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Natural gas hydrate (NGH) is a prospective clean energy. Conventional development method of the NGH reservoir by pressure drop in vertical well faces the problems of limited stimulated reservoir volume, low production efficiency, and low conductivity. At present, pressure drop production in various well types and stimulation treatments such as hydraulic fracturing in fracable NGH reservoirs have become the focus. The effects of pressure drop production in vertical well, horizontal well, radial wells, and hydraulic fracturing in vertical and horizontal wells in NGH reservoirs were simulated with HydrateResSim. We simulated the productivity of NGH reservoir for 1000 days with several types of development mode. The productivity is highest in horizontal well staged fracturing (5 stages), followed by pressure drop production in horizontal well, fractured vertical well (single fracture), and radial well (16 holes). The worst effect is obtained through production by pressure drop in single vertical well. The optimal effects were obtained with the fracture spacing of 58 m, the fracture half-length of 100 m and the conductivity of 30 μm2 cm. The horizontal well multi-cluster fracturing significantly enlarges the stimulated area and provides channels for gas migration and enhances productivity. [ABSTRACT FROM AUTHOR]

The deep geothermal energy produced from Enhanced Geothermal System (EGS) has a great development prospect because of enormous potential and environmental friendliness. EGS process involves a complex thermal-hydraulic process, and fractures in EGS are main channels for fluid flow and heat transfer, the understanding of which is crucial to the sustainable utilization of geothermal reservoirs. In this paper, a 3D thermal-hydraulic coupled numerical model is proposed to describe the interaction of fluid flow and heat transfer. Besides, the EGS with multiple fracturing horizontal wells (MFHW) is adopted to evaluate the effect of multiple hydraulic fractures on geothermal energy extraction performance. The MFHW with multiple stimulated fractures could increase fluid flow path and heat exchange area significantly, thereby enhance the heat recovery ability. Firstly, we analyzed the evolution of temperature and flow fields in EGS and compared the MFHW EGS with conventional vertical EGS. Secondly, the effects of fracturing parameters, including the fracture number, fracture length, and fracture conductivity, on heat extraction performance were investigated. Finally, the cost for drilling and hydraulic fracturing in MFHW EGS was calculated. The results indicate that MFHW EGS has a higher cumulative thermal production and a better heat extraction performance than that of conventional vertical EGS. For the optimization of hydraulic fracture parameters, the cumulative thermal production firstly increases and then decreases as the fracture number increases, the cumulative thermal production curve exists an inflection point of fracture number. Longer fracture length and higher fracture conductivity could enhance the cumulative thermal production, but the output growth slows down gradually. Considering economic cost, the best fracture parameters for MFHW EGS in this paper are the fracture number of 7, the fracture length of 300 m, and the fracture conductivity of 350 μm2•cm, respectively. The research provides a better study for multiple fracturing horizontal wells (MFHW) EGS and helps to optimize fracture parameters and geothermal reservoir management, which is conductive to improve the geothermal energy efficiency. • An EGS with multiple fracturing horizontal well (MFHW) was put forward. • A 3D MFHW thermal-hydraulic coupled model was established. • The MFHW EGS was compared with conventional vertical EGS. • The optimal fracture parameters for MFHW EGS were concluded. [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]

