Study On Percolation Mechanism And Productivity Model Of Tight Sandstone Gas Reservoir

Low permeability tight sandstone gas reservoir has small pore throat,high water saturation and complicated percolation mechanism.Therefore,the research chooses the typical zone in the southeast of Ordos Basin as the research.Based on the macroscopic basic geological understanding,the microscopic and macroscopic characteristics of the reservoir are studied by using scanning electron microscopy,casting thin sections,gas-water phase permeability,high-pressure mercury injection,gas-water visual displacement model and rock deformation as the experimental methods,combined with rock mechanics,seepage mechanics,reservoir physics,fractal theory,oil and gas field development engineering and other multidisciplinary knowledge.The percolation law of the reservoir is analyzed,and a set of productivity theories for vertical wells and fractured wells with the same gas-water production in the process of quantitative analysis of pressure reduction are established,and formed a method to classify and evaluate the developed reservoirs.The results were achieved:（1）The two kinds of sandstone are developed in the layer of the study area including quartz and lithic quartz,with an average porosity of 8.41%and an average permeability of0.45×10^-3μm²,which are typical tight sandstone reservoirs.The reservoirs less than0.1×10^-3μm² account for a relatively high percentage（36.0%）,resulting in high water cut in gas reservoirs.（2）The reservoir mainly develops dissolution intergranular pores,point-like and sheet-like throats.The pore throat is mainly small pores–micro throats.The pore throat structure can be divided into four categories.The properties of pore throat structure become worse,the pore throat radius decreases,the heterogeneity increases,the starting pressure gradient increases,and the slippage effect is obvious from I to IV.（3）The residual water mainly occurs in three forms:cluster formed by bypass flow,isolated corner and membrane.Type I pores are mainly dominated by membrane residual water,with the minimum irreducible water saturation,the maximum isotonic point,the minimum pressure difference of water phase flow node,the widest gas-water two-phase area,and the maximum pressure drop of pressure relief production.The proportion of cluster and corner residual water increases from I to III.The transition type of gas-water displacement is uniform→reticular→finger,and the displacement efficiency decreases in turn.Type IV pore has the longest pressure relief time,the highest residual water,the smallest pressure drop and the smallest recovery.（4）Based on the stress sensitivity method and the rock nonlinear elastic method,the deformation models for predicting porosity and permeability under different stresses are established respectively,with the relative errors of<2.25%for porosity prediction and<0.50%for permeability prediction.At the same time,the relationship between irreducible water saturation,water film thickness and gas-water two-phase permeability curve considering the rock tortuosity under different effective stress are established.The research shows that the irreducible water saturation increases with the effective stress,the water film thickness increases,and the relative permeability of gas-water two phases changes slightly.（5）The productivity models under different conditions（unstable,pseudosteadystate flow and stable）of Darcy and non-Darcy（high-speed non-Darcy,low-speed non-Darcy considering starting pressure gradient and low-speed non-Darcy considering slippage effect）under different stresses are established.When the formation pressure decreases by 10%,the vertical well productivity decreases by 51.9～82.3%,and the fractured vertical well productivity decreases by 42.9%～69.3%.When the factors else are the same,the fractured gas wells increased production by 1.58 to 3.17 times.Compared with the productivity equation only considering permeability sensitivity,the gas well productivity model considering porosity and permeability sensitivity is closer to the production.（6）The influencing factors of IPR of gas well productivity in the depressurization process are analyzed.Compared with un-fractured gas wells,the fractured gas wells are less affected by stress sensitivity,rising water saturation,and decreasing gas-to-water ratio.The productivity of gas wells with different types of pore structure is calculated.The results show that the pore structure decreases the productivity of gas wells from type I to IV,and the decrease range increases.However,fracturing increases the gas well productivity.（7）Six parameter evaluation systems reflecting reservoir characteristics including porosity,permeability and start-up pressure gradient are established.The evaluation chart of tight gas reservoir development in the southeast area of Ordos Basin is formulated and verified by production data.The model tends to be more reasonable when the reservoir is evaluated by combining dynamic parameters.Combined with percolation model experiment,the production performance of different types of gas wells is analyzed,which is consistent with the results of pressure relief extraction experiment.Comprehensive analysis shows that a reasonable development plan should be formulated before production,and production pressure difference should be strictly controlled to maximize gas well productivity.