Search

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. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing, a production enhancement technique, is widely used in the development of unconventional oil and gas reservoirs. The formation of a complex network of hydraulic fractures that connect with natural fractures is crucial for hydraulic fracturing in unconventional reservoirs. However, the current understanding of vertical fracture propagation behavior under the influence of variations in the mechanical properties of interbedded rock is insufficient to meet the requirements for simulations of unconventional reservoirs under complex geological conditions. In this study, a three-dimensional discrete grid method was employed to establish a model of three-dimensional fracture propagation. Numerical simulations were conducted to investigate the vertical growth of fractures while considering the influence of bedding planes. The effects of formation factors (stress, Young's modulus) and bedding planes (cohesion, density) on the height growth of hydraulic fractures were explored. The results indicated that the interbedded stress contrast, Young's modulus contrast, and bedding planes collectively controlled the height growth of hydraulic fractures. The height of hydraulic fractures decreased with increasing minimum horizontal principal stress of adjacent layers. When the minimum horizontal principal stress of adjacent layers exceeded the vertical stress, hydraulic fractures gradually deflected into the horizontal plane. Adjacent layers with large values of Young's modulus promoted the height growth of hydraulic fractures, while adjacent layers with small values of Young's modulus inhibited the height growth of hydraulic fractures. The presence of bedding planes further suppressed height growth, and the degree of suppression was related to the cohesion and density of the bedding planes. Weaker cohesion and higher density resulted in greater suppression. The results of this study provide a reference for the design and optimization of hydraulic fracturing treatments in unconventional oil and gas reservoirs. [ABSTRACT FROM AUTHOR]

Methane deflagration fracturing is a new reservoir stimulation method that serves the efficient development of shale gas reservoirs. However, the propagation law of deflagration fractures is still unclear. In this paper, a numerical model considering the effect of stress wave impact and gas drive of deflagration fracturing was established based on the continuum–discontinuum element method (CDEM). The correctness of the numerical model was verified by comparing it with a laboratory experiment, the steady and unsteady analytical solutions of gas flow, and the approximate solution of fracture propagation. Then, numerical simulations of methane deflagration fracturing in vertical wells and horizontal wells under different factors were carried out to analyze the fracture mechanism. The results indicate that deflagration fracturing in vertical wells can break through the stress concentration around the borehole; the initial radial fractures are formed under the action of stress wave impact and then propagate substantially under the driving action of high-pressure gas. The in-situ stress difference affects the deflagration fracture propagation and makes the half-fracture length in the direction of maximum principal stress larger than that in the direction of minimum principal stress. The more significant the stress difference is, the more noticeable this deviation will be. When the deflagration peak pressure is high, the reservoir burst degree is large, which is conducive to enlarging the stimulation range of deflagration fracturing. Staged deflagration fracturing in horizontal wells can form 5–8 obvious fractures perpendicular to the horizontal borehole in each explosion section. A large cluster spacing and explosion section length are conducive to expanding the stimulation scope. Moreover, the propagation of deflagration fractures will be induced by the natural fractures, and the natural fracture with a considerable length or a slight angle between the dip angle and the propagation direction of deflagration fractures is more likely to be activated. • The deflagration fracturing for shale gas reservoir stimulation is proposed. • The numerical model considers the effects of stress wave and gas drive. • The propagation law of deflagration fracture is revealed by numerical simulation. • Deflagration fracturing can break the stress concentration to form complex fracture. [ABSTRACT FROM AUTHOR]

