory characteristics of decreased stress, which can provide an early warning for the instability and failure of rock masses. The undulated height of sinusoidal fractures significantly affects the fracture initiation and propagation of specimens. Fractures occur and develop from the prefabricated fractures at low undulated height (≤10 mm) to multiple ways under the high undulated height (>10 mm); that is, fractures occur at the prefabricated fractures and the end of specimens simultaneously. With the increased undulated height, the fractures expand from parallel to the slope of the prefabricated fractures to perpendicular to the prefabricated fractures. The fracture propagation direction of sinusoidal-fracture rock masses with different undulated heights is mainly from the middle of the slope surface of prefabricated fractures to the end. It is the main direction of the shear failure of sinusoidal prefabricated fractures, and the monitoring of weak planes should be strengthened in the actual slope engineering.

An irregular lattice spring model: uniform elasticity, grid refinement and isotropic crack propagation

The Born lattice spring model (LSM) is implemented on an irregular network. This irregular LSM is directly mapped to linear elasticity theory such that the model can capture an elastically uniform system where the direction of resultant fracture propagation is no longer influenced by a regular lattice structure. Furthermore, this allows the density of nodes to be varied throughout the simulation while maintaining the correct elastic mapping. For the first time, the LSM can be refined around areas of interest such as heterogeneities or crack tips, while capturing the spatial cooperative effects of isotropic fracture propagation in heterogeneous materials.

Experimental study on hydraulic fracturing behavior of frozen clayey silt and hydrate-bearing clayey silt

There are critical problems in the field trials of natural gas hydrate (NGH) production, such as low production rate, limited recovery range and short period of stabilized production. Reservoir stimulation is a potential means to promote gas recovery from NGHs since previous studies have proved that gas production of other unconventional gas reservoir can be highly enhanced by hydraulic fracturing. However, few studies pay attention to the hydraulic fracturing behavior of clayey silt NGH reservoir. In this work, the effects of sediment properties and fracturing fluid parameters on fracture initiation, propagation and morphology were experimentally investigated by hydraulic fracturing of frozen clayey silt and methane hydrate-bearing clayey silt. The results indicated that the uneven distribution of ice in sediments led to non-uniform fracture propagation phenomenon, such as single-wing fracture and fracture deflection. The fracturing fluid with injection rate of 90 mL/min was conducive to the formation of complex fractures in sediments, which indicated that that increasing injection rate could greatly increase the effect of reservoir reconstruction. When the axial pressure (8 MPa) was 2 MPa higher than the confining pressure, the fracture propagated in the direction perpendicular to the minimum principal stress, which revealed the magnitude of minimum in-situ stress difference controlling the fracture propagation direction. In addition, this work showed the fracability of clayey silt hydrates even with low saturation, which revealed the feasibility of hydraulic fracturing of marine methane hydrate reservoir.