Experimental Study On The Properties Of Surrounding Rock Of Tectonic Coal Shaft Wall Under The Action Of Drilling Fluid Circulation

Coalbed methane resources are widely distributed in soft structural coal seams in China,and the evaluation and control of the wellbore stability of structural coal seams are the key issues to determine the efficient exploitation of structural coalbed methane.In the process of structural coal seam drilling,the effect of drilling fluid will have an important impact on the stability of the wellbore.It has important practical significance and theoretical value for safe drilling of tectonic coal seams and coalbed methane exploitation to carry out research on the influence of drilling fluid circulation times and types on the physical and mechanical properties,permeability characteristics and acoustoelectric response characteristics of the surrounding rock of the tectonic coal body at the radial scale of the tectonic coal seam.On the basis of simulating the circulation of drilling fluid in the drilling process of tectonic coal formation,the property test experiment of surrounding rock of tectonic coal shaft wall after drilling fluid circulation was carried out,the influence and mechanism of drilling fluid circulation times and types on the engineering properties of surrounding rock of tectonic coal shaft wall are analyzed.The main conclusions are:(1)According to the characteristics of the tectonic coal formation and the drilling process,three types of water-based drilling fluid formulations were designed and optimized in the orthogonal test.The three types of drilling fluids all meet the performance requirements for drilling in the tectonic coal formation,and have the characteristics of high p H,low filtration properties and high dynamic-plastic ratio.The coal rock expansion rate test proves that the selected drilling fluid has good sealing and stable wellbore ability.(2)The physical properties,seepage properties,mechanical properties and acoustic and electrical response characteristics of wellbore tectonic coal samples under the conditions of different drilling fluid cycles were obtained by testing.(1)The density of the surrounding rock samples of the tectonic coal shaft wall increases gradually,and the wave velocity first decreases and then increases with the increase of the number of cycles.(2)The permeability coefficient of the surrounding rock samples of the tectonic coal shaft wall increases first and then decreases with the increase of the number of cycles.(3)With the increase of the number of cycles,the peak strength of the surrounding rock of the tectonic coal shaft wall shows a trend of first decreasing and then increasing;the peak strain variation has a large discreteness;most of the failures of the samples are at a certain angle to the axial direction.The cohesion of the surrounding rock of the tectonic coal shaft wall first decreased and then increased,and the internal friction angle decreased gradually.(4)Under uniaxial compression,the minimum resistivity of the surrounding rock samples of the tectonic coal shaft wall increases first and then decreases with the increase of the number of cycles,and the maximum value of the acoustic emission energy basically decreases first and then increases.Internal cracks are mainly shear failures.And under variable-angle shearing,the maximum value of acoustic emission energy first increases and then decreases,and the internal cracks of the samples are mostly shear failures.(3)According to the characteristics of drilling fluids Y-1,Y-2,and Y-3 with high p H value,low fluid loss and high dynamic-to-plastic ratio,the physical properties,seepage properties,mechanical properties and acoustic and electrical response characteristics of wellbore tectonic coal samples under the conditions of different drilling fluid cycles were obtained by testing.(1)The density of the surrounding rock in the tectonic coal shaft wall after the three types of drilling fluid circulation increases obviously,and the increase rate is Y-1>Y-3>Y-2;the wave speed increases to a certain extent.(2)The decrease rate of permeability coefficient of surrounding rock of tectonic coal shaft wall is Y-1>Y-3>Y-2.(3)The peak stress and cohesion of the surrounding rock of the tectonic coal shaft wall are reduced,and the reduction rate is Y-1>Y-3>Y-2,and the reduction rate of the internal friction angle is Y-2>Y-1>Y-3.(4)Under uniaxial compression,in the stage of rapid decrease of resistivity of surrounding rock of tectonic coal shaft wall,the overall decreasing speed is Y-1>Y-2>Y-3.After the drilling fluid circulation,the maximum energy generated by the acoustic emission event is Y-1 < Y-2 < Y-3,and the internal crack of the sample is mainly shear failure;and under the condition of variable angle shearing,the internal crack shear failure and tension-shear composite failure of the sample are distributed.(4)From the perspective of drilling fluid circulation times and drilling fluid types,the change mechanism of engineering properties of surrounding rock of tectonic coal shaft wall under the action of drilling fluid circulation is analyzed.(1)With the increase of the number of cycles,the scouring,filtration,adsorption and hydration effects of drilling fluid on the surrounding rock of the tectonic coal shaft wall during the circulation process become more and more obvious,which reduces the mechanics of the surrounding rock of the tectonic coal shaft.strength,permeability coefficient decreased.(2)The high p H environment of drilling fluid Y-1 makes the mineral components in the surrounding rock of the tectonic coal shaft dissociate,the cement dissolves,the mutual connection of the tectonic coal particles is weakened,and the mechanical strength of the surrounding rock of the tectonic coal shaft wall decreases most significantly;The low filtrate provided by the high viscosity carboxymethyl cellulose in the drilling fluid Y-2 combined with the vegetable glue molecules makes the infiltration of the drilling fluid filtrate less,so that the damage to the surrounding rock of the tectonic coal well wall is small,the mechanical strength is reduced to the smallest degree,and the permeability coefficient is significantly reduced.There are 65 figures,31 tables and 101 references in this thesis.