Review of the Effect of Diagenetic Evolution of Shale Reservoir on the Pore Structure and Adsorption Capacity of Clay Minerals

Abstract

This Review summarizes the diagenetic evolution of clay mineral structure and pores in shale gas reservoirs. In the early diagenetic stage and in period A of the middle diagenetic stage, the intergranular and intragranular pores of clay minerals were altered by mechanical and chemical compaction, and V L (Langmuir volume) shows a positive linear correlation with clay content. In period B of the middle diagenetic stage and late diagenetic stage, intergranular pores almost disappeared, clay adsorption capacity significantly decreased, and there was a lack of apparent correlation between V L and clay content. Experimental and molecular simulations of clay adsorption capacity in shale are limited due to several drawbacks. (i) Shale samples in different diagenetic stages were characterized by strong heterogeneity, which affected the reliability of the evolution results of clay adsorption capacity. (ii) The method of selecting clay-rich shale samples, extracting clay minerals from shale, or applying pure clay minerals ignored the difference of diagenetic background, the damage of pores and structure of clay minerals, or the support of brittle minerals to shale pore system. (iii) Molecular simulation mostly employed a simplified plate model or single clay mineral model, which oversimplified the structure and diversity of clay minerals in real shale gas reservoirs. Therefore, innovative experimental simulations on the effect of diagenetic evolution on pore structure and adsorption capacity of clay minerals are necessary, which need to comprehensively weigh the combined effect of time and space on the reservoir diagenetic evolution. Besides, the visualization of shale structure, adsorption process, and CH 4 adsorption behavior of the shale constituents may help our understanding. Furthermore, a more accurate model of the molecular structure of clay minerals is indispensable, especially the dynamic evolution model with different water contents.

Water Adsorption and Its Pore Structure Dependence in Shale Gas Reservoirs

“…The pore structure and adsorption capacity of kaolinite is obviously better than that of the other two. , Illite and chlorite are the final products of “smectite-chlorite” and “smectite-illite” evolution sequences during the diagenesis of clay minerals. This evolution is a chemical compaction process with a denser sheet structure, poorer pore development, smaller interlayer spacing, and lower type and number of interlayer cations. , Consequently, their individual fitting with the water adsorption amount does not show a significant relationship (Figure ), but due to its strong hydrophilicity, it still makes an important contribution to the water vapor adsorption process. Additionally, their contribution is greater in primary adsorption than that in secondary adsorption, which is similar to that of total clay content, due to the hydrophily.…”

Pore System Diversity and Control Factors of High-Overmature Shale─Taking the Lujiaping and Longmaxi Formation Shales as an Example

“…Second, clay mineral particles easily fill the mineral interior and between mineral particles, thus reducing the reservoir space. 60,61 Figure 17 shows that the macropore and micropore PV and mesopore and micropore SSA of the shale samples of the Lujiaping Formation have no obvious correlation with clay minerals. There is a weak negative correlation between macropore SSA and clay minerals.…”

Research on the Shale Porosity–TOC Maturity Relationship Based on an Improved Pore Space Characterization Method

“…B. The diagenetic evolution of a specific study area can be further analyzed using the porosity-TOC maturity relationship [49]. C. Shale maturity can be predicted using the porosity-TOC maturity relationship, as TOC and porosity are calculated from logging data; this will improve the understanding of shale oil-rich mechanisms and sweet spot prediction [50].…”