Abstract The authors have justified and physically simulated methods of finding poroperm properties as function of effective stresses s in geomaterials of different types. The related test bench is designed and manufactured; it includes a measurement cell to be filled with a granular material, a loading facility and a recording system for pressure, flow rate and stress σ m applied to the cell. At each stage of loading, seepage tests were carried out at various input pressure p n of gas. By the stationary measured flow rates σ mn , using the back analysis, the empirical permeability dependence k(s) is determined and approximated by the two-parameter exponential function. Under fixed σ m the cell was pumped out and connected with a reservoir filled with known quantity of air. By the settled equilibrium pressure in the cell-reservoir system, the porosity φ m is calculated. The tests with fine-grained sand show that the coefficient characterizing the permeability-effective stress dependence something like 0.2 MPa –1 ; the permeability-porosity relation can be described by an exponential function; and the Kozeny-Carman equation is fulfilled at a good accuracy in this case. The technology of making specimens of preset shape from artificial geomaterials (graded sand, mixed with cryogel) having poroperm properties as well as deformation and strength characteristics similar to high-permeable reservoir rocks (as laboratory tests have shown) is developed and trialed. The test bench is manufactured for the physical simulation of mass transfer processes in cylindrical specimens with central hole under nonuniform stress state. From the back analysis of the test data, the empirical dependence of permeability on effective stress in artificial geomaterials is determined. 1 Introduction Justification of optimal flow charts for hydrocarbon extraction, interpretation of well logging data, development of methods to stimulate productive capacity of reservoirs–this is just a short list of problems solvable using information on poroperm properties of rocks (Hsu et al. 2019; Fjaer et al. 2008).

Mud loss/mud gain, known as the breathing (ballooning) effect, presents a major problem in drilling because of the difficulty in distinguishing lost circulation and kicks. Based on the analysis of the geological characteristics, the geometric model is established. Then the mathematical model is established by coupling the discrete fractures and matrix, and it solved by the finite element method. The effect of formation characteristics and drilling fluid parameters on the breathing effect is investigated and analysed systematically. The results show breathing effect is prone to observe in the sandstone reservoir with the interlayer of silty shale, which has the obvious time-dependent effect. Compared with the formation with a single fracture or matrix, formation with a fracture network will result in severe breathing effect. The existence of fractures reduces the integrity of wellbore rock and changes the distribution of pore pressure around the wellbore. Matrix with high permeability will trigger a more severe breathing effect. However, Young's modulus has little effect on breathing effect. The degree of breathing effect will not deteriorate with the further aperture increase of fracture. The trend of breathing effect in different fracture's aperture goes consistent approximately when the aperture of fracture is larger than 0.3mm. Compared with the viscosity of drilling fluid, the density of drilling fluid has a greater impact on the breathing effect. Low-density drilling fluid and moderately high viscosity of drilling fluid can reduce the impact of breathing effect.

Abstract To solve various technological problems of mining, such as, prevention of air inflow into coal-seam degassing holes and steam entry into underground galleries when thermoshaft production of heavy oil is performed. There are developed the chemical compositions to form the membranes' impermeable and drainage elements. Also, there is provided the stand's description and laboratory studies' results which are the compositions' influence on permeability of porous medium. A method of creating membranes by hydraulic fracturing of the rocks, in which to get a transverse fracture, shear loading of the walls along the wellbore axis is used. A comparative analysis of this method with directional hydraulic fracturing on the basis of slot initiator is performed. In the article calculations of influence of the fracture pressure and fracture propagation from the wellbore and fracture radii are presented. The technological advantages of the developed method of hydraulic fracturing for the creation of impermeable membranes are represented. 1. Introduction In mining the impervious membranes are used to protect a mine working from water and gas inflow and to create edges of the metals leaching zone and etc. Among these purposes there is prevention from air filtering from the mine working into degassing boreholes. The sealing quality estimates the efficiency of the preliminary coal beds degassing. There is a problem of the air inflow into methane withdrawal point through the rock mass. A well-known solution addressing the problem is to apply insulating compound onto the mine working surface. Its uneven surface causes great specific consumption of expensive chemical reagents. However, isolation of a surface part around the degassing borehole head does not interfere with the air filtering through the disturbed rock bed around the mine working made out of its non-isolated parts. More effective solution is impervious membranes created inside of the rock mass with the help of one or several hydraulic fracturing fractures parallel to each other and located across an uncased borehole. This paper suggests several schemes for the membranes made of drainage and impermeable elements. In order to obtain the elements a fill fluid and insulating compound have been created. The work presents results of the laboratory research on the fill fluid and insulating compound's influence on porous medium permeability.

