Hydraulic Fracture Propagation Research Articles

Influences of clean fracturing fluid viscosity and horizontal in-situ stress difference on hydraulic fracture propagation and morphology in coal seam

The viscosity of fracturing fluid and in-situ stress difference are the two important factors that affect the hydraulic fracturing pressure and propagation morphology. In this study, raw coal was used to prepare coal samples for experiments, and clean fracturing fluid samples were prepared using CTAB surfactant. A series of hydraulic fracturing tests were conducted with an in-house developed triaxial hydraulic fracturing simulator and the fracturing process was monitored with an acoustic emission instrument to analyze the influences of fracturing fluid viscosity and horizontal in-situ stress difference on coal fracture propagation. The results show that the number of branched fractures decreased, the fracture pattern became simpler, the fractures width increased obviously, and the distribution of AE event points was concentrated with the increase of the fracturing fluid viscosity or the horizontal in-situ stress difference. The acoustic emission energy decreases with the increase of fracturing fluid viscosity and increases with the increase of horizontal in situ stress difference. The low viscosity clean fracturing fluid has strong elasticity and is easy to be compressed into the tip of fractures, resulting in complex fractures. The high viscosity clean fracturing fluids are the opposite. Our experimental results provide a reference and scientific basis for the design and optimization of field hydraulic fracturing parameters.

Quantitative characterization and fracture morphology in reservoirs with various lithologies: An experimental investigation

Accurately quantifying the development of hydraulic fractures is pivotal for further elucidating the relationship between reservoir physical properties and hydraulic fracture propagation. In this research, we centered on reservoirs featuring diverse lithologies, procured downhole full-diameter cores, conducted triaxial hydraulic fracturing experiments, and employed CT scanning and electron microscopy to characterize fracture morphology. The primary findings are summarized as follows: sandstones specimens typically exhibit bi-wing fractures inclined at a certain angle to σH, with internal microfractures predominantly comprised of corrugated shear fractures traversing mineral grains. Conversely, conglomerate specimens display a higher prevalence of branching fractures, with fracture zones forming in areas of gravel densities and weak heterogeneous matrices, along with the development of multiple microfractures. Volcanic rock specimens fractures primarily propagate via deflection following NF activation, showcasing two modes of microfracture propagation: through and alongside the NF. Quantitative parameters of fracture morphology indicate that the fractal dimension ranges from 2.02 to 2.08 for sandstone fractures, 2.19 to 2.22 for conglomerate fractures, and 2.11 to 2.13 for volcanic rock fractures. Notably, the NF filler in volcanic rock undergoes erosion by fracturing fluid, accumulating non-uniformly on the surface of rock crystals, thereby augmenting fracture tortuosity. Additionally, the fracture surface area of conglomerates surpasses that of sandstones and volcanic rocks by 1.3 times, with the stripping rate of conglomerate fracture surfaces exceeding 10%. Furthermore, the average normal width of conglomerate fractures surpasses 2 mm, whereas volcanic rock and sandstone measure 1.6 mm and 1.1 mm, respectively. Notably, sandstone fractures exhibit uniform normal width distribution, while conglomerate fractures, especially those around gravels and eroded by fracturing fluid, manifest larger widths. Moreover, NF influenced volcanic rock fractures also exhibit wider widths. Elucidating the quantitative attributes of fracture morphology in lithologically diverse reservoirs holds significant implications for formulating hydraulic fracturing and targeted optimization strategies aimed at enhancing fracturing efficacy.

