Hydraulic Fracture Propagation Research Articles

Research on the influence of rock fracture toughness of layered formations on the hydraulic fracture propagation at the initial stage

Deep underground rocks exhibit significant layered heterogeneity due to geological evolution and sedimentation. Rock fracture toughness, as one of the important indicators of hydraulic crack propagation, also exhibits heterogeneous distribution. In order to investigate the influence of non-uniform fracture toughness of layered rocks on hydraulic crack propagation, this paper establishes a planar three-dimensional hydraulic crack propagation model. The model is numerically solved using the 3D displacement discontinuity method (3D-DDM) and the finite difference method. The calculation results indicate that when the distribution of the fracture toughness of layered rocks changes from uniform to non-uniform, the fracture morphology develops from a standard circular crack to an elliptical crack. When the difference of the rock fracture toughness between adjacent rock layers and the middle rock layer (pay zone) is large enough, the fracture morphology will develop towards a rectangular shape. In addition, when the fracture toughness of rock layers is non-uniformly distributed, the hydraulic crack not only rapidly expand in the softening layer (rock layer with lower fracture toughness), but also slowly propagate in the strong layer (rock layer with higher fracture toughness). However, the propagation speed in the softening layer is much faster than that in the strong layer. The results indicate that the heterogeneity of rock fracture toughness has an important impact on the morphology, propagation speed, and direction of hydraulic fractures.

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.

Numerical insight into hydraulic fracture propagation in hot dry rock with complex natural fracture networks via fluid-solid coupling grain-based modeling

Abundant natural fractures (NFs) in hot dry rock (HDR) reservoirs serve as a crucial element in hydraulic fracturing technology and reservoir production. Nevertheless, the interaction mechanism between hydraulic fractures (HFs) and natural fracture networks (NFNs) remains poorly understood. This study combines the novel hydro-grain-based model (hydro-GBM) and discrete fracture network (DFN) model to investigate the effects of in-situ stress, fracture number, and fracture length on hydraulic fracturing behaviors in mineral-scale granite with NFNs. Results indicate that as the complexity of NFs increases, the stress shadow effect is amplified during HF propagation, while the spatial arrangement of NFs governs the propagation of secondary paths. Additionally, a downward trend is observed in average activity levels of acoustic emission (AE) events and proportions of large events within NFs and matrix. Damage and activation degrees in both NFs and matrix decrease with an increase in the number of NFs. The inability of main hydraulic paths to propagate over long distances can be attributed to the dispersion of fracturing fluid and the rapid decay of pressure within densely distributed, short NFs. These numerical findings shed light on the process of HFs activating NFNs and provide valuable insights for enhancing heat extraction efficiency in HDR reservoirs.

Study on Composite Fracture Characteristics and Hydraulic Fracturing Behavior of Hard Rock

To investigate the influence of non-singular terms (T stress) in the stress field on the composite fractures of hard rock Type I–II, such as rock splitting failure and hydraulic fracture propagation, this study focused on hard rocks in metallic mines. Through splitting tests and hydraulic fracturing experiments, the impact of T stress on the characteristics of Type I–II composite fractures in hard rocks was analyzed. Utilizing the generalized maximum tangential (GMTS) stress criterion considering T stress, the stress intensity factors of hard rock Type I–II composite fractures with different pre-existing crack angles were predicted. The critical fracture pressure expression for hard rocks was derived based on the maximum tangential stress (MTS) criterion. The results indicate that the GMTS criterion, considering T stress, is more suitable for describing the characteristics of Type I–II composite fractures under rock-splitting loads. However, under hydraulic fracturing, T stress has a minor influence on the fracture characteristics of hard rock hydraulic fractures. Therefore, when predicting the critical fracture pressure of hydraulic fractures, T stress can be disregarded. This study provides a scientific basis and guidance for hard rock hydraulic fracturing engineering.

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.