The temperature of a deep shale reservoir may reach more than 100°C, and the effect of thermal shock on shale hydraulic fracturing has rarely not been considered in previous studies. Based on mesoscopic damage mechanics and the finite element method, a thermo-hydro-mechanical-damage (THMD) coupling model considering temperature, seepage, stress, and damage fields was constructed to investigate the effects of reservoir temperature, convective heat transfer coefficient (h), in-situ stress difference and bedding plane angle (α θ) on shale hydraulic fracturing. The results show that multiple hydraulic fractures (HFs) can occur under thermal shock and that HFs control the distribution of seepage, temperature, and stress fields. Reservoir temperature, in-situ stress difference and α θ are primary factors affecting hydraulic fracturing, whereas h is a secondary factor. When the reservoir temperature rises from 50°C to 150°C, the initiation and breakdown pressures decrease by 65.5% and 16.7%, respectively. HFs cross the bedding plane more easily, and fracture complexity is obviously enhanced. A higher h is favourable for slightly reducing the initiation and breakdown pressures, but it has little influence on the fracture complexity. Once the in-situ stress difference is low, there is a high fracture complexity, but HFs are more easily captured by bedding planes to limit the propagation of fracture height. When the in-situ stress difference is high, HFs are more likely to form bi-wing fractures. Whether α θ is too large or small, it is not conducive to improving the fracture complexity. In this study, when α θ is 30°, HFs and bedding planes intersect to form a fracture network. Essentially, thermal shock plays a key role in reducing the initiation pressure and forming multiple HFs during the fracturing process, and fracture propagation mainly depends on the injection pressure. The results can serve as reasonable suggestions for the optimization of shale hydraulic fracturing. [ABSTRACT FROM AUTHOR]

The stimulation method of the marine natural gas hydrate (NGH) reservoir through hydraulic fracturing has been proposed to resolve the problem of the low production capacity in the conventional development method of pressure drawdown. Nevertheless, due to the strong plasticity and high argillaceous siltstone content of the marine NGH reservoir, conventional small-particle-size proppant cannot form effective support for fractures after fracturing because of serious embedding in the reservoir. To solve this problem, the large-size phase change proppants were developed in this study. First, an epoxy resin curing system that can reduce curing time to 40 min in low temperature and humid environment was developed. Then, the epoxy resin and curing system was emulsified, and through the optimization of the emulsification process, the particle size of the proppant can be controlled in 0.5–4.5 mm and the cementation between the proppant particles during the curing process can be prevented. Finally, the proppant performances were evaluated. The performance evaluation shows that the cured proppants have regular structure and good compressive strength, and the emulsion proppants have good transport capacity. Their large sizes provide effective propping effects for fractures generated in weakly cemented clayey silt marine NGH reservoirs. [ABSTRACT FROM AUTHOR]

• A coupled THMD model for thermal reservoir fracturing propagating simulation was built. • The model accuracy was validated through case study, theoretical model and experiment. • The effect of different parameters on HDR hydraulic fracturing under thermal stress was explored. The hot dry rock (HDR) hydraulic fracturing is a complex physical process coupling the effects of seepage, stress, temperature, and damage. The high temperature and brittleness of the HDR leads to the great thermal stress, and the rock is possibly thermally damaged, thus promoting hydraulic fracture (HF) extension and significantly improving the permeability around the HF. In this paper, a thermo-hydro-mechanical-damage (THMD) coupling model is established based on elastic thermodynamics, Biot's classic seepage mechanics and mesoscopic damage mechanics, and its accuracy is evaluated through case study and verification with theoretical models and experiments. The evolution of multi-physics during hydraulic fracturing of HDR is studied, and the effects of rock thermophysical parameters, temperature difference, rock heterogeneity, Young's modulus, permeability, and injection rate on HF extension in the HDR are investigated. The results show that initially, due to the severe temperature variation near the borehole, the higher thermal expansion coefficient leads to the greater thermal tensile stress and facilitates rock damage, thus reducing the fracture pressure. The research results provide theoretical basis and technical support for fracturing design of geothermal system. [ABSTRACT FROM AUTHOR]

