Fracture Propagation Behavior Research Articles

Analysis of Fracturing Expansion Law of Shale Reservoir by Supercritical CO2 Fracturing and Mechanism Revealing

The rapid expansion of reservoir fractures and the enlargement of the area affected by working fluids can be accomplished solely through fracturing operations of oilfield working fluids in geological reservoirs. Supercritical CO2 is regarded as an ideal medium for shale reservoir fracturing owing to the inherent advantages of environmental friendliness, excellent capacity, and high stability. However, CO2 gas channeling and complex propagation of fractures in shale reservoirs hindered the commercialization of Supercritical CO2 fracturing technology. Herein, a simulation method for Supercritical CO2 fracturing based on cohesive force units is proposed to investigate the crack propagation behavior of CO2 fracturing technology under different construction parameters. Furthermore, the shale fracture propagation mechanism of Supercritical CO2 fracturing fluid is elucidated. The results indicated that the propagation ability of reservoir fractures and Mises stress are influenced by the fracturing fluid viscosity, fracturing azimuth angle, and reservoir conditions (temperature and pressure). An azimuth angle of 30° can achieve a maximum Mises stress of 3.213 × 107 Pa and a crack width of 1.669 × 10−2 m. However, an apparent viscosity of 14 × 10−6 Pa·s results in a crack width of only 2.227 × 10−2 m and a maximum Mises stress of 4.459 × 107 Pa. Additionally, a weaker fracture propagation ability and reduced Mises stress are exhibited at the fracturing fluid injection rate. As a straightforward model to synergistically investigate the fracture propagation behavior of shale reservoirs, this work provides new insights and strategies for the efficient extraction of shale reservoirs.

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.

Cracking behavior of brittle materials under eccentric decoupled charge blasting

The decoupled charge structure is often used in controlled blasting. However, in actual engineering scenarios, boreholes are typically horizontal or inclined. The explosive is close to the borehole wall because of gravity action, resulting in an eccentric charge structure. In this study, concentric and eccentric decoupled charge blasting tests were conducted on 250 mm × 250 mm × 150 mm cuboid concrete specimens. The blast-induced fracture propagation behavior was evaluated using digital image correlation (DIC) technology, and surface fracture networks were extracted by image processing to investigate the fractal characteristics. A three-dimensional finite element model was established and verified using experimental data to reveal the influencing mechanism of the coupling medium and eccentricity coefficient on concrete fractures from the energy perspective. Finally, the borehole wall pressure (BWP) and displacement distribution with different charge structures and coupling media are analyzed theoretically, and the application of an eccentric charge structure in smooth blasting is discussed. The results reveal that nonequivalent BWP and displacement are produced in eccentric charge blasting, with a more evident crushed zone appearing on the coupled side owing to the higher energy utilization efficiency of the explosive. Sand had the largest fracture volume fraction ratio between the coupled and decoupled sides, effectively controlling the damage level on the decoupled side. The proposed eccentric decoupled charge-blasting scheme produced directional damage effects in the field test, maximizing the failure of the excavation rock while reducing the damage level to the reserved rock.

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.

In-situ laboratory study on influencing factors of pre-SC-CO2 hybrid fracturing effect in shale oil reservoirs

Supercritical CO2 (SC-CO2) fracturing, being a waterless fracturing technology, has garnered increasing attention in the shale oil reservoir exploitation industry. Recently, a novel pre-SC-CO2 hybrid fracturing method has been proposed, which combines the advantages of SC-CO2 fracturing and hydraulic fracturing. However, the specific impacts of different pre-SC-CO2 injection conditions on the physical parameters, mechanical properties, and crack propagation behavior of shale reservoirs remain unclear. In this study, we utilize a newly developed "pre-SC-CO2 injection → water-based fracturing" integrated experimental device. Through experimentation under in-situ conditions, the impact of pre-SC-CO2 injection displacement and volume on the shale mineral composition, mechanical parameters, and fracture propagation behavior are investigated. The findings of the study demonstrate that the pre-injection SC-CO2 leads to a reduction in clay and carbonate mineral content, while increasing the quartz content. The correlation between quartz content and SC-CO2 injection volume is positive, while a negative correlation is observed with injection displacement. The elastic modulus and compressive strength exhibit a declining trend, while Poisson's ratio shows an increasing trend. The weakening of shale mechanics caused by pre-injection of SC-CO2 is positively correlated with the injection displacement and volume. Additionally, pre-injection of SC-CO2 enhances the plastic deformation behavior of shale, and its breakdown pressure is 16.6% lower than that of hydraulic fracturing. The breakdown pressure demonstrates a non-linear downward trend with the gradual increase of pre-SC-CO2 injection parameters. Unlike hydraulic fracturing, which typically generates primary fractures along the direction of the maximum principal stress, pre-SC-CO2 hybrid fracturing leads to a more complex fracture network. With increasing pre-SC-CO2 injection displacement, intersecting double Y-shaped complex fractures are formed along the vertical axis. On the other hand, increasing the injection rate generates secondary fractures along the direction of non-principal stress. The insights gained from this study are valuable for guiding the design of pre-SC-CO2 hybrid fracturing in shale oil reservoirs.

