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

Abstract

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. • We studied fracture propagation and thermal effects of modified zipper fracturing. • Crack length has significant impacts on heat extraction rather than crack width. • Critical length determines if crack propagation distinctly improve heat extraction. • Thirty-meter is the best fracture spacing for heat transfer enhancement in reservoir. • Injection flow rate controlling is an effective way to optimize fracture morphology.