• An improved thermal-hydraulic-mechanical coupling model was established. • Evaluation index system for geothermal energy development effect was put forward. • Evaluation method for EGS based on AHP and FCE was proposed. • Well arrangement patter and development parameters were optimized. In the paper, the thermal-hydraulic-mechanical (THM) coupling model is improved by considering more factors, and a comprehensive evaluation method is proposed to optimize the heat extraction way. The thermal extraction characteristics in the Enhanced Geothermal System (EGS) and their influence factors were modeled by several researchers, and various evaluation and optimization methods were established. However, by comparing various thermal-hydraulic-mechanical (THM) models, we find that these factors of THM models are not considered comprehensively, and current EGS evaluation focuses on the individual index, the multi-objective evaluation index system of EGS has not been proposed, and a comprehensive evaluation system considering multiple indexes has not been established yet. In this study, a thermal-hydraulic-mechanical coupling model, considering matrix, fractures and changes in physical parameters of rock and fluid, was established for optimizing the EGS. The evaluation index system, including the operation life, the thermal breakthrough time, the average heat production rate, the total heat recovery factor and the re-injection factor, were presented. And the comprehensive optimization method based on analytic hierarchy process (AHP) and fuzzy comprehensive evaluation (FCE) of EGS were proposed. The model is verified by two analytical solutions. Moreover, the model was solved by finite element method (FEM) to optimize various programs of developing the EGS. In this paper, the performances of different geothermal extraction methods are compared and the different extraction parameters are optimized. The results show that the optimized geothermal mining method has better effect than that before optimization. The orthogonal test shows the optimal case of 5 fractures, the fracture permeability of 10D, the well spacing is 400 m and the production pressure draw-down of 20 MPa. The research results provide an accurate and effective method to optimize the development mode of geothermal resources, which is conductive to improve the geothermal energy efficiency. [ABSTRACT FROM AUTHOR]

The simulation of hydraulic fracturing by the conventional ABAQUS cohesive finite element method requires a preset fracture propagation path, which restricts its application to the hydraulic fracturing simulation of a naturally fractured reservoir under full coupling. Based on the further development of a cohesive finite element, a new dual-attribute element of pore fluid/stress element and cohesive element (PFS-Cohesive) method for a rock matrix is put forward to realize the simulation of an artificial fracture propagating along the arbitrary path. The effect of a single spontaneous fracture, two intersected natural fractures, and multiple intersected spontaneous fractures on the expansion of an artificial fracture is analyzed by this method. Numerical simulation results show that the in situ stress, approaching angle between the artificial fracture and natural fracture, and natural fracture cementation strength have a significant influence on the propagation morphology of the fracture. When two intersected natural fractures exist, the second one will inhibit the propagation of artificial fractures along the small angle of the first natural fractures. Under different in situ stress differences, the length as well as aperture of the hydraulic fracture in a rock matrix increases with the development of cementation superiority of natural fractures. And with the increasing of in situ horizontal stress differences, the length of the artificial fracture in a rock matrix decreases, while the aperture increases. The numerical simulation result of the influence of a single natural fracture on the propagation of an artificial fracture is in agreement with that of the experiment, which proves the accuracy of the PFS-Cohesive FEM for simulating hydraulic fracturing in shale gas reservoirs. [ABSTRACT FROM AUTHOR]

• In fluence of stress shadow on multi-clusters fracturing was analyzed. • The proppant transport and placement were considered in the model. • Multiple cluster hydraulic fracturing in multiple horizontal wells was simulated. Multiple cluster hydraulic fracturing is widely used in the development of shale reservoir. The morphology of multiple fractures propagating from multiple cluster is complex due to the stress shadow effect. The design and operation of multiple cluster hydraulic fracturing in horizontal wells require adequate knowledge of the effect of different factors, including rock properties, in-situ stresses and fluid properties on the fracture morphology. In this paper, lattice simulation, a new particle based computational method was used to investigate the multiple cluster hydraulic fracturing in shale. The results showed that stress anisotropy and cluster spacing play an important role on geometry of the propagating fracture. The middle hydraulic fracture is restricted to propagate when cluster spacing is decreased. Simultaneous, two-step and sequential fracturing scenarios in a single horizontal well were simulated. The simultaneous and sequential fracturing showed to mainly affect the fracture propagation morphology with little effect on fracture length, while the middle fracture is shorter than its two side fractures in two-step fracturing. In two horizontal models, the simultaneous and sequential fractures showed similar morphology during multiple cluster hydraulic fracturing. In term of stimulated reservoir volume (SRV), zipper fracturing showed the largest SRV during multiple cluster hydraulic fracturing in two horizontal wells. [ABSTRACT FROM AUTHOR]

