Fine Oil Reservoir Description And Distribution Law Of Remaining Oil In Low-Permeability Sandstone Reservoir

Based on the drilling information, core analysis, production characteristics and oil testing datas, by dint of unions of conventional test analysis of physical property, scanning electron microscope, X-ray and casting sheet, study on the reservoir petrology, diagenesis and diagenetic facies are carried out. Combined with conventional and constant speed mercury injection test, research on the qualitative, quantitative evaluation of pore throat structure including pore throat type, size, distribution, sorting and other characteristics was expanded. Also quantitative research microscopic seepage characteristics using oil-water phase infiltration and T₂ spectrum distribution pattern obtained from NMR. On the basis of calculation of the effective petrophysical limit, quantitative evaluation of classification of reservoir has been launched. It can be applied to compartmentalize reservoir quality coinciding with practical production. Finally with the help of true sandstone micro model for water-oil displacement, residual oil patterns are established in macroscopic and microscopic ways. Emphatically analyzing effect of pore structure distribution law of remaining oil is summarized. Acquired knowledge is as follows:?1? Single nose-like structures which succeed well one by one of Chang-2 and Chang-6 oil bearing beds of Yanchang formation in Jiyuan oil-field were fundamentally stable. The slope is less than 1-2 degrees. Evolution direction of pores is conteolled by complex diagenesis and abundant diagenetic phenomena. Sandstones of them is in the A mesogenetic stage, especially partial sandstones of Chang-6 formation is in the B mesogenetic stage.?2? Uneven developed throat including its size, distribution and sorting et al is the primary cause of complex pore structure of low or extra low permeability reservoir. The stronger influence of throat on permeability, the higher degree of water enrichment. The greater the contribution rate of porosity to permeability, the lower the degree of oil-water differentiation, the higher the oil saturation, the greater the oil rich area. The proportion of area controlled by pore and throat commonly can represent the ability of oil and water mixture.?3? The high pressure mercury injection has high identification accuracy especially for the pore system which is less than 0.1 ?m, and the Constant speed mercury intrusion has high identification accuracy especially for the pore system which is more than 0.1 ?m. The better the physical properties of the reservoir, the more the missing part of the pore and throat. Thus the calculated recovery ratio is lower than the actual. The combination of these two can characterize the microscopic pore reservoir's structural features in a quantitative and effective way.?4? Features of seepage characteristics is often as low oil relative permeability, narrow two-phase infiltration zone and strong interference degree of oil and water as well as low movable fluid saturation and oil displacement efficiency. The heterogeneity of micro pore throat structure is the main factor that affects the oil water displacement characteristics and the distribution of remaining oil. Although permeability of simples is the same or close, characteristics of water flooding and oil displacement efficiency may also be different.?5? Due to the strong heterogeneity of low or ultra-low permeability reservoirs, movable fluid content in the pore throat whose size is the same is not not necessarily the same. As a whole with reservoir physical property becoming better number of intergranular pores and holes surface pore increases, pore throat sorting becomes uniform as well. Remaining movable fluid in a good reservoir is more, while remaining potential is greater too.?6? On the basis of the limit of petrophysical lower limit values of an effective reservoir defined, factors affecting the quality of low ?ultra-low? permeability reservoir has been analysed. Results show that sedimentation is the basis while effects of diagenesis and diagenetic facies are considerable. Specially seepage characteristics controlled by pore structure are the most important factors affecting the quality of reservoir. Comprehensive score of reservoirs is calculated according to the weight of each influence factor with method of fuzzy mathematics and then relationship between sedimentary facies, diagenesis, diagenetic facies, filtration ability and reservoir types are ensured ultimately. Meanwhile together with oil-bearing reservoirs and actual production of oil wells at the beginning reservoir is divided into 4 categories:high quality reservoir of highest production capacity, medium reservoir whose oil-bearing and production are medium to good, difference reservoir whose oil-bearing and production are poor and non-effective reservoir.?7? Besides sedimentary facies and sandstones rhythm, intercalary strata shield and types of channeling also impact on macro distribution characteristics of residual oil. Abundance of remaining oil reserves in high quality reservoirs which develop stacked sandstones one by one and large thickness of oil layer is bigger. Remaining oil saturation of individual sand body isolated docking and connected sand body with large differences in physical properties is higher too. Channeling with high degree of enrichment of remaining oil in the upper part has high oil displacement efficiency.?8? Complex micro pore and throat structure is the main internal cause of residual oil. Reservoirs with a high degree development of pore structure and good connectivity and sorting performe as low residual oil and high displacement. Due to its bigger original base coefficient of storage space the remaining oil abundance is higher and remaining potential is bigger as well. As a result of large differences in pore throats oil presence in reservoirs with good physical property and strong heterogeneity difficulty discharges from pores. The upper formation of high permeability zone is not affected by water flooding, and the enrichment degree of remaining oil is high. Reserviors with low residual potential Often express as Less storage space, poor physical properties and low homogeneity of pore structure.