Genetic mechanism of permeability anisotropy in conglomerate reservoir and its controlling factors

Abstract

Abstract The near-source, multistream, and rapidly changing sedimentary environments of conglomerate reservoirs lead to reservoir characteristics such as strong heterogeneity and complex modal pore structure, which create unique seepage pathways and strong permeability anisotropy, which in turn affect the displacement mode and oil recovery of crude oil in pores and throats. Thus, it is necessary to clarify the genetic mechanism and controlling factors of this permeability anisotropy, providing the basis and premise of sustainable and efficient development of conglomerate reservoirs. In this paper, a typical conglomerate reservoir from the Kexia Formation in the 7th Block of the Karamay oilfield (Xinjiang, China) was selected as the research object to solve the permeability anisotropy problem. According to the sealed coring data, eight conglomerate samples were cut preliminarily in the field and prepared finely in the laboratory to make standard cube samples with side lengths of 4.5 cm, and then these samples were investigated using casting thin section, computed tomography scanning, and constant velocity mercury injection experimental methods to determine the genetic mechanism of permeability anisotropy and its controlling factors at macro- and microscales. The results showed that sedimentation and hydrodynamic conditions controlled the distribution of permeability anisotropy, the separation, rounding, and arrangement of mineral particles, the development of sedimentary structural planes, and the different types of packing textures, which affected the seepage paths and resistance in different directions, resulting in great differences in permeability anisotropy. The permeability in the direction parallel to water flow was the largest, followed by the direction perpendicular to water flow, and the permeability in the vertical sedimentation direction was the lowest because of the combined influences of the tortuosity of seepage, the connectivity of pore throats, and the sedimentary structural planes. In addition, diagenesis in later stages can reduce the absolute value of permeability in different directions and the strength of permeability anisotropy by changing the micropore structure of the conglomerate reservoir, but it will not change the distribution of permeability anisotropy. Based on these observations, a generalized linear mixed model was used to analyze the micropore structure parameters and standard deviation of permeability in different directions from the eight conglomerate samples, and the results showed that the relative sorting coefficient had the greatest influence on the standard deviation of permeability, and the uniformity of pore-throat size distribution was the main microscale controlling factor on the strength of permeability anisotropy. When the pore-throat size distribution was less uniform, the difference in the amplitude of permeability anisotropy was more obvious. The average capillary radius, seepage tortuosity, and maximum pore-throat radius showed less importance than relative sorting coefficient, so they are considered secondary factors. The other micropore structure parameters showed some influence on the permeability anisotropy, but the degree of influence was limited.