Pore Structure Characteristics And Reservoir Evaluation Of Chang 8 Tight Sandstone Reservoir In Ansai Area

Micropore structure is the key to affect oil and gas filling and migration,controls oil and gas bearing capacity of reservoir and determines reservoir storage and seepage capacity.In this study,Chang 8 tight sandstone reservoir in Ansai area of Ordos Basin is taken as an example.Through core photos,casting thin sections,CT scanning,high pressure mercury injection,constant velocity mercury injection,nuclear magnetic resonance and other technical methods,the pore throat structure and seepage capacity of tight sandstone reservoir in Ansai area are qualitatively and quantitatively analyzed.Finally,according to the parameters of micropore structure,the reservoir classification standard is established and the reservoir comprehensive evaluation is carried out.Eventually this study concludes the following understanding.(1)The Chang 8 reservoir group in Ansai area of Ordos Basin mainly develops lacustrine delta deposits.The lithology is mainly dark mudstone and dark gray fine sandstone,and underwater distributary channel sand body is more developed.Iron calcite accounted for the largest proportion of interstitial materials,followed by hydromica and chlorite,and the overall structural maturity of the study area was higher.(2)The compaction and pressure solution in the study area are relatively developed,and cementation and metasomatism are common.The cementation types are mainly carbonate cementation and clay mineral cementation,and the clay minerals are mainly chlorite and illite.The cementation methods are mostly film cementation and pore cementation.The dissolution in the study area is strong,mainly in feldspar particles.The existence of compaction pressure solution and cementation metasomatism will make the reservoir physical properties worse,and the dissolution and a small amount of chlorite film filling pores are beneficial to the reservoir physical properties and seepage capacity.(3)The overall pore throat is small,the development degree is weak,and the seepage ability is general.The pore throat size is mainly concentrated in the range of 0.01 μm～1 μm,and mainly develops transitional-mesopore throat.The pore throat sorting is poor,and the average rapex value is 0.216 μm,which is small on the whole,reflecting poor pore throat connectivity of reservoirs,it is known that the pore throat combination in the study area is characterized by fine throat-small pore type.(4)Based on NMR experiments,the overall bound water content in Ansai region is high,and the movable fluid porosity is small.Micropores are mainly developed,and the movable fluid space is small.The main pore throat radius of the representative well is between 0.00095μm and 0.1797μm,which also proves that the pore throat structure of the reservoir in the study area is complex,the micropores are developed in large numbers,and the reservoir connectivity is poor.(5)According to the oil-water phase permeability experiment,the range of water saturation in the two-phase co-permeability area of the study area is narrow,with an average of 12.89%,and the average of small porosity is only 9.59%.The oil displacement efficiency in the final period is generally higher than that in the anhydrous period.The permeability is positively correlated with the oil displacement efficiency.The oil displacement efficiency is general,but the water flooding effect is remarkable.(6)Finally,the multi-parameter standard is used to evaluate Chang 8 reservoir in Ansai area:Chang 811 is dominated by Ⅲa reservoir and Ⅱ reservoir is supplemented;chang 812 is dominated by Ⅲa reservoir,Ⅲb reservoir is complementary;chang 821 and Chang 822 are Ⅲa and Ⅲb reservoirs,and the reservoir quality of Chang 811 and Chang 812 is relatively good.