Mode I fracture propagation and post-peak behavior of sandstone: Insight from AE and DIC observation

Microcracks generated at the crack tip, particularly in quasi-brittle materials such as sandstone, give rise to a non-linear region known as the fracture process zone (FPZ), which cannot be analyzed using linear elastic fracture mechanics (LEFM). To gain insights into the nonlinear fracturing behavior of rock, this study conducted three-point bending experiments on sandstone beams with prefabricated notch. Stable mode-Ⅰ fracture propagation during the post-peak stage was achieved by the monotonic increment of crack mouth opening displacement (CMOD) measured by a clip gauge. The acoustic emission (AE) and digital image correlation (DIC) techniques were utilized to visually track the detailed crack propagation process from both micro and macro perspectives. Nonlinear fracture parameters, including FPZ length, were identified by AE and DIC measurements and compared with the theoretical calculation. A nonlinear fracture criterion relating applied load, equivalent crack length, and stress intensity factor were established, from which the theoretical CMOD-P curve at the post-peak stage were obtained and validated with experimental measurement from MTS machine. The results reveal that microcracks formulating the FPZ materialize in the pre-peak stage, with the majority emerging post-peak to facilitate the propagation of the traction-free crack and FPZ in the loading direction, as evidenced by AE locations and DIC displacement gradient. Throughout the crack propagation process, over 65 % of microcracks are estimated to be tensile mode based on both AF/RA and polarity analysis, aligning with anticipated outcomes from macroscopic three-point bending fracture tests. Energy dissipation primarily occurs within the FPZ to facilitate separation of crack faces, with the dissipated energy region advancing towards the beam boundary. By integrating AE source locations and DIC displacement gradient, the FPZ length of the 3-point-bending sandstone specimens is determined to be lp = 8.1 mm, slightly exceeding the theoretical FPZ lengths calculated by the Irwin model (lp = 7.0 mm) and the Dugdale-Barenblatt model (lp = 7.6 mm). Using a non-linear fracture criterion that incorporates the FPZ and employing the in-time equivalent crack length measured by AE and DIC as an intermediate variable, the theoretical applied load-crack length curve during the post-peak stage is calculated, showing close agreement with load cell measurements with a negligible error of approximately 2 %. Furthermore, the theoretical post-peak CMOD vs. load curve aligns with experimentally obtained results, demonstrating a non-linear crack propagation speed at a constant CMOD rate, wherein crack propagation speed decreases as crack length increases.

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.

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.

Influence of non-intersecting cemented natural fractures on hydraulic fracture propagation behavior

Fractures in deeply buried or formerly deeply buried rocks commonly contain mineral deposits having varying degrees of strength. Understanding how cement deposits in fractures interact with hydraulic fractures (HFs) is important for hydraulic fracture design for practical engineering applications such as enhanced geothermal fluid circulation or oil and gas production. Using discrete element methods, we describe a fully fluid-solid coupling numerical model simulating hydraulic fracture propagation to investigate the influence of cemented natural fractures (CNFs) that do not intersect the predicted path of hydraulic fractures on HF propagation morphology. The fracture morphology depicted by the numerical model aligns well with analytical solutions, indicating the reliability of the simulation results. Using this model, we delved into the effects of fracture cement strength and position, fracturing fluid viscosity, and induced stress field on the interactive propagation patterns of fractures. The simulation results indicate that the HF is captured by weakly non-intersecting CNF, causing the activation of the CNF and the development of cracks primarily caused by tensile failure. As the HF approaches the CNF, the induced tensile stress region at the fracture tip offsets toward the CNF, resulting in fracture reorientation. When the cementing strength ratio between pre-existing natural fracture and reservoir matrix is less than 0.5, it is beneficial to activate CNF and enhance reservoir permeability. The seepage of low-viscosity fracturing fluid into the reservoir significantly enhances the ability of the HF and non-intersecting CNF to communicate with each other, thereby providing favorable pathways for oil and gas flow. The research findings offer valuable insights into fracture network connectivity and permeability enhancement, contributing to enhanced reservoir stimulation strategies.

