Research On The Law Of Gas Drilling Horizontal Wellbore Purification

Gas drilling effectively protects the reservoir and horizontal well greatly expands the seepage area.Gas horizontal drilling combines the advantages of gas drilling and horizontal well,which is an effective development mode in the low pressure,low permeability and low abundance tight gas reservoir.One of the restriction factors is the problem of hole cleaning in the horizontal well which influences some important problems as drilling efficiency,drilling cost,wellbore quality and drilling safety.Compared with conventional drilling technology,the cutting-carried ability of gas is poor and the force situation of particles in horizontal well is different from the one in vertical well,which lead to the essential change of cuttings migration in gas horizontal drilling.Therefore.the further study of hole cleaning law is greatly significant to the generalization and application of the gas horizontal drilling technology.Based on aerodynamics and gas-solid two phase fluid mechanics and making multipurpose use of computational fluid dynamics(CFD),model calculation and the visual experiment,the hole cleaning law in gas horizontal drilling is studied and the distribution of annular flow field in horizontal annulus is by the thorough analysis.The cuttings migration law and the cuttings bed dynamic migration law in gas horizontal drilling are researched.Main achievements are as follows:(1)Using the bit experimental apparatus to drill the rock,through the screening experiment,the particle size distribution after the roller bit breaking rock is got,which will provide the basis for the research of cuttings migration mechanism and the visual experiment.(2)The flow field of the horizontal annulus under different conditions as diameter extension,variable diameter and eccentric annulus is analyzed and simulated with the CFD software.And the flow law of horizontal annulus is concluded.(3)The motion model of cutting grain is established by the analysis of the force on cuttings.The influence of some factors as grain size,gas injection rate,eccentricity and the rotating speed on cutting grain and particle group is concluded.(4)The motion model of the particle on cuttings bed and the calculation model of the cutting bed height are established,and analyze the influence of some factors as grain size,gas injection rate,eccentricity and the rotating speed on the height of cuttings bed.Based on visual cuttings transport experiment,the paper verifies the migration feature of particles on cuttings bed and migration regularity of cuttings.(5)The paper proposes a design scheme of hole cleaning short section in gas horizontal drilling which could change the flow field,repeatedly crush the rock and lift particles physically.The influence of the short section on the flow law is obtained by the use of the CFD software.The result indicates that the helical flow field will occurred when gas flows through the short section which is helpful to the cleaning efficiency.And it has a greater impact on annular flow field and migrates a longer distance when the short section rotates.The research can effectively guide the measures of horizontal hole cleaning as well as the design of flow parameters of horizontal wells,so it has a positive significance for ensuring the safety and extending the horizontal wellbore effectively in the progress of gas horizontal drilling.