Abstract Ischia Island (Italy) experienced slope instabilities during the Holocene that occurred at different scales, from shallow mass movements up to large rock and debris avalanches. These events were strictly related to volcano-tectonic activity and the presence of a well-developed hydrothermal system and mobilized significant volumes of greenish alkali-trachytic tuffs (Mt. Epomeo Green Tuff, MEGT). Ongoing gravity-induced slope deformations in the Mt. Nuovo region also involve the MEGT over an area of 1.2 km2. In order to constrain geometry and mechanism of this phenomenon, and to highlight possible interactions between the thermo-baric field of the hydrothermal system and stress-strain conditions related to the Mt. Nuovo slope deformations, laboratory tests were carried out to assess and characterize the mechanical behaviour of the MEGT. Physical properties of the MEGT were first measured, including porosity, permeability, and elastic wave velocity. Mechanical characterization was then performed using a combination of uniaxial, tensile, and triaxial experiments on as-collected samples and samples thermally stressed in the laboratory. The obtained results reveal MEGT to have a high porosity and permeability, and that it deforms in a compactant (i.e. ductile) manner at very low confining pressure (i.e. depth). The triaxial tests allow us to derive failure envelopes for MEGT under dry and water-saturated conditions, defining the range of stress conditions for failure. 1. Introduction Flank instabilities represent the most common indirect geo-hazard related to volcanic activities. Destabilization is usually produced by a combination of predisposing and triggering factors. Although magma emplacement or earthquake shaking are the most common triggers able to produce external transient forcing that could interact with inner pressures or accelerate deformations [1, 2], hydrothermal systems may also play a significant role, influencing slope stability in two ways: alteration can weaken or strengthen the materials involved as function of PT conditions and the chemical composition of fluids [3] and transiently increasing the stress field within the slope [4, 5, 6]. Such collapses can in turn trigger subsequent hazardous events in a sort of "domino effect" leading to the occurrence of explosive eruptions, triggered by the sudden decompression of shallow magma reservoirs or hydrothermal systems [7, 8] or, in the case of marine volcanoes, destructive tsunami waves [9]. The case study of a gravity-induced slope deformation affecting the edge of Ischia resurgent caldera is presented here; to account for a well-constrained mechanical behavior of the involved lithologies, in view of future multiphysics numerical modelling, a suite of mechanical laboratory tests was carried out.

Abstract In Germany geothermal energy production plays an important role in the area of renewable energies. The Upper Jurassic (Malm) aquifer beneath the Molasse Basin is predestinated for hydro-geothermal energy. As part of the research program PrognosPermae several laboratory tests were performed on Malm limestones and dolomites. The rock samples were characterized by uniaxial compressive strength (UCS), tensile strength, Young's Modulus, Poisson's ratio, bulk density, porosity and ultrasonic p- and s-wave velocities. Additionally water-permeability was measured in a triaxial cell under varying stress conditions. Ultrasonic wave velocities were investigated on dry and saturated samples, free to leak. The aim of this study is to ascertain a correlation between the lithology of the target horizon, permeability and the ultrasonic wave velocities. Furthermore, the influence of different stress levels on water-permeability of the specimens is investigated. 1. Introduction Planning the exploitation of hydro-geothermal systems requires geophysical parameters from seismic and borehole measurements as well as the quantification of rock (hydro-) geomechanical properties in the laboratory. Many studies have already been performed on carbonate rocks (e.g. [1–6]). Nevertheless since geomechanical properties of carbonates depend strongly on depositional, diagenetic and tectonical processes it is essential to investigate these properties for the present conditions. One of the main hydro-geothermal energy targets is located in the South German Molasse Basin. The aim of the presented research project is to develop a workflow to improve the potentiality diagnosis of the Upper Jurassic (Malm) aquifer. Therefore (hydro-) geomechanical parameters of carbonate samples were examined. For the study borehole and seismic data of the investigation area close to Kirchweidach are available. Three wells, one of them a side-track, were drilled until a target depth of around 3,800 m (TVD). The characteristics of the Malm reservoir in a depth of 3,500 to 3,800 m is qualified by cuttings, thin sections and geological maps. For laboratory tests analog outcrop material with similar properties was chosen. To cover the range of possible rock formations detected in the cuttings limestone and dolomite of the Franconian Alp were selected. This work presents the first results of the experimental laboratory program.

