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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]

• 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]