Hydraulic Fracture Propagation Model Research Articles

Distinct element modeling of hydraulic fracture propagation with discrete fracture network at Gonghe enhanced geothermal system site, northwest China

Accurate prediction of hydraulic fracture propagation is vital for Enhanced Geothermal System (EGS) design. We study the first hydraulic fracturing job at the GR1 well in the Gonghe Basin using field data, where the overall direction of hydraulic fractures does not show a delineated shape parallel to the maximum principal stress orientation. A field-scale numerical model based on the distinct element method is set up to carry out a fully coupled hydromechanical simulation, with the explicit representation of natural fractures via the discrete fracture network (DFN) approach. The effects of injection parameters and in situ stress on hydraulic fracture patterns are then quantitatively assessed. The study reveals that shear-induced deformation primarily governs the fracturing morphology in the GR1 well, driven by smaller injection rates and viscosities that promote massive activation of natural fractures, ultimately dominating the direction of hydraulic fracturing. Furthermore, the increase of in situ differential stress may promote shear damage of natural fracture surfaces, with the exact influence pattern depending on the combination of specific discontinuity properties and in situ stress state. Finally, we provide recommendations for EGS fracturing based on the influence characteristics of multiple parameters. This study can serve as an effective basis and reference for the design and optimization of EGS in the Gonghe basin and other sites.

Effects of orthogonal cleat structures on hydraulic fracture evolution behavior

Hydraulic fracturing operation as an effective enhancing coalbed gas production method is widely used in ultra-low permeability coal seam. However, complex geo-stresses and high heterogeneity between natural cleats structure lead to difficulty predicting hydraulic fracture patterns. Fracture evolution behavior for fracturing operation in coal seams requires a better understanding. In this study, a 2D model of hydraulic fracture propagation was built based on the cohesive zone model of finite element method. The effect of orthogonal cleat system, in-situ stress, dig angle and construction parameters on fracture geometries were main investigated. The main conclusions were as follows: (1) According to the interaction types between hydraulic fracture and cleat system, ladder-shaped fracture and H-shaped fracture geometry was summarized. The difference between them was whether there were continuous and small pressure fluctuation stages. (2) When the horizontal stress difference coefficient was lower and larger than 3/12, fracture geometry was prone to present Η shape and ladder shape respectively. Besides, the dimensionless fracture length and the dimensionless fracture extension aspect ratio of fracture were increasing with larger horizontal stress difference coefficient. (3) The favorable condition for fracture extension was that the dig angle was 45°. Hydraulic fracture tended to propagate along face cleats direction. (4) Larger fracture fluid displacement was beneficial to form more balanced hydraulic fracture geometry and promote large extension scale. As fracture fluid viscosity increased, the fracture geometries transformed from ladder shape to H shape.

Numerical simulation of multiple hydraulic fracture propagation in heterogeneous coal reservoirs based on combined finite-discrete element method

Multi-stage fracturing in Horizontal well increases the permeability of coalbed methane by generating multiple fractures. However, the heterogeneity of coal reservoirs is a crucial factor that cannot be ignored in the study of multiple hydraulic fracture propagation. Therefore, we established a two-dimensional model for multiple hydraulic fracture propagation based on the combined finite-discrete element method (FDEM) and assigned a Weibull distribution function to the heterogeneity of the physical parameters of the cohesive elements in the model. The objective was to simulate and study the fracture propagation law of multi-cluster fracturing in horizontal wells in heterogeneous coal reservoirs. The research results indicated that: 1) as the heterogeneity of the coal reservoir weakened, the deflection angle of the main fracture increased. More secondary fractures were generated in the coal reservoir, leading to significant discontinuity. 2) The fracturing disturbance area was always concentrated at the tip of the main fracture, with a double wing shape. However, the fracturing disturbance areas at the tips of multiple main fractures could easily converge, with a square shape; 3) It is recommended to use a moderate injection rate and increase the perforation spacing appropriately when hydraulic fracturing is carried out in coal reservoirs with a heterogeneity coefficient m=5.

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.