Abstract Field measurement of rock mass permeability is essential that numerous factors influence its directional magnitude. Lugeon test is a popularly conducted field instrument in order to measure hydraulic conductivity of a rock mass. Discontinuity orientation, spacing and discontinuity surface quality, infill presence and type play essential role in permeability of the rock mass in addition to rock material itself. Geological Strength Index ( GSI ) is a parameter used in Gen. Hoek-Brown failure criterion and supporting empirical equations in order to estimate rock mass strength and deformability parameters. Frequently used Rock Quality Designation ( RQD ) and GSI and Lugeon values were combined in order to generate a relation among them. The proposed relationships are produced by interpretation of geotechnical core logging and Lugeon test results. 1. Introduction Groundwater has great influence on rock engineering structures. Difficulties may include construction operations, rock mass deformation and stability. Hydraulic conductivity, pore water pressure, water pressure acting along the joints are all important as well as water sensitivity of the rock material. Understanding of groundwater condition is crucial for surface structures on rock masses, foundations, slopes [1, 2, 3]. For underground rock structures, information about ground water pressure and water inflow rate is essential [4, 5] since it strongly influences operational issues as well as stability of the structure and supports. Operational issues may include pump selection and infrastructure design for water discharge, foreseeing a need for grouting, water sealing. Permeability character of ground plays important role for also surface structures such as dams and foundations [6, 7]. 2. Hydraulic conductivity For water flow through a saturated granular material, Darcy's law [8] is widely accepted: equation Total flow ( Q in m 3 /s) is directly proportional to the cross-sectional area of flow ( A in m 2 ), hydraulic conductivity (or called as coefficient of permeability in m/s) and hydraulic gradient i . i is a ratio of pressure difference (Δ h ) along a particular flow length ( l in m) can be represented as: equation In addition to the intrinsic permeability of the geological material represented by k i , hydraulic conductivity, K is dependent on the fluid properties which are unit weight γ and viscosity μ: equation

Abstract Extensive rock grouting has been carried out during the construction phase of the Cityringen Metro Project. The main goal of the rock grouting has been to reduce the permeability in the rock by filling joints, cracks and fissures, in order to minimize the groundwater inflow during excavation of caverns, pump sumps and TBM break in/out through the stations/shafts. Both horizontal and vertical grouting has been performed above and below the water table, with unique challenges and advantages/disadvantages. The grouting has been performed as up-stage, where the grout hole is drilled to full depth and then grouted from the bottom and up in stages using a pneumatic packer. The split-spacing method has been used to expand the grouting pattern if needed after primary grouting campaign has been finished and tested. The article describes, through case histories, the achieved results, challenges and limits of the applied methods, including future recommendations. The rock characteristics of the Copenhagen limestone, in regards to the performed rock grouting, are also discussed. 1. Introduction The Cityringen Metro is expected to be running in 2019 and will consist of two single tracks in twin tunnels, each approximately 16.5 km in length. A total of 17 underground stations, 4 cross-over facilities and 3 construction and ventilation shafts have been made when the project is finished. The Cityringen is built entirely in the urban area of Copenhagen; most of the stations are located in congested areas, surrounded by historical building. One of the most sensitive activities during construction is the break-in/-out of the station/shafts. It is mandatory to ensure a dry or at least a controllable groundwater inflow during the break-in/out phases. Between various technical solutions, the contractor developed a grouting plug to reduce the inflow of water, which has proved efficient in the Copenhagen limestone. 2. Geology The Geology of the Copenhagen area consists of Quaternary Deposit overlying the Paleogene Deposits, primarily consisting of Danian Limestone and Locally of Selandian Greensand [1]. The Copenhagen Limestone Formation is divided in 3 stratigraphically units: Upper Copenhagen Limestone (UCL), Middle Copenhagen Limestone (MCL) and Lower Copenhagen Limestone (LCL). The tunneling and excavation activities are entirely in UCL and MCL.