In order to study the effects of different parameters in hydraulic fracturing on formation temperature after low-temperature fracturing fluid is injected into high-temperature reservoir, in this paper, considering (1) osmotic pressure and capillary pressure (2) microthermal effect (heat conduction, heat convection, heat expansion, viscous dissipation) (3) the coupling processes of temperature (T), hydraulic, (H) mechanical (M) and chemical (C) in hydraulic fracturing process, and the coupled temperature prediction model of oil-water two-phase THMC based on discrete fractures is established. The effects of fracturing fluid temperature, reservoir temperature, dimensionless conductivity, Young's modulus, injection rate, cluster spacing, and proportion of branch fracture area on reservoir and fracture temperature during the pumping and shut-in stages of shale oil reservoir were studied. The results show (1) For formations with different reservoir temperatures, the temperature drop at the fracture was approximately 97% from the start of the operation to the end of the 30-day shut-in. The saturation change caused by fracturing fluid filtration is much larger than the temperature change caused by heat transfer, and it is found that the temperature and saturation change mainly occur in the early shut-in period (2) Imbibition accelerates the filtration loss of fracturing fluid. During pumping, the bottom hole temperature with imbibition considered is lower than that without imbibition considered. The temperature curve with imbibition considered begins to deviate about 10 min after pumping, but the final temperature tends to be the same, (3) When the fracturing fluid is pumped at high pressure, the fracture and matrix will be deformed, which will improve the porosity and permeability of the near well zone, thus speeding up the heat transfer rate. The high Young's modulus inhibits the increase of permeability. Compared with the young's modulus of 40 GPa and 20 GPa, the increase of permeability decreases by 9% (4) When the well was shut in for 30 days, the cluster spacing was greater than 20 m, and the temperature change around the fracture did not affect the clusters on both sides. The spacing between the clusters was 5 m, and the perforation clusters on both sides had a significant effect on the temperature. The temperature of the reservoir between the two adjacent clusters dropped from 393 K to 385 K. When there is a branch fracture, the small cluster spacing is equivalent to increasing the complexity of the fracture and accelerating the initial heat transfer rate. (5) High injection temperature, high dimensionless conductivity, low Young's modulus and low pumping rate are conducive to the heat transfer of fracturing fluid in the formation. • Based on the discrete fracture model, the temperature prediction model of oil-water two-phase flow was established. Considering the capillary force and osmotic pressure, the temperature (T) -hydraulic (H) -mechanical (M) -chemical (C) was realized. The finite element method is used to solve the model. • The effects of multiple factors on reservoir and fracture temperature during fracturing pumping and shut-in stages are analyzed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Hydraulic fracturing simulation experiments of shale outcrops were first carried out. [•] Fracture morphology was observed for the first time by high-energy CT scanning. [•] The effects of multiple factors on fractures propagating in shale play were studied. [•] CT scanning images were combined with internal fractures photographs for analysis. [•] Hydraulic fracturing of horizontal well was simulated for shale specimens. [Copyright &y& Elsevier]

• 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]

To understand the fracture behavior for variable density in-plane perforations on a horizontal well, true triaxial hydraulic fracturing experiments were performed with acoustic emission monitoring. The experimental results indicate that the variable density in-plane perforations can not only create transverse fractures but also arrest fracture propagation toward undesirable zones. The in situ stress and treatment parameters play significant roles in the fracture morphology and breakdown pressure. Under a large horizontal principal stress difference, there is a high possibility for simple transverse fracture creation, which results in a lower breakdown pressure. Moreover, the decrease in fracturing fluid viscosity can increase the fracture complexity and increase in pump rate can increase breakdown pressure and induce the complex fracture geomtetry. The acoustic emission (AE) characteristics and cutting fracture morphology demonstrate that not all perforations can be successfully initiated during the fracturing process. Because of the stress interaction from side perforations, fracture initiation and propagation from the middle perforation are strongly suppressed. Although the middle fracture is not fully initiated and propagated, it is still beneficial for the side hydraulic fracture connection and propagation in one plane. Secondary and axial fractures can still be observed, primarily due to the stress interaction between neighboring perforations and misalignment of the perforation tunnel orientation with the principal stress direction. In addition, the creation of complex fractures tends to occur using a high pump rate, which relates to tense fluid distribution competition from each perforation tunnel. Fracture initiation may follow an order but mostly tends to initiate from side perforations. Because of the stress interaction, hydraulic fractures from the side perforation can sometimes deviate from the perforation tunnel direction, twist and kink, and finally, generate nonplanar fractures. A large horizontal principal stress difference is recommended and beneficial for simple, straight and planar fracture creation during hydraulic fracturing stimulation with variable density in-plane completion for the horizontal well. • The application of local in-plane perforations can control the fracture path and complexity. • Tri-axial fracturing experiments were conducted on tight rock samples with local in-plane perforations. • The change in fracturing fluid viscosity and pump rate can alter the fracture path between perforations. • The connectivity of neighboring fractures can lower the breakdown pressure. [ABSTRACT FROM AUTHOR]