Geometric characteristics of diverting fractures for multi-stage dynamic temporary plugging and diverting fracturing in fractured reservoir

Temporary plugging and diverting fracturing (TPDF) technology constitutes a pivotal stimulation methodology in the advancement of unconventional resources, given its efficacy in establishing communication with natural fractures (NFs). Presently, the comprehension of TPDF is primarily confined to laboratory-scale experiments, with a notable dearth of accurate knowledge regarding the diverting law and geometric characteristics of diverting fractures at the field scale. This study introduces a hydro-mechanical-damage model for hydraulic fracture (HF) propagation, developed through the derivation and refinement of the phase field method. The model integrates the dynamic flow distribution process among multi-perforation clusters under stress interference, alongside the construction of a dynamic transportation and plugging model for the temporary plugging agent in TPDF. Subsequently, a coupled model encompassing multi-cluster and multi-stage dynamic TPDF is established using the finite element method. Utilizing this model, an exploration of the diverting fracture's opening and propagation mechanisms, as well as the impact of stimulation and reservoir factors on geometric characteristics, is conducted at the field scale. The results show that the TPDF model has high accuracy, and for the first time, it realistically simulates the gradual increase and decrease in fracturing pressure in the field after temporary plugging. The diverting fracture propagation is affected by the stress interference from the initial HF (ini HF), NF, and horizontal stress difference. The energy required to open NFs to form a diverting fracture is higher than that to cross NFs after temporary plugging, and there is a strong positive correlation between the energy rise and the approach angle and the horizontal stress difference. The use of multi-scale plugging agents can promote a more uniform distribution of diverting fractures. When the NF angle is near 60°, the diverting fracture has a larger fracture control volume. The NF distribution and the interaction between NF and HF are of great significance to the opening position of diverting fractures in TPDF. According to the propagation pattern of diverting fractures, they can be divided into five categories. The temporary plugging agent can alleviate the directional propagation of HFs caused by NF induction. The alleviating effect is not only controlled by the interaction mode between HF and NF but also related to the NF density. The propagation distance of the diverting fracture formed by the inferior fracture will be about 50% farther compared with the superior fracture. The diverting fractures are mainly interfered by the stress of the HFs in the same perforation cluster at the early stage and mainly interfered by the HFs of different perforation clusters at the middle and late stages.

Mechanisms of fracture propagation from multi-cluster using a phase field based HMD coupling model in fractured reservoir

Natural fractures (NFs) are common in shale and tight reservoirs, where staged multi-cluster fracturing of horizontal wells is a prevalent technique for reservoir stimulation. While NFs and stress interference are recognized as significant factors affecting hydraulic fracture (HF) propagation, the combined influence of these factors remains poorly understood. To address this knowledge gap, a novel coupled hydro-mechanical-damage (HMD) model based on the phase field method is developed to investigate the propagation of multi-cluster HFs in fractured reservoirs. The comprehensive energy functional and control functions are established, while incorporating dynamic fluid distribution between multiple perforation clusters and refined changes in rock mechanical parameters during hydraulic fracturing. The HMD coupled multi-cluster HF propagation model investigates various scenarios, including single HF and single NF, reservoir heterogeneity, single HF and NF clusters, and multi-cluster HFs with NF clusters. The results show that the HMD coupling model can accurately capture the impact of approach angle (θ), stress difference and cementation strength on the interaction of HF and NF. The criterion of the open and cross zones is not fixed. The NF angle (α) is not a decisive parameter to discriminate the interaction. According to the relationship between approach angle (θ) and NF angle (α), the contact relationship of HF can be divided into three categories (θ=α, θ<α, and θ>α). The connected NF can increase the complexity of HF by inducing it to form branch fracture, resulting in a fractal dimension of HF as high as 2.1280 at angles of ±45°. Inter-fracture interference from the heel to the toe of HF shows the phenomenon of no, strong and weak interference. Interestingly, under the influence of NFs, distant HFs from the injection can become dominant fractures. However, as α gradually increases, inter-fracture stress interference becomes the primary factor influencing HF propagation, gradually superseding the dominance of NF induced fractures.

Study on rock fracture mechanism and hydraulic fracturing propagation law of heterogeneous tight sandstone reservoir.

