Study on Diversion Channel Characteristics and Reducing High Resistance Body during Large-scale Hydraulic Fracturing in Coal Seam

Author:Lv Shuai Feng

Supervisor:wang sheng wei


Degree Year:2019





Tectonic coals of the No.3 coal seam in the Xinyuan Coal Mine of northern Qinshui Basin are developed widely,and coal and gas outburst disasters are prone to occur during production.At present,measures including underground hydraulic caving and gas fracturing technology for gas extraction are taken to eliminate outburst before roadway tunneling.However,these local gas control technologies need a large amount of engineering work,long construction period and high cost,and can not be carried out at the same time with roadway tunneling,thus seriously affecting the coal mine production efficiency.The practice of large-scale hydraulic fracturing on the ground effectively reduced the risk of gas outburst,which undoubtedly opens up a new way for gas controlling in the soft and broken coal seam.Because the spatial distribution of coal seam fractures is highly heterogeneous,and the vertical structure of coal seam is more complex,at present,the research on macroscopic characteristics of fracture system and its influence on hydraulic fracture propagation is insufficient.Especially,there is a lack of engineering practice and research on large-scale hydraulic fracturing for outburst control in coal seam.Based on the hydraulic fracturing diversion channel,identification of natural fractures in the coal seam,hydraulic fracture propagation mechanism,characterization of high resistance body and fracturing parameters optimization are studied.The natural fracture system and hydraulic fractures provide the main diversion channel for the fracturing fluid.Firstly,the spatial characteristics of the natural fractures are comprehensively identified at multiple scales by observation in the coal mine and laboratory experiments.The visible fracture system and spatial combination model of coal structure are established.The predominant orientations of exogenous fractures in coal seam are NNE and NW directions.The tectonic coal is mainly located in the middle and lower part of the coal seam.The degree of deformation of micro-fractures is controlled by the coal structure.In addition,the direction and density of subsurface exogenous fractures in coal seam have a good correspondence with the direction and scale of surface joints,which provides a new method for predicting the dominant orientation of exogenous fractures in the coal seam.At the same time,the propped hydraulic fractures of coalbed methane wells excavated in the underground are continuously tracked and observed,thus complex propagation morphology types of hydraulic fracture such as horizontal shape,inverted “T” shape and asymmetric “I” shape are obtained and there genesis are analyzed.The influence mechanism of natural fractures on hydraulic fracture propagation is also revealed.Then,based on the gas control effect of hydraulic fracturing,the concept of fluid high resistance body,which is opposite to the diversion channel and has hindrance effect on hydraulic fracture propagation and fracturing fluid filtration,is put forward.As to the high resistance body of tectonic coal,and hydraulic fractures are easy to initiation,but the extension length is short,which is not conducive to the large-scale diffusion of fluids.For the high resistance body of fault with big angle,makes fluid divert and hydraulic fracture expand along fault plane.The conductivity of fault high resistance bodies is improved dramatically after fluid diversion asymmetric diagnosed from the injected fluid volume and energy according to fracturing construction curves.Finally,the fracturing technology measures with large displacement and fluid volume,auxiliary use of diverting agent and permeating agent,are put forward to reduce high resistance bodies,and good application effects have been achieved in field tests.The borehole gas drainage quantity multiplied after fracturing.This study clarifies the spatial relationship between diversion channel and high resistance body,and their interaction with fluids under large-scale hydraulic fracturing conditions for the first time,which lays a scientific foundation for optimizing fracturing operation parameters to serve specific objectives.For regional gas control in coal mine,large-scale hydraulic fracturing opens up another new engineering technology measure besides exploitation of protective layer and in-seam gas extraction.As far as hydraulic fracturing technology is concerned,constant validation of new service objects can promote the progress and upgrading of hydraulic fracturing technology and better serve other unconventional oil and gas development.