3D X-ray study reveals how rock grains move and stress builds

A team of researchers from Johns Hopkins uses an innovative X -ray imaging approach to reveal how compression reshapes the tiny spaces and constraints in sandstone – inscriptions which could predict how this common rock used for fuel tanks behaves under deep underground pressure. The results appear in the Journal of Geophysical Research: Solid Earth.

Geologists have never been able to see in three dimensions exactly how individual grains move and stress accumulates inside the rocks – so far.

“We want to understand how the forces are transmitted through the rocks and how this transmission changes as you increase the strength and end up breaking the rock,” said Ryan Hurley, Associate Professor of Mechanical Engineering and Researcher at the Hopkins Extreme Materials Institute. “Why? Because these processes govern everything that happens in the earth’s crust, of our artificial activities such as stimulation of the oil reservoir to natural events such as earthquakes.”

In their survey, the Hurley team has used tools, some of which are used for medical imaging, to look inside Nugget sandstone, generally found in the American West, examining its complex network of pores, cereals and pockets, which dictate how a breakup occurs.

The researchers used a series of X -ray measurement techniques: X -ray tomography, which provides 3D images of the structure of a rock; 3D X -ray diffraction, which can see constraints in each grain through a rock; and high -energy diffraction microscopy with near field, which shows the orientations of the crystals of each grain in a rock. These methods were used by scientists of materials to study metals, but Hurley said that her group is among the very rare – and the first – to use them all in tandem and specifically to study the rocks.

“At the start, we used these techniques to study simple materials composed of monocrystal collections, but now we use all the techniques together to build a complete image of the structure of a rock, crystalline texture and strength transmission during mechanical loading,” he said.

Hurley explained that the most determining factors that dictate the quantity of stress that a rock can take before rupture is the texture – the crystalline orientation of the grains – and the structure – where the grains and the gaps are.

The researchers saw what they called “the evolution of stress” in the rock, observing that behavioral links existed between rocks and non -cohesive granular materials such as sand and gravel, both having reactions similar to external stresses. In addition, as the sandstone was compressed, his pores closed towards the force and opened laterally, which caused the cracking of the sample but does not separate completely.

“These studies reveal that under stress rocks can, in certain circumstances, behave like interaction grain collections, presenting similarities with the transmission of interparticular strength in granular materials,” said Hurley. “This suggests a link that has been proposed for more than a decade but not, so far, confirmed.”

Hurley said that this work provides proof of concept to use the same series of techniques to study the relationship between the texture, structure and mechanics of rocks in exquisite details.

“With the progress of the X-ray in situ capacities emerging in the coming years, we intend to use these techniques to study larger samples in stress conditions relevant to applications, such as those present in underground reservoirs or close to defects,” he said. “We also plan to develop and validate the” digital twins “of our samples and to study how the alterations of the structure or texture change the mechanics, so that our results can apply as largely as possible.”