Pore Structure Fractal and Graded Evaluation in Tight Sandstone Reservoirs:A case study of the Huangliu Formation in the Ledong area,Yinggehai Basin
[Objective]Tight sandstone reservoirs have small pore throats,various pore throat types,and strong het-erogeneity.Clarifying the micro morphology of pore throat in tight sandstone reservoirs and dividing the grading of res-ervoir pore structure are important for evaluating the reservoir quality.Multi angle and semi quantitative evaluation of reservoir capacity and flow characteristics is helpful for clarifying the types of reservoirs with high productivity poten-tial.[Methods]Taking the tight sandstones of the Huangliu Formation in the Ledong area of Yinggehai Basin as an example,this study uses the experimental methods of casting thin section,scanning electron microscope(SEM),and high-pressure mercury injection(HPMI).By analyzing the reservoir diagenesis,dividing the reservoir pore throat types,and calculating fractal dimension parameters,the pore structure characteristics are clarified.To analyze the influence of reservoir microstructure parameters on macro physical properties,the pore throat radius scale and the tight sandstone reservoir types are divided,and research on fractal dimension calculation and grading evaluation of reservoir pore structure is performed.[Results]The HPMI experimental results show that the pore-throat radius of tight sandstone reservoirs in the Huangliu Formation has a bimodal distribution,the displacement pressure is be-tween 0.14-2.75 MPa,and the average pore throat radius is between 0.08-1.10 µm.For a better reservoir type,the proportion of the large pore-throat is higher.Using the HPMI fractal dimension(D)to evaluate the pore-throat struc-ture characteristics of the Huangliu Formation reservoirs,when the fractal dimension is closer to 3,the pore throat ra-dius is smaller,the throat is more complex,and the reservoir quality worsens.Owing to the double peak distribution of the pore-throat radius in the Huangliu Formation tight reservoirs,the mercury inlet curves display segmentation.Therefore,the fractal dimensions of large pore throat(Dmax)and small pore throat(Dmin)are calculated in this study.The Dmax value has the strongest correlation with porosity and permeability.The pore structure characteristics of large pore-throat are one of the important factors affecting the reservoir quality.Based on the intersection of the two linear fitting curves in the lg SHg-lgPc diagram and whether mercury is injected into the pore space,the tight sandstone is di-vided into three levels:large,small,and isolated pore-throats.The closer the fractal dimension is to 2,the more con-nected the large pore-throat spaces are in the reservoirs,and the correlation between large pore-throat porosity and permeability is higher.Therefore,the large pore-throat can more effectively characterize the reservoir quality and pore throat structure.Based on the grading characteristics of pore throats,diagenetic facies,and the complexity of pore structure,four types of models for grading evaluation of pore structure are established.Type Ⅰ is the small pore-throat dominant reservoirs with strongly cemented diagenetic facies,and the proportion of large pore-throat porosity is less than 45%.The D value ranges from 2.70-2.80,and the Dmax value is 2.75-2.90,with an isolated point-single point connecting bundle throat and tight reservoir.Type Ⅱ is the large to small pore-throat continuous reservoirs,with weakly cemented and dissolved diagenetic facies.Large pore-throat porosity accounts for 45%-70%.The D value is 2.60-2.80,and the Dmax value is 2.70-2.85,in a single-point and multi-point connecting curved sheet throat,low permeability reservoir.Type Ⅲ is the large pore-throat dominant reservoirs,with strong dissolution diagenetic facies.Large pore-throat porosity accounts for more than 70%.The D value distribution is 2.55-2.60,and the Dmax value is 2.55-2.65,in a network connection thick throat,high-quality reservoir.Type Ⅳ is the single peak reservoir with large pore-throat and strongly compacted diagenetic facies.Large pore-throat porosity accounts for 50%-60%.The D value distribution is 2.70-2.80,and the Dmax value is 2.65-2.75,in a multi-point connection of tube bundle throat,low permeability reservoir.Using the reservoir permeability,porosity,and Dmax parameters,through Fisher's discrim-inant method,new parameters F1 and F2 are obtained,and the reservoir type discrimination criteria were estab-lished.Finally,the logging prediction model of reservoir type is established.Comparing the reservoir type with gas logging response and gas saturation,under the same geological background,the gas logging response in type Ⅲ,type Ⅱ,and type Ⅰ reservoirs is shown to gradually decrease.Type Ⅲ reservoir has strong reservoir and seepage ca-pacity,high gas production efficiency,and good exploration value.Type Ⅳ reservoirs can be used as a barrier in sand bodies,and type Ⅰ reservoirs have poor reservoir capacity.[Conclusion]Using reservoir diagenesis,pore structure characteristics,and fractal dimension parameters,the tight sandstone reservoir quality can be semi-quanti-tatively classified,providing a new idea for tight sandstone reservoir evaluation and classification,and clarifying the control effect of reservoir pore structure on gas-water distribution.
万方数据
