Microscopic Migration Mechanism Of Coal Fines In Propped Fracture

Coalbed methane is an integral part of unconventional natural gas resources,and has important exploration and development prospect.Fracturing and other reservoir stimulation can improve the development efficiency of coalbed methane.in our country.A large number of practices show that a large amount of coal fines will be produced after fracturing.The damage of reservoir and propped fractures caused by coal fines blockage has a serious impact on the normal drainage of coalbed methane.A lot of researches have been done on the generation and migration of coal fines in strata and propped fracture.But these studies focus on the macro scale.At present,the microscopic migration mechanism of pulverized coal in propped fracture has not been scientifically revealed.In view of this,this paper applies theory and experiment to study the microscopic migration of partial coal fines in fractures by combining the field practice.The purpose is to reveal the microscopic migration mechanism of coal fines in propped fractures,So as to provide theoretical support for the prevention and control of coal powder in the field.(1)Based on the basic theory of coal petrology,geomechanics and geology of coalbed gas development,the general characteristics of the fractured target coal seam are understood in the target area of Eastern Guizhou and Western Yunnan.The characteristics of the target block are soft,weak water and multiple thin layers.Coal in the target area is mainly undeformed coal.Coal in the target area is mainly semibright coal and clarain coal.The cleats in coal are developed.Coal is mainly composed of organic matter,but inorganic minerals in coal is mainly composed of clay minerals,quartz and calcite.(2)Based on microdynamics,the mechanical mechanism of coal fines migration is analyzed.Based on rock mechanics,the destruction of coal and the formation mechanism of coal fines are understood,coal fines is the result of the interaction of geological and engineering practice The size distribution of coal fines in the stratum is larger.Size distribution curve of coal fines has a certain degree of fluctuation,and have two wave crest.The hydrodynamic force(Fd and Ff),electric double layer repulsion force(FDL)and Born short-range repulsion force(FB)of coal fines are acted as the motivity of coal fines.The gravity(FG)and Van der Waals' force(Fads)of coal fines hindered the movement of coal fines.(3)By analyzing the physical properties of coal at different levels in the target area,it is found that the relationship between the degree of coal meta morphism and the absolute value of zeta potential is LC3#coal>SH15#coal>HP5#coal.The zeta potential of coal fines particles in different fluids is also different.The absolute value of them is:1%coal fines dispersant solution>distilled water>1%KC1 solution.(4)Through indoor simulation experiment,design experiment scheme and preparation of experimental samples,simulate the migration process of coal fines in underground cracks.The influence of fluid type,coal fines source,flow rate and coal fines size on the movement of coal fines in natural crack and fracturing propped fracture is analyzed by orthogonal design.Research shows that there are following knowledge in natural crack.?The residual coal fines is partitioned.It can be divided into condensed zone,dispersed area and vacant zone.The addition of coal fines dispersant and increase of particle size can reduce the area of condensed zone.? Flow rate has the greatest influence on the migration of coal fines in crack.The amount and flow rate of coal fines is proportional to the degree of coal meta morphism and liquid flow,is inversely proportional to coal particle size.The addition of KCl or coal fines dispersant can promote the migration of coal fines,and KCl is more.There are the following conclusions in the fracture support crack.?The residual coal fines is more dense at the exit of the crack.?A small amount of bubbles gathered around the coal fines particles in the proppant filling zone.? Coal particle size has the greatest influence on the migration of coal fines in propped fracture.And the amount and flow rate of coal fines is proportional to the degree of coal meta morphism,liquid flow and coal particle size.The addition of KCl or coal fines dispersant can promote the migration of coal fines.?The higher the coal rank coal from the average particle size is small.And the greater the flow out of the larger mean particle size of coal fines.The addition of dispersant is favorable for the migration of small particle coal.(5)In view of the migration mechanism of pulverized coal,from the angle of coal control fines,it is suggested that the coal seam with high degree of coalification should be considered for fracturing.The coal fines dispersing and KCl agent is added in the fracturing or flowback process.Increase the flow of injection and reproducing liquid properly.