Experimental test and theoretical calculation of the fracture height limit of gas pipe flow to Darcy flow

摘要

Low-speed flow experiments in which ultra-fine copper tubes are used to simulate micro-fractures in carbonate strata are conducted to analyze the variations of gas flow state in fractures of different fracture heights,determine flow state transition limit and transition interval,and establish the calculation method of flow state transition limit.The results show that the ideal Hagen-Poiseuille flow is the main form of gas flow in large fractures.Due to the decrease of fracture height,the gas flow in the fracture changes from Hagen-Poiseuille flow with ideal smooth seam surface to non-Hagen-Poiseuille flow,and the critical point of the transition is the boundary of flow state transition.After the fracture height continues to decrease to a certain extent below the boundary of the flow state transition fracture height,the form of gas flow gradually changes to the ideal Darcy flow,thus the transition interval of the gas flow state in the closing process of fracture can be determined.Based on the three-dimensional microconvex body scanning of the fracture surface,the material properties of fracture and properties of fluid in the fracture,a method for calculating the boundary of flow state transition is established.The experimental test and theoretical calculation show that the limit of the fracture height for the transition from pipe flow to Darcy flow is about twice the sum of the maximum height of the microconvex bodies on the upper and lower sides of the fracture.