Abstract
The process of primary migration, which controls the transfer of
hydrocarbons from source to reservoir rocks, necessitates the existence
of fluid pathways in formations with inherently low permeability.
Primary migration starts with the maturation of organic matter that
produces fluids which increase the effective stress locally. The
interactions between local fluid production, microfracturing, stress
conditions, and transport remain difficult to apprehend in shale source
rocks. Here, we analyze these interactions using a coupled
hydro-mechanical model based on the discrete element method. The model
is used to simulate the effects of fluid production emanating from
kerogen patches contained within a shale rock alternating kerogen-poor
and kerogen-rich layers. We identify two microfracturing mechanisms that
control fluid migration: i) propagation of hydraulically driven
fractures induced by kerogen maturation in kerogen-rich layers, and ii)
compression induced fracturing in kerogen-poor layers caused by fluid
overpressurization of the surrounding kerogen-rich layers. The relative
importance of these two mechanisms is discussed considering different
elastic properties contrasts between the rock layers, as well as various
stress conditions encountered in sedimentary basins, from normal to
reverse faulting regimes. Results show that the layering causes local
stress redistribution that controls the prevalence of each mechanism
over the other. When the vertical stress is higher than the horizontal
stress in kerogen-rich layers, microfractures propagate from kerogen
patches and rotate toward a direction perpendicular to the layers.
Microfracturing in kerogen-poor layers is more pronounced when the
confinement in these layers is higher. Those mechanisms were shown to be
representative of Draupne formation.