Evaluation and optimization of heat extraction in enhanced geothermal system via failure area percentage

It is quite difficult to quantitatively measure heat extraction in a fractured enhanced geothermal system (EGS) reservoir and its impacting factors. This study thus aims to define a characteristic parameter that is failure area percentage for evaluating and optimizing geothermal extraction. Through establishing a thermal-fluid-solid coupling model, the operation lifecycle and economy of an EGS case was evaluated by the proposed parameter. A sensitivity indicator was developed to measure the quantitatively effection of various factors on heat extraction. The results show that heat extraction increases with the three growing-up factors: fracture network complexity, pressure difference, and rock permeability. However, too large value of these factors are adverse to geothermal system smooth operation. Based on the sensitivity indicator analysis, heat extraction is the most sensitive to the variation of fracture network complexity. This study would provide a guidance for optimization of EGS designing

Modified zipper fracturing in enhanced geothermal system reservoir and heat extraction optimization via orthogonal design

Modified zipper fracturing in horizontal wells is an effective tool for promoting the energy productivity of hot dry rock based enhanced geothermal system. However, the following two questions are not fully understood: 1) what extent of heat extraction from geothermal reservoir can be improved; 2) how to obtain the optimized fracture morphology meeting heat extraction requirements. We established a two-dimensional horizontal wells hydraulic fracturing model to study the fracture propagation and effects of modified zipper fracturing, and developed a length index to evaluate the impacts of fracture morphology on reservoir heat transfer behavior. The critical length, which is half of the interval between two horizontal wells, determines whether fracture propagation has effects on heat extraction or not. The orthogonal design is used to study the sensitivity of factors affecting hydraulic fracturing and heat extraction. Fracture spacing has insignificant influence on morphology, and therefore we set it as 30 m which is the optimal spacing. According to different formation parameters (in-situ stress difference and elastic modulus), controlling fracturing fluid injection rate is the best choice to optimize fracture morphology. These research results would supply a guidance for designing hydraulic fractures to meet the maximum heat extraction in enhanced geothermal system. © 2020 Elsevier Lt