Study On Percolation Mechanism And Reservoir Evaluation Of Xujiahe Low Permeability Sandstone Gas Reservoirs In Central Sichuan Basin

Xujiahe low permeability sandstone gas reservoirs in central Sichuan basin produced a lot of water in the development process. Aiming at this issue, the microscopic pore structures of reservoir and percolation mechanism in reservoir pressure are studied, and a new reservoir evaluation method is formed combined with static and dynamic reservoir characteristics. The following key findings are achieved:(1) Pores less than 0.1 micrometer diameter are accounted for 30%-70% of pore volume in Xujiahe low permeability sandstone gas reservoirs; Throat is not only gas flow channel, but also an important gas storage space. This pore structure determines the tight and high water saturation reservoirs.(2) The concept of movable water is proposed, which divides raw water into bound water and movable water; Movable water saturation NMR test and well logging interpretation method are proposed.(3) Method of using reservoir movable water saturation to predict water production feature of gas well is established:gas well where movable water saturation is below 6% does not produce water; For movable water saturation between 6% to 8%, gas well yields a few water; For movable water saturation between 8% to 11%, gas well yields a lot of water; When movable water saturation is greater than 11%, gas well produces water severely. The results can be used to predict gas wells water production features during the development, guiding to a reasonable development plan, and to achieve effective water control purposes.(4) The gas slippage effect under the reservoir conditions can be neglected in low permeability sandstone gas reservoir and stress sensitivity is weak. More than 60% water saturation, the gas phase flow showed liquid nonlinear seepage characteristics; At about 40% water saturation, gas flow characteristic is gradually showed; less than 30% water saturation, gas flow characteristics is significantly appeared, and high velocity will produce turbulent flow phenomenon.(5) Gas and water relative permeability is not only a function of water saturation, but also the function of pressure gradient. Gas relative permeability decreases with pressure gradient, while the water relative permeability increases with the pressure gradient. A gas-water two-phase flow model of low permeability sandstone gas reservoir is established. Future gas production dynamics of gas well is predicted based on this new model.(6) According to the static and dynamic features of low permeability sandstone gas reservoirs, a reservoir evaluation parameter system including six parameters is established:permeability, porosity, gas saturation, mainstream throat radius, threshold pressure gradient and mobile water saturation. A new type of multi-parameter comprehensive reservoirs evaluation method for low permeability sandstone gas reservoirs is formed. This result lays a solid foundation for reservoir evaluation and selection for Xujiahe low permeability sandstone gas reservoirs in Sichuan basin.