Pore Structure And Tracer-containing Fluid Migration In Connected Pores Of Wufeng And Longmaxi Shales From Western Hubei And Eastern Chongqing Regions

West Hubei-Eastern Chongqing regions are important areas for shale gas exploration and development in South China,and a large amount of natural gas production from organic Wufeng and Longmaxi shales.In particular,the discovery of Fuling shale gas field in Chongqing City strongly proved the huge shale gas resource and the commercial success of shale gas development in the study area.The characteristics of geometry and topology are key factors for efficient exploration and development of shale gas.However,the study area has experienced several large-scale tectonic movements,which leads to the complex pore structure and pore connectivity,as well as the obscure controlling factors in Wufeng and Longmaxi shales,collectively limiting the sweet spot evaluation and large-scale development of natural gas.In addition,the effects of pore connectivity,wettability on fluid migration within pore spaces and shale gas production are poorly documented,which are important for assessing the shale gas production,understanding the shale gas enrichment and the implementation of efficient shale gas development plan.In this thesis,we focused on the organic-rich shales from Wufeng and Longmaxi formations in West Hubei-Eastern Chongqing regions.We first investigated the geological settings and geochemical characteristics,and used complementary experiments,to characterize the multiscale pore structure of the black shales and discusse the main controlling factors for pore structure with fractal theory.Finally,based on the wettability characteristics,we conducted tracer-containing API brine imbibition and diffusion to examine the relationships between pore connectivity,wettability and fluid migration within these black shales.This work can be beneficial to provide theoretical and practical importance for shale gas exploration.The main conclusions can be summarized as follows:1?A combination of complementary tests,such as SEM,low pressure gas adsorption,MICP and SANS,were conducted to characterize the complex pore structure,including the pore types,pore structure parameters,as well as the pore structure differences between Wufeng and Longmaxi shales.?1?High-resolution SEM images show that modified organic matter pores are the dominant pore types in Wufeng and Longmaxi shales,and the pores are often irregular,faveolated,bubble-like,elliptical and slit-like in shapes with a wide and multi-modal pore size distribution.The pores in Wufeng shales are often heterogeneously distributed in organic matter with crescent and slit-like shapes with a narrow and mono-modal distribution for the pore size;?2?Based on different theory models,pore structure parameters?surface area,pore size distribution and porosity,etc.?are determined by low pressure gas adsorption and MICP.Wufeng shales have abundant micropores,with larger pore volume and surface area values than Longmaxi shales;?3?Using Katz-Thompson model,permeability values for Wufeng and Longmaxi shales are between 10^-15m² and 10^-21m².Pore tortuosity values increase with respective to critical pressure to show the typical low permeability characteristics.The different inflection points indicate the different pore structures and pore channels between Wufeng and Longmaxi shales;?4?a combination of MICP and SANS tests indicates that abundant closed pores are developed,and the fraction of closed pores in shales with higher TOC content is obviously larger.2?Many factors can affect the pore structure within Wufeng and Longmaxi shales,such as external factors?tectonic stress and sedimentary environment?and internal factors?TOC content and mineral composition?.?1?The distribution and thickness of shales with high TOC content are macroscopically controlled by tectonic stress variation and sedimentary environment,which indirectly controls the development of pore space in shales;?2?Positive relationships are observed between TOC content and micropore surface area,BET surface area,micropore volume,mesopore volume.This suggests that the pores in Wufeng and Longmaxi shales are dominated by nanoscale organic matter pores?dominated by micropores and mesopores?and inorganic pores with larger pore size are not well developed;?3?Because the quartz in Wufeng and Longmaxi shales are biogenic in origin,resulting in the clearly positive relationships between quartz content and micropore volume,mesopore volume and BET surface area.Thus,shales with higher quartz contents trend to have larger micropore and mesopore surface area and pore volumes.In contrast,negative relationships exist between clay content and micropore volume,mesopore volume and BET surface area.Clay-rich shale with a lack of enough rigid solids during diagenesis allowed for stronger compaction,pores destroy and collapse,resulting in the rapid decrease of porosity.3?Wufeng and Longmaxi shales are strongly oil-wet and moderately strong water-wet,exhibiting mixed-wet characteristics.The difference of contact angles between samples are strongly related to the TOC content,pore types and mineral composition,as well as their distribution pattern.Spontaneous fluid?DI water and n-decane?imbibition tests show that,compared to vertical samples,parallel samples have much larger imbibition slopes,suggesting DI water is more likely to migrate along the beddings and fluid movement of vertical bedding is restricted.4?A combination of highly restricted pathways,the presence of sparsely connected hydrophilic pore network,complex pore structure and mixed-wet characteristics has limited the tracer-containing API brine movement within the Wufeng and Longmaxi shales,which is of great significance to improve the efficiency of shale gas extraction.?1?Tracer-containing API brine imbibition and diffusion show that the high concentrations of non-sorbing ReO₄^-,strong sorbing Eu³⁺and weak sorbing Cs⁺are mainly limited within several millimeters range in the shale edge,indicating the very limited edge-connected pore connectivity.The effective diffusion coefficient values range from 0.4×10^-13 to 8.0×10^-13m²/s and the maximum fitted tortuosity is about 40000,suggesting the high tortuosity and the presence of sparsely connected hydrophilic pore network in Wufeng and Longmaxi shales.Compared to non-sorbing ReO₄^-and strong sorbing Eu³⁺,the migration distance for weak sorbing Cs⁺is obvious larger,suggesting the weak retardation from cation exchange between Cs⁺and negative ions in clay minerals;?2?The non-sorbing ReO₄^-tracer only occupies connected pore spaces and does not interact with the shale matrix.The actual migration distance for strong sorbing Cs⁺,weak sorbing Eu³⁺and natural gas are also related to the delay factor,resulting in a much larger migration distance than the geometrical tortuosity;?3?In the process of hydraulic fracturing and subsequent production,the hydrocarbon production will drop rapidly in the first few years.It is necessary to attract sufficient attention from the oil and gas industry to apply targeted engineering technology to overcome the constraints of low pore connectivity and mixed wettability on hydrocarbon movement,and finally improve the development efficiency and economic benefit.