4D imaging of fracturing in organic-rich shales during heating

TL;DR

This study uses 4D imaging and modeling to reveal how fractures develop in organic-rich shales during heating, driven by kerogen decomposition and internal pressure buildup, affecting fluid escape pathways.

Contribution

It introduces a combined experimental and numerical approach to understand fracture formation in shales during thermal decomposition, highlighting the role of kerogen breakdown.

Findings

Cracks nucleate at ~350°C during heating.

Fractures propagate parallel to bedding and coalesce.

Kerogen decomposition causes internal pressure leading to fracturing.

Abstract

To better understand the mechanisms of fracture pattern development and fluid escape in low permeability rocks, we performed time-resolved in situ X-ray tomography imaging to investigate the processes that occur during the slow heating (from 60\degree; to 400\degree;C) of organic-rich Green River shale. At about 350\degree;C cracks nucleated in the sample, and as the temperature continued to increase, these cracks propagated parallel to shale bedding and coalesced, thus cutting across the sample. Thermogravimetry and gas chromatography revealed that the fracturing occurring at ~350\degree;C was associated with significant mass loss and release of light hydrocarbons generated by the decomposition of immature organic matter. Kerogen decomposition is thought to cause an internal pressure build up sufficient to form cracks in the shale, thus providing pathways for the outgoing hydrocarbons. We show that a 2D numerical model based on this idea qualitatively reproduces the experimentally observed dynamics of crack nucleation, growth and coalescence, as well as the irregular outlines of the cracks. Our results provide a new description of fracture pattern formation in low permeability shales.