📄 earth_science

Contribution of matrix hydraulic conductivity to groundwater flow in highly fractured high-porosity rock with low swelling properties and minimal fracture infill

Through in situ hydraulic testing and stochastic inverse analysis at the Horonobe Underground Research Laboratory, this study demonstrates that in highly fractured, high-porosity mudstones with low swelling properties, both high matrix hydraulic conductivity and fractures significantly contribute to groundwater flow regardless of fracture density.

Original authors: YUSUKE OZAKI

Published 2026-07-08

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: YUSUKE OZAKI

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Earth's crust as a giant, ancient sponge. Usually, when we think of water moving through rock, we picture it rushing through big cracks and fissures, like water flowing through a broken pipe. The rock itself (the "matrix") is often seen as solid and tight, barely letting a drop of water through.

However, this study by Yusuke Ozaki and his team at the Japan Atomic Energy Agency looked at a very specific type of "sponge" deep underground in Hokkaido, Japan. This rock is special: it's full of tiny holes (high porosity) and has been squashed by tectonic forces, creating a dense network of cracks (shear fractures). But here's the twist: unlike many other rocks, this one doesn't swell up when wet, and the cracks aren't clogged with clay or mud.

The researchers wanted to solve a mystery: When water moves through this rock, is it just zooming through the cracks, or is it also soaking through the rock itself?

The Experiment: The "Step-Up" Test

To figure this out, the team drilled holes into the rock and performed a "step-up" water injection test. Think of it like turning on a garden hose.

They started with a gentle trickle of water.
Then they turned the knob slightly to increase the flow.
They kept doing this, step by step, up to a strong jet.

If the rock were just a collection of cracks that opened up under pressure (like a door swinging open), the relationship between the water flow and the pressure would change drastically. But, the pressure rose in a perfectly straight, predictable line. This told them the rock's "plumbing" wasn't changing; the cracks stayed open and stable, and the rock itself wasn't swelling.

The Detective Work: The "Flow Dimension"

To understand how the water was moving, the team used a mathematical tool called the "Generalized Radial Flow" model. They looked for a number called the "flow dimension."

Dimension 1 (Linear): Imagine water flowing down a single, narrow hallway. It's a straight line.
Dimension 2 (Radial): Imagine water spreading out like a ripple in a pond. It's a flat circle.
Dimension 3 (Spherical): Imagine water exploding outward in a perfect bubble, filling a sphere.

In many fractured rocks, scientists expect water to flow in "hallways" (Dimension 1) or "ripples" (Dimension 2) along the cracks.

The Big Discovery

The team tested different sections of the rock. Some sections had lots of cracks, some had a few, and one section had absolutely zero cracks.

Here is what they found:

The "No-Crack" Section: Even in the section with no fractures, the water moved! It didn't just sit there. The flow dimension was close to 3, meaning the water was spreading out in a sphere, soaking through the rock itself.
The "Crack" Sections: Even in the sections packed with cracks, the water didn't just rush through the cracks in a straight line. The flow dimension was still high (between 2.2 and 3).

The Analogy: The Crowded Party

Think of the rock as a crowded party room.

The Cracks are the wide aisles between the tables where people can walk fast.
The Rock Matrix is the crowd of people standing shoulder-to-shoulder.

In most rocks, if you drop a message (water) in the room, it only travels fast if it finds the aisles. If the aisles are blocked or narrow, the message gets stuck.

In this specific rock (the Koetoi Formation), the researchers found that the "people" (the rock matrix) are actually moving around quite easily too! Even if you drop the message in a spot with no aisles, it still spreads out quickly because the crowd itself is porous and permeable. The water isn't just using the "highways" (cracks); it's also using the "local streets" (the rock itself).

Why This Matters

For a long time, scientists assumed that in this type of rock, the cracks were the only thing that mattered for water flow. They thought the rock itself was too tight to let water through.

This paper proves that assumption wrong. In this specific high-porosity, highly fractured rock:

Both the cracks and the rock itself are major highways for water.
The water doesn't just follow a single path; it spreads out in all directions, using both the cracks and the tiny pores in the rock.

The Takeaway

The researchers concluded that because the rock and the cracks work together so well, we can treat a small chunk of this rock (just a few meters wide) as a single, uniform block with "average" properties. This is great news for building computer models to predict how water (and potentially radioactive waste) moves underground. It means we don't have to map every single tiny crack to get a good prediction; the rock itself does a lot of the heavy lifting.

In short: The rock isn't just a cracked shell; it's a sponge that breathes water through its skin, not just its cracks.

Technical Summary: Contribution of Matrix Hydraulic Conductivity to Groundwater Flow in Highly Fractured High-Porosity Rock

Problem Statement
Reliable numerical modeling and spatiotemporal upscaling of groundwater flow in fractured rock are critical for the safety assessment of deep geological repositories for high-level radioactive waste. In typical mudstones with low swelling properties and minimal fracture infill, flow is generally controlled by fracture hydraulic connectivity, which is dependent on the mechanical stress state (specifically effective normal stress). However, the Koetoi Formation at the Horonobe Underground Research Laboratory (URL) presents a unique case: it is a highly fractured, high-porosity mudstone that exhibits high fracture hydraulic connectivity regardless of the stress state, despite having ductility indices (DI > 2, DIu > 0.15) that typically suggest limited connectivity.