Sensitivity analyses of random cave groups on karst tunnel stability based on water–rock interaction using a novel contact dynamic method

This paper concentrates on the sensitivity and dynamic simulation of randomly distributed karst cave groups on tunnel stability and connectivity extended ratio based on water–rock interaction using a novel contact dynamic method (CDM). The concept of karst cave group connectivity extended ratio during tunneling and water inrush is proposed. The effects of cave shape and spatial distribution on Qiyueshan tunnel are investigated. Tunnel deformation and damage index, and connectivity extended ratio with uniform random karst cave groups are evaluated. The results demonstrate that the connectivity extended ratio is verified as a crucial judgment in predicting the safe distance and assessing the stability of the tunnel with the karst cave group. CDM model captures the fracture propagation and contact behavior of rock mass, surface flow, as well as the bidirectional water–rock interaction during the water inrush of Qiyueshan tunnel with multiple caves. A larger cave radius and smaller minimum distance between the cave and tunnel increase the deformation and damage index of the surrounding rock. When the cave radius and cave area ratio increase, the failure pattern shifts from overall to local failure. These findings potentially have broad applications in various surface and subsurface scenarios involving water–rock interactions.

Determination of the Thickness and Fracture Toughness of Shale Interfacial Transition Zone Near Various Micromineral Aggregates Using Nano-Scratch Tests

Summary Heterogeneously distributed micromineral aggregates (MMAs) are common in shale reservoirs. The interfaces between these MMAs and the surrounding minerals significantly affect fracture propagation behaviors during shale oil exploitation. In this paper, the concept of interfacial transition zone (ITZ) between the MMAs (e.g., striped barite, pyrite, calcite, apatite blocks, and bedding plane) and the surrounding mineral in shale is introduced. Due to the small thickness of the ITZ, its thickness and mechanical properties are very difficult to determine by traditional standard methods. To address this issue, this paper proposes a method combining nano-scratch tests, scanning electron microscopy (SEM), and quantitative evaluation of minerals by SEM (QEMSCAN) techniques to investigate the thickness and fracture toughness of the ITZ near typical MMAs. The results show that the thickness of the ITZ determined by the transverse force FT and scratch depth (-d2) varies from 3.2 μm to 17.3 μm. In addition, the fracture toughness of both MMAs and ITZs is characterized by high heterogeneity ranging from 0.1 MPa·m0.5 to 2.1 MPa·m0.5. Moreover, a formula evaluating the fracture toughness of the ITZ is proposed taking into account the type and content of minerals in the ITZ. A strong linear relationship is observed between the thickness of the ITZ and the average fracture toughness ratio. In addition, a relationship is established between the fracture toughness of the ITZ and MMAs, and an evaluation equation is derived. This study is helpful for understanding the characteristics (i.e., mainly thickness and fracture toughness) of the ITZ within shales.

Fracture analysis of chopped carbon fiber sheet molding compound composite under tensile loading via in-situ μXCT

This paper investigated the crack initiation in a chopped carbon fiber sheet molding compound (SMC) composite under different tensile loads by integrating synchrotron micro-X-ray computed tomography (μXCT) and micromechanics analysis. It was found that the varied distribution of fiber chips in the SMC composite led to distinct fracture propagation behaviors. With the loading increasing, the crack primarily occurred within the fiber chips, and the delamination exited when the crack propagated along the interface between adjacent layers. To understand the location and sequence of crack initiation in SMC composites, the sample were divided into identical cubes. After tracking the microstructure of each cube, the corresponding interfacial debonding strength as well as Young's modulus were predicted. Predictive modeling of crack initiation characteristics was conducted assuming homogeneous material deformation under small strain. The predicted outcomes closely matched with the experimental findings.

Fracture initiation and propagation behaviors of thinly interbedded tight sandstone reservoirs by various hydraulic fracturing methods

This paper proposed three new fracturing methods, slick water with an integrated variable viscosity fracturing, supercritical CO2 shock fracturing and pulse hydraulic fracturing. To verify the efficacy of the new methods, we conducted laboratory true-triaxial fracturing experiments by using outcrops collected from the Lower Shihezi formation, Ordos Basin. The results indicate that supercritical CO2 shock fracturing is observed to have the lowest breakdown pressure compared with other methods.