• A new model is proposed to study the multi-cluster fracturing. • The influence of different factors on the fracture propagation is studied. • The accuracy of model is verified by experiment. Multi-cluster fracturing in horizontal wells is a key technology for successful development of ultra-low permeability reservoir. The propagation of hydraulic fracture during multi-cluster fracturing is complicated, especially in shale reservoir with multiple natural fractures. The design and operation of multi-cluster fracturing requires adequate understanding of influence of different factors on hydraulic fracture propagation. Up to now, many scholars have studied the hydraulic fracture morphology in multi-cluster fracturing, but few have analyzed the effect of natural fractures on hydraulic fracture propagation during multi-cluster fracturing. In this paper, a new computationally version of the particle-based model is established by Xsite to study the fracture propagation in multi-cluster fracturing with natural fractures. The tensile strength and rock toughness are calculated, and tri-axial experiments are performed to verify the accuracy of model. Simulation results show that cluster spacing and in-situ stress difference have a significant influence on the length of the hydraulic fracture and the morphology of fracture. The length of middle fracture increases with the increase of the cluster spacing, but decreases with the increase of the in-situ stress difference during multi-cluster fracturing with three natural fractures. The enhancing of cluster spacing can reduce the deflection of left and right fractures, and the increase of the in-situ stress difference can improve the ability of middle fracturing penetrating the natural fracture. Three fracturing sequence of synchronous fracturing, two-step fracturing and sequential fracturing is simulated. The left and right fractures can always penetrate the natural fracture with different fracturing sequence. But the middle fracture shows different morphology of arresting by natural fracture (during synchronous fracturing), partially penetrating the natural fracture (during two-step fracturing) and directly penetrating the natural fracture (during sequential fracturing). Different cement strengths of natural fracture are analyzed. The increase of strength of natural fracture can enhance the ability of hydraulic fracture penetrating the natural fracture. Hydraulic fracture is arrested by weak natural fracture (shear strength of 0.5 MPa), resulting in natural fracture opening. As hydraulic fracture intersecting with strong natural fracture (shear strength of 20 MPa), the hydraulic fracture can penetrate the natural fracture directly without natural fracture opening. [ABSTRACT FROM AUTHOR]

Highlights • An effective method for simulating wormhole in fractured formation was proposed. • The effects of three typical fractures on wormhole propagation were studied. • The local control domain around the fracture was found. • A step-by-step calculation method was proposed in this paper. Abstract Wormholes can effectively connect the fracture system as the dominant channels when carbonate matrix is acidified, thereby increasing the distance of acidification. It is critical to clarify the wormhole propagation law in carbonate reservoirs so as to optimize the acidification design. Previous studies highlight the influences of matrix as well as single simple fracture on wormhole propagation. However, the works characterizing impacts of single and multiple fractures with different geometry are insufficient. In this paper, the two-scale continuum model and pseudo-fracture model were combined to study the wormhole with different fractures. It was found that the fractures parallel to injection direction can concentrate fluids and thus accelerate the acid penetrating the formation, while the perpendicular ones will disperse fluids, thereby slowing down the breakthrough. The effect of straight fractures and arc fractures on wormholes are similar. When the inclination angle is less than 60°, straight fracture and arc fracture can be regarded as parallel fracture of corresponding length. When the inclination angle is greater than 60°, they can be regarded as superposition of parallel fracture and perpendicular fracture. As for the circular and polygonal fractures, only the parts near the outlet impact the subsequent wormhole propagation trajectories. The analysis of flow field shows that there is a control domain of about 3· L ·cos(θ) around the fractures. When there is no intersection of control domains of multiple fractures, there is no interference between them. There is no need to consider all fractures in the formation. Based on this, the step-by-step calculation method proposed in this paper is suitable for acidizing large-scale formation with complex fracture network. [ABSTRACT FROM AUTHOR]

