Assessment of Mudrock Brittleness with Micro-scratch Testing

TL;DR

This paper introduces a micro-mechanical scratch test method to quantify mudrock brittleness, providing a more direct and consistent measure than traditional correlations with mineral content or elastic properties.

Contribution

It develops a systematic micro-scratch testing approach to measure mudrock brittleness, validated with glass and plastics, and applies it to various mudrock samples for the first time.

Findings

Brittleness ranking: Woodford, Eagle Ford, Marcellus, Mancos, Vaca Muerta.

Eagle Ford exhibits mixed brittle/ductile failure.

Dolomite content correlates more strongly with brittleness than other minerals.

Abstract

Mechanical properties are essential for understanding natural and induced deformational behavior of geological formations. Brittleness characterizes energy dissipation rate and strain localization at failure. Brittleness has been investigated in hydrocarbon-bearing mudrocks in order to quantify the impact of hydraulic fracturing on the creation of complex fracture networks and surface area for reservoir drainage. Typical well logging correlations associate brittleness with carbonate content or dynamic elastic properties. However, an index of rock brittleness should involve actual rock failure and have a consistent method to quantify it. Here we present a systematic method to quantify mudrock brittleness based on micro-mechanical measurements from the scratch test. Brittleness is formulated as the ratio of energy associated with brittle failure to the total energy required to perform a scratch. Soda lime glass and polycarbonate are used for comparison to identify failure in brittle and ductile mode and validate the developed method. Scratch testing results on mudrocks indicate that it is possible to use the recorded transverse force to estimate brittleness. Results show that tested samples rank as follows in increasing degree of brittleness: Woodford, Eagle Ford, Marcellus, Mancos, and Vaca Muerta. Eagle Ford samples show mixed ductile/brittle failure characteristics. There appears to be no definite correlation between micro-scratch brittleness and quartz or total carbonate content. Dolomite content shows a stronger correlation with brittleness than any other major mineral group. The scratch brittleness index correlates positively with increasing Young's modulus and decreasing Poisson's ratio, but shows deviations in rocks with distinct porosity and with stress-sensitive brittle/ductile behavior (Eagle Ford).