Hydraulic fracturing technology is an effective way to develop tight sandstone reservoirs with low porosity and permeability. The tight sandstone reservoir is heterogeneous and the heterogeneity characteristics has an important influence on fracture propagation. To investigate hydraulic fracture performance in heterogeneous tight reservoir, the X-ray diffraction experiments are carried out, the Weibull distribution method and finite element method are applied to establish the uniaxial compression model and the hydraulic fracture propagation model of heterogeneous tight sandstone. Meanwhile, the sensitivity of different heterogeneity characterization factors and the multi-fracture propagation mechanism during hydraulic fracture propagation is analyzed. The results indicate that the pressure transfer in the heterogeneous reservoir is non-uniform, showing a multi-point initiation fracture mode. For different heterogeneity characterization factors, the heterogeneity characteristics based on elastic modulus are the most sensitive. The multi-fracture propagation of heterogeneous tight sandstone reservoir is different from that of homogeneous reservoir, the fracture propagation morphology is more complex. With the increase of stress difference, the fracture propagation length increases. With the increase of injection rate, the fracture propagation length increases. With the increase of cluster spacing, the propagation length of multiple fractures tends to propagate evenly. This study clarifies the influence of heterogeneity on fracture propagation and provides some guidance for fracturing optimization of tight sandstone reservoirs.

Fracture propagation laws of staged hydraulic fracture in fractured geothermal reservoir based on phase field model

Hydraulic fracturing is widely used in geothermal resource exploitation, and many natural fractures exist in hot dry rock reservoirs due to in-situ stress and faults. However, the influence of natural fractures on hydraulic fracture propagation is not considered in the current study. In this paper, based on the phase field model, a thermo-hydro-mechanical coupled hydraulic fracture propagation model was established to reveal the influence of injection time, fracturing method, injection flow rate, and natural fracture distribution on the fracture propagation mechanism. The results show that fracture complexity increases with an increase in injection time. The stress disturbance causes the fracture initiation pressure of the second cluster significantly higher than that of the first and third clusters. The zipper-type fracturing method can reduce the degree of stress disturbance and increase fracture complexity by 7.2% compared to simultaneous hydraulic fracturing. Both low and high injection flow rate lead to a decrease in fracture propagation time, which is not conducive to an increase in fracture complexity. An increase in the natural fracture angle leads to hydraulic fracture crossing natural fracture, but has a lesser effect on fracture complexity. In this paper, we analyzed the influence of different factors on initiation pressure and fracture complexity, providing valuable guidance for the exploitation of geothermal resources.

Criteria for discriminating layer-penetration fracturing considering pressure drop and interfaces

Accurately predicting fracture height growth in a stratified reservoir presents a significant challenge. In this study, we have developed a semi-analytical prediction model for fracture height growth that accounts for pressure drop effects and considers the influence of bedding interfaces, all based on the principles of equilibrium height theory. The methodology involves solving intricate nonlinear equations and generating fracture height profiles, systematically investigating the effects of various factors: in-situ stress, fracture toughness, fluid density, and perforation location on fracture height. Sensitivity analysis of the model has revealed findings: (1) The presence of a stress barrier leads to a substantial increase in the induced stress required for fracture height growth. In particular, the stress barrier inhibits growth on one side while promoting it on the opposite side. (2) The influence of the fracture toughness barrier on fracture height growth, while significant, is overshadowed by that of the stress barrier. An increase in the fracture toughness of the rock decelerates the rate of fracture height growth but simultaneously raises the required induced stress for layer penetration. (3) Increasing the density of the fracturing fluid exerts a dual effect, slowing the growth rate of the upper fracture tip while accelerating the advancement of the lower fracture tip. Higher fluid density, under constant perforating pressure, results in a noticeable reduction in overall fracture height. (4) Fracture growth from the low-stress layer to the high-stress layer required an increase in induced stress. The research findings not only provide essential data on fracture height profiles, enhancing the applicability of the pseudo-3D hydraulic fracture propagation model, but also contribute to the refinement of governing equations within hydraulic fracture modeling.

A phase‐field model for hydraulic fracture nucleation and propagation in porous media

AbstractMany geo‐engineering applications, for example, enhanced geothermal systems, rely on hydraulic fracturing to enhance the permeability of natural formations and allow for sufficient fluid circulation. Over the past few decades, the phase‐field method has grown in popularity as a valid approach to modeling hydraulic fracturing because of the ease of handling complex fracture propagation geometries. However, existing phase‐field methods cannot appropriately capture nucleation of hydraulic fractures because their formulations are solely energy‐based and do not explicitly take into account the strength of the material. Thus, in this work, we propose a novel phase‐field formulation for hydraulic fracturing with the main goal of modeling fracture nucleation in porous media, for example, rocks. Built on the variational formulation of previous phase‐field methods, the proposed model incorporates the material strength envelope for hydraulic fracture nucleation through two important steps: (i) an external driving force term, included in the damage evolution equation, that accounts for the material strength; (ii) a properly designed damage function that defines the fluid pressure contribution on the crack driving force. The comparison of numerical results for two‐dimensional test cases with existing analytical solutions demonstrates that the proposed phase‐field model can accurately model both nucleation and propagation of hydraulic fractures. Additionally, we present the simulation of hydraulic fracturing in a three‐dimensional domain with various stress conditions to demonstrate the applicability of the method to realistic scenarios.