Multiple hydraulic fracture propagation simulation in deep shale gas reservoir considering thermal effects

Deep and ultra-deep shale gas will gradually become the focus of unconventional oil and gas resources in the petroleum industry. In the development of shallow shale reservoirs, the multi-cluster hydraulic fracturing of horizontal wells is the commonly applied technique to create a network of fractures and achieve reservoir stimulation. However, for deep shale reservoirs, the behavior of multiple hydraulic fracture propagation is affected by stress interference between fractures. On the other hand, it is also influenced by the temperature disturbances caused by the injection of low-temperature fracturing fluids into high-temperature formations and the resulting thermal effects. Currently, most research on multiple hydraulic fracture propagation in shale only considers the interaction between fluids and solids while neglecting thermal effects. Therefore, based on the displacement discontinuity method, finite volume method, and finite difference method, this study established a numerical model for the non-planar propagation of multi-cluster hydraulic fractures in deep shale, considering the thermal effects. And the model was validated through numerical solutions of the temperature field and double-fracture propagation. Moreover, the number of fracture clusters, pumping rate, and formation temperature were selected as influencing factors, and the differences in multiple hydraulic fracture propagation with and without considering thermal effects were analyzed. The results show that as the formation temperature increases, the thermal effects become more significant, leading to greater differences in the morphology of multi-cluster fractures. Moreover, the thermal effects are more significant when the formation temperature exceeds 90 °C. Additionally, appropriately reducing the number of fracture clusters or increasing the pumping rate can to some extent mitigate stress interference between fractures and promote uniform fracture propagation. The findings of this study contribute to understanding the influence of thermal effects on multiple hydraulic fracture propagation and provide theoretical basis for the efficient development of deep shale gas.

The influence of bedding interface strength on the vertical propagation of hydraulic fractures

Bedding interfaces in unconventional oil and gas may influence the vertical propagation path of hydraulic fractures. In this article, the cohesive elements were used to describe the tensile and shear damage of bedding interfaces. The vertical propagation law of hydraulic fractures under different stress, bedding interface tensile strength, and shear strength conditions was calculated. The simulation result revealed three types of hydraulic fractures: “H”-shaped, “fishbone”-shaped, and “I”-shaped fractures. The large vertical stress can cause difficulties in damaging bedding interfaces. When the vertical stress difference lies between 0 and 5 MPa, complex fishbone-shaped fractures can easily form. However, when the vertical stress exceeds 5 MPa, “I”-shaped fractures are more likely to occur. In case the vertical stress difference is less than 0 MPa, hydraulic fractures may encounter obstacles in crossing the bedding interface, resulting in the formation of “H”-shaped fractures. A larger tensile strength of the bedding interface will promote hydraulic fractures to pass through the bedding interface and make it easier to form “I”-shaped fractures, promoting the growth of fracture height, which promotes fracture height growth. On the other hand, a low shear strength in the bedding interface can cause shear slip, resulting in the formation of “H”-shaped fractures, which inhibits the growth of fracture height. Tensile shear mixed damage typically happens at the bedding interface. This type of damage can restrict the vertical propagation of hydraulic fractures, but it can also make the fractures more complicated.

Numerical analysis of hydraulic fracture propagation in deep shale reservoir with different injection strategies

Numerical analysis of hydraulic fracture propagation in deep shale reservoir with different injection strategies

A Dynamic Propagation Numerical Model of Hydraulic Fracture Interactions with Pre-Existing Complex Natural Fractures

The propagation of hydraulic fractures is highly influenced by the geological structure of the reservoir in unconventional reservoirs, such as natural fractures. In this paper, a new fluid–solid coupling dynamic model was built which presents the failure mechanism of hydraulic fracture with pre-existing simple and complex natural fractures. The cohesive element method and the maximum principal stress fracture criterion were used in the new model. An analysis was conducted to investigate the impact of various factors, including encounter angle, in situ stress, elastic modulus, and Poisson’s ratio, on the propagation of hydraulic fractures. The simulation results indicate that the encounter angle and the in situ stress are the main factors affecting the fracture morphology. When the encounter angle and the in situ stress difference are small, hydraulic fractures propagate along natural fractures. When the elastic modulus is small, it is advantageous for the advancement of both hydraulic and natural fractures. The Poisson’s ratio has a slight effect on the fracture propagation pattern.

