Wellbore Multiphase Flow Analysis Of Coalbed Methane Gas Drilling And Process Parameters Optimization

Coalbed methane is a kind of unconventional natural gas, with clean and pollution-freecharacteristics, which is abundant in our countury.Coalbed methane plays an important role inthe energy structure reform of our country. Because the coal reservoir has the characteristicsof low mechanical strength, low porosity, easy to collapse and easy to be polluted, thegas-filled underbalanced drilling technology, is more and more applied in the coalbedmethane exploration to improve coalbed methane recovery.However, coalbed methane miningtechnology is not very mature in our country now.There is a lot of key technology,such as theoptimal injection volume in the process of underbalanced drilling, injection pressure andcuttings migration rule remain to be continuously explored and researched.The research object of this paper is Zheng1-H3well, located in qinshui basin, Jinchengcity, with gas injected from a cave vertical well.In order to optimize drilling processparameters of this well, following work has been done in this paper:a. Theoretical research and calculation of a one-dimensional model of the wellbore: First,it was assumed that the solid phase was evenly distributed in the liquid phase; Second,on thebasis of the homogeneous flow model, the wellbore pressure drop calculation formula witheccentricity, density formula, gas rate formula and calculation formula of annular returnvelocity were derived in this paper;Third, in Excel,VBA was used to program and calculatethe mixture density, wellbore pressure, gas volume fraction and gas superficial velocity.b.CFD simulation of actual three-dimensional wellbore model and drilling fluid flow insidethe drill string:First, wellbore annulus flow concentric computational domain andcomputational domain with eccentricity of0.8,and computational domain in the drilling stringwere built.After the computational domains were meshed,they were guided into the CFXsoftware,in which,drift model was selected to simulate the three-phase flow in the wellboreand ordinary single-phase turbulent model was selected to simulate the flow of drilling flowin the drilling string.After boundary conditions were set,CFX started to slove the flow underdifferent working conditions.Second,after the solving, the flow characteristics (phasedistribution, pressure distribution, density,velocity) of multiphase flow in the wellbore werestudied and the computed results were compared with the calculation results of one-dimensional model to point out the weakness of one-dimensional model and the necessityof CFD simulation.Third, how pressure and density change with the gas injection volume andwater injection volume was studied.Then cuttings migration rule in the wellbore wasresearched,including cuttings superficial velocity, cuttings concentration changing with theamount of gas injection and liquid injection;Fourth,researches of technological parameterswere done on gas injection pressure, liquid injection pressure changing with gas injectionvolume, liquid injection volume and drilling speed, and the effects of the drill stringeccentricity on the multiphase flow in wellbore.Finally, optimized parameters of gas injectionvolume,gas injection pressure,liquid injection pressure were got under different liquidinjection volume.c. Field experiment validation: Some of the values calculated by CFD simulation werecompared with field experiment datas.The result of the comparison showed that the CFDsimulation was correct and feasible in the range of permissible error.