Hydraulic fracturing phase-field model in porous viscoelastic media

The mechanical behavior of shale exhibits pronounced time-dependent nonlinearity. Existing phase-field models are inadequate for evaluating the fluid-driven fracture propagation process in porous viscoelastic media. In this study, a new viscoelastic phase-field model for fluid-driven fracture propagation was introduced based on a thermodynamically consistent framework using the Maxwell–Wiechert model to describe viscoelastic constitutive behavior, with the bulk and shear moduli expanded using the Prony series. Volumetric deviatoric splits were used for both elastic and viscous energies. The fluid flow in both the fracture and the matrix adhered to Darcy's law. Additionally, the phase-field driving force considers the combined influence of rock elastic, viscous, and fluid energies. The numerical calculation iteration format was constructed using finite element discretization and the Newton–Raphson (NR) iterative method. A staggered iteration algorithm was applied to address the displacement, pressure, and phase-field problems. This method not only accurately simulates mechanical cyclic loading but also models the extension of multiple hydraulic fractures in homogeneous reservoirs and hydraulic fracture propagation in fractured reservoirs.

A hydro-thermo-damage-mechanical fully coupled cohesive phase-field model for fracking in quasi-brittle thermo-poroelastic media

A hydro-thermo-damage-mechanical fully coupled cohesive phase-field method is developed for modelling of fracking in quasi-brittle thermo-poroelastic media. A new sigmoid interpolation function is proposed to characterize the transitional behaviour from porous media to cracks for permeability, specific heat capacity, and thermal conductivity. A fluid continuity equation with the Darcy-Poiseuille law and an energy conservation equation with the Fourier’s law are then used to simulate fluid flow and heat transfer in cracking porous media, respectively. The fluid pressure and temperature of fluids and solids are fully incorporated into the governing equation of the phase-field regularized cohesive zone model to efficiently simulate complicated quasi-brittle hydro-thermal fracking, without the need of remeshing, crack tracking or auxiliary fields. The resultant displacement–pressure-temperature-damage coupled multiphysics system of equations is solved using a staggered Newton–Raphson iterative algorithm within the finite element framework. The method is first validated by five two-dimensional thermally or hydro-thermally induced fracture examples with analytical solutions, experimental data, and published numerical solutions. Three-dimensional fracking and fluid circulation problems in a vertical five-spot wellbore system with complex natural fracture networks, are then simulated as an application to energy harvesting in enhanced geothermal systems (EGS). It is found that the present method is capable of accurately and robustly modelling complicated hydraulic fracture propagation and fluid circulation problems and has the potential to be used for fracking design and optimization of EGS.

Numerical study of hydraulic fractures propagation in deep fracture-cavity reservoir based on continuous damage theory

Natural fractures and cavities are the primary spaces for oil and gas accumulation in fracture-cavity carbonate reservoirs. Establishing the connection between these spaces and the wellbore through hydraulic fracturing treatment is important for oil and gas extraction from such reservoirs. Due to the discontinuity and heterogeneity of the existing natural fracture-cavity system, anticipating the viability of hydraulic fracturing treatment is troublesome. A new method to simulate the hydraulic fracturing propagation in fracture-cavity reservoirs is proposed based on the continuous damage theory. The method considers the random spatial distribution of fractures and cavities and can simulate the arbitrary expansion of hydraulic fractures in the three-dimensional direction. Based on this method, the influence of different geological and engineering factors on the propagation patterns of hydraulic fractures in the fracture-cavity reservoirs is investigated. It is found that the increase of reservoir burial depth significantly limits the propagation ranges of hydraulic fractures. The propagation modes of hydraulic fractures encountering natural fractures change with increasing burial depth, undergoing a transition from “penetrate and deflect” to ”defect” and then to ”penetrate”. The reduction of horizontal stress difference increases the complexity of hydraulic fractures, but it is not conducive for hydraulic fractures to connect more natural fractures and cavities. The increase in fracturing pump rate is significantly beneficial for hydraulic fractures to connect more natural fractures and cavities. The viscosity of fracturing fluid has a significant impact on the morphology of hydraulic fracture propagation, which undergoes a transition from simple to complex, and then to simple with the change of the fracturing fluid viscosity from low to high. either too high or too low viscosity of the fracturing fluid is not conducive to the connection of more natural fractures and cavities by hydraulic fractures. The obtained conclusions can provide a reference for the design of hydraulic fracturing treatment for fracture-cavity carbonate reservoirs.

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.

