Research On Gas-Water Two-Phase Seepage Of Fractured Horizontal Wells In Shale Gas Reservoir Based On Semi-Analytic Method

As an important field for the increasing of natural gas reserves and production in China,shale gas has great development potential.The multi-stage multi-cluster fracturing technology is the main way to produce stimulated reservoir volume in shale gas reservoirs.Due to the unique storage characteristics of shale gas reservoirs,the gas seepage in the reservoir is complicated.And the gas-water two-phase seepage has existed for a long time,which has a great impact on the production of shale gas wells.At present,the models of shale gas seepage mainly focus on single-phase gas seepage in different occurrence mode.The shale gas desorption and multi-scale flow in multi-porosity mediu,the coupling of matrix seepage and seam flow,and the gas-water seepage after fracturing have not been fully considered in these models.The two-phase seepage mechanism and the productivity prediction of the stimulated shale gas reservoir are inaccurate,which restricts the development of shale gas reservoirs.Therefore,the following research is carried out on the gas-water two-phase seepage in fractured horizontal wells in shale gas reservoir:(1)Based on the characteristics of shale gas reservoir and gas occurrence modes,the multi-scale migration of shale gas in multiple porous mediu is analyzed.Considering the comprehensive effects of water saturation,stress sensitivity and effective gas viscosity,the apparent permeability model of the matrix is established according to the distribution frequency of organic pores and inorganic pores.(2)The fracture network apparent permeability model is established based on fractal theory and cubic law,and the two-phase seepage characterization parameters(effective permeability,water saturation,etc.)are analyzed by phase permeability curve in stimulated zone.(3)Based on the source function of the closed formation,considering the effects of fracture interaction,high-speed non-Darcy flow and water saturation,the multi-scale seepage in matrix and the gas-water two-phase flow in fracture network is coupled by applying the spatial discretization technique,superposition principle,Young-Laplace equation,Forchheimer equation,material balance equation and other theories.The two-phase seepage theory and productivity calculation method of shale gas fractured horizontal well are established,and the factors affecting the productivity are comprehensively analyzed.The following conclusions and understandings are obtained through the research of this paper:(1)The shale reservoir has multiple pore structure and high water saturation.The flow is the single-phase gas seepage with irreducible water in matrix area,while the flow is the gas-water two-phase seepage in the fracture network.(2)There are viscous flow,slip flow,Knudsen diffusion and gas desorption,surface diffusion in the organic pores,while slip flow and viscous flow are the main seepage modes in the inorganic pores with water film on the surface.With the decreasing of pore diameter,the contribution of viscous flow to gas flow is gradually weakened,and the contribution of diffusion to flow is gradually enhanced.The higher the water saturation and the stronger the stress sensitivity effect,the greater the decrease of capillary permeability.(3)Considering the influence of pore distribution in shale gas reservoirs,the apparent permeability model at core scale is established.Studies have shown that the stronger the stress-sensitive effect is,and the higher water saturation of the reservoir is,the lower the apparent permeability of matrix is.The longer the average length of the fracture is,the larger the opening is,the greater the absolute permeability of fracture network is.The effective permeability of gas phase increases with the decreasing of water saturation,while the effective permeability of water phase decreases gradually.(4)According to the gas-water two-phase unsteady seepage model of horizontal well with stimulated reservoir volume,the average formation pressure of shale gas reservoir gradually decreases,and the apparent permeability of reservoir increases.The daily gas production and daily liquid discharge of gas wells show a "L" type,and the production is rapidly decreasing at the initial stage.The analysis of production shows that the gas production and water production are W-shaped along the main hydraulic fracture.and the difference of fracture flow at different positions is small in the initial stage of production.The interference between fractures increases and the difference increases gradually over time.(5)The high-speed non-Darcy effect in the fractures adds the flow pressure drop of gas.The average formation pressure and daily production of the reservoir decrease rapidly when the effect of shale gas desorption is not taken into account,and the cumulative gas production is also less.Fracture network parameters mainly affect the initial production of fractured horizontal wells.The initial water saturation of fracture network,width of stimulated zone and fracture network permeability are the main factors affecting the amount of liquid production.The greater the initial water saturation and the width of the stimulated zone are,the lower the permeability of fracture network is,and the slower the water saturation decrease in the stimulated zone.The water saturation decline rate of the fracture network at different locations is different,the water saturation of stimulated zone at both ends of the horizontal section decreases rapidly,while the water saturation at the middle stimulated zone decreases slowly.The results of this study can provide a theoretical basis for productivity prediction and backflow parameter optimization of horizontal fractured wells in shale gas reservoirs.