Abstract
AbstractQuantification of risk to seal integrity in CCS, or gas extraction from hydraulic fracturing, is directly affected by the accessibility of organic pores within organic rich mudrocks. Knowledge of the host organic matter's mechanical properties, which are influenced by depositional environment and thermal maturity, are required to reduce operational risk. In this study we address the effect of both depositional environment and maturity on organic matter Young's modulus by means of Atomic Force Microscopy Quantitative ImagingTM, which is a nondestructive technique capable of nanomechanical measurements. Shales from varying marine depositional environments covering kerogen Types II (Barnett), IIS (Tarfaya), and II/III (Eagle Ford/ Bowland) are analyzed to capture variance in organic matter. The findings show organic matter has a Young's modulus ranging between 0.1 and 24 GPa. These marine shales have a bimodal distribution of Young's modulus to some degree, with peaks at between 3–10 and 19–24 GPa. These shales exhibit a trend with maturity, whereby Young's modulus values of <10 GPa are dominant in immature Tarfaya shale, becoming similar to the proportion of values above 15 GPa within the oil window, before the stiffer values dominate into the gas window. These peaks most likely represent soft heterogeneous aliphatic rich kerogen and stiff ordered aromatic rich kerogen, evidenced by the increase in the stiffer component with maturity and correlated with 13C NMR spectrocopy. These findings enable increased realism in microscale geomechanical fracture tip propagation models and may allow direct comparison between Young's modulus and Rock‐Eval parameters.
Highlights
Quantification of mudrock geomechanics has become increasingly important in order to inform the design of hydraulic fracturing treatments
In this study we address the effect of both depositional environment and maturity on organic matter Young's modulus by means of Atomic Force Microscopy Quantitative ImagingTM, which is a nondestructive technique capable of nanomechanical measurements
Samples of the Bowland and Eagle Ford Carol were selected across intervals known to have elevated Total organic carbon (TOC) in order to ensure that a representative amount of organic matter Young's modulus data could be obtained during the Atomic Force Microscopy (AFM) QITM analysis
Summary
Quantification of mudrock geomechanics has become increasingly important in order to inform the design of hydraulic fracturing treatments. Journal of Geophysical Research: Solid Earth there has been a drive to improve the complexity and reliability of subsurface geomechanical models for both unconventional resources and potential carbon storage. This has yielded benefits of better constraining sweet spots in the unconventional plays, where the shale is oil or gas window mature, and of the right mineralogic composition (Dehua & Fangzheng, 2012; Khair et al, 2013), along with improvements in models for the large-scale geologic storage of CO2 (Rutqvist, 2012; Zoback, 2010). Over the last 5 years numerous new approaches have been taken to model mechanical properties and fracture propagation in shales such as Eagle Ford (Marongiu-Porcu et al, 2015) and Woodford (Bennett et al, 2015)
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