Development of CASRock for modeling multi-fracture interactions in rocks under hydro-mechanical conditions

The interaction between multiple fractures is important in the analysis of rock fracture propagation, fracture network evolution and stability and integrity of rocks under hydro-mechanical (HM) coupling conditions. At present, modeling the mechanical behavior of multiple fractures is still challenging. Under the condition of multiple fractures, the opening, closing, sliding, propagation and penetration of fractures become more complicated. In order to simulate the HM coupling behavior of multi-fracture system, the paper presents a novel numerical scheme, including mesh reconstruction and topology generation algorithm, to efficiently and accurately represent fractures and their propagation process, and a potential function-based algorithm to address contact problem. The fracture contact algorithm does not need to set contact pairs and thus is suitable for complex contact situations from small to large deformations induced by HM loading. The topology of fracture interfaces is constructed by the dynamic adding algorithm, which makes the mesh reconstruction more rapid in the modeling of fracturing process, especially in the case of multiple fractures intersections. The numerical scheme is implemented in CASRock, a self-developed numerical code, to simulate the propagation process of rock fractures and the interaction of multiple fractures under the condition of HM coupling. To verify the suitability of the code, a series of tests were performed. The code was then applied to simulate hydraulic fracture propagation and fracture interactions caused by fluid injection. The ability of this method to study fracture propagation, multi-fracture interaction and fracture network evolution under hydro-mechanical coupling conditions is demonstrated.

Research on fracture propagation of hydraulic fracturing in a fractured shale reservoir using a novel CDEM-based coupled HM model

Shale reservoirs are usually buried deep, and contain lots of complex natural fractures (NFs) under the influence of tectonic stress and faults. Previous studies usually focus on small-scale reservoir models and there is also a lack of research that completely focuses on NF characteristics. Using the continuous-discontinuous element method (CDEM), a Hydro-Mechanical (HM) coupling model was constructed to explore the propagation mechanism of hydraulic fracture (HF) and the effects of NF characteristics on HF propagation. The results show that as the HF approaches the NFs, the NFs preferentially tend to undergo shear failure, resulting in a decelerated HF propagation rate and an increased injection pressure. After water enters the NFs, the HF rapidly propagates, so the fracture area swiftly increases and the HF has a more tortuous fracture morphology. The NF orientation (α) and the internal friction angle (φ) exhibit a pronounced and distinct effect on HF propagation. Specifically, when α is 30°∼60°, the HF propagation is more tortuous and complex, so it is easier to form a larger transverse reconstruction range. When the NF density (η) is no less than 0.12, the HF tends to open the NFs and form a more tortuous morphology, but a higher η does not necessarily imply a higher fracture complexity. A longer NF (l > 12 m) benefits to improve the longitudinal reservoir reconstruction scope, but has little impact on the overall reconstruction effect. In the presence of stress shadow effect, it is improbable for the HF to completely open the intersecting NFs; instead, it typically opens only a segment or one of them. Consequently, it may be challenging to establish a fracture network with the simultaneous development of primary and branching fractures. Additionally, some water leakage into the inactive NFs gives rise to a decline in both the fracture length and fracture area. The findings can offer theoretical insights for refining the fracturing design in shale reservoirs with NFs.

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

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

Variations in the present-day stress field and its implications for hydrocarbon development in the Hassi Terfa field, Hassi Messaoud, Algeria

Understanding the contemporary stress field and its variations is important in hydrocarbon production. These fluctuations are significant for borehole stability and trajectory, hydraulic stimulation, fault reactivation, and optimal well completion. In this study, we provide the first geomechanical model to determine the orientation and magnitude of the present-day stress of the Hassi Terfa field using geophysical logs to understand the origin of the spatial variation of the stress state. The breakouts from acoustic image logs provide an average maximum horizontal stress (SHmax) orientation of N115°, consistent with the Africa–Eurasia plate motion, which imposes the largest first-order stress source. However, our analysis demonstrates local stress perturbations near faults. This may mean that the stress field in the Hassi Terfa is superimposed by a third-order source of stress (<100 km). Our model demonstrates variations in stress magnitude with depth interpreted to be lithology-dependent, which act as a natural barrier against fracture propagation. We propose a new approach to quantify these perturbations via the stress regime index R′, using the magnitude of the principal stress components. The results demonstrate different layer-to-layer faulting regimes. These perturbations will limit the vertical propagation of hydraulic fracturing to the overlying formation, which enables it to access a larger reservoir volume and improve production. In general, a strike–slip regime with a slight normal slip component is interpreted for the study area. We develop the first-ever quantitative stress map using the relative principal stress magnitude parameter (Aϕ) to present the lateral variation of the stress regime throughout the Saharan platform. Results demonstrate a consistent strike–slip to normal/strike–slip regime with a uniform SHmax orientation. This map will help operators achieve optimal well direction and optimize hydraulic stimulation. Our results have considerable implications for hydrocarbon production in the central and southeastern Saharan platform.

