Characteristics And Sensitivity Of Chang 8₁ Reservoir In Maling Area,Ordos Basin

The research object of this paper is Chang 8₁ reservoir in Maling area of Ordos Basin,which is a typical low-porosity,ultra-low-porosity and ultra-low permeability tight sandstone reservoir.Taking Chang 8₁ reservoir in Maling area of Ordos Basin as an example,this paper describes the basic characteristics of the reservoir from four aspects:regional geological background,reservoir physical characteristics,petrological characteristics and pore throat structure characteristics by means of X-ray diffraction,scanning electron microscopy,thin section analysis and mercury injection experiments,and on the basis of defining the basic characteristics of the reservoir.The sensitivity of low permeability reservoirs in the study area is studied and analyzed by water flooding experiment,and suggestions and measures to restrain the sensitivity of reservoirs are provided for the next production and construction.Through the experimental study of reservoir physical properties,petrological characteristics,pore structure characteristics and reservoir sensitivity in the study area,the following achievements and understandings are obtained:The main lithology of Chang 8₁ reservoir in Maling area is lithic feldspathic sandstone and feldspathic lithic sandstone.The composition of debris is mostly quartz and feldspar,and the vol?me of debris is small.The cement in the interstitial mainly includes clays,carbonates and siliceous materials.The clay minerals are mainly illite and chlorite,and the kaolinite content is less.The diagenesis in the study area is mainly compaction-pressure dissolution and cementation.The study area is greatly affected by compaction.The pores are mainly mixed pores.Types include residual intergranular pores and feldspar dissolved pores.The main types of throats in the study area are mainly flaky throat and sieved throat.The Chang 8₁ reservoir sandstone in the Maling area is in the early stage of the middle diagenetic stage A to B.The sensitivity analysis of the study area shows that the reservoir speed sensitive damage rate range is from 0.5%to 50.45%,with an average of 19.45%.The reservoir has weak speed sensitivity and the critical water injection rate is 13.6m/d.The damage rate of distilled water is 12.77%⁷3.42%,with an average of 34.43%.The reservoir belongs to medi?m and weak water sensitivity as a whole.The salinity damage rate in the study area was between 5.92%and 40%,and the average salt sensitivity index was 23.09%.The reservoir had weak salt sensitivity and the salinity threshold was 36750mg/L.The damage rate of reservoir acid sensitivity is between-35%and 80%,with an average of 16.24%.The reservoir has no-weak acid sensitivity;The alkali sensitivity damage rate is between 33.5%and 60.08%,and the average alkali sensitivity index is 43.41%.It is moderately alkali sensitivity and has a critical pH of 9.0.The clay mineral type and content and pore structure in the study area are the most direct controlling factors of reservoir sensitivity..The water sensitivity and alkali sensitivity of the study area are strong,so it is necessary to focus on the protection and treatment of reservoir damage caused by alkali sensitivity and water sensitivity.