Evaluation Study On Flow Conductivity Of Complex Fractures Of Coal Bed

Hydraulic fracturing is commonly adopted as a stimulation method in the development of coal bed methane.It is used to produce cracks or a network of cracks with high flow conductivity,to increase the bottom flow area and to reduce the flow resistance of gas into the well.It is well-known that coal bed is often characterized by very complicated properties like low porosity and permeability,low closure pressure and elastic modulus,high Poisson’s ratio,high softness and brittleness and high intricacy of the crack network,so the fracture patterns occurred after the fracturing can be very complicated,which thus renders it really difficult to accurately evaluate the flow conductivity afterwards.Therefore,it is necessary to propose a new experimental approach and a new theoretical model applicable to the evaluation and analysis of the flow conductivity of complex fractures in coal bed methane wells.The paper has firstly conducted the experiments,under the same sand-paving content,on horizontal fractures,T-shaped fractures and 工-shaped fractures of coal bed to evaluate their individual flow conductivity.Then we have analyzed the effects of closure pressure and closure time etc.on the flow conductivity of coal bed fractures and we have obtained the changing patterns of flow conductivity of different patterns of coal fractures.Based on 3D elastic mechanics and space geometry,a rhombohedra filling pattern is adopted to arrange the proppants.Considering the tortuosity of the arrangement of the sphere particles and the compaction effect caused by the deformation and imbedding of proppants,we have accordingly established three theoretical models for horizontal fractures,T-shaped fractures and 工-shaped fractures,with which we have analyzed the effects of factors like the closure pressure of fractures,Poisson’s ratio and the elastic modulus of coal as well as the proppants,diameter of the proppants and the paving density of sand on the flow conductivity of coal fractures.We have also comparatively analyzed the distinction in flow conductivity of different patterns of fractures of coal.The results show that 1)closure pressure,proppants diameter,paving density of sand can exert a noticeable impact on different patterns of coal fractures;2)whereas the Poisson’s ratio of both coal and proppants places a much weaker influence on the flow conductivity of coal;3)the more complicated the fracture pattern,the more noticeable the change of flow conductivity subject to relevant factors.This paper has proposed an experimental evaluation method for the analysis of flow conductivity under different fracture patterns,considering the effects of fracture patterns on flow conductivity;and has established corresponding theoretical calculation models for different facture patterns of coal.The results obtained have provided a new thinking and anew approach to evaluating the flow conductivity of fractures with complex patterns,as well as a theoretical foundation for production evaluation of coal bed methane wells on site.