How does the heterogeneous interface influence hydraulic fracturing?

Under the influence of the nonlinear fluid-solid coupling, hydraulic fracture exhibits various propagation modes (such as toughness- or viscosity-dominated), which stem from the competition between the solid deformation and fluid flow. Based on the homogeneous assumption, the basic theoretical analysis has divided toughness and viscosity scales in the tip region. However, regarding more realistic and complex geological conditions, like layered heterogeneity, knowledge of fluid-driven fracture propagation is still unclear. This work establishes a theoretical model and solving approach to reveal the multiscale asymptotic behavior during hydraulic fracture passing through the heterogeneous interface (i.e., discontinuous elastic properties). The influence of material discontinuity, regarded as the remote force, in the near-tip, intermediate, and far-field scales is analyzed by the asymptotic analysis and validated by the numerical solutions. Notably, solutions at the intermediate scale manifest as individual feature owing to the heterogeneity: as the crack in front of the interface, just a specific transition solution governed by the property and position of the interface appears; once the crack tip passes the interface, the interface-governed transition solution and interface solution occur simultaneously and interact as the crack tip moves away from the interface. Such multiscale property results in the interface-governed fluid-solid interaction in the tip region, and finally leads to changes in interface failure and propagation mode. On the one hand, the criterion for interface failure should be modified by simultaneously incorporating the heterogeneity of solid domain and multiscale nature of tip solution, especially for viscosity- or interface-dominated propagation regimes. On the other hand, the propagation mode in a heterogeneous domain is controlled by two characteristics: traditional l/lmk for toughness-viscosity competition associated with c/Lμ for material discontinuity effect proposed in the present work. These insights provide the theoretical foundation for modeling hydraulic fracture propagation in layered heterogeneous domains.

Numerical simulation of hydraulic fracture propagation after thermal shock in shale reservoir

The scale of propagation of hydraulic fractures in deep shale is closely related to the effect of stimulation. In general, the most common means of revealing hydraulic fracture propagation rules are laboratory hydraulic fracture physical simulation experiments and numerical simulation. However, the former is difficult to meet the real shale reservoir environment, and the latter research focuses mostly on fracturing technology and the interaction mechanism between hydraulic fractures and natural fractures, both of which do not consider the influence of temperature effect on hydraulic fracture propagation. In this paper, the hydraulic fracturing process is divided into two stages (thermal shock and hydraulic fracture propagation). Based on the cohesive zone method, a coupled simulation method for sequential fracturing of deep shale is proposed. The effects of different temperatures, thermal shock rates, and times on the scale of thermal fractures are analyzed. As well as the effects of horizontal stress difference and pumping displacement on the propagation rule of hydraulic fractures. The results show that the temperature difference and the thermal shock times determine the size and density of thermal fractures in the surrounding rock of the borehole, and the number of thermal fractures increases by 96.5% with the increase of temperature difference. Thermal fractures dominate the initiation direction and propagation path of hydraulic fractures. The main hydraulic fracture width can be increased by 150% and the length can be increased by 46.3% by increasing the displacement; the secondary fracture length can be increased by 148.7% by increasing the thermal shock times. This study can provide some inspiration for the development of deep shale by improving the complexity of hydraulic fractures.

