Yoshiaki Mizuta

Experimental determination of elastic constants of Oshima granite, Barre granite, and Chelmsford granite

Employing polyhedral specimens, longitudinal and shear wave velocities were measured in various directions of propagation and polarization. Sound velocities showed orthorhombic elasticity in all of the rocks. With the assumption of orthorhombic elasticity the nine stiffness constants of all samples were determined by the sound velocities under atmospheric pressure and Kelvin-Christoffels equation. Twenty-one stiffness constants of Oshima granite, determined without assuming any symmetry, also showed orthorhombic features. Directions of the symmetry axes agreed well with the orientation of the preexisting cracks. Akaikes Information Criterion showed that the orthorhombic model with nine nonzero elastic stiffnesses was better than the model having 21 nonzero elastic stiffnesses for Oshima granite. The polyhedrons of two granitic rocks were loaded under hydrostatic pressure. All components of the stiffness constants increased with pressure. Under pressure of more than 120 MPa, two granitic rocks were approximately isotropic. The results show that oriented microcracks are mainly responsible for the orthorhombic elasticity of the granitic rocks and also indicate that dry oriented cracks can not be a cause for the anisotropic elasticity of granites at depths of more than 6–8 km.

Influence of fluid viscosity on the hydraulic fracturing mechanism

Cubical granite specimens were fractured by borehole pressurization of 1 cP water, 80 cP oil and via a urethane sleeve. Viscous oil tends to generate thick and planar cracks with few branches, while water tends to generate thin and wavelike cracks with many secondary branches. While penetrating fluids extended cracks rapidly, pressurization via a urethane sleeve led to stepwise crack extension. Fault-plane solutions of AE (Acoustic Emission) events indicated that shear-type mechanisms were dominant during water injection and sleeve pressurization, whereas tensile-type mechanisms were dominant during oil injection. These results could be helpful in optimizing stimulation treatments in the petroleum industry.

Three-dimensional elastic analysis by the displacement discontinuity method with boundary division into triangular leaf elements

The authors have developed a computing system for 3-D elastic analysis by the distribution of constant displacement discontinuities over a boundary divided into triangular leaf elements. Use of the triangular element allows the extension of the Displacement Discontinuity Method (DDM) to include the boundary value problems with any structural boundary in three dimensions, although the method is not very useful unless the boundaries include faults/cracks. The DDM for the 3-D solution of the various “expressions of influence coefficients”, which are theoretically derived through integration over a triangular element and its partial differentiations, is described. The solution procedure is also briefly presented. The accuracy of the results, which are numerically calculated by the procedure, is demonstrated by comparison with the strict solutions of excavation-induced displacements and stresses around both a spherical cavity and a penny-shaped crack in two kinds of stress fields, respectively.

Three dimensional stress determination by hydraulic fracturing for underground excavation design

Abstract The authors have developed a hydraulic fracturing system for three dimensional stress determination. The calculation procedure introduces new interpretations based on information obtained through laboratory hydraulic fracturing of intact rocks. Hydraulic fracturing data incorporated in the system must include at least both the fluid pressure at steady flow in a longitudinal fracture artificially produced around the borehole and the shut-in pressure in a transverse fracture which is apt to develop along a natural joint. Stress states in impermeable rock can then be evaluated by the system without assuming the direction of principal stresses. In this paper, the principle of the new method for three dimensional stress determination and an outline of the measuring system are described first, and then the results are reported of in situ measurements at several sites where the new system was applied.