Abstract A fast computational code is presented that is dedicated for the elastic analysis of three-dimensional excavations and cracks in rocks. The problem is solved on the boundaries that are discretized with a new triangular leaf constant displacement discontinuity element with one collocation point. The creation of the new triangular element was inspired from Mindlin's special version of grade-2 or strain-gradient elasticity theory (second gradient of displacement, g2). This element is characterized by a much better measure of the average stress at the center of gravity of the triangular element compared to that of the classical elasticity element close to regions with stress or strain gradients (e.g. notches, cracks etc). In a verification stage, the accuracy of the computational algorithm for the pressurized penny-shaped and mixed-mode elliptical crack problems that have analytical solutions is demonstrated. More specifically, it is shown that the average error of the crack tip Stress Intensity Factor predicted by the gradient modified method for nine discretizations of varying density is around 3.5 % with a maximum error of 5 %, while the constant displacement discontinuity element displays errors varying around 14 %. Moreover, the new method preserves the simplicity and hence the high speed of the constant displacement discontinuity with only one collocation point per element, but it is far more efficient compared to it, especially close to the crack tips and corners of excavations where the displacement and stress gradients are highest. 1. Introduction It is almost certain that any planned underground excavation in the scale of 10 1 m or more will transect a fault or persistent joint. The problem in the design phase is to examine the effect of the faults on the behavior of the rock mass during excavation and then to optimize the design of the underground openings and pillars. In Geophysics there remains the reasonable trend to explain earthquake mechanisms by means of dislocation or fracture mechanics models. Also, in petroleum engineering as well as in Rock Mechanics, the hydraulic fracturing technique where a pressurized mode-I crack propagates from a shallow or deep borehole, is widely used for permeability enhancement and measurement of in situ stresses, respectively. There many more problems involving threedimensional excavations and fractures that should be attacked with computational methods capable to tackle in a formal and accurate manner crack tip or corner singularities.

Abstract Shale, a fine-grained sedimentary rock, is composed of highly compacted clay particles, nanometric porosity and silt-sized particles of other minerals. In this paper, a combined use of X-Ray micro-CT and nanoindentation tests is proposed for the qualitative and quantitative characterization of this material. The individual methods are utilized at different scales of observation: nanoindentation at nano and micro level while the X-ray microtomography at the meso scale. For the purpose of mechanical morphology evaluation, the grid nanoindentation measurements are carried out resulting in the overall histograms of indentation properties, i.e. indentation modulus and hardness. Tests are performed at different values of applied load and in two directions: parallel and normal to the plane of isotropy, x 1 and x 3 , respectively. The deconvolution technique is used for the segmentation of the material constituents with respect to mechanical properties. Based on the results obtained from nanoindentation testing at nano scale, the shale matrix composition is identified, namely: the number of constituents, their volume fractions as well as the mechanical properties. In addition, at micro scale, the homogenized parameters of the intact shale material are determined. On the other hand, based on the X-ray microtomography imaging results, the geometry of the shale structure at meso scale is investigated. The studies focus on observation of the evolution of 3D crack network in the samples of shale under uniaxial compression at different levels of load. The research includes three series of scans for eight samples, previously subjected to loading. Based on the acquired images, some statistical morphological measures are calculated, i.e. volume fraction of cracks, tortuosity and crack aperture spatial distribution. In addition, the effective parameters of anisotropic permeability and their changes induced by different levels of loading are estimated using Kozeny-Carman formula. 1. Introduction Shale, a fine-grained sedimentary rock, is composed of highly compacted clay particles, nanometric porosity and silt-sized particles of other minerals. It is a multi-phase material with multi-scale internal structure. Modern technologies as nanoindentation and computed microtomography give a possibility to investigate morphological and mechanical properties of shales at very small scales of observation. Nanoindentation method was developed by Doerner and Nix [1] and later by Oliver and Pharr [2] and it allows to determine the hardness of the investigated material as well as its elastic modulus from depth sensing indentation load-displacement data (DSI) [1]. It was successfully applied for characterization of mechanical properties of shales [3, 4, 5] and other types of rock [6, 7, 8]. Computed microtomography technique is a modern method for nondestructive 3D imaging of material internal structure. The technique was successfully used to verify the porosity of soils [9, 10] and rocks [11, 12]. A number of research concerns the analysis of failure mechanism and the impact of anisotropy on the failure of rock samples [13, 14, 15].

ABSTRACT: The permeability for two types of sandstone, Berea sandstone and sandstone from Otway Paaratte formation, under different confining pressure are measured and porosity is investigated 3D micro-focus X-ray CT and 3D medial axis analysis. The permeability is determined using flow pump test and the porosity is determined by mercury intrusion porosimetry. The total porosity of Berea sandstone and Otway sandstone are around 17.4% and 25.0%, respectively. The permeability of both sandstones with 18 MPa of an effective confining pressure were approximately 70% of that with 10 MPa effective confining pressure. However, the permeability of Berea sandstone is higher than that of Otway sandstone by one order. In order to investigate this curious result, we performed 3D medial axis analysis and conclude that the larger porosity does not mean the higher permeability and the lower of throat/pore ratio has lower permeability. 1 INTRODUCTION Important factors in the long-term and stable CO 2 geological storage are injection capacity, storage capacity and seal capacity etc. Therefore the porosity and permeability of the target formation for geological storage of CO 2 should be measured and considered important. In general the higher porosity, the higher permeability, but it is not always. In other words, the permeability of a rock is related to the porosity, but also to the shapes of the pores in the rocks and their level of connectedness etc. Porosity is a measure of the void spaces in a rock, and it can be defined as a fraction of the volume of voids over the total volume. Voids can be obtained by its geometry information and spatial information. Permeability is a measure of the ability of a porous rock or fractured rock to allow fluids to pass through it. Therefore the permeability of the rocks reflects physical quantity of fluid or ease of fluid flow passing through voids or fractures. That can be expressed in units of cm/sec and m 2 . The former is known as permeability or hydraulic conductivity and the latter is intrinsic permeability, which is the property of rocks that is an indication of the ability of fluids to flow through rocks.