Hydraulic fracturing in layered heterogeneous shale: The interaction between adjacent weak interfaces

Shale is known as the typical layered heterogeneous rock with multiple weak interfaces between different layers, and how to depict the propagation characteristic of fluid-driven fracture in such complex geological structures is critical for hydraulic fracturing engineering. Traditional investigations only focus on the intersecting behaviors of hydraulic fracture (HF) at a single weak interface. Herein, we reveal the propagation of HF in shale formation based on the extended finite element method (XFEM), where multiple weak interfaces and different rock properties are carefully considered in the sandwiched layered structure (SLS). The crack tip singularities at the weak interfaces due to the material discrepancy are characterized by the eigen-function expansion, and the unified enrichment function is constructed to evaluate the stress intensity factors (SIFs) directly without extra computations. The corresponding energy release rate (ERR) criteria are employed to determine the HF propagation direction. By the comprehensive analysis with various dip angles and material/structural properties, the propagation of HF moving across the layered heterogeneous shale is regarded as the interaction between adjacent weak interfaces. For the front weak interface, the inhibition effect could be seen in the fracture penetrating capacity by the rear interface, which reflects contrasting trends due to large material discrepancies of various weak interfaces. The propagation behavior at the rear weak interface is affected by the facilitation effect from the front interface, and the HF pre-offset at the front interface greatly enhances the facilitation effect on fracture penetrating through shale formations.

Study on the induced effect of bedding weakness in deep shale gas reservoir on hydraulic fractures propagation

Study on the induced effect of bedding weakness in deep shale gas reservoir on hydraulic fractures propagation

Effects of Geo-Stress and Natural Fracture Strength on Hydraulic Fracture Propagation in a Deep Fractured Tight Sandstone Reservoir.

Hydraulic fracturing technology has been widely used in the tight reservoir reconstruction. Unfortunately, with the deepening of mining depth and the increase of geo-stress, the propagation mechanism of medium-pressure fractures in the reservoir is significantly different from that of conventional shallow reservoirs. Based on the combined finite discrete element method, this paper conducts numerical simulation research on deep tight sandstone reservoirs in the west. The discrete fracture network modeling method is used to establish a tight sandstone reservoir model with natural bedding, and the influence of geo-stress difference and natural fracture strength on hydraulic fracture propagation law in a high geo-stress environment is discussed in detail. The results show that the difference between geo-stress and the strength of natural fractures has a significant effect on the shape and expansion of hydraulic fractures under the high geo-stress conditions. The greater the difference in ground stress, the more obvious the tendency of the main fractures of the reservoir, and the shorter the branch fractures. With the increase of natural fracture strength, the changes in propagation pressure, fracture length, area, and width, which can be fitted with a linear function with a goodness of fit as high as 0.99. In addition, the morphological results of hydraulic fractures in the simulation are not only affected by the constitutive parameters of the model but also may be affected by the randomness of the natural fracture network, thus, showing a certain degree of dispersion. Therefore, it is extremely necessary to build a reservoir fracturing model in a specific area based on more detailed geological monitoring data to guide actual construction. The above achievements have certain reference significance for the field operation of deep tight sandstone reservoirs.

Propagation and complex morphology of hydraulic fractures in lamellar shales based on finite-discrete element modeling

We explore the controls of stress magnitude and orientation relative to bedding on the resulting morphology/topology of hydraulic fractures using a combined finite-discrete element method (FDEM). Behavior is shown conditioned by the ratio of principal stresses λ=σ3/σ1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda ={\\sigma }_{3}/{\\sigma }_{1}$$\\end{document} and relative inclination of the bedding. When the lateral pressure coefficient (λ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda$$\\end{document}) is less than 0.67, hydraulic fractures predominantly initiate as tensile fractures along the wellbore, aligning with the maximum principal stress direction. Conversely, for λ≥0.67\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda \\ge 0.67$$\\end{document}, shear cracks are favored to initiate for the minor stress difference, leading to a less predictable initiation and extension direction. Simultaneously, diminished stress differences correspond to elevated reservoir breakdown pressures, displaying a linear correlation with lateral pressure coefficients and little influenced by equivalent bedding orientation. Bedding plane orientation significantly impacts the mode and morphology of hydraulic fracture propagation. Bedding parallel to the direction of the minimum principal stress (σ3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\sigma }_{3}$$\\end{document}) favors layer-penetrating and bifurcated fractures, whereas inclined bedding facilitates the emergence of numerous steering-type and capture-type fractures. Especially at steeper inclinations (β=60∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta =60^\\circ$$\\end{document}), hydraulic fractures readily extend along the bedding surface, inducing macroscopic shear slip failure. Under high-stress disparities, the breakdown pressure exhibits greater sensitivity to bedding inclination, and its influence pattern aligns with the variations in tensile strength, typically reaching maximum and minimum values at bedding inclination angles of 0° and 60°, respectively.