Effects of elastic anisotropic models on the prediction of horizontal stresses and hydraulic fracture geometry

Abstract Stress barriers play a key role in the propagation of hydraulic fractures. They are local maxima in the stress field that constrain vertical fracture propagation. The development of stress barriers is influenced by rock mechanical properties, pore pressure and tectonic stresses. However, stress prediction models are highly sensitive to available geophysical measurements and assumptions made on rock constitutive models. We compare stress estimations performed with elastic isotropic, anisotropic and viscoplastic models using Thomsen's notation ( ɛ , δ , γ ) to quantify anisotropy and its effects on hydraulic fracture geometry. Using a single depth for principal stress calibration, we compare stress distributions and simulate hydraulic fracture geometries along simple vertical and horizontal well sections. Prediction errors stemming from isotropic models along anisotropic intervals increase when tectonic stresses increase. Errors generated by either over- or underestimation of δ increase for tectonically passive environments, while errors generated by either over- or underestimation of ɛ increase for tectonically active environments. Additional corrections, but also uncertainties, can be introduced by considering viscoelastic rock behaviour. Because of stress shadowing and fracture interaction, the risk of stress barrier underestimation is higher when estimating hydraulic fracture geometries along the various stages of horizontal wells.

Experimental Investigation of Fracture Propagation in Clayey Silt Hydrate-Bearing Sediments

More than 90% of the natural gas hydrate resources are reserved as marine clayey silt sediments. It is of great significance to efficiently develop a clayey silt hydrate. At present, there are problems of low single well production and small depressurization range in its production test, which is still a long way from commercial exploitation. The combination of hydraulic fracturing technology and other methods such as depressurization method is regarded as one of the potential technical means to achieve the commercial exploitation of the hydrate. However, compared with shale gas reservoirs and coalbed methane reservoirs, clayey silt hydrate reservoirs have special mechanical properties, resulting in unique hydraulic fracturing processes. Therefore, it is necessary to study the fracture initiation and propagation laws of clayey silt hydrate reservoirs. To this end, we carried out large-scale (30 × 30 × 30 cm) true triaxial hydraulic fracturing experiments using a simulated material with similar mechanics, porosity, and permeability to clayey silt hydrate-bearing sediments. The effects of completion method, fracturing method, and fracturing fluid displacement on hydraulic fracture propagation of clayey silt hydrate-bearing sediments were studied. The results showed that a perforated completion can significantly increase the fracture reconstruction area and decrease the fracture initiation pressure compared to an open hole completion. Due to the small horizontal stress difference, it is feasible to carry out temporary plugging fracturing in clayey silt hydrate reservoirs. Temporary plugging fracturing can form steering fractures and significantly improve fracture complexity and fracture area. Increasing the fracturing fluid displacement can significantly increase the fracture area as well. When conducting fracturing in clayey silt hydrate-bearing sediments, the fracturing fluid filtration area is obviously larger than the fracture propagation area. Therefore, it is recommended to use a high-viscosity fracturing fluid to reduce the filtration of the fracturing fluid and improve the fracturing fluid efficiency. This study preliminarily explores the feasibility of temporary plugging fracturing in clayey silt hydrate reservoirs and analyzes the effect of completion methods on the propagation of fracturing fractures, which can provide a reference for the research conducted on the fracturing stimulation of clayey silt hydrate reservoirs.

2D Phase-Field Modelling of Hydraulic Fracturing Affected by Cemented Natural Fractures Embedded in Saturated Poroelastic Rocks

AbstractThe interaction between a propagating hydraulic fracture (HF) and a pre-existing natural fracture (NF) embedded in saturated poroelastic rock formations is studied numerically in 2D plane–strain configurations. In this study, the phase-field method is further developed to be employed for modelling the HF propagation and the evolution of tensile and shear failure in geo-materials as gradient-type diffusive damaged zones. The shear slippage and dilation mechanisms inside the cemented NF are modelled using a Mohr–Coulomb–Griffith failure criterion that fitted in the framework of the phase-field fracture using appropriate energy functionals. The most important factors controlling the HF–NF interaction outcome are the approaching angle, differential in-situ stress, and hydro-mechanical characteristics of the NF. It is found out that higher tensile and shear strengths of the cemented NF are in favour of the crossing outcome when the differential stress is high enough to mobilise the resisting shear stresses against the slippage. Small hydraulic aperture (low hydraulic conductivity) for the NF is also in favour of the crossing outcome which helps to restrict the pressurised region local to the HF tip, lowering the possibility of shear slippage in the NF and the HF’s diversion. It is also concluded that the injection rate and the viscosity of fracturing fluid are operative factors to be adjusted for increasing the chance of crossing, a critical element for successful operation of hydraulic fracturing for effective use of subsurface energy resources.