A novel method for multiscale digital core reconstruction based on regional superposition algorithm

To characterize matrix pores and microfractures which are widely developed in unconventional reservoirs and also to analyze flow law in multiscale flow region, some related research has been carried out in this study. Based on the statistical characteristics of microscale and nanoscale scanning images of cores, digital cores of microfractures are reconstructed by an improved Voronoi method and the digital cores of matrix pores are reconstructed by the quartet structure generation set (QSGS). By means of regional superposition algorithm, the digital cores of microfractures are firstly magnified in each divided region and then integrated with the corresponding digital cores of matrix pores. After the superposition is completed in whole regions, numerical model after superposition is reduced to the original size of the digital core of microfractures. Finally, the multiscale digital core is reconstructed. Based on reconstructed multiscale digital core, flow simulation considering the permeability of matrix is carried out by the gray lattice Boltzmann method (GLBM). The results show that by constraining the generation areas of random points, the improved Voronoi method can better reflect the geometric characteristics of specific microfractures and the average relative errors of the two-point function and variation function are 13.36% and 2.52% respectively compared with the original microfractures. The multiscale digital core reconstructed by the regional superposition algorithm integrates the anisotropy of matrix pores while retaining the original characteristics of microfractures. With the increase of permeability of matrix, pressure drop range in the flow simulation region expands, and the average velocity at the outlet of microfractures increases. Compared with the case not considering the permeability of the matrix, the maximum increase of average velocity at the outlet of microfractures can reach 48.37% when the permeability of the matrix is set as 0.75 mD. When the flow direction in the microfractures is transverse, compared with the multiscale digital core with transverse connectivity in the matrix, the multiscale digital core with longitudinal connectivity in the matrix has higher flow velocity at the outlet of microfractures, and the average increase of velocity is up to 1.73%.