Study On Formation Water Distribution Of Tight Sandstone Gas Reservoir In The Northern Tianhuan,Ordos Basin

Oil,gas and water distribution has always been a difficult and hot issue in exploration and development,especially the tight sandstone gas reservoir with complex gas-water relationship,there are two kinds of fluids with different phase states in the strongly heterogeneous low-porosity and permeability reservoirs,the distribution law is usually very complex,and it is often difficult to characterize the gas distribution in the reservoirs.Therefore,the study on the distribution law of formation water can be used as an important means to obtain the distribution of natural gas.The northern part of Tianhuan area is located in the northwest of ordos basin.The hydrocarbon generation intensity of the upper Paleozoic hydrocarbon source rocks in the area is low,the reservoir of member8 of Shan1-He8 is dense and heterogeneous,and the gas-water relationship of the gas reservoir is very complex,which seriously restricts the exploration and development of natural gas.Therefore,it is of great significance to study the formation water distribution in this area.In this paper,the key techniques of casting thin section observation,scanning electron microscopy,high pressure mercury injection,constant velocity mercury injection,nuclear magnetic resonance,gas flooding physical simulation were applied,based on the two macro background of low hydrocarbon generation intensity and tight reservoir in the study area,the densification process of diagenetic transformation and formation water geochemical characteristics,pore throat microstructure and microscopic occurrence state of formation water,gas and water macroscopic distribution characteristics and control factors,distribution law of formation water are systematically discussed from micro and macro dimensions.The geochemical characteristics and genesis of formation water are clarified,four kinds of micro-occurrence states of formation water are put forward and their modes and classification criteria are established,the main controlling factors of gas-water distribution and the mechanism of accumulation are identified,the gas-water distribution model and the mechanism model of gas-water extraction are established,which laid a foundation for efficient gas reservoir exploration,the main results are as follows:1.The tight sandstone in the study area undergo a densification process of"compaction leading,cementation worsening,and dissolution adjustment",and the current porosity is 7.48%.Compaction concentration and calcium-bearing mineral dissolution are the main causes of high-salinity Ca Cl₂ formation water,while cementation and adsorption are the main causes of lossing Mg²⁺and Na⁺.The increase of formation water salinity is a process in which the concentrations of Ca²⁺,Na⁺and Mg²⁺continue to increase,the percentage of Ca²⁺increases,and the percentage of Na⁺and Mg²⁺decreases.Reservoirs with low formation water salinity and low Mg²⁺percentage have weak compaction and cementation during diagenesis and relatively high permeability,while reservoirs with high formation water Ca²⁺percentage usually have high volcanic rock debris content,weak compaction resistance and relatively low permeability,and generally have relatively high Na⁺percentage in such reservoirs.2.Based on the nuclear magnetic resonance test of gas-driven water,it is proposed that formation water in the tight sandstone reservoir in the study area has four microscopic occurrence states:bound water,capillary water,free water and adsorbed water.Bound water occurs in the intergranular and intercrystalline pores controlled by the tiny pore throat,capillary water occurs in the dissolution and intergranular pores with medium and small throat,free water occurs in the dissolution and intergranular pores with large throat,adsorbed water occurs on the surface of the macropores,the cutoff values of pore throat radius are 0.1?m,0.26?m and 0.28?m,the cutoff values of permeability are 0.21×10^-3m²,0.51×10^-3m² and 0.55×10^-3m²,and the cutoff values of porosity are 5.86%,7.99%and8.18%,respectively.In the study area,the microscopic type of occurrence of formation water is controlled by pore throat less than 0.1?m,free water account for 50%,capillary water account for 18%,bound water account for 30%,and adsorbed water account for2%,the residual water saturation is controlled by starting pressure gradient,and the residual water saturation is 32%.3.The distribution characteristics of gas and water are analyzed,and the main controlling factors of gas and water distribution and the accumulation mechanism are identified in the area with low hydrocarbon generation intensity.The source rock controls the longitudinal gas-water macroscopical distribution,the sand body size microstructure controls the local gas-water relation,the sand body structure has a certain control effect on the gas-bearing saturation,and the better reservoir physical property is necessary for gas enrichment,the near-source sand bodies generally contain gas,the high quality reservoirs are enriched,the remote source small pressure difference charging and the large pore throat superimposed molding sand bodies are coupled to control the reservoir formation.4.Based on the characteristics of micro-occurrence and macro-distribution of gas-water,a micro-macro-coupling gas-water distribution model and gas-water extraction mechanism model are established.From Shan1 up to He8,the gas reservoir presents a gradual change distribution patterns of gas layer to water-bearing layer,gas-water layer and water layer,the gas content changes from the general sand body to the high part of the superposed sand body,the gas saturation decreases and in turn appear bound water,adsorbed water,capillary water and free water.The dynamic relationship model of natural gas charging intensity,reservoir pore throat radius,microscopic type of occurrence and water saturation control the mechanism of gas and water generation.