Research On Flow Model Of CBM Production System

CBM well productivity simulation and prediction is the basis for formulating CBM development plans.The complex morphological fractures caused by the cleats and the absorption and desorption complicate the productivity simulation of fractured horizontal wells in CBM reservoirs.The unsteady characteristic of CBM production is obvious,and the flow in the wellbore impacts formation productivity.If ignoring this,the calculation accuracy will be affected.Therefore,the establishment of an integrated reservoirwellbore model,which can simulate the complex fracture-horizontal well flow system and the unsteady-state flow is of great significance for the realization of intelligent CBM procuction and the CBM productivity improvement.Based on the source function theory and Newman product method,combined with the principle of mirror reflection and superposition,a seepage mathematical model that can simulate the complex fracture-horizontal well flow system in the closed reservoir is established.The model has good modeling flexibility,and can simulate the seepage process in systems of fractures and horizontal well sections with any bending shape.This model has good scalability and can be used to develop integrated gas reservoir-wellbore model.According to the adsorption and desorption characteristics of CBM reservoirs,the gas-water two-phase flow equation is established.A CBM pseudo-pressure function is proposed and the relationship between pseudo-pressure and density is established.Therefore,the comprehensive diffusion equation for CBM reservoirs is derived.Based on the complex fracture-horizontal well flow model,the complex fracture-horizontal well productivity calculation model for CBM reservoirs is deduced to solve the productivity calculation problem under the complex seepage conditions.Compared with the simulation results of commercial numerical simulation software CMG,the established model is proved to be accurate.The effects of fracture angle,fracture length and bottom hole flow pressure(BHP)on the productivity of fractured horizontal wells are analyzed.It is founded that the angle between the fracture and the horizontal section should be 45°?90°and the optimal fracture length should meet the plane penetration ratio equal to 0.5.The fluctuation of BHP has a great influence on the production of CBM reservoirs.When calculating productivity,considering the effect of the wellbore flow on BHP can further improve model calculation accuracy.Depend on the production characteristics of CBM wells,the unsteady characteristics of the flow in the wellbore are summarized.The net inflow of gas and liquid in the wellbore is not zero,and the flow directions of gas and liquid t in the liquid column section can bedifferent.A wellbore flow model which can simulate wellbore flow at different production stages is established.The model distinguishes the liquid rate through the annulus from the wellhead water production.With continuous wellhead production and dynamic liquid level,the bottom hole flow pressure can be calculated,as well as the formation water production and gas production.Combined with the CBM well productivity model,this model can be used to simulate the production process and calculate wellhead production.Using the wellbore as the inner boundary of the productivity model,a flexible boundary setting algorithm for CBM fractured horizontal wells is proposed.Gas nozzle size and other artificially controllable surfacethe working condition parameters are used as the model boundaries.Multi-node comprehensive analysis methods are used between different boundaries.Thereby,integrated CBM reservoir-wellbore simulation is obtained.The model can simulate the entire production process of CBM wells,calculate the gas production and water production at the wellhead,as well as dynamic liquid level,casing pressure,bottomhole flow pressure,formation water production,formation gas production and other parameters.Compared with the horizontal well productivity model with constant pressure boundary,it is found that the integrated model can better simulate the production of CBM wells and give a more realistic simulation effect.The flow model of the CBM production system established in this paper is simple to calculate and reliable.And this model has advanced simulation functions and is suitable for the optimization of CBM reservoir development schemes and the optimization design of lifting schemes.It can promote the digital gas field technology of CBM reservoirs.