Role of Organic Matter on Nanoscale and Microscale Creep Properties of Source Rocks
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2018 American Society of Civil Engineers. Studying the time-dependent behavior of gas shale formations is important for understanding the mechanical performance of source rocks in response to drilling, hydraulic fracturing, and pressure depletion due to production, among others. Compared with nonsourcerock shales, the role of the organic matter (OM) on creep rates of gas shale formations remains to be uncovered, which is the focus of this paper. By means of a hybrid experimental-modeling approach involving creep nanoindentation and microindentation and first-order modeling within the framework of the correspondence principle of viscoelasticity, it was found that organic matter drives creep rates of shale rocks, whereas the confining texture of the inorganic phases decelerate the creep rate. By means of micromechanical textural models used to analyze nanoindentation results, it is shown that creep in source rocks entail changes in packing density related to total organic carbon (TOC)-dependent energy-dissipation mechanisms in the microstructure: dilation in organic-poor and overmature source rocks, and compaction in organic-rich mature ones. Finally, a comparison of microindentation results with first-order creep homogenization modeling that considers the organic matter as the dominant creeping phase reveals that the relevant creep rate of organic-rich shale is situated within two asymptotes defined by texture: an upper bound defined by the self-consistent scheme representing mature disordered systems, and a lower bound defined by a continuous creeping matrix of immature OM with rigid clay inclusions. These morphologies relate to the connectivity of OM and, thus, to maturity.