• The reason for the poor effect of acid fracturing with retarded acid in dolomite reservoir was found. • The correlation between the acid-rock reaction control mode and etching morphology was analyzed. • The effects of different factors on the conductivity of fractures were studied. • Two uneven acid injection approaches were discussed. Retarded acids are often used for acid fracturing in carbonate reservoirs to reduce acid leakoff. Compared with limestone reservoirs, dolomite reservoirs are seen with lower fracture conductivity using retarded acid. This study focuses on the acid etching morphology of fracture, and investigates influential factors and improvement methods of fracture conductivity. Our analysis shows the etching morphology has no direct dependency upon the controlling regime of acid-rock reaction. It is determined by reaction rates. Low conductivity is caused by the slow reaction between dolomite and retarded acid, and difficulties in creating irregular etched grooves on fracture surfaces. A small amount of dolomite in reservoirs does not impact conductivity, and yet dolomitic contents over 80% will lead to tremendous reduction of conductivity. It is feasible to form the irregular etching morphology by reducing the flow rate and the thickening agent concentration while alternately injecting two types of acid solutions into reservoirs. [ABSTRACT FROM AUTHOR]

Slickwater fracturing can create complex fracture networks in shale. A uniform proppant distribution in the network is preferred. However, proppant transport mechanism in the fracture network is still uncertain, which restricts the optimization of sand addition schemes. In this study, slot flow experiments are conducted to analyze the proppant placement in the complex fracture system. Dense discrete phase method is used to track the particle trajectories to study the transport mechanism into the branch. The effects of the pumping rate, sand ratio, sand size, and branch angle and location are discussed in detail. Results demonstrate that: (1) under a low pumping rate or coarse proppant conditions, the dune development in the branch depends on the dune geometry in the primary fracture, and a high proportion of sand can transport into the branch; (2) using a high pumping rate or fine proppants is beneficial to the uniform placement in the fracture system; (3) sand ratio dominates the proppant placement in the branch and passing-intersection fraction of a primary fracture; (4) more proppants may settle in the near-inlet and large-angle branch due to the size limit. Decreasing the pumping rate can contribute to a uniform proppant distribution in the secondary fracture. This study provides some guidance for the optimization of proppant addition scheme in the slickwater fracturing in unconventional resources. [ABSTRACT FROM AUTHOR]

Temporarily plugging staged fracturing (TPSF) can effectively increase the complexity of fracture network in naturally fractured reservoirs. In different geological and engineering conditions, how to evaluate the stress interference of multiple fractures and interaction of natural fractures should be investigated further. To realize the temporarily plugging, new perforation elements were developed, and modified pore pressure cohesive zone (PPCZ) model was combined to simulate the interaction between hydraulic fractures and natural fractures. All simulated results are validated by comparing with analytical model. Results show that lateral fractures can really break through the suppression from previous fractures and enhance the complexity of fracture network when using TPSF. Then, case studies are presented to investigate the influence of cluster spacing, horizontal stress difference and number of perforation clusters. The results reveal that, when increasing the cluster spacing, fractures propagate uniformly and influence less; when the spacing reaches at a certain degree (40 m), the complexity of fracture network will not change. Higher stress difference results in simpler fracture network, which owns a straighter propagation trajectory. Further, more perforation clusters lead to stronger interference among multiple fractures, and more divergence and coalescence will take place. [ABSTRACT FROM AUTHOR]