Simulation and optimization of unstable dynamic propagation of multiple fractures in the shale formation

Multi-cluster hydraulic fracturing of horizontal wells is a well-adopted technique with high efficiency to increase the production of tight and shale formations. However, the stress shadows among clusters pose challenges to the synchronous propagation of hydraulic fractures during multi-cluster fracturing. In order to explore the fracture propagation mechanism and characteristics under the influence of stress shadow in low-spacing staged multi-cluster fracturing, a three-dimensional hydraulic fracturing model was generated using a lattice-based method. This model considered the impact of geological and engineering parameters on the propagation behavior of multiple fractures in shale formation. A variable pumping approach is adopted, where the fracturing fluid is initially injected at a high rate and then transitioned to a lower rate. Afterward, a method was proposed to quantitatively assess the extent of fracturing in a specific area (i.e., the stimulated area), considering the impact of stress shadow within a single stage. The simulation results demonstrated significant differences in the fracture stimulation area due to the influence of each parameter in the case of uncontrollable geological factors and controllable engineering factors. An increase in both Young’s modulus and stress anisotropy of the reservoir results led to a corresponding increase in the total fracture stimulation area. As the principal stress orientation increased, the fracture stimulation area gradually decreased. In terms of operational parameters, the stimulated area of hydraulic fractures gradually decreased as the fracture spacing increased. With increasing injection rate, the stimulated area initially expanded and then decreased, peaking at an injection rate of 0.04 m3/s. These findings can provide valuable insights into the propagation behavior of multi-cluster hydraulic fractures under uncertain parameters, with significant implications for future engineering applications.

Investigation into hydraulic fracture propagation behavior during temporary plugging and diverting fracturing in deep coal seam

In this study, we conducted indoor temporary plugging and diverting fracturing (TPDF) experiments on samples of natural deep and shallow coal seams using a small-scale true triaxial fracturing simulation system. By integrating high-precision CT scanning technology and crack reconstruction techniques, and relying on the quantitative characterization of the crack complexity coefficient, we investigated the differences in internal structures between deep and shallow coal seam samples and the variations in the expansion of TPDF cracks. Furthermore, the study primarily focused on exploring the influence of temporary plugging agent (TPA) parameters (quantity and particle size) on the expansion patterns of TPDF cracks in deep coal seam coal-rock samples. The experimental results reveal that in shallow coal samples, artificially induced fractures are notably longer and extend to various surfaces of the samples. Conversely, in deep coal samples, the expansion of artificially induced fractures is influenced by well-developed cleavage and cleats. The crack complexity coefficient of artificial fractures in deep coal samples is 1.54 higher than that in shallow coal samples, with an increased crack width of 98 μm. However, the expansion distance of the fractures is shorter and does not extend to the S3 and S6 surfaces. Increasing the dosage of the TPA is advantageous for inducing frequent redirection of fractures and communication with previously untouched areas. When the dosage of the TPA is increased from 30 g/L to 60 g/L, the expansion distance of artificial fractures significantly increases, extending to various surfaces of the samples. The crack complexity coefficient increases by 1.1, and the crack width enlarges by 84 μm. Appropriate particle sizes of the TPA can effectively form seals. However, small particle sizes (200/400 mesh) struggle to seal initial fractures, while large particle sizes (10/20 mesh) can lead to excessive wellhead blockage, causing rapid shut-in pressure, destructive fracturing in deep coal samples, and the generation of a large amount of coal fines. This study holds significant guidance for the optimal selection of process parameters in the temporary plugging hydraulic fracturing of deep coal rock.

