Nobuo Hirano

Drilling to gabbro in intact ocean crust

Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.

Beyond‐laboratory‐scale prediction for channeling flows through subsurface rock fractures with heterogeneous aperture distributions revealed by laboratory evaluation

The present study evaluates aperture distributions and fluid flow characteristics for variously sized laboratory-scale granite fractures under confining stress. As a significant result of the laboratory investigation, the contact area in fracture plane was found to be virtually independent of scale. By combining this characteristic with the self-affine fractal nature of fracture surfaces, a novel method for predicting fracture aperture distributions beyond laboratory scale is developed. Validity of this method is revealed through reproduction of the results of laboratory investigation and the maximum aperture-fracture length relations, which are reported in the literature, for natural fractures. The present study finally predicts conceivable scale dependencies of fluid flows through joints (fractures without shear displacement) and faults (fractures with shear displacement). Both joint and fault aperture distributions are characterized by a scale-independent contact area, a scale-dependent geometric mean, and a scale-independent geometric standard deviation of aperture. The contact areas for joints and faults are approximately 60% and 40%. Changes in the geometric means of joint and fault apertures (µm), em, joint and em, fault, with fracture length (m), l, are approximated by em, joint = 1 × 102 l0.1 and em, fault = 1 × 103 l0.7, whereas the geometric standard deviations of both joint and fault apertures are approximately 3. Fluid flows through both joints and faults are characterized by formations of preferential flow paths (i.e., channeling flows) with scale-independent flow areas of approximately 10%, whereas the joint and fault permeabilities (m2), kjoint and kfault, are scale dependent and are approximated as kjoint = 1 × 10−12 l0.2 and kfault = 1 × 10−8 l1.1.

High Resolution Modeling Of Aperture Structure And Flow Path In Rock Fracture

In this paper, we describe a numerical method to model aperture structures and flow paths in single rock fractures based on actual fracture surface geometries and actual fracture permeability. Fracture surfaces were measured using a CCD laser displacement sensor (resolution: 10 μm) with 250 μm square mesh for 100 mm × 150 mm single tensile fractures in granite samples. Fracture permeability was also measured under 10–100 MPa confining pressure conditions. Aperture structures and flow paths were modeled numerically using the actual fracture surface geometries so that the model’s permeability consistent with actual fracture permeability. Channeling flows were clearly observed at all conditions because of heterogeneous aperture structures. The results also suggested that fracture permeability could be overestimated if based on the conventional parallel plate model using an arithmetic mean value of local apertures.

Evaluation of Fluid Flow Path in a Single Fracture Undergoing Normal Stress and Shear Offset

We report on an evidence of anisotropy in flow path in a single fracture on the basis of measurements of a contact area in a fracture with shear offset of up to 5 mm undergoing normal stress of up to 90 MPa. An area of contact in a fracture undergoing normal stress is measured by using a pressure‐sensitive sheet inserted between fracture surfaces. This method allows simple measurements of contact area in the stressed conditions. Our results show that contact area increases with an increase in normal stress, on the contrary, contact area decreases with an increase in the offset. These dependencies in a contact area suggests that an open area, i.e., aperture area, which is illustrated as an area except for contact area in a fracture, can change associated with normal stress even under the condition of 100 MPa. In addition, we observed that contact area distributes in the direction perpendicular to the direction of offset, which indicates an anisotropy in aperture distribution that become possible flow path ...

Evaluation of Aperture Structure and Fluid Flow in Shear Fracture of Granite using High‐Resolution Numerical Modeling Technique

Aperture structure and fluid flow of shear fractures with different shear displacement (1 and 5 mm), which were created in granite under different normal stress (20 and 60 MPa) for a shear plane, were evaluated for 10–100 MPa confining pressure, using high‐resolution numerical modeling technique. Aperture structure was heterogeneous and fluid flow occurred in preferential flow paths for all the shear fractures under the confining pressure. For shear fractures created under 20 MPa normal stress, aperture structure was characterized by contact points clustered in a few specific regions, and which caused clear channels where fluid flow occurred preferentially, despite of different shear displacement. On the other hand, for a shear fracture with 1 mm shear displacement, which was created under 60 MPa normal stress, aperture structure was characterized by no clear channel although large clustered contact points were observed, and which caused tortuous flow paths. Additionally, for a shear fracture with 5 mm sh...

