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Abstract The SRV fracturing is the core of commercial exploitation of shale gas, and the design philosophy of the SRV fracturing is gradually introduced into the stimulation of conventional low permeability reservoir and unconventional reservoir. However, in reservoirs with poorly developed natural fractures or high horizontal principal stress difference, it is difficult to achieve the multi-fracture or fracture network by conventional hydraulic fracturing, and effectively stimulating reservoir needs other assisting methods. This paper aims to present a method of guiding the multi-directional hydraulic fractures by multiple radial slim holes drilled in spatial positions, and creating multi-fractures and even complex fracture network. In this paper, a 3D hydraulic fracturing extended finite element numerical model in the vertical well assisted by planar multi-radial slim holes was established, and the effect of radial slim hole number, diameter, length, vertical density, azimuth and phase angle, Poisson's ratio, horizontal principal stress difference and rock Young's modulus, fracturing fluid viscosity and injection rate, reservoir permeability, etc. on guidance of radial slim holes were investigated. Then, we conducted the true tri-axial hydraulic fracturing test with the artificial core, and the feasibility of the technology was verified. The results show that decreasing azimuth and phase angle and increasing hole diameter, length and vertical density strengthen the guiding effect of planar multi-radial slim holes, and increasing Poisson's ratio, reservoir permeability, and fracturing fluid injection rate and viscosity enhance the guidance effect of radial slim holes. Increasing horizontal principal stress difference and reservoir Young's modulus weaken the radial slim holes guiding effect. The physical modeling experiment proves that controlling generation of complex multi-fractures is feasible by rationally arranging the spatial positions of radial slim holes. The spatial arrangement of four planar radial slim holes with phase angle of 90°, azimuth of 45° and vertical density not less than 2 holes/m in the vertical well is a scientific scheme. With the reasonable treatment scheme, creating 6 main fractures is possible. The research results provide theoretical support for multi-fractured stimulation assisted by planar multi-radial slim holes, and help the design of well completion parameters and fracturing treatment parameters of the technology. Highlights • The new multi-fractured stimulation technique for hydraulic fracturing was put forward. • Stimulation experiments of radial slim holes-assisted hydraulic fracturing were carried out. • The effects of multiple factors on the guiding strength of radial slim holes were studied. • The best technical solution for radial slim holes-assisted hydraulic fracturing was proposed. [ABSTRACT FROM AUTHOR]

Under the influence of tectonic stress and faults, there are many natural fractures (NFs) in hot dry rock (HDR) reservoirs. However, the effect of NFs on the propagation of hydraulic fractures (HFs) has often not been considered in previous studies. A coupled thermo-hydro-mechanical-damage (THMD) model by the finite element method is constructed to investigate hydraulic fracturing and production performance in a fractured geothermal reservoir. The results show that thermal stress and injection pressure jointly promote rock initiation and propagation. HF can propagate tortuously and deviate from the maximum horizontal stress direction under the influence of NFs. HF and open NFs constitute the main channel of heat transfer, which determines the heat extraction performance. The fluid viscosity (μ) and injection flow rate (q in) are the main factors affecting the fracturing and production performance. A higher μ and q in can significantly increase HF length and activate the NFs along the propagation path, so the fracture area and production temperature have a distant ascension. The horizontal stress difference (Δ σ) and NF number (n) are secondary factors affecting the fracturing and production performances. A higher or lower Δ σ and n are not conducive to forming a long HF and enhancing the production temperature. A larger fracture area or higher fracture complexity does not necessarily lead to better production performance. For the development mode of HDR using two injection wells and one production well, it is beneficial to enhance the production temperature to give priority to ensuring the uniform propagation of HF. The results can provide a theoretical basis for the optimal design of an enhanced geothermal system in a fractured geothermal reservoir. • A coupled THMD model for the fracturing and production is constructed and verified • HF propagation under the influence of NFs and the production performance are studied • The main controlling factors affecting the fracturing and production are determined • The uniform propagation of HF is beneficial to improve the production performance [ABSTRACT FROM AUTHOR]

The clear mechanism of hydraulic fracture propagation in glutenite reservoirs with high heterogeneity is still not obtained, thus it is difficult to carry out the design of fracturing plan effectively. Based on the characteristics of the glutenite reservoirs, a coupled flow-stress-damage (FSD) model of hydraulic fracture propagation with gravels is established. This model is experimentally verified and the research on the influence of rock physical parameters and gravel property on the hydraulic fracture propagation is conducted. It is shown that as the gravel tensile strength increases, the hydraulic fracture is prone to propagate around the gravel, where the fracture deflection always occurs; as the gravel Young's modulus increases, there is high probability that hydraulic fracture propagates around the gravel, with more obvious fracture deflection; the matrix permeability influences fracture propagating morphology when encountering gravel and total fracture length; the horizontal geostress difference seriously impacts the fracture deflection; as the fracturing fluids injection displacement increases, the fracture is prone to deflect when encountering gravel; the low viscosity fracturing fluids result in the shorter fracture; the larger gravel increases the possibility of fracture deflection; in case of smaller gravel sizes, the increasing gravel content has a big influence on fracture deflection, and the increasing content of large gravel complicates the fracture morphology, resulting in the fine branched fractures; for the well rounded gravel, the fracture propagation around the gravel is prone to occur, and the fracture is not prone to deflect. Compared with the conventional sandstone reservoir, the glutenite reservoirs have higher breakdown and extension pressures, which fluctuate due to the gravel; the larger gravel size results in higher extension pressure. In this paper, a simulation method of hydraulic fracture propagation in the glutenite reservoirs is introduced, and the result provides the theoretical support for prediction of fracture propagation morphology and plan design of hydraulic fracturing in the glutenite reservoirs. [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]