Laboratory study on acid fracturing performance in high temperature carbonate reservoirs

In this article, we investigate acid fracturing, a promising method to increase the effectiveness of fractures in high temperature carbonate reservoirs. In order to determine the performance of acid fracturing in carbonate reservoirs, we conducted a lab scaled true triaxial acid fracturing experiment on high temperature carbonate rock. The fracture initiation, propagation behavior, and conductivity under the action of acid solution were analyzed. The 3-D micro computed tomography was applied to illuminate the fracture network patterns; coefficient of stress difference, acid etching time, and acid concentration were also analyzed. Results show acidification exhibits the lowest breakdown pressure compared with no-acidification, reduced by 58%.

Main Control Factors of Fracture Propagation in Reservoir: A Review.

The fracture distribution and internal control factors after the fracturing of unconventional oil and gas reservoirs determine the reservoir reforming effect to a large extent. Based on the research of global scholars on the influencing factors of fracture propagation, comprehensive theoretical model, and numerical simulation, this Review systematically discusses the influence of internal geological factors and external engineering factors of unconventional oil and gas reservoir on fracture propagation behavior and summarizes the current problems and development trends in fracture research. The results show the following: (1) The fracture propagation is a comprehensive process constrained by lithology and mineral composition, water saturation, nonhomogeneity, natural weak surface, and ground stress. (2) External engineering factors have a meaningful control effect on fracture propagation; the type and temperature of fracturing fluids can also change the mechanical properties of different rocks, thus affecting the fracture propagation pattern. (3) The existing fracture propagation models have certain limitations, and their computational reliability still needs to be further verified. (4) Numerical simulation can break through the limitations of physical simulation, but different simulation methods have different shortcomings and applicability. In the future, we should focus on: (1) finding parameters to quantitatively characterize heterogeneity at the 3D level, which is an important direction to study the effect of heterogeneity on fracture propagation; (2) introducing computerized methods to establish a geological model that considers multiple factors and combining it with numerical simulation software to study fracture propagation; (3) considering the characteristics of fluid-liquid-solid phase comprehensively, establishing a suitable THL coupling equation; (4) how the interaction mode of fracturing fracture is combined with the natural fracture geometry, and how the fracture is affected by fracturing engineering parameters such as fluid injection rate and viscosity of fracturing fluid; and (5) geology-engineering dynamic integration, which is an important direction to be carried out in the future.

Study of mixed-mode fracture in functionally graded material using an adaptive phase-field fracture model

This work systematically investigates the mixed-mode fracture propagation in functionally graded materials. The crack trajectory and load–displacement responses were effectively simulated using the mixed-mode phase field approach in combination with an adaptive mesh refinement technique. The mesh refinement is carried out using quadtree decompositions, and the elements with hanging nodes are handled as n−sided polygons. The adaptive implementation is validated by solving benchmark problems from the literature. The adaptive technique demonstrates greater computational efficiency by significantly reducing the number of elements without compromising the accuracy of the solution. This study provides insight into how fracture propagation behavior is influenced by gradient profile, material gradation, crack location, energy release rate, and displacement configuration in uni-directional and bi-directional functionally graded material. Remarkably, it is found that the peak load on the load–displacement graph changes noticeably, and the fracture path remains constant despite variations in the critical energy release rate. Notably, the results show that materials with lower fracture toughness have an early failure when studying a center fractured plate. This thorough examination deepens our understanding of the propagation of mixed-mode cracks and provides insight into improving the analysis and design methodologies for functionally graded materials.

The influences of perforating phase and bedding planes on the fracture deflection in laminated shale

The perforating phase leads to complex and diverse hydraulic fracture propagation behaviors in laminated shale formations. In this paper, a 2D high-speed imaging scheme which can capture the interaction between perforating phase and natural shale bedding planes was proposed. The phase field method was used to simulate the same conditions as in the experiment for verification and hydraulic fracture propagation mechanism under the competition of perforating phase and bedding planes was discussed. The results indicate that the bedding planes appear to be no influence on fracture propagation while the perforating phase is perpendicular to the bedding planes, and the fracture propagates along the perforating phase without deflection. When the perforating phase algins with the bedding planes, the fracture initiation pressure reserves the lowest value, and no deflection occurs during fracture propagation. When the perforating phase is the angle 45°, 60° and 75° of bedding planes, the bedding planes begin to play a key role on the fracture deflection. The maximum deflection degree is reached at the perforating phase of 75°. Numerical simulation provides evidence that the existence of shale bedding planes is not exactly equivalent to anisotropy for fracture propagation and the difference of mechanical properties between different shale layers is the fundamental reason for fracture deflection. The findings help to understand the intrinsic characteristics of shale and provide a theoretical basis for the optimization design of field perforation parameters.