Experimental Apparatus for Measurement of IR and Raman Spectrum at High Temperatures and Pressures

In order to investigate effects of molecular behavior of water at interface on the rock surface, high temperature‐pressure cell (up to 400 °C and 50 MPa) was developed to measure infrared and Raman, which was attached with optical microscopy, therefore, in situ and microscopic measurements were enable at high temperature and pressure conditions. Infrared properties of thin film water and interfacial water at rock surface were measured. As a result of infrared spectroscopic measurements of thin film water, the broad peak at ca. 3360 cm−1 of infrared spectra, attributed to OH stretching mode of water molecular, was observed at room temperature and pressure. Continuous shift of the OH vibration mode was obtained from room to hydrothermal conditions. These results indicate that changes of molecular structure of the water were detected by using newly designed high temperature‐pressure cell. Compared with the result of infrared properties of thin film water, infrared properties of interfacial water at rock surf...

Three Dimensional Numerical Analysis of Fluid Flow through Fractured Rock Core Using X-Ray Computed Tomography

A three dimensional numerical modeling coupled with X-ray Computed Tomography (CT) for fracture flow was applied to fractured granite core samples. One of the samples had an artificial single fracture, and the others had natural multiple fractures. A relationship between CT value and fracture aperture (fracture aperture calibration curve) was obtained by X-ray CT scanning for a fracture aperture calibration standard with varying the aperture from 0.1 to 0.5 mm. As a result, a linear relationship was obtained between CT value and fracture aperture.

Experimental And Numerical Analysis Of Flow Path Change In Rock Fracture Under Hydrothermal Condition

A hydrothermal flow‐through experiment was performed for an artificially created single tensile fracture of granite. Water that had dissolved granite (Si concentration: 250 ppm) was injected into the fracture under hydrothermal conditions (effective normal stress: 10–13 MPa, temperature: 150 °C) during 450 hours. Non‐monotonic changes of fracture permeability were observed. Fracture permeability decreased significantly only during first 150 hours. Si concentration of water produced from the fracture was increased monotonically. However, the Si concentration was smaller than that of water injected into the fracture. A numerical modeling using experimental data was also performed for hydrothermal flow in the fracture. Aperture structures for the beginning and the end of the experiment and resulting hydrothermal flow were determined using experimentally obtained fracture surface geometries and fracture permeability. For the beginning of the experiment, developments of preferential flow paths (channeling flow...

OH Stretching Vibration Changes of Water at Water‐Rock Interface under High Temperatures and Pressures: An In‐Situ Study using Infrared Spectroscopy

The author has recognized an error in the section of the text regarding induced Compton scattering. The paper has been withdrawn pending a revision to this section.In order to investigate effects of molecular behavior of interfacial water on rock surface, high temperature‐pressure cell (up to 400 °C and 50 MPa) was developed to measure infrared (IR) and Raman spectra, which was attached with optical microscopy, therefore, in situ and microscopic measurements were enable at high temperature and pressure (HT & HP) conditions. IR spectra of water on metal, interfacial water on quartz surface and synthetic quartz solution were measured. As a result of IR spectroscopic measurements of water on metal, the broad peak at ca. 3400 cm−1, attributed to OH stretching vibration of water molecules, was observed at high temperature and pressure conditions. Continuous shift of the OH vibration mode was obtained from room to hydrothermal conditions. Compared with the result of IR properties of water on a metal, IR properties of water on a quartz surface exhibit different trend: the peak position shifted to higher wavenumber with increasing temperature and slightly shifted with press...

Numerical evaluation of anisotropic fluid flow in sheared rock fractures

Anisotropic fluid flow in single sheared fractures in granite is investigated numerically under normal stresses up to 90 MPa that are the extended stress conditions of the previous studies. Aperture distributions of the sheared fractures under the normal stresses generated numerically on the basis of direct measurements of contact area suggest anisotropy of connectivity in fracture aperture even under the normal stresses. Numerical simulation of fluid flow using the generated aperture distribution shows not only anisotropy in fracture permeability and that in preferential flow paths, but also the normal stress dependency in the anisotropy of fracture permeability. An investigation on contact ratio reveals the anisotropy and the normal stress dependency in aperture connectivity, which supports the results of flow simulation. These results suggest importance of the flow anisotropy in discrete fractures for investigating three‐dimensional flow properties in fractured rocks at a greater depth.