Induced hydraulic fractures in underground block caving mines using an extended finite element method

Block caving has become an attractive underground rock mining method that requires effective preconditioning of the orebody to initiate the progressive rock collapse by its weight. Hydraulic fracturing is commonly used in cave mining as a preconditioning method to induce artificial fractures in the orebody rock mass. This paper presents the numerical model of hydraulic fracture propagation in underground caving mines. An extended finite element method based on phantom nodes is adopted to simulate the propagation of multiple fractures along arbitrary paths. A poroelastic constitutive model governs the behavior of orebody rock mass, while a cohesive zone model controls the hydraulic fracture propagation. The adopted method is validated against analytical solutions for penny-shaped hydraulic fracture. The stress shadowing effect on hydraulic fracturing is investigated using representative rock properties from the “El Teniente mine”, obtained by calibrating a hydraulic fracturing field test. Several analyses of multiple hydraulic fractures considering a sequential fracturing scheme are simulated to investigate the influence of in-situ stresses, fracture spacing, injection flow rate, and Young's modulus and permeability of the matrix. The numerical results show that small fracture spacing, in-situ stress, and rock permeability locally change the principal stress magnitude, resulting in a more extensive fracture propagation. Also, the injection of new fractures induces stress shadows that reinitiate the propagation of the previous ones. This behavior is reduced by increasing the fracture spacing and rock permeability. Finally, this paper provides insights into the main parameters that affect fracture extension, which can be used to optimize the preconditioning process of underground block caving mines.

Hydraulic Fracture Propagation and Proppant Transport Mechanism in Interlayered Reservoir

Hydraulic fracture is crucial for assuring well production from unconventional reservoirs. For the optimization of hydraulic fracture geometry and the ensuing production of an interlayered reservoir, vertical hydraulic fracture propagation path has been analyzed. However, an effective fluid channel cannot be formed if the proppant is unable to reach the area where the fracture propagates. This paper presents a numerical model using the lattice-based method to investigate the hydraulic fracture propagation and proppant transport mechanism in interlayered reservoirs. The hydraulic fracture propagation model was simulated under different geological and fracturing engineering factors. The results indicate that interlayer Young’s modulus and horizontal stress anisotropy are positively correlated with longitudinal propagation and proppant carrying ability in interlayered formations. The fracturing injection rate has an optimal solution for fracture propagation and proppant carrying since a too low injection rate is unfavorable for fracture penetration of the interlayer, while a too high injection rate increases fracture width instead of further fracture penetration. In conclusion, attention is drawn to fine particle size proppants used in multi-layer reservoirs for fracturing fluid to carry proppants as far as possible to obtain maximum propped area.

Novel phase field model of hydraulic fracture propagation in poroelastic media and numerical investigation of interaction between hydraulic fracture and natural fracture

Novel phase field model of hydraulic fracture propagation in poroelastic media and numerical investigation of interaction between hydraulic fracture and natural fracture

Numerical investigation of poroelastic effects during hydraulic fracturing using XFEM combined with cohesive zone model

Hydraulic fracturing is an effective technology used to stimulate hydrocarbon production from unconventional reservoirs. Numerical simulation of the hydraulic fracturing process plays a key role in the design of hydraulic fracturing. However, most of existing models are overly simplified by neglecting the poroelasticity of rock matrix, which may have a substantial effect on the growth of hydraulic fractures, especially in oil and/or water saturated formations. In this study, we developed a fully coupled finite element model for hydraulic fracture propagation in permeable formations by combining XFEM technique with cohesive zone model, and used it to investigate the effects of poroelasticity on the geometry evolution of single fracture which initiated from an injection point. Fluid flow within the fractures, Darcy flow within the rock matrix, hydraulic fractures propagation, and fluid leak-off into the formation are simultaneously taking into account in the model. Then, the model is validated by comparing the results with available analytical solutions. To understand and quantify the poroelastic effects on the propagation of hydraulic fracture, several cases with different matrix permeability, leak-off coefficients, and bulk modulus of pore fluid are performed. The simulation results show that the total volume of leakage is controlled by the combined action of matrix permeability and leak-off coefficient. The fracture aperture and length decrease with the increase of matrix permeability or leak-off coefficient, while as the bulk moduli of pore fluid increases, the fracture aperture and length tend to increase.

