Functional analysis of multi-scale fractures within a single fracture cluster in multi-horizontal wells

Defining the fracture function at each scale in the reservoir helps to analyze and optimize the thermal recovery properties of the reservoir. This study aims to reveal the functionalities of fractures at different scales by simulating one single fracture cluster coupled in multi-scale fractures. The geometric layout and parameter characteristics of the main fractures, branch fractures, and micro-fracture domain are investigated to speculate on their functions and influence on heat extraction performance. The results display that the main fractures and micro-fracture domain serve as the predominant channel for fluid flow. The branch fractures exert control over the flow transition from the main fractures to the micro-fracture domain, thereby influencing the generation of preferential flow. Micro-fracture domain is the primary way of fluid filtration loss. When the thickness of micro-fracture domain increased from 0 m to 20 m, the production flow rate increased from 2 kg/s to 4.2 kg/s, and the water loss ratio increased from 1 % to 4.9 %. However, expanding the micro-fracture domain can significantly enhance production flow rate and heat recovery rate at the expense of increased water loss ratio. Notably, preferential-flow-induced thermal breakthrough will not occur prematurely in this process. The permeability of branch fractures exerts the most substantial influence on the overall heat extraction, owing to its greatest impact on reservoir lifespan. These findings would provide valuable insights for optimizing fracturing morphology in multi-horizontal wells.