Distribution Characteristics And Formation Mechanisms Of Gas-Water Distribution In The Tight Sandstone Gas Reservoirs

Tight sandstone gas has been brought into attention as an important exploration and development target in unconventional resources,which is characterized by complex gas-water distribution compared with conventional sandstone gas.The complex gas-water distribution strongly restricts the exploration and development of tight sandstone gas.Therefore,it is of great significance to investigate the characteristics,controlling factors,and formation mechanisms of the complex gas-water distribution for the tight sandstone gas exploration and development.In this paper,the nuclear magnetic resonance(NMR),pressure-controlled mercury porosimetry(PCP),Micro-CT(X-CT),and physical simulation for the migration and accumulation of unconventional hydrocarbon are performed on the Upper Paleozoic Permain Shanxi and Lower Shihezi Formation in the Daniudi area of the Ordos Basin to investigate the characteristics,controlling factors,and formation mechanisms of the gas-water distribution of tight sandstone gas from macroscale to microscale.The types and characteristics of gas-water distribution in the reservoir scale and its macroscale geological controlling factors are firstly revealed by the combined research of sedimentology,well logging,and natural gas geology.The formation processes,critical criterias,and gas accumulation characteristics of gas-water distributions are then analyzed by physical simulation experiments of gas flooding under the formation conditions.Thirdly,the microscopic controlling factors are revealed by the studies on the pore structures and microscale gas-water occurrence characteristics,which are analyzed by the combination of the NMR,PCP,and X-CT.In the last,the macroscale formation process and microscale-core scale-reservoir scale formation mechanisms of gas-water distributions are elaborated by combining the hydrocarbon generation history,gas migration period,and dynamic variation mechanisms on the gas-water interface.Tight sandstone gas reservoirs are characterized by three main types of gas-water distributions,including gas water inversion distribution,residual water distribution,and edge and bottom water distribution.The gas water inversion exhibits variation from gas layer to gas-water mixed layer to water layer from bottom to top in the well scale and exhibits change from gas zone to gas-water transition zone to water zone from tectonic lower parts to higher parts in the section and plane scale.The residual water distribution and edge and bottom water distribution show the changes from gas-water mixed layer to gas layer and water layer to gas layer in the longitudinal of single well,respectively.The residual water distribution is dominated by gas zone with scattered gas-water transition zone in the tectonic lower parts in section and plane,while the edge and bottom water distribution shows a wide distribution of water zone in the lower parts.Hydrocarbon generation intensity,reservoir quality,and differential pressure between source rock and reservoir rock are three macroscopic geological controlling factors for gas-water distribtution.The hydrocarbon generation intensity is the material controlling factor with the hydrocarbon generation intensity of 25×10~8m~3/km~2 controlling the area of the gas-bearing zone.The reservoir quality serves as the storage controlling factor with prosity of 7.2%and permeability of 0.41 mD controlling the boundary between gas-bearing zone and water zone.The differential pressure between source rock and reservoir rock(DPSR)acts as the dynamic controlling factor with 18.5 MPa identified as the critical pressure criteria between the gas-bearing zone and water zone.On the premise that hydrocarbon generation intensity controls the areas and gas-bearing characteristics of tight gas sandstone reservoirs,the types and styles of gas-water distribution are coupled controlled by reservoir quality and DPSR.Core scale analyses indicate that the permeability and charging pressure coupled control the formation processes and formation criterias of gas-water distributions,determining the types of gas-water distribution and their gas saturation growth processes and degrees.The results also indicate that the gas water inversion is more likely to form in the low permeability reservoir with permeability of 0.38 mD acting as the upper formation limit.The gas water inversion is characterized by low gas saturation and rapid gas saturation growth model.Greater permeability is favorable for the formation of residual water distribution and edge and bottom water distribution,which are characterized by rapid to stable and slow to stable saturation growth patterns and high gas saturation.Pore scale analyses suggest that the pore structure and microscopic fluid occurrence characterisitics control the fluid flow characteristics and fluid flow capacity,determining the formation criterias and gas-bearing capacities of different types of gas-water distributions.The critical charging pressure of gas water inversion and critical formation pressure criterias of residual water distribution and edge and bottom water distribution decrease with the increasing pore and throat radii,pore-throat connectivity,and movable capillary water porosity.It can be inferred that reservoirs with macropores and medium-coarse throats dominant pore structure is more likey to form residual water distribution and edge and bottom water distribution with high gas saturaiton,while the meso-micropores and medium-fine throats dominant pore structure is favorable for the formation of gas water inversion distribution,which is generally characterized by low gas saturation.The formation processes of different gas-water distributions are deduced by the comparative analyses of gas charge period,hydrocarbon generation history,formation processes in core scale,and dynamic variation mechanisms on the gas-water interface,in which the gas water inversion mainly experiences three evolution stages,including gas charging stage with the formation of gas water inversion in the early Juassic(210?200Ma),gas-water boundary upward movement stage with the expansion of gas water inversion in the early Jurassic to late Cretaceous(200?100 Ma),and gas water inversion stabilization stage in the late Cretaceous(100 Ma)to present,while the residual water distribution and edge and bottom water distribution experience the gas charging stage in the early Juassic,gas stable migration stage with formatin of conventional gas-water distribution in the early Jurassic to late Cretaceous,and conventional gas-water distribution stabilization stage in the late Cretaceous to present.Combinding the results of macroscale,core scale,and pore scale analyses,the formation mechanisms of gas-water distributions can be concluded as follow,the pore structure and microscopic fluid occurrence characteristics determine formation criterias and gas-bearing capacity,charging pressure and permeability coupled control the formation processes,types,and gas accumulation degrees,and DPSR and reservoir qualities coupled control the distribution characteristics and distribution patterns.