Transverse penny-shaped hydraulic fracture propagation in naturally-layered rocks under stress boundaries: A 3D phase field modeling

The ground (tectonic) stress and layered structures of rocks are the among the main factors that influences the hydraulic fracturing behavior. The purpose of this study is to examine the suitability of the phase field model (PFM) in simulating transverse penny-shaped hydraulic fracture propagation and to investigate the phase field feature for the transverse penny-shaped hydraulic fracture in layered rocks under stress boundaries. A phase field model for 3D transverse penny-shaped hydraulic fracture propagation in naturally-layered rocks considering the effect of initial stress field is established. The mathematical model is based on Biot poroelasticity theory and the phase field fracture theory, while the governing equations are solved by using the finite element method in a staggered manner. The established PFM is validated experimentally and analytically by 2D and 3D examples. At last, the influences of the initial stress field, stiffness contrast and inclination angle of the layer interface on the penny-shaped fracture evolution in naturally-layered rocks are investigated by using the PFM. The study indicates that the phase field model has excellent feasibility and practicability in predicting penny-shaped hydraulic fractures. The layer inclination has nearly no effect on the penny-shaped fracture evolution in naturally-layered rocks. The stress ratio Sv/Sh on the boundaries has a significant effect on propagation of the penny-shaped hydraulic fracture. With the increase in Sv/Sh, the hydraulic fracture deflects and propagates along the direction of the maximum in-situ stress. For a high Sv/Sh, branching scenarios can be observed. The stiffness contrast of the rock layers determines whether the penny-shaped hydraulic fracture can penetrate into the adjacent layer. The predictions on the effects of the initial stress field, stiffness contrast and inclination angle of the layer interface provide new understanding of penny-shaped hydraulic fracture propagation in underground geological environment.

Study on hydraulic fracture extension of volcanic formation in J230 area, Xinjiang

It is key of “network fracturing” stimulation to correctly recognize the rock failure pattern during hydraulic fracturing treatment. In this paper, basic characteristics of the Carboniferous volcanic reservoirs in J230 area of Xinjiang oilfield are overviewed. Triaxial in-situ stresses of the formation are interpreted by conventional well-logging data, and the natural fissure characteristics are interpreted by imaging well logging information, and intersection of geometry relationship between hydraulic fracture and cross natural cracks is described on approaching angle. Current experimental achievements of hydraulic fracture penetration cross or extension along natural fissure are summarized, mechanics criterion under the different action manner are established for opening and shear failure of natural fractures. Taking Carboniferous volcanic reservoir of J230 area as an example, a hydraulic fracture propagation model is obtained by calculation results under the action of hydraulic-natural cracks synergy. It is shown that the approaching angle in the formation is mainly 50°–60°, shear sliding failure will be firstly induced along natural-hydraulic fracture interface and then natural fissure will open because of going net pressure. With further increase of the net fluid pressure, hydraulic fracture propagated along the natural fissure because of tangential failure and gradually turns to the direction of maximum horizontal principal stress, which is conducive to the formation of network fractures in stimulated reservoir treatment.

Investigation of the Vertical Propagation Pattern of the 3D Hydraulic Fracture under the Influence of Interlayer Heterogeneity

The low permeability and thinly interbedded reservoirs have poor physical properties and strong interbedded heterogeneity, and it is difficult to control the hydraulic fracture (HF) height and width during hydraulic fracturing, which affects the effect of HF penetration and sand addition. In this work, a three-dimensional fluid–solid fully coupled HF propagation model is established to simulate the influence of interlayer heterogeneity on vertical HF height and HF width, and the relationship between HF length and HF width under different treatment parameters is further studied. The results show that, in thin interbedded strata, the high interlayer stress contrast, high tensile strength, and low Young’s modulus will inhibit the vertical propagation of HFs. The interlayer heterogeneity results in the vertical wavy distribution of HF width. Under the high interlayer stress contrast, Young’s modulus, and tensile strength, the HF width profile becomes narrow and the variation amplitude decreases. The HF length decreases and the HF width increases as the injection rate and fracturing fluid viscosity increase. This study is of great significance for clarifying the vertical propagation pattern in thinly interbedded reservoirs, optimizing the treatment parameters, and improving the effect of cross fracturing and proppant distribution.

