Research On Gas & Water Coupled Flow In Formation And Wellbore Of GB Low Permeability Reservoir

Low permeability gas reservoir has become an important gas supply source in China.Compared with conventional gas reservoirs,the geological features of low permeability gas reservoir are more special and their seepage mechanism is more complicated.GB gas reservoir is the main gas zone of natural gas production in China,and it is also a typical large-scale karst palaeogeomorphology of low permeability gas reservoir.The characteristics of GB gas reservoir are deposition stability,stratification distribution,low porosity and low permeability.Gas-water two-phase co-permeation zones are wide so that the gas wells can still have productivity after water breakthrough.However,it will lead to the increase of flow resistance and wellbore effusion,causing the deviation in in the productivity evaluation of the gas well and the inaccuracy of the prediction of the wellbore pressure drop,which will affect later gas production process.It leads to the increase in the low production rate and low efficiency of the gas wells from year to year.Therefore,it is urgent to study the production capacity and wellbore pressure drop of gas reservoir.This paper firstly analyzed the general characteristics of GB low permeability gas reservoir and summarized the status of development of gas reservoir.Next,the critical liquid-carrying model and flow pattern chart of gas wells was modified,and a comprehensive gas-water two-phase pressure drop model which was suitable for effusion and self-injection conditions was established.Then,the gas-water-two-phase well productivity model considering starting pressure gradient,gas slip,stress sensitivity,skin effect and high-speed non-Darcy effect was derived.Finally,a coupling model for the formation and wellbore based on the pipe flow model and the productivity model was established.The main achievements and understanding can be summarized as follows:(1)The high irreducible water saturation caused the gas wells to have no anhydrous gas recovery period and the wider gas-water two-phase co-permeation zone ensured that the gas well was not easily run over by the fluid.The production time of gas well can be extended by the implementation of drainage and gas extraction process.(2)The critical liquid carrying model was modified according to the data of the gas well on the verge of the fluid accumulation,and the prediction accuracy of the model was as high as 90.91%.The criterion for the flow pattern of annular flow was modified,the criterion for the flow pattern of slug flow and bubble flow pattern was established,and the flow pattern chart was modified.(3)Aiming at the phenomenon of fluid accumulation in the gas well,a pressure drop model which was suitable for the liquid accumulating well was established.Combining with the modified flow pattern chart,an integrated pressure drop model which was suitable for both liquid and self-injection production states was established.The relative error of the liquid accumulating well and the self-injection production well was 9.74%and 10.67%respectively.(4)Considering the starting pressure gradient,gas slippage,stress sensitivity,skin effect and high-speed non-Darcy effect,a gas-water two-phase gas well productivity model based on the definition of gas-water two-phase pseudo pressure was established.The calculations of the practical examples showed that the relative error between the productivity model of gas-water two phase well and the well test productivity was only 4.64%,which indicated that the model had good accuracy,and the water production had great influence on the productivity of gas well.(5)Gas-water two phase pressure drop model and productivity model were used to establish the coupling flow model.Combined with gas well production dynamic data,gas reservoir material balance equation,water saturation equation and gas water relative permeability curve,gas reservoir engineering parameters could be obtained only by gas well production data based on bottom hole flow pressure as the fitting standard,with the help of optimization algorithm.(6)The program was compiled using Visual Basic language.This program can achieve the prediction of wellbore pressure drop,gas well productivity evaluation and gas reservoir engineering parameter solution for liquid accumulating well and the self-injection production well.(7)The analysis of the influence factors of gas well productivity showed that gas well productivity was positively related to formation pressure,reservoir thickness and slip factor,and negatively related to water-gas volume ratio,start-up pressure gradient and reservoir stress sensitivity index.Besides,the production water-gas volume ratio,reservoir stress-sensitive index had a greater impact on gas well productivity,while the start-up pressure gradient and slip factor had less effect on gas well productivity.