Design and productivity evaluation of multi-lateral well enhanced geothermal development system

Abstract

Enhanced geothermal development system is an effective means to develop hot dry rock geothermal resources, and its reasonable structural design is crucial to the efficient exploitation of hot dry rocks. In this paper, a multi-lateral well enhanced geothermal development system, which is composed of one low-permeability thermal reservoir, three multi-lateral injection wells, three multi-lateral production wells and three artificial fractures, was designed based on the traditional dual-well development model. Then, a 3D hydrothermal coupling numerical evaluation model was established by using the local heat balance method, and its accuracy and reliability were verified according to Lauwerier analytical theory of fractured flow and heat transfer. Finally, the influence of reservoir parameters, well layout parameters, and injection-production parameters on the heat production performance of this enhanced geothermal development system were explored based on the water-rock coupling mechanism inside the reservoir in the process of thermal production. And the following research results were obtained. First, the “rock invasion effect” of the cold front in the fractures of the multi-lateral well development system is stronger than that in the matrix. Increasing the reservoir permeability can improve the internal heat transfer effect, but also enhances the “invasion performance” of cold front. Second, the annual thermal energy extraction of the system gradually decreases over time and it also decreases with the increase of the injection temperature. It increases with the increase of the reservoir permeability in the early stage of the operation, but decreases greatly after the completion of the thermal breakthrough. Third, the system's thermal energy extraction rate and production mass flow rate are less influenced by reservoir porosity and injection temperature, but they decrease with the increase of the vertical spacing of injection-production wells. Fourth, the production temperature decreases greatly with the increase of the reservoir permeability, increases with the increase of the reservoir porosity, and decreases with the decrease of the vertical spacing of injection-production wells. Fifth, the heat production performance of the system in the early stage can be improved by increasing the length of the production well, and the heat production capacity of the system can be enhanced to a certain degree by increasing the injection-production pressure difference, but excessive pressure difference will impact the service life of the reservoir seriously.