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As main heat exchange channel in enhanced geothermal system, the evolution of hydraulic conductivity in fracture is significance for efficient heat mining. For the thermal stress or thermal cracking spontaneously induced by the temperature difference between low-temperature fluid and hot rock in heat mining stage, it is necessary to explore the damage mechanism along EGS fracture and the corresponding permeability evolution. Firstly, the long-term permeability tests under high temperature (50–200 ℃) were conducted by the self-developed high temperature seepage experimental device. Then, a coupled THM-D model was constructed to describe the damage distribution along fracture. Combined with experimental and simulation results, relationship between the thermal stress/cracking and the evolution of fracture permeability is revealed. The results indicate that during high-temperature (200 ℃) experiments, the fracture permeability first increases rapidly under the low-temperature induced thermal stress/cracking, then decreases due to the blockage effect induced by the debris particles generated in thermal cracking along fracture. The enhancement of injection velocity and heterogeneity are all conducive to the emergence of thermal cracking in matrix along fracture. Simultaneously, high confining pressure has a negative effect on the migration of debris particles of thermal cracking, which contribute to prevent the blockage of debris particles. [ABSTRACT FROM AUTHOR]

As main heat exchange channel in enhanced geothermal system, the evolution of hydraulic conductivity in fracture is significance for efficient heat mining. For the thermal stress or thermal cracking spontaneously induced by the temperature difference between low-temperature fluid and hot rock in heat mining stage, it is necessary to explore the damage mechanism along EGS fracture and the corresponding permeability evolution. Firstly, the long-term permeability tests under high temperature (50–200 ℃) were conducted by the self-developed high temperature seepage experimental device. Then, a coupled THM-D model was constructed to describe the damage distribution along fracture. Combined with experimental and simulation results, relationship between the thermal stress/cracking and the evolution of fracture permeability is revealed. The results indicate that during high-temperature (200 ℃) experiments, the fracture permeability first increases rapidly under the low-temperature induced thermal stress/cracking, then decreases due to the blockage effect induced by the debris particles generated in thermal cracking along fracture. The enhancement of injection velocity and heterogeneity are all conducive to the emergence of thermal cracking in matrix along fracture. Simultaneously, high confining pressure has a negative effect on the migration of debris particles of thermal cracking, which contribute to prevent the blockage of debris particles. [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]

10.3390/pr10050939 11 Pan Y., Xu Y., Yang Z., Wang C., Liao R. Shale Gas Productivity Prediction Model Considering Time-Dependent Fracture Conductivity. Hydraulic fracturing is a technique that can provide space for oil and gas flow by pumping fracturing fluid into a reservoir to fracture rock and filling proppant to create fractures or fracture nets. References 1 Zou Y., Gao B., Ma Q. Investigation into Hydraulic Fracture Propagation Behavior during Temporary Plugging and Diverting Fracturing in Coal Seam. [Extracted from the article]

The tight sandstone oil reservoirs characterized by the low porosity and permeability must be hydraulically fractured to obtain the commercial production. Nevertheless, the post-fracturing production of tight oil reservoirs is not always satisfactory. The influence mechanism of various factors on the fracture propagation in the tight oil reservoirs needs further investigation to provide an optimized fracturing plan, obtain an expected fracture morphology and increase the oil productivity. Thus, the horizontal well fracturing simulations were carried out in a large-scale true tri-axial test system with the samples from the Upper Triassic Yanchang Fm tight sandstone outcrops in Yanchang County, Shaanxi, China, and the results were compared with those of fracturing simulations of the shale outcrop in the 5th member of Xujiahe Fm (abbreviated as the Xu 5th Member) in the Sichuan Basin. The effects of the natural fracture (NF) development degree, horizontal in-situ stress conditions, fracturing treatment parameters, etc. on the hydraulic fracture (HF) propagation morphology were investigated. The results show that conventional hydraulic fracturing of the tight sandstone without NFs only produces a single double-wing primary fracture. The fracture propagation path in the shale or the tight sandstone with developed NFs is controlled by the high horizontal differential stress. The higher stress difference (<12MPa) facilitates forming the complex fracture network. It is recommended to fracture the reservoir with developed NFs by injecting the high-viscosity guar gum firstly and the low-viscosity slick water then to increase the SRV. The low-to-high variable rate fracturing method is recommended as the low injection rate facilitates the fracturing fluid filtration into the NF system, and the high injection rate increases the net pressure within the fracture. The dual-horizontal well simultaneous fracturing increases the HF density and enhances the HF complexity in the reservoir, and significantly increases the possibility of forming the complex fracture network. The fracturing pressure curves reflect the fracture propagation status. According to statistical analysis, the fracturing curves are divided into types corresponding to multi-bedding plane (BP) opening, single fracture generation, multi-fracture propagation under variable rate fracturing, and forming of the fracture network through communicating the HF with NFs. The results provide a reference for the study of the HF propagation mechanism and the fracturing design in the tight sandstone reservoirs. [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]

The radial well fracturing has been applied and achieved effective output. However, the fracture morphology and propagation law of it are still remain unclear, which limits the development and application of it. A 3D numerical model is established by ABAQUS to solve the current problems. The stage of fracture propagation is simulated by the extended finite element method. The conclusions stated that in situ stress field distribution was changed during the radial well fracturing and the induced stress field along the radial well is established at the same time. The propagation of major fracture is orientated by the induced stress in a certain area. The guiding function of fracture orientated propagation is evaluated by included angle η between the axis of radial well and equivalent fracture. The smaller the value of η is, the better effect of fracture orientated propagation is. The increase of horizontal stress difference and decrease of vertical density of radial wells are unfavorable to decrease the value of η. The failure pressure grows up with the azimuth angle of radial well increasing. The increasing of elastic modulus of formation is not conducive to the effect of radial well fracturing. The increasing of Poisson’s ratio of formation is conducive to the failure pressure and maximum fracture width, but not as helpful as to decrease the value of η. Larger diameter of radial well is better for fracture oriented propagation. Operation parameters of fracturing fluid viscosity and construction displacement have less effect on the value of η. Also, the degree of different parameters influence on value of η, failure pressure and maximum fracture width is ranked by the grey correlation method. The results can be referred to prediction of fracture propagation and design of radial well completion or fracturing. [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]