TY - BOOK

T1 - A fundamentals mineralogical investigation of downhole cements within the context of Underground Hydrogen Storage

AU - Sammer, Thomas

N1 - no embargo

PY - 1800

Y1 - 1800

N2 - Underground hydrogen storage (UHS) offers significant potential for storing renewable energy, but the feasibility of this process also depends on the integrity of the cement sheaths used in boreholes when exposed to hydrogen. The integrity of these cements is crucial for ensuring both economical and safe storage of hydrogen. However, knowledge about the influence hydrogen might have on downhole materials and especially the cement sheath is scarce. In the geoenergy sector, downhole cements must be designed with low permeability and high mechanical durability, qualities that are deeply influenced by the material's microstructure. Enhancing these properties requires a detailed understanding of how different additives and preparation techniques tailor and respectively influence the microstructure once investigated in the lab. Advancements in high-resolution Scanning Electron Microscopes (SEM) have underscored the need for meticulous sample preparation to preserve fine microstructural features. To assess the impact of hydrogen exposure on the commonly used class G cement, hydrothermal autoclave experiments were conducted. These experiments, coupled with X-ray diffraction, scanning electron microscopy, and nitrogen adsorption/desorption techniques, provided insight into the potential changes in the mineralogical phase composition and microstructure of the cement. The results indicate only minor alterations, such as the decomposition of monosulphate and the formation of ettringite, with the pore size distribution and overall porosity showing minimal change. These findings suggest that class G cement retains its structural integrity and phase composition under hydrogen exposure, reinforcing its suitability for UHS. Addressing the demands of the geoenergy industry also requires exploring how modifications to class G cement can enhance its performance. The approach used in this study involved the incorporation of carbon black and silica fume to modify the microstructure. These additives were found to create a denser microstructure with reduced porosity while maintaining a similar pore size distribution. The introduction of carbon black resulted in a lower Young¿s modulus, indicating increased plasticity, while silica fume increased both the mechanical strength and permeability of the cement with the second being undesirable. In addition to exploring different mix formulations, the study of microstructural characteristics at the nanoscale has been greatly advanced by comparing different SEM sample preparation techniques. Specifically, state of the art resin-embedded polishing and novel Broad Ion Beam (BIB) milling were evaluated for their ability to preserve and reveal fine structural details in hydrated cement paste. SEM analysis, supported by image processing for porosity and pore geometry, and Atomic Force Microscopy (AFM) for surface roughness assessment, revealed that BIB milling was particularly effective in exposing nanoscale features such as gel porosity and the acicular morphology of calcium silicate hydrates. This technique produced pores with higher aspect ratios, while resin-embedded polishing achieved smoother vertical surfaces and more circular pores, indicating a better vertical surface roughness. These findings highlight the strengths and limitations of each preparation technique, offering critical insights into the preparation of cement samples for high-resolution SEM analysis. In conclusion, this study provides a comprehensive understanding of the factors that influence the performance and durability of downhole cements, whether in the context of UHS or the broader geoenergy industry. The research demonstrates how careful cement slurry formulation and advanced analytical techniques can be combined to understand, evaluate and optimize cement performance in demanding environments.

AB - Underground hydrogen storage (UHS) offers significant potential for storing renewable energy, but the feasibility of this process also depends on the integrity of the cement sheaths used in boreholes when exposed to hydrogen. The integrity of these cements is crucial for ensuring both economical and safe storage of hydrogen. However, knowledge about the influence hydrogen might have on downhole materials and especially the cement sheath is scarce. In the geoenergy sector, downhole cements must be designed with low permeability and high mechanical durability, qualities that are deeply influenced by the material's microstructure. Enhancing these properties requires a detailed understanding of how different additives and preparation techniques tailor and respectively influence the microstructure once investigated in the lab. Advancements in high-resolution Scanning Electron Microscopes (SEM) have underscored the need for meticulous sample preparation to preserve fine microstructural features. To assess the impact of hydrogen exposure on the commonly used class G cement, hydrothermal autoclave experiments were conducted. These experiments, coupled with X-ray diffraction, scanning electron microscopy, and nitrogen adsorption/desorption techniques, provided insight into the potential changes in the mineralogical phase composition and microstructure of the cement. The results indicate only minor alterations, such as the decomposition of monosulphate and the formation of ettringite, with the pore size distribution and overall porosity showing minimal change. These findings suggest that class G cement retains its structural integrity and phase composition under hydrogen exposure, reinforcing its suitability for UHS. Addressing the demands of the geoenergy industry also requires exploring how modifications to class G cement can enhance its performance. The approach used in this study involved the incorporation of carbon black and silica fume to modify the microstructure. These additives were found to create a denser microstructure with reduced porosity while maintaining a similar pore size distribution. The introduction of carbon black resulted in a lower Young¿s modulus, indicating increased plasticity, while silica fume increased both the mechanical strength and permeability of the cement with the second being undesirable. In addition to exploring different mix formulations, the study of microstructural characteristics at the nanoscale has been greatly advanced by comparing different SEM sample preparation techniques. Specifically, state of the art resin-embedded polishing and novel Broad Ion Beam (BIB) milling were evaluated for their ability to preserve and reveal fine structural details in hydrated cement paste. SEM analysis, supported by image processing for porosity and pore geometry, and Atomic Force Microscopy (AFM) for surface roughness assessment, revealed that BIB milling was particularly effective in exposing nanoscale features such as gel porosity and the acicular morphology of calcium silicate hydrates. This technique produced pores with higher aspect ratios, while resin-embedded polishing achieved smoother vertical surfaces and more circular pores, indicating a better vertical surface roughness. These findings highlight the strengths and limitations of each preparation technique, offering critical insights into the preparation of cement samples for high-resolution SEM analysis. In conclusion, this study provides a comprehensive understanding of the factors that influence the performance and durability of downhole cements, whether in the context of UHS or the broader geoenergy industry. The research demonstrates how careful cement slurry formulation and advanced analytical techniques can be combined to understand, evaluate and optimize cement performance in demanding environments.

KW - unterirdische Wasserstoffspeicherung

KW - Mineralogie

KW - Bohrlochzement

KW - Bohrlochintegrität

KW - Underground Hydrogen Storage

KW - Well Integrity

KW - Mineralogy

KW - Downhole Cement

KW - Hydrogen Interaction

KW - Geoenergy Industry

M3 - Doctoral Thesis

ER -