Grain size effects in sediment compaction: an augmented continuum theory based on grain-scale fracture mechanics

Most of the world’s energy comes from fossil fuels hosted in the shallow crust[1]. Now and for the foreseeable future, the safe and sustainable use of these resources calls us to understand the physics of crustal rocks, developing new paradigms to characterize, simulate and predict their response across scales, i.e. from the scale of few interacting grains to that of a sedimentary basin (Fig. 1). To pursue such goals, major challenges derive from the complex nature of geomaterials, i.e. natural solids exhibiting heterogeneity at all scales. Microscopic features like grains, pores and cracks are indeed crucial to define the macro-scale properties that are relevant for petroleum sciences, such as strength of cores[2], reaction-transport constants[3] and geometry of fault systems[4]. Yet, despite the role played by such attributes, the theories currently used to explain the mechanics of reservoir compaction lack a fundamental basis to warrant the use of petrophysical data in quantitative analyses, forcing us to recur to phenomenology and empiricism. It is therefore arguable that the feedbacks between petrophysical properties and macroscopic response will not be captured unless new hypotheses will be formulated to explain the role of microstructural attributes. This project aims to provide new contributions towards this long-term goal by formulating new methods to interpret evidences about the compaction of granular rocks. For this purpose, we propose to link two scales crucial for petroleum sciences: (i) the scale of a Representative Elementary Volume (REV), at which macro-scale quantities are defined and (ii) the scale of a grain, were the processes that lead to reservoir compaction are originated. To accomplish such goals we propose to interpret seemingly incoherent evidences about the role of grain sorting and grain size. Our purpose is to formulate and validate mechanistic hypotheses about the interplay between isolated fracture events at the grain-scale and the collective comminution of assemblies of particles. As a result, the overarching goal of this research is to bridge the physics of single-grain failure with the macroscopic processes that control sediment compaction at the reservoir scale.