Study On The Mechanism And Key Technology Of Fracture Leakage Plugging While Drilling In Coal Mining Area

With increased drilling activity in deeper in coal mining area,the stratum drilled into has become more and more complicated.Large fracture apertures may cause severe leakage while drilling the fractured formation.Wellbore strengthening via building a stable plugging zone in the fracture is the key to deal with leakage during drilling in coal mining area.Focusing on the core issue of how to establish a stable plugging zone inside the fracture,the thesis starts with the analysis of the characteristics of the lost formation and the law of the fractured leakage in the coalbed methane wells drilled in Qianxi coal mining area,researches on theory of the formation process and loading capacity of the plugging zone,as well as the prediction model of the loading capacity of the sealing wall under different failure modes.The influencing factors and strengthening methods of the loading capacity of the plugging zone,as well as the high concentration bridge plug composite sealing technology,are studied via the method of combining of comprehensively theoretic analysis,numerical simulation,experiment of lab model and field engineering application.The following research results have been obtained:（1）The calculation model of fracture mechanical aperture using loss volume and pressure gradient in fracture as parameters is established based on the Navier-Stokes equations,and the relationship between the particle size distribution of the Lost Circulation Material（LCM）and the plugging location is calculated through the analysis of experimental data via Gray correlation method.The relationship between the coefficient of particle drag force and particle Reynolds number has been researched using particle sedimentation test and data fitting method.And then,the particle size distribution is established to meet the conditions of the crack entry and the accumulation for bridging based on the analysis of the screen shape and the results of the laboratory experiments.（2）Based on the macro and micro stress analysis of the plugging zone and the calculation of stress intensity factor at the tip of the fracture,the prediction model of the loading capacity of the plugging zone under the conditions of shear instability,sliding instability and fracture deformation instability is optimized.The results have shown that the physical and mechanical properties,geometric parameters,initial position,length and porosity of the LCM are the main factors affecting the loading capacity of the plugging zone.（3）The strengthening methods for plugging zone are studied via numerical simulation and similar simulation,while the influences of particle gradation,particle concentration of LCM and the sealing pressure difference of sealing on plugging zone’s formation and shape are qualitatively analyzed by PFC^3D.The relationships between the loading capacity of the plugging zone and the main factors are studied,including the particle size distribution,the type and concentration of LCM,the length and dosage of the fiber material,and the plugging technology,based on the long crack plugging simulation experimental apparatus developed by this project,which could monitor the dynamic change of the plugging process in real time.The results have shown that the appropriate distribution of D₅₀ and D₉₀ of LCM is conducive to forming the seal,and D₁₀ of LCM has a significant impact on the loss volume during plugging.The polypropylene fiber,which accounts for 2.0%of the quality ratio of LCM and is about twice of the width of the fracture aperture,can effectively improve the integrity and loading capacity of the plugging zone.The concentration of LCM has a significant impact on the loading capacity and the optimum value for the experiment condition is15%because of the higher value may cause plugging at the entrance of the fracture.（4）Formulas of a kind of thixotropic cement slurry with early strength and a kind of mud to cement（MTC）friendly with the mud have been optimized to solve the low loading capacity problem in wide cracks.（5）The high concentration bridge plugging technology and high concentration bridge compounded plugging technology have been raised based on theoretical analysis and laboratory investigation.And field tests have been conducted in several wells in Qianxi coal mining area.The experiment results have shown that the technologies can effectively block fracture leakage.The research result of this thesis contributes a more scientific and convenient plugging method for lost circulation caused by fractures in coal mining areas,and has some reference value for further studies and application in related field.