Study On Control Mechanism Of Water Inrush From Horizontal Bottom Of Coalbed Methane

Through strata fracturing technology of horizontal well in the roof is the key technology for coal-bed methane recovery.This technology reduces the difficulty of drilling horizontal wells in coal seams,and can achieve good effect of volume modification and formation of complex fracture networks.In the process of achieving efficient,safe and green development of coal-bed methane,horizontal well roof fracturing technology faces three major problems: First,the micro-pores and micro-cracks cause the fracturing fluid to penetrate and reduce in the rock formation that influence of the cracking pressure and the crack extension;Secondly,Effect of geometric properties of natural fractures on the initiation pressure of horizontal wells;Finally,how to control the hydraulic crack can penetrate the roof to reach the target horizon without penetrating the floor to connect the underground aquifer.These problems have become an urgent problem to be solved in the development of coal-bed methane.It is of great significance to carry out theoretical research on horizontal well roof cross-layer fracturing for realizing efficient exploitation of coal-bed methane and reducing the occurrence rate of floor water inrush accident.Guided by the basic theories of elasticity,fracture mechanics,hydraulics and rock mechanics,this paper theoretically studies various influencing factors of horizontal well cross-layer fracturing.The theory of controlling through strata fracture penetration is analyzed,and a new model for calculating initiation pressure and fracture extension is established.In terms of crack initiation pressure,the mechanism of additional stress induced by the concentration of perforation is proposed for the fixed-hole perforation method.It is considered that the additional stress can change the stress state around the well-bore.The theoretical formula for the penetration force due to the penetration of fracturing fluid is derived,which acts on the fractured formation in the form of a volumetric force.The geometrical characteristics of natural cracks are studied.It is believed that natural cracks should participate in the calculation of cracking pressureas an independent physical index,and the theoretical expression of crack characteristics is derived.Combining these research results,a new cracking pressure is derived and verified by the on-site construction data.In the extension of hydraulic fracture penetration.Three kinds of pressure drop methods in the process of hydraulic fracture penetration are analyzed,namely: the law of pressure drop of perforation orifice,the law of pressure drop in a single stratum,and the law of pressure drop between multiple strata.Using the classical PKN crack model and fracturing fluid loss theory,the relationship between the hydraulic fracture extension radius and the fracturing construction time is derived.It is found that the mathematical model of crack propagation through fracture is represented by a piecewise function.This theoretical model was also verified by on-site construction data.The main factors affecting the extension of hydraulic cracks were ascertained: Poisson's ratio,elastic modulus,formation depth,crack model width,and fluid loss coefficient.By rationally arranging these parameters,the purpose of controlling the extension of the penetration crack can be achieved.