Adaptive phase-field modeling of fracture propagation in bi-layered materials

We study fracture propagation in bi-layered materials using an adaptive phase-field method. The method combines a phase-field representation of fracture with spatial adaptivity to provide mesh refinement in the smeared zone of damage, where higher accuracy is needed to resolve the solution. We place a particular emphasis on studying the competition between crack arrest, deflection, and penetration at the interface of two dissimilar materials. To explore this competition, we perform tension tests on various models with different combinations of material and geometric properties. We begin by validating the adaptive phase-field model through a uniaxial tension test. The material mismatch and locations of the initial damage for this example are chosen such that the fracture propagation behavior is identical to that in a homogeneous material. Next, we independently vary the mismatches in elastic stiffness and critical energy release rate to study their influence on crack growth. Our observations show that both mismatches contribute to an effective toughening of the model, but in different ways. Critical energy release rate mismatch causes crack deflection along the interfaces, while elastic stiffness mismatch results in retardation of crack growth. We also note that for a specific combination of material mismatch and location of initial damage, crack nucleation in the adjacent material is preferred over propagation from the existing notch. Finally, through models with inclined interfaces, we demonstrate the effect of the relative orientation between the approaching crack and the material interface on crack penetration, deflection, and nucleation. The numerical results from this study provide valuable insight into the various failure mechanisms in bi-layered materials.

Characteristics of hydraulic fracture penetration behavior in tight oil with multi-layer reservoirs

The tight oil reservoirs of Fuyang and Gaotaizi in the Daqing Oilfield exhibit characteristics such as thin layers and distributed barriers, which pose challenges for maintaining drilling accuracy within the desired geological sweet spot. Hydraulic fracturing has emerged as the optimal method to connect production wells with target layers effectively. Understanding the behavior of hydraulic fracture propagation in multiple thin layers with significant differences in mechanical properties is crucial for assessing the extent of effective drainage. In this study, a finite element numerical simulation model incorporating cohesive elements is employed to investigate and quantify the effects of stress differences and rock mechanical parameters on fracture penetration. The results indicate that the stress difference between the reservoir and barrier layers plays a key role in controlling hydraulic fracture penetration and height. Moreover, the flow rate and viscosity of the fracturing fluid have a noticeable impact on fracture progression. Hydraulic fractures tend to propagate along the interface of layers with low flow rate and fluid viscosity, thereby hampering the penetration performance in those layers. To achieve a comparable penetrated fracture length in low permeability reservoirs, it is necessary to increase the volume of fracturing fluid in high permeability reservoirs. Furthermore, the barriers layers, which possess relatively weaker mechanical properties, can constrain the width development of hydraulic fractures. This limitation may result in the risk of excessive liquid without sufficient proppant (commonly known as “over liquid but no sand”). Therefore, appropriate measures should be implemented to increase the net pressure and avoid encountering this complex situation.

Experimental investigation on fracture growth for integrated hydraulic fracturing in multiple gas bearing formations

The key to extracting multiple natural gases from coalbed methane, shale and other tight gases through integrated hydraulic fracturing in coal measure strata (CMS) is determining whether hydraulic fracture can connect different gas-bearing zones. In this paper, several groups of combination samples comprised of artificial rocks were prepared. True triaxial hydraulic fracturing experiments were conducted to stimulate the hydraulic fracture propagation and penetration behavior. The effects of in-situ stress, injection rate, natural fracture and well type on fracture growth were studied. The main results were as follows: (1) Fracture geometries in CMS exhibited strong asymmetric, nonplanar extension and unilateral expansion features in the vertical plane. (2) Higher values of the vertical stress difference coefficient and injection rate led to larger vertical fracture extension and an increased likelihood of fracture connection to adjacent gas-bearing layers. (3) The behavior of the hydraulic fracture varied significantly between vertical and horizontal wells. In vertical wells, the main fracture presented a "+” shape and fully communicated natural coal cleats system, while in horizontal wells, fracture height growth was limited, resulting in fracture patterns that presented vertically as “H” or “T” shape due to the hydraulic fracture's tendency to turn direction when propagating to natural weak planes. The study found that the fracture vertical extension ability increased with rising vertical stress difference coefficient. (4) Natural fractures could induce fracture turning and branching, which improved fracture network complexity but also caused fracturing fluid loss and hydraulic energy dissipation. Therefore, choosing shale or coal layer with developed natural fractures as fracturing layer was unhelpful for integrated fracturing operation. Finally, higher fracturing fluid viscosity, injection rate and gradient sanding could enhance fracturing operation efficiency. These results could provide theoretical guidance for integrated fracturing operation in CMS.