Stress-induced crack path in Aji granite under tensile stress

The double-torsion test using Aji granite was carried out to investigate the interaction between stress-induced crack path and mineral grains. Crack velocities were controlled at range 10−7 m/s to 10−1 m/s. After the stressed specimens were dyed, we checked the crack path by thin section analysis, using an optical microscope. The stress-induced crack path was divided into two types, transgranular and intergranular cracks, and each path was subdivided with respect to mineral grains. In spite of the extensive range of crack velocities, the ratios between the transgranular and intergranular crack lengths did not change. The crack paths were all jagged, and often showed detour around the grain boundary when faced with obstacles like hard grains or preexisting cracks. That is to say, quartz grain played an important role as an obstacle. Feldspar grain could change the crack path because of its cleavage plane. Biolite grain had a serious effect on the path even if its constitution ratio is very small. Fractal dimensions of the crack paths were calculated by three methods, as indicators of surface roughness. The fractal dimensions were shown in a slight trend with the change of crack velocity. This trend can be explained from the point of limited cracking rate in stress corrosion.

Double fracture method ofin situ stress measurement in brittle rock

SummaryA new technique, the Double Fracture Method ofin situ stress measurement, is described with regard to its theoretical principles and field instrumentation. The method differs from the hydraulic fracture method in that the loading fluid is prevented from penetrating into the induced fractures. In the new method, the loading pressure is raised to create a set of two mutually perpendicular microfractures on the borehole boundary. The principal stresses and their orientation in the plane normal to the borehole can be deduced from the diametral deformation of the borehole. Laboratory and field measurements validating the method are discussed.

AN IMPROVED METHOD OF PREDICTING UNDERGROUND CLIMATE

Abstract The authors previously proposed a practical method for precalculating the rate of flow, temperature and humidity of air currents in underground networks. In this method, however, water vaporization at the airway wall was only treated approximately. Since that time, a drop in wall temperature of the wet portion of the airway has been observed and a more accurate method of precalculation is therefore proposed. Furthermore, in order to perform the calculation by this method, the authors have proposed a new method for determining the wetness factor. Applicability and accuracy of this method were examined by comparing the calculated results with measured results. It has been found that the values calculated agree well with those measured in the field. In particular, the accuracy of prediction of humidity is much greater than that achieved using the earlier method.

EFFECT OF GRAIN SIZE IN GRANITIC ROCKS ON HYDRAULIC FRACTURING MECHANISM

Hydraulic fracturing experiments ere conducted by employing four 20-cm-cubic granitic rock specimens of different grain size. Water was injected into a vertical hole in the specimen, bored normal to a rift plane along which mirco cracks preferentially oriented. In the two horizontal directions, 12Npa and 6Mpa confining pressures were applied to the specimen. Due to water injection, a horizontal crack along a rift plane was created in the specimens having larger grains, while a vertical crack along the direction of the maximum confining pressure was created in those having smaller grains. In addition, fault plane solutions of acoustic emission indicated that dominant micro fracturing mechanism was shear in the specimens having larger grains while it was tensile in those having smaller grains. These results demonstrate that grain size significantly affects the orientation of crack extension and the micro fracturing mechanism in hydraulic fracturing of granitic rocks.

Field Experiments and Data Processing for Continuous Measurements by the GPS Displacement Monitoring System.

The authors are developing a real-time displacement monitoring system by using the Global Positioning System to assess the stability of large slopes. This paper presents the results of field experiments for long-term continuous measurements and discusses the stability and properties of the data obtained by the system. The measured values include random errors, deviations, and largely scattered errors. In order to choose the acceptable data from the original data, including the above errors, a data processing method is proposed in this paper. Applying the proposed method, the system can provide stable and reliable data.Field experiments for displacement measurements were also conducted to verify the applicability of the system to continuous displacement monitoring. It is concluded that the system can detect at least 5 mm displacements in the horizontal direction and 10 mm displacements in the vertical direction in real time. The accuracy is better than that of the conventional off-line GPS method.

Studies on hydraulic fracturing stress measurement assisted by water jet borehole slotting

Abstract This study offers feasibility for direct detection of the maximum principal stress by hydraulic fracturing test assisted by water jet borehole slotting. Investigation previously made by the authors, performance tests of the abrasive water jet system designed by them, laboratory hydraulic fracturing tests and numerical simulations carried out by them are described.