The Research On Gas Channel Failure Mecha-Nism And Repair Technology In Coalbed Methane Well

The problem that coal seam products powdered coal has been an important problem putsbounds to coalbed methane exploration. Field research was made to actual production ofHancheng and Sanjiao Ordos Basin, on this basis, we analyzed reasons for the debasementeven evanish of gas production after coalbed methane well operation, researched the systemof gas channel preserve, dredge and restorative procedure, which consists of the generation,locomotion, deposition block and drainage key technique of powdered coal. This is veryimportant to the output increase and stable production period extension of coalbed methanewell, establishes the theoretic foundation for the realization of stable, continuous, long-termproduction of coalbed methane in China.This is key to realize coalbed methane commercialexploitation.Via analyzing reasons for the debasement of gas production after coalbed methanewelloperation, influence model of main factors to gas production of coalbed methane well wasestablished, differentiation to the main reasons for the debasement of gas production aftercoalbed methane well operation was made, concluded that powdered coal is the main reasonfor the debasement even evanish of gas production after coalbed methane well operation. Inorder to research powdered coal formation， this article researched the formation anddestruction of gas channel.(1)Formation mechanism of coalbed methane well gas channelwas analyzed; gas channel flow conductivity mathematical model was built and the influenceof gas channel flow conductivity to single well daily capacity was got; gas channel modelafter powdered coal deposition block was established, the influence of powdered coal to gaschannel permeability, flow conductivity and gas deliverability was analyzed.(2)Based onmechanism that gas channel destruction creats powdered coal, condions of gas channeldestruction creats powdered coal in immediate vicinity of wellbore and far wellbore. Bottomhole pressure buildup model of well off and well workover was built,and based on this modeland different hydrous coal mechanical strength test, well off and well workover regime andreflow production regime without gas channel destruction was established, so that coalbedmethane wells can product efficiently and steadily.In order to research the movement of powdered coal in gas channel, based on mechanicalprinciples, we got overrolling start condition, suspension transport condition of powdered coal grain which has dropped out in gas channel, and critical condition that proppant can’t be bledoff. Porosity and permeability model of powdered coal dropout, movement and block wasbuilt. The influence of powdered coal displacement to coal porosity and permeability wasanalyzed. Based on this permeability model, powdered coal moderate blowout theory wasresearched, powdered coal moderate blowout model and powdered coal carrying formationfluid velocity window model was built, so that powdered coal grain that has dropped out canbe bled off moderately, and flow matrix can be dredged, permeability increased so that perwell production can be raised. Based on the concept that friction drag is minimal in gaschannel, moderate powdered coal carrying formation fluid critical velocity was given. Basedon porous media model and equivalent circular tube model, near wellbore and far wellboregas channel powdered coal grain movement mathematical model was established respectively,the movement rule of powdered coal in gas channel was analyzed.Considering the influence of gas channel wall to powdered coal grain subsidence, thedeposition model of powdered coal grain in the gas channel was built, powdered coalsubsidence terminal velocity was got. Based on gas channel equivalent model in nearwellbore and far wellbore, the sedimentary rule of powdered coal in gas channel of coalbedmethane well in drainage and production period and well off period, and the influence factorto powdered coal deposition in gas channel was analyzed. Current character after powderedcoal deposition and the influence of powdered coal deposition to coal reservoir porosity andpermeability was analyzed. Powdered coal block mathematical model was established, theinfluence of drawdown pressure and initial powdered coal thickness to equivalent damageradius. Based on powdered coal dropout and blockade, the model of powdered coal grainmovement in gas channel was established. Based on parameters of coal reservoir and actualphenomenon, coal reservoir blockage can be diagnosed. Based on the model of loosepowdered coal amount and model of loose powdered coal grain diameter, gas channelblockage can be predicted. Based on the mechanical analysis of modifiable powdered coalblockage grain, the critical condition of modifiable blockage was got, the foundation of gaschannel dredge was established.