Anisotropic Mechanical Behaviors of Shale Rock and Their Relation to Hydraulic Fracturing in a Shale Reservoir: A Review

Shale gas is an important supplement to the supply of natural gas resources and plays an important role on the world’s energy stage. The efficient implementation of hydraulic fracturing is the key issue in the exploration and exploitation of shale gas. The existence of bedding structure results in a distinct anisotropy of shale rock formation. The anisotropic behaviors of shale rock have important impacts on wellbore stability, hydraulic fracture propagation, and the formation of complex fracture networks. This paper briefly reviews previous work on the anisotropic mechanical properties of shale rock and their relation to hydraulic fracturing in shale reservoirs. In this paper, the research status of work addressing the lithological characteristics of shale rock is summarized first, particularly work considering the mineral constituent, which determines its physical and mechanical behavior in essence. Then the anisotropic physical and mechanical properties of shale specimens, including ultrasonic anisotropy, mechanical behavior under uniaxial and triaxial compression tests, and tensile property under the Brazilian test, are summarized, and the state of the literature on fracture toughness anisotropy is discussed. The concerns of anisotropic mechanical behavior under laboratory tests are emphasized in this paper, particularly the evaluation of shale brittleness based on mechanical characteristics, which is discussed in detail. Finally, further concerns such as the effects of bedding plane on hydraulic fracturing failure strength, crack propagation, and failure pattern are also drawn out. This review study will provide a better understanding of current research findings on the anisotropic mechanical properties of shale rock, which can provide insight into the shale anisotropy related to the fracture propagation of hydraulic fracturing in shale reservoirs.

Numerical simulation of the simultaneous propagation of multiple hydraulic fractures based on expanded finite element method

Hydraulic fracturing technology with horizontal wells is a highly efficient approach to stimulate the unconventional reservoir by creating lots of fractures to increase the oil and gas recovery rate. However, some field monitoring data indicate that multiple fractures could not propagate uniformly due to stress interference, which leads to a lower stimulation volume and production capacity. Therefore, investigating the stress interference mechanism between multiple fractures is important to obtain a higher production capacity for unconventional reservoirs. In this study, a two-dimensional XFEM (expanded finite element method) model was established to investigate the influence of stress interference on the fracture propagation of multiple fractures in horizontal wells. The different injection parameters, completion parameters, and fracturing patterns were investigated by sixteen simulation cases. The results show that first the influencing degree of stress interference from different parameters is fracture spacing, injection flow rate, number of fractures, and fluid viscosity, respectively. Second, compared to the simultaneous fracturing and sequential fracturing, the zip fracturing pattern (sides fractures created first and then mid-fracture) could create more uniform fractures in the reservoir. Third, a small fracture spacing and a higher injection flow rate could lead to a faster stress interference, but the fluid viscosity and the number of fractures have little effect on the occur timing of stress interference. This study could provide some meaningful perspective on the field fracturing design in horizontal wells.

Experimental study of hydraulic fracture propagation with multi-cluster in-plane perforations in horizontal wells

Tri-axial fracturing studies were carried out to understand the impact of lateral mechanical parameters on fracture propagation from multiple in-plane perforations in horizontal wells. Additionally, the discussion covered the effects of geology, treatment, and perforation characteristics on the non-planar propagation behavior. According to experimental findings, two parallel transverse fractures can be successfully initiated from in-plane perforation clusters in the horizontal well because of the in-plane perforation, the guide nonuniform fishbone structure fracture propagation still can be exhibited. The emergence of transverse fractures and axial fractures combined as complex fractures under low horizontal principal stress difference and large pump rate conditions. The injection pressure was also investigated, and the largest breakdown pressure can be also found for samples under these conditions. The increase in perforation number or decrease in the cluster spacing could provide more chances to increase the complexity of the target stimulated zone, thus affecting the pressure fluctuation. In a contrast, the increase in fracturing fluid viscosity can reduce the multiple fracture complexity. The fracture propagation is significantly affected by the change in the rock mechanical properties. The fracture geometry in the high brittle zone seems to be complicated and tends to induce fracture reorientation from the weak-brittle zone. The stress shadow effect can be used to explain the fracture attraction, branch, connection, and repulsion in the multiple perforation clusters for the horizontal well. The increase in the rock heterogeneity can enhance the stress shadow effect, resulting in more complex fracture geometry. In addition, the variable density perforation and temporary plugging fracturing were also conducted, demonstrating higher likelihood for non-uniform multiple fracture propagation. Thus, to increase the perforation efficiency along the horizontal well, it’s necessary to consider the lateral fracability of the horizontal well on target formation.

