Study On Choke Size Determination Method For Coalbed Methane Wells After Fracturing Flowb Ack

The coalbed methane(CBM)has become one of the most important unconditional energy,the development of which has aroused great attention and been supported by our government,as a result,a wide prospect will present in future.The coal seam is featured as low permeability,and a great deal of coal fines produced in the development of CMB.If coal fines deposited in the fracture,the flow conductivity will decrease rapidly.The model for calculating the choke size is based on the fracture-wellbore-nozzle system and theory of coal fines starting and proppant flowback,which indicates great significance for optimization of the choke size.The main content in this paper presents as follows.Firstly,the model for the free coal fines starting,suspending flow in the fracture and coal fines elevation in solid-liquid flow was founded in consideration of drainage for free coal fines.The minimum coal fines starting and suspending flow critical velocity was calculated through the moment balance.Secondly,analyzing forces loaded of the proppant in and after the fracture closure,the maximum critical velocity can be calculated on the basis of moment balance.Thirdly,coal fines and flowback fluid sufficiently mix in wellbore,as a result,the fluid properties are changed,the critical velocity of flowback fluid is affected by the mutual interference among solid particles in flow process.The critical velocity carrying the coal fines out was computed by modeling the liquid-solid flow in the wellbore with modification of flowback fluid in different pattern of flow.Fourthly,based on the fitted 3D crack propagation,the flowback control equation was set up with the principle of material balance.And consider calculating the pressure in the fracture and filter loss in different time of the flowback.According to the idea of system analysis,combine the pressure balance in wellbore and choke size,the model and method for calculating the choke size were established.Lastly,the choke size was calculated by numerical simulation with the production data and fracturing construction data of the Qinshui basin.Finally,the fracturing fluid flowback system was optimized.