A Review of Research Progress on Characterization Technology of Nano Organic Pore Structure in Shale

The development and utilization of shale gas has become an important way to ensure national energy security and achieve global carbon neutrality goals. Shale gas reservoir is the direct target layer of shale gas exploration and development, because its shale nanopore throat system, dominated by organic pores, has the characteristics of integrated source storage, low porosity, low permeability and non-darcy flow. The evaluation of shale reservoir evaluation needs to break through the bondage of traditional inorganic pore evaluation ideas and the bottleneck of nanoscale characterization technology, and adopt higher precision and resolution experimental technology to characterize nanopores and depict organic pores.

To understand the research progress on characterization technology of nano organic pore structure in shale and explore the evolution mechanism of organic pore formation, its experimental technical bottlenecks and possible solutions.

According to the experimental types and principles, shale nanopore analysis can be divided into two types: physical experiment technology and imaging analysis technology. The former mainly carries out quantitative physical experiment analysis of rock physical properties, while the latter carries out microscopic in-situ observation and image analysis of pore structure.

Since organic pores were first discovered in the Barnett shale in North America in 2009, significant progress has been made in studying shale characterization techniques and developmental features. (1)Multi-scale and multi-type nanopore characterization techniques were established. Among them, the quantitative microstructure characterization technologies are dominated by mercury-adsorption joint measurement method and pulse attenuation method. Quantitative parameters of shale physical properties and full aperture distribution ranging from 0.35nm to 10000nm and permeability of < 1μD can be accurately obtained. High-resolution microscopy scanning based on field-emission SEM and microscopy-CT forms a multi-scale structure reconstruction technology of nanopore, and thus 2D-3D image information is available. (2)The formation and evolution of organic pores are controlled by organic matter type, rock evolution and other factors. It is necessary to reveal the internal connection between the influencing factors and the physical evolution law of organic matter molecular structure, which is the key to identify the heterogeneity of shale reservoirs. Formation and keeping of organic pores is in the process of decomposition and condensation reaction competition space effect. (3)Previous studies established a series of kerogen and asphalt structure model for the molecular organic pores evolution mechanism, providing the theoretical basis for research on the genetic mechanism and evolution law of organic pores at the molecular level. Transmission electron microscope and atomic force microscope are used to observe molecular space arrangement and microstructure internal form in stereo. These techniques enable the understanding of the mechanisms of organic pore formation and preservation at the nanoscale.

In-situ structure imaging and component scanning technology, reservoir description, composition analysis and digital core integration, are developed to the structure and composition, pore permeability and brittle integration dynamic evaluation. A leap from microstructural analysis to macroscopic big data prediction is realized, meeting the needs of efficient exploration and development of shale gas geology and engineering integration.