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Among the reservoir fracture characterization parameters, fracture aperture determines the seepage capacity of dense sandstone reservoirs and is a key parameter in the evaluation of fracture effectiveness, and small changes in aperture often determine whether a fracture serves as a channel or a bottleneck for fluid flow. Taking the dense sandstone of the Ordos Basin as an example, this project proposes a set of fracture aperture evaluation methods based on "geologic analysis - fracture surface measurement - fracture filling simulation - fine macroscopic elastic‒plastic mechanics experiment - geomechanical modeling". By rough discrete crack geometry modeling, the true geometric shape of underground cracks is restored, the deformation process of cracks is reconstructed, and the influence of different factors on crack opening is analyzed in detail and comprehensively. A quantitative relationship between crack opening and its main controlling factors is established, and three-dimensional deterministic modeling of the fracture aperture is realized. Finally, the reliability of the model is verified via dynamic and static data, and a theoretical calculation model for the fracture aperture under multiple constraints, such as in situ stress, rock mechanics parameters, fracture occurrence, fracture scale, fracture filling characteristics, and fracture surface characteristics, is proposed. The results of the study show that the fracture aperture increases with increasing fracture spacing when the spacing of the fractures is less than 2 m. When the spacing of the fractures is greater than 2 m, the aperture of the subsurface fractures is essentially unaffected by the spacing of the fractures. Controlled by the type of in situ stress in the study area, the fracture aperture increases with increasing fracture dip angle. As the horizontal principal stress difference increases, the fracture aperture decreases; when the ground stress difference is greater than 10 MPa, the control of the average fracture aperture by the ground stress difference weakens. The average effective fracture aperture was largest in the uniform type of filling; in the uniform type of filling style, strip type of filling and cluster type of filling, the average effective fracture aperture was largest when the fracture filling rate was 12–14%, 36% and 26%, respectively. Among the different fillings, the average effective fracture aperture was the largest for the siliceous fillings, followed by the calcareous fillings, whereas the average effective fracture aperture was the smallest for the muddy fillings. With increasing roughness, the fracture closure rate decreases, whereas the average fracture aperture shows a two-stage change; when the JRC is less than 20, the fracture aperture basically remains the same; when the JRC is greater than 20, the fracture aperture increases with increasing roughness. The reservoir fracture aperture is positively correlated with the Young's modulus and negatively correlated with the Poisson's ratio. The research results not only enrich and improve the basic theory and methodology system of fracture research in tight sandstone reservoirs but also provide new ideas and methods for fracture characterization and prediction research and provide a scientific basis for the characterization of fracture effectiveness in reservoirs and the efficient development of oil reservoirs.
| Country | China |
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