Simulation and regulation of fractures asynchronous initiation-propagation for deep shale gas horizontal well fracturing

The intensive cluster fracturing technology is widely applied in deep shale reservoirs. However, field data show that the fracture initiation time and propagation length of each cluster are different, which causes significant non-uniformity and affects the fracturing results. In this paper, a hydraulic fracture asynchronous initiation and propagation model for deep shale gas is established based on mechanics theory, which can consider the influence of in-situ stress variation and stress interference effect on the fracture asynchronous initiation and propagation behavior. According to the model design parameters, deep shale hydraulic fractures are regulated and optimized in combination with the non-uniform flow-limiting perforation technology to achieve uniform fracture propagation. The results show that, the non-uniformity of each fracture is obvious using conventional uniform perforation. However, with non-uniform perforation, each cluster perforation number is adjusted, the difference of initiation time and each fracture propagation velocity is significantly reduced, and the length of all fractures tend to be basically the same. And the field monitoring data are in good agreement with the calculation results, which verifies the reliability of the model. This study can reduce design blindness of non-uniform flow-limiting perforation technique, and has an important theoretical guidance for the optimal design of fracturing.

A 3D peridynamic fluid–solid coupling damage model of hydraulic fracture propagation

AbstractPeridynamics provides a new model and method for describing material failure and fracture development. In this study, a three‐dimensional peridynamic fluid–solid coupling model of hydraulic fracture propagation was established. The model considered the effective stress and transverse isotropic characteristics of shale and was used to simulate the propagation of the hydraulic fracturing of bedding shale. The damage evolution influence on the fracture propagation behavior, as well as the interactions between the fractures and bedding, were investigated through a simulation. The damage evolution had discontinuous characteristics. In addition to the incomplete damage zone adjacent to the complete damage zone, there were nonadjacent and incomplete damage zones. The fracture and propagation of hydraulic fractures followed the same trends of damage evolution. Moreover, the simulation and related experimental results confirmed the existence of a large amount of mesodamage in the region far from the fracture, and the concept of “remote damage” in hydraulic fracturing was proposed. Finally, the damage evolution of the propagating fractures and bedding was simulated. The simulation showed that bedding hinders the propagation of main fractures in the height direction but promotes the complexity of fracture propagation. Overall, this study provides a new theoretical understanding and research ideas for an in‐depth understanding of hydraulic fracture geometry and the accurate calculation of Stimulated Reservoir Volume.

Numerical Simulation on Hydrofracture Propagation in Fractured-Vuggy Unconventional Reservoirs

The unconventional reservoirs such as carbonate formation develops complex and diverse storage space structures, and it is composed of large-scale cavity, dissolved vug, and fractures. The carbonate reservoir is highly heterogeneous. Acid fracturing of carbonate reservoir is completed through the complex mechanical mechanism of interaction between vug and hydraulic fracture (HF). We use the equivalent method of reducing the rock strength by acid etching and serious fluid leakoff during interaction of HF and vug to establish a finite element (FE) model of HF propagation during acid fracturing in the fractured-vuggy carbonate reservoir. The model considers the effect of serious fluid leakoff during interaction between HF and vug, mechanism of interaction between HFs and the fracture-vug system, and change in acid etching intensity. Then, we carry out numerical simulation on impacts of injection rate, fluid viscosity, leakoff behavior in fractures and vugs, and natural fracture (NF) approaching angle on HF propagation in acid fracturing and compare the characteristics of injection pressure, fracture pressure, and HF size. It is suggested that the acid fracturing treatment should be operated by increasing the acid solution viscosity to reduce fluid leakoff, injecting fracturing fluid and acid fluid alternatively, increasing injection rate, and injecting fibers and ceramics when small pressure drop occurs during the HF interacts with the fracture-vug. When a large pressure drop occurs, it is suggested that the middle-low viscosity acid be injected at a low rate to etch the carbonate rock and enhance the fracture conductivity. HF propagates under higher pressure when the NF approaching angle is smaller.