Fracture propagation morphology and parameter optimization design of pre-CO2 hybrid fracturing in shale oil reservoirs, Ordos Basin

Shale reservoirs typically exhibit the characteristics of large burial depth, low permeability, and well-developed bedding. Despite the implementation of water-based fracturing techniques with high displacement, attaining the optimal reservoir stimulation effect remains a formidable challenge. Consequently, in recent years, a novel method of hybrid fracturing method using pre-CO2 + sand-carrying slick water has been proposed. Within this framework, the meticulous design of fracturing operation parameters stands out as a pivotal factor influencing the efficacy of reservoir stimulation. This paper presents a coupled 3-D hydraulic fracture propagation model, investigating the influence of injection displacement, injection volume, and three-stage pre-injection fluid volume ratio on the fracture propagation. Two quantitative evaluation metrics, namely fracture surface area S and fracture transverse plane fractal dimension D, are precisely defined. Additionally, real-time analysis of bottom-hole pressure is conducted to further comprehend the dynamics of the process. Numerical investigations have revealed that both pre-CO2 injection displacement and volume exert a substantial influence on fracture half-length and complexity. Furthermore, the proportion of hybrid fracturing fluid demonstrates a notable impact on fracture height and complexity. The design of injection parameters does not exhibit a significant effect on the formation breakdown pressure. This study quantitatively evaluates fracture propagation under different working conditions from the perspective of fracture morphology, and serve as practical guidance for optimizing the process parameters design for hybrid fracturing in shale oil reservoirs, Ordos Basin.

Numerical study of multiple hydraulic fractures propagation in poroelastic media based on energy decomposition phase field methods

This paper proposes a novel phase-filed model for simulating hydraulic fracturing in poroelastic media under complex stress conditions. The main theoretical contribution lies in the fact that a generalized strain energy density decomposition is incorporated to hydraulic fracturing phase field model, which unifies hydrostatic-deviatoric and spectral decompositions and predicts material failure through classical Mohr-Coulomb failure criterion. Based on Biot’s poroelastic theory, an indicator function is used to distinguish between fluid flow in fractures and in pores. The fully coupled equations are solved by combining staggered schemes with Newton-Raphson methods. The proposed model is validated by comparing laboratory experimental results with numerical results. In order to demonstrate the influence of introducing hydrostatic-deviatoric and spectral energy decompositions, we simulated propagation of multiple hydraulic fractures and coalescence of hydraulic fractures with natural fractures. Finally, the effects of stress ratios (σx/σy) on multiple hydraulic fractures are analysed. It is found that when the stress ratio is 2, a more complex hydraulic fracture network is formed in the porous media.

