Research On Two-Phase Flow Mechanism And Mathematical Model Of Water-Bearing Tight Gas Reservoirs

Tight gas is the unconventional natural gas resource with great potential.There are nano and micrometer sized pore throats developed in the tight reservoir,strong heterogeneity make the gas-water distribution complicated,the law of fluid flow no longer accords with Darcy flow,unclear understanding of gas-water flow law.Tight gas reservoirs are often developed by horizon well fracturing,after fractured can form complex fracture networks in the reservoirs,the problem of complex flow in multiple media exists in fluid flow,it is difficult to accurately predict the two-phase production of water-bearing tight gas fractured horizontal wells,the law of pressure propagation in the production progress is unclear.This paper takes Daniudi gas field,in the Ordos Basin as target block,through laboratory experiment,theoretical analysis and numerical simulation combined method,the occurrence mechanism of water phase in water-bearing tight gas reservoirs and gas-water two-phase multiscale nonlinear seepage is studied,the gas-water two-phase and multi-region coupling productivity prediction model and dynamic supply boundary propagation model of fractured horizontal wells are established.Mainly carried out the following research work:(1)The capillary force curve is obtained on the basis of the pressure pump experiment,the pore throat radius of the study block is mainly between 0.38μm0.99μm,and the reservoir types are divided into three categories according to the shape of capillary force curve.Based on the distribution range of pore throat radius the precision of CT scanning was determined and digital core reconstruction was carried out on the basis of CT scanning to quantitatively characterize,the microscopic pore structure of tight reservoir,which showed that the roar had more influence on the porosity of reservoir than the porosity.(2)Based on the NMR curve and the flow condition of water phase,the water phase can be divided into:movable water,constrained water and bound water.In the process of micro-visualization experiment of gas water flooding,with the increase of displacement velocity,the proportion of medium porosity in matrix reservoir is the highest(57.25%),and that of large porosity in micro-fracture reservoir is the highest(62.86%).According to the result of microscopic visualization experiment,there are four types of constrained water in water-bearing tight gas reservoirs:angular,membranous,columnar and cluster.With the increase of displacement velocity,a small amount of angular and columnar constrained water can be driven out.Based on the results of matrix model and fracture model)the gas phase preferentially forms the dominant channel in the pore throat with uniform size.The constrained water saturation is high,mainly cluster constrained water and columnar constrained water.The existence of micro-fractures can connect more small pores,and the pores of all levels in the reservoir can be used evenly,which plays a positive role in the flow of fluid in the reservoir,however,there are still some pores that cannot be used.(3)The bubble method was used to conduct the threshold pressure gradient experiment.With 0.001 MPa/m as the limit of threshold pressure gradient,permeability and threshold pressure gradient was inversely proportional,water saturation and threshold pressure gradient were proportional.The stress sensitivity test was carried out by the method of constant confining pressure changing internal pressure,the permeability loss rate increased with the increase of water saturation.When the water saturation is 30%,the permeability loss rate of matrix core is 74.73%,and that of fracture core is 90.77%,when the water saturation is 70%,the permeability loss rate of both types of cores exceeds 95%.Compared with the stress-sensitivity test results of matrix core,it can be found that the stress sensitivity of reservoir with fractures is stronger.Based on the threshold pressure gradient and stress-sensitive experimental data,water saturation was introduced into the original equation to provide a basis for the establishment of the subsequent productivity model.(4)Based on the complex flow field morphology and fluid flow law of fractured horizontal wells,the gas-water two-phase production prediction model of fractured horizontal wells in tight gas reservoirs with low water saturation and highwater saturation is established.Low water saturation is regarded as quasi-singlephase gas flow,while high water saturation is regarded as gas-water two-phase flow.The analytical mathematical models of gas-water two-phase nonlinear seepage in the main fracture region,the fracture network region and the matrix region were established considering stress sensitivity,slippage effect,threshold pressure gradient,capillary force and relative permeability,the calculation equation of boundary pressure of each region was obtained based on the principle of material balance,the three-region coupled gas-water two-phase productivity model was obtained by equivalent seepage resistance method,the average pressure distribution model is derived by using the area weighting method,the dynamic supply boundary propagation calculation method is obtained by combining the material balance and steady-state sequential replacement method.The model was verified by actual production data,and the fitting accuracy of daily gas volume and daily water volume reached 90.56%and 85.17%.(5)By comparing the influencing factors under different water saturation,the main controlling factors of productivity under different water saturation were obtained.Under the condition of low water saturation,production pressure difference is the main controlling factor of productivity,and the production system of step pressure control can be adopted in the production process.Under high water saturation,water saturation is the main controlling factor of productivity.In the early stage of production,attention should be paid to water production to prevent liquid accumulation near the wellbore.It should also be noted that the influence of matrix permeability on production under different water saturations is not the most important.However,when considering the stress sensitivity,the cumulative gas production under the two water saturations decreases by more than 13%,indicating that the influence of stress sensitivity on productivity under both conditions cannot be ignored.This paper reveals the water-phase occurrence mechanism of water-bearing tight gas reservoirs,studies the nonlinear flow law of water-bearing tight gas reservoirs,and forms the gas-water two-phase productivity prediction method of fractured horizontal wells in water-bearing tight gas reservoir,which provides important theoretical basis and technical support for the development of fractured horizontal wells in water-bearing tight gas reservoirs.