Why and How Subsurface Barrier Materials Fail Under Geomechanical Load

The carbon capture and storage (CCS) is by far the only technology which can currently reduce the presence of CO₂in the atmosphere on a significant scale. CO₂ storage in depleted oil and gas reservoirs is also not likely to lead to technological difficulties. However, the CCS projects have been limited to a few industrial field-trial types of project, while the large scale industrial deployment is still in the future, [Global CCS Institute, 2015&2019]. One of the major concerns is potential gas leakage to water aquifers or atmosphere due to caprock integrity and leaky wellbores, or simply - failure in barrier materials.

By definition, barrier materials are low permeability rocks or engineered materials, designed to have petrophysical properties that prevent migration of fluids through them. In this respect, shale rock and wellbore cement qualify as barriers at ambient conditions. Once the subsurface pressurization occurs due to CO2 injection, and physical and chemical subsurface equilibrium is out of balance, barrier materials can fail.

This paper presents results from a comparative study of micro/nano geomechanical properties of shale rocks and wellbore cement. Atomic Force Microscopy, Indentation, Scanning Electron Microscopy and Energy Dispersive Spectroscopy were used to provide an insight into susceptibility of barrier materials to fracture initiation/propagation.

Based on the data it can be concluded that geochemistry does influence geomechanical response of barrier materials. In addition, failure at atomic scale is a good indicator for behavior of barrier materials at macro scale, and can be used to predict potential leakage in the field. However, most importantly, by understanding the mechanisms and properties that influence fracture mechanics of nature’s barrier materials, such as shale rocks, we can design and engineer more robust barriers for wellbore construction and abandonment.