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Fractal Characterization Of Micro Pore Structure And Study On Flow Properties Of Low Permeability Reservoirs

Posted on:2023-10-23Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y X XiaFull Text:PDF
GTID:1520306827952479Subject:Earth Exploration and Information Technology
Abstract/Summary:PDF Full Text Request
Oil and gas in low permeability reservoirs are important supplements to conventional oil and gas resources.Vigorously developing low permeability oil and gas resources is an effective way to alleviate the current resource shortage,adjust the energy structure,and ensure national energy security.Different from conventional oil and gas reservoirs,low permeability reservoirs have complex microscopic pore structure and strong heterogeneity,resulting in complex distribution and flow of fluids in pore space.Simple pore model is difficult to describe the permeability difference between different reservoirs,which creates obstacles to clarify the seepage characteristics of low permeability reservoir.How to accurately characterize the complex microscopic pore structure of low permeability reservoir,and then clarify its seepage characteristics are the key scientific questions that need to be solved to improve the development efficiency of low permeability reservoir.To solve these scientific questions,this thesis utilizes fractal structure parameters to characterize the pore structure characteristics of reconstructed porous media based on fractal geometry theory,establishes the relationship between fractal dimension and pore size distribution,and reveals the relationship between the complex microstructure of porous media and quantitative fractal structure parameters.Secondly,the spontaneous imbibition experiment of tight sandstone reservoir samples is carried out.Through the experiment,the dynamic characteristics of spontaneous imbibition affected by the pore structure are revealed,and the key factors affecting the imbibition capacity of low permeability reservoirs are discussed.Then,the permeability of low-permeability reservoirs is obtained by LBM simulation and laboratory experiments,and the influence of microstructure on permeability is quantitatively analyzed by the calculated fractal structure parameters.And based on the 3D visualization equipment,the gas seepage experiment at the pore scale is carried out to clarify the gas-water flow characteristic in low-permeability reservoirs.Finally,the research on the microstructure,mineral composition and wetting characteristics of clayey-silt samples of natural gas hydrate reservoirs is carried out.The creeping permeability model under different seepage pressures is established.Based on in situ CT scanning and seepage equipment,the gas displacement experiment is carried out to clarify the main controlling factors of gas seepage characteristics.The main achievements and understandings are as follows:(1)It is difficult to characterize the complex pore structure of porous media with a single fractal parameter.The complex characteristics,heterogeneity and anisotropy of the pore structure are well characterized by the three fractal structure parameters:fractal dimension,lacunarity,and succolarity,respectively.The fractal dimension calculated by the pore size distribution is highly consistent with the fractal dimension calculated by the box counting method,which proves that the tight sandstone has a self-similar fractal pore interval.(2)The imbibition curves of tight sandstones are different in terms of total imbibition mass and early imbibition slope.The NMR T2 spectrums during the imbibition process show a bimodal feature.The distribution and amplitude of the peaks of T2 spectrum characterize the relationship between the imbibition process and the microscopic pore development characteristics.The number,complex characteristics and connectivity of pores are the main controlling factors for spontaneous imbibition in tight sandstones.(3)The number,heterogeneity and connectivity of pores are the main controlling factors of tight sandstone permeability.In the gas-water seepage process,there is a starting pressure gradient,and the gas saturation changes rapidly at first and then stabilizes.In the early stage,the gas mainly displaces the water in the large pores in the center of the core,and in the later stage,it mainly displaces the water in the small pores in the edge.The contribution of pore and throat of different sizes to gas seepage under different displacement pressures varies greatly,and the heterogeneity of pore distribution has an important influence on the dynamic changes of gas-water distribution in the seepage process.(4)The clayey silt samples from natural gas hydrate reservoirs in South China Sea are characterized by loose and unconsolidated structure.The pore space is mainly composed of small pores at the microscopic scale.There are many clay minerals,and the samples have strong water-wet characteristics.Seepage pressures and pore structure characterization parameters show an obvious negative correlation.The established creeping permeability model predicts that the permeability decreases with the increase of reservoir production pressure.Average throat length,fractal dimension and succolarity are the main controlling factors of gas permeability in clayey-silt reservoirs.In this study,it is found that the fractal structure parameters are effective in characterizing the complex microscopic pore structure of low permeability reservoirs,and analyzing the differences in seepage characteristics between different samples in different reservoirs.Through fractal structure parameters,the bridge between macroscopic properties and microscopic structures is built,and the effect of microstructure on macroscopic seepage characteristics is clarified.
Keywords/Search Tags:Low permeability reservoirs, Microstructure, Flow properties, Spontaneous imbibition, Fractal theory
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