While high fracture density and void spaces at fracture intersections are known contributors to flow, the Koetoi Formation also possesses a rock matrix with unusually high hydraulic conductivity (10⁻¹¹ to 10⁻⁹ m/s) due to its high porosity. It remains unclear whether this high matrix hydraulic conductivity significantly contributes to groundwater flow alongside fractures, or if flow is dominated solely by fracture networks. Clarifying the role of the rock matrix is essential for selecting appropriate modeling approaches and guiding model upscaling efforts.

Methodology
To address this uncertainty, the authors conducted in situ constant-rate stepwise hydraulic injection tests in the Koetoi Formation at the Horonobe URL.

Experimental Design: Four 20-meter boreholes (250-MIG1-1 to 4) and a fifth closely spaced borehole (250-MIG1-5) were drilled. Monitoring sections were partitioned based on fracture density, ranging from a section with no fractures to sections containing up to five fractures.
Hydraulic Testing: Constant-flow-rate step injection tests were performed to isolate hydraulic responses from hydromechanical (HM) coupling. Injection rates were increased stepwise (5 to 40 mL/min) to verify that hydraulic conductivity remained constant (linear pressure response), ensuring that fracture apertures were not altered by the test.
Data Analysis: The authors employed a Generalized Radial Flow (GRF) model, which unifies flow regimes from linear to spherical using a flow dimension parameter ( $n$ ).
Inverse Analysis: A stochastic inverse analysis using Markov chain Monte Carlo (MCMC) methods (specifically the Differential Evolution Metropolis sampler) was applied. This approach allowed for the simultaneous estimation of hydraulic conductivity ( $K_f$ ), specific storage ( $S_{sf}$ ), test zone compressibility, and flow dimension ( $n$ ), while quantifying parameter uncertainties and correlations. Strong priors were applied to specific storage based on theoretical and in situ measurements to ensure physically meaningful results.

Key Results

Flow Dimension: The estimated flow dimensions across all monitoring sections, including the fracture-free section, were consistently high, ranging from 2.2 to 3.0. A flow dimension near 3 indicates spherical flow, suggesting that flow is not restricted to linear fracture channels.
Fracture-Free Section: In a monitoring section containing no fractures (Section 4 of 250-MIG1-2), hydraulic pressure responses were observed. The analysis uniquely identified a flow dimension of approximately 3 and a hydraulic conductivity of ~10⁻⁹.⁵ m/s. This confirms that groundwater flow occurred through the rock matrix alone in the absence of fractures.
Hydraulic Conductivity: Estimated hydraulic conductivities in fractured sections were higher than in the intact matrix but remained within the same order of magnitude (10⁻¹⁰ to 10⁻⁸ m/s) as the matrix.
Hydromechanical Coupling: The linear relationship between injection rate and hydraulic pressure confirmed that HM coupling was negligible during the tests; fracture apertures did not change significantly under the applied stress conditions.
Representative Elementary Volume (REV): The variation in estimated hydraulic conductivity converged within a few meters, suggesting that the REV for hydraulic conductivity in this formation is on the scale of a few meters.

Key Contributions

Quantification of Matrix Contribution: The study provides direct in situ evidence that high matrix hydraulic conductivity contributes significantly to groundwater flow in the Koetoi Formation, even in highly fractured rock. Flow is not dominated solely by fracture networks or fracture intersections.
Validation of Modeling Approach: The successful application of the GRF model combined with stochastic inverse analysis demonstrated that flow dimensions >2 can be reliably identified even in complex, highly fractured environments, provided that parameter uncertainties are rigorously quantified.
Upscaling Guidance: The finding that the REV converges at a scale of a few meters supports the use of effective physical properties for modeling groundwater flow at this site, facilitating model upscaling for larger-scale safety assessments.

Significance and Claims
The paper claims that the traditional assumption—that flow in low-swelling mudstones is controlled exclusively by stress-dependent fracture connectivity—does not fully apply to the Koetoi Formation. Instead, the high porosity of the rock matrix creates a high-conductivity pathway that operates concurrently with fractures.

The authors emphasize that clarifying the role of the rock matrix is essential for selecting appropriate numerical models. The results suggest that models assuming flow is restricted to discrete fracture networks may be insufficient for this formation. Instead, a dual-porosity or continuum approach utilizing effective parameters derived from the high matrix conductivity and high flow dimensions is more appropriate.

The study concludes that while both fractures and the rock matrix contribute to flow, the specific roles of these features in solute transport remain to be determined. The authors note that subsequent tracer tests and detailed core sampling are planned to further elucidate the individual contributions of fractures and the matrix to solute transport, but the current hydraulic tests confirm the matrix's role in fluid flow.