The mechanical properties of kerogen, the organic constituent of shale source rocks, change as it becomes progressively buried under sediment over geologic time. While these changes are due to both mechanical and chemical mechanisms, the individual impact of these mechanisms is poorly understood. In this work, we use atomistic models to isolate how the elastic properties of kerogen are affected by one of these mechanisms: changes in density due to mechanical compaction. We use atomistic models of kerogen at four different maturity levels – immature, top of the oil window, middle-end of the oil window, and over-mature. At each maturity level, we construct representative kerogen structures at densities ranging from 0.9 gm/cm3 to 1.5 gm/cm3 using molecular dynamics simulations. Subsequently, the elastic moduli are calculated at 0 K, 300 K, and 500 K using molecular statics and molecular dynamics simulations.
Kerogen exhibits an amorphous structure with a short-range order up to 6 Å and no discernable long-range order. Increases in kerogen density upon burial are accommodated by proportional increases in the stacking of poly-aromatic islands present in its structure. We show that the increased stacking leads to the formation of π-π stacking bonds, which correlates to the increases in the elastic moduli. We also find that Poisson's ratio measured from atomistic simulations changes linearly with changes in density but is invariant to changes in chemical composition. For all of these properties, the values measured via simulation show good agreement with results from nano-indentation, atomic force microscopy (AFM), and ultrasonic measurements.
These results are useful for several reasons. First, they provide an estimate of Poisson's ratio for kerogen over a range of densities and maturities. This estimate is useful in AFM and nano-indentation experiments, where Poisson's ratio is difficult to measure but is needed to calculate Young's modulus from the reduced modulus. Second, the results demonstrate how atomistic modeling can be applied to gain new insight into the relationship between kerogen structure and its mechanical properties. Third, the agreement between the elastic moduli measured via simulation and experiment shows that atomistic methods can be utilized to accurately characterize kerogen, which is important for building accurate rock models for hydraulic fracturing simulation. Finally, the atomistic models of kerogen developed in this work, constrained by their mechanical properties, can be employed to study other processes such as crack propagation and surface adsorption.
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