Hydraulic fracture initiation and propagation in deep coalbed methane reservoirs considering weak plane: CT scan testing

Hydraulic fracturing technology in coal reservoirs helps increase coalbed methane (CBM) production, and the development of hydraulic fractures (HFs) is influenced by in situ stress and original fractures. Therefore, in this study, triaxial HF tests and computed tomography (CT) were used to determine HF initiation and propagation in coal, considering weak structural planes and in situ stress. The experimental results show that weak structural planes control HF initiation and propagation in coal under a deep-stress environment. Most HFs occurred along bedding planes, followed by exogenetic fractures. The influence of the horizontal stress difference coefficient k, which is equal to the maximum horizontal stress minus the minimum horizontal stress and divided by the minimum horizontal stress, on the propagation of the HF was studied. When k increases to 1.04, the HF becomes perpendicular to the direction of the minimum principal stress. When k < 1.04, the main controlling factor of HF propagation is a weak structural plane, and when k ≥ 1.04, the main controlling factor of HF propagation is in situ stress. Based on the CT reconstruction model, the average aperture distributions of the discrete bedding fractures and exogenetic fractures of coal in this experiment were calculated to be 0.0947 mm and 0.1940 mm, respectively. This determines the different strength properties of exogenetic fractures and bedding planes, and determines the priority initiation position of HF in coal. During the HF initiation and propagation, the average energy along the exogenous fracture was lower than that along the bedding planes. On this basis, this study predicts that the main factor controlling HF propagation in a deep coal seam in the Qin-Shui Basin, China, is the weak structural plane in the coal seam.

Numerical analysis of 3D nonplanar hydraulic fracture propagation in fractured-vuggy formations using a hydromechanical coupled XFEM approach

In this paper, a hydromechanical coupled XFEM approach is developed for simulating 3D nonplanar hydraulic fracturing, fracture–vug interactions and natural crack intersection problems. The extended finite element method associated with a high-order tip enrichment function is used for describing rock deformation, and the finite element method is chosen for discretizing fluid flow at the fracture surface. Rock deformation and fluid flow are coupled and solved by the Picard iteration method. The hybrid explicit–implicit representation method is adopted to represent 3D nonplanar fracture surfaces, and the displacement correlation method is chosen for extracting the mixed-mode stress intensity factor. A new mixed-mode failure criterion is introduced for determining 3D nonplanar fracture growth. Analytical solutions and experimental investigations are employed to verify the validity and effectiveness of our model. The simulations of fracture–vug interactions show that fracture surface growth on the side without vugs is constrained, while vugs can aggravate the tortuousness of the fracture surface on the side with vugs. In addition, four new intersection patterns (penetration, partial arrest, partial diversion, full arrest, and vertical propagation) are found in the 3D nonplanar fracture intersection simulations. The numerical examples highlight the considerable impacts of vertical growth and the mixed-mode effect on hydraulic fracturing.

Experimental Investigation into the Process of Hydraulic Fracture Propagation and the Response of Acoustic Emissions in Fracture–Cavity Carbonate Reservoirs

Fracture–cavity carbonate reservoirs account for a considerable proportion of oil and gas resources. Because of the complicated relationships between cavities, fractures and pores in these reservoirs, which are defined as cavity clusters, fracturing technology is employed to enhance their hydrocarbon productivity. However, almost all previous studies have just considered the effect of a single natural cavity or fracture on the propagation of a hydraulic fracture; therefore, the mechanism by which a hydraulic fracture interacts with a cavity cluster needs to be clarified. In this study, cavity clusters with different distributions were accurately prefabricated in synthetically made samples, and large-scale simulation equipment was employed to systematically perform fracturing experiments considering different horizontal differential stress levels. Meanwhile, the hydraulic fracture propagation behaviors were comprehensively analyzed through fracture morphology, fracturing curves, the complexity of the fracture network and acoustic emission monitoring. It was found that a natural fracture with a smaller approach angle is favorable in guiding a hydraulic fracture to a cavity. The fracturing curves were divided into the following four types: frequent fluctuations with “step-like” shapes, great fluctuations with slightly lower closure pressure, fluctuations with obviously lower closure pressure, and little fluctuations with obviously lower closure pressure. And different cavity cluster distributions play a dominant role in the complexity of generated hydraulic fracture networks. In addition, AE energy was used to judge the ease of crossing the cavity. The above findings indicated that for the actual exploration and exploitation of carbonate reservoirs, the geological exploration of different fracture–cavity structures in reservoirs would be required, and targeted fracturing engineering designs need to be carried out for different fracture–cavity carbonate reservoirs.