ABSTRACT: The pore pressure evolution induced by excavation of drifts is analyzed by means of a fully coupled hydro-mechanical finite element simulation. Special emphasis is given to anisotropic initial stress state and anisotropy of the elastic parameters of the rock. The main goal is to simulate the main trends of the pore pressure evolution with a simple model taking into account the influence of the face advance and the inherent anisotropy of the material. It is shown that depending upon the permeability of the rock and the degree of anisotropy of the material, strong overpressures can be induced before the passage of the face. 1 INTRODUCTION Field monitoring during the excavation progress of drifts in the Underground Research Laboratory (URL) of The French National Radioactive Waste Management Agency (Andra) in Callovo Oxfordian claystone, showed an anisotropic pore pressure change around drifts excavated following the major horizontal principal stress. It is observed that, the amplitudes of pore pressure change in the ground depend upon several factors, such as the orientation of the drift regarding major principal stress, the distance to the face and to the drift wall. Moreover, the influence of the excavation face on the pressure field ahead the face, is observed on a distance of about 20 m (i.e. about four diameters) (Noiret et al. 2011, Armand et al. 2011, 2014). It should be noted that, at the main level of M/HM URL (i.e., −490 m) the major principal horizontal stress is about 16.0 MPa and is oriented in the direction N155°E10°, while the minor principal horizontal total stress is very close to the vertical one (≈12.0 MPa) (Wileveau et al. 2007). Therefore, the initial stress state is quasi isotropic (in the plane of their section) for the drifts following the direction of the major horizontal principal stress. Interpretation of observed pore pressure variations around excavation is a challenging task, especially in formations like claystones with a very low permeability (< 10 19 m 2 ). Recent studies have been carried out on the pore-pressure evolutions induced by excavation: Analysis of measurements around undergrounds excavations in claystones (Corkum & Martin 2007, Marshall et al. 2008, Noiret et al. 2011, Armand et al. 2011,2014); and some numerical analysis as for example: coupled modeling of the thermal impact on the hydro-mechanical behavior of the rock mass (Bian et al. 2012), unsaturated hydro-mechanical modelling of excavation (Charlier at al. 2013) and coupled modeling of the excavation damaged zone based on strain localization analysis (Pardoen et al. 2015, Salehnia etal. 2015).

ABSTRACT: Rock masses serve a vital function as natural flow barriers for geological disposal of radioactive waste; therefore, information on rock permeability is essential. Highly accurate measurement of permeability requires understanding of how temperature changes in the surrounding environment influence measurement results. We performed permeability measurement under conditions with dramatic changes of temperature in the surrounding environment to investigate the influence of such changes on the experimental results. Measurement of permeability with no temperature change was also conducted as reference. All measurements were conducted using the transient pulse method, and the sample material used was Toki granite obtained from Gifu Prefecture in central Japan. We found that temperature changes in the surrounding environment remarkably affected the pressure in reservoirs upstream and downstream, the pressure difference between them, and the confining pressure; all increased when temperature increased for our experimental system. Notably, pressure difference was affected immediately. This difference directly relates to estimation of permeability. As temperature changes significantly affect pressure difference, it is essential to minimize temperature changes in the surrounding environment so as to achieve accurate measurement of rock permeability. 1 INTRODUCTION Rock masses are used as natural flow barriers for geological disposal of radioactive waste. Hence, rock masses with low permeability supply favorable environment for the disposal. In situ hydraulic tests using bore holes are carried out for estimating the permeability of rock masses including fractures and faults in the rock mass scale (Takeuchi et al. 2013), while permeability tests in laboratory can be applied to evaluate the permeability of intact rocks in the core size scale. In the latter case, it is desirable that permeability tests are conducted in calm surrounding environment without temperature variation and vibration in order to evaluate the hydraulic constants (hydraulic conductivity and specific storage) of low permeability rocks such as granite and shale. Temperature variation in the surrounding environment during test causes temperature change in experimental equipment and fluids used there by heat transfer, subsequently yield the change of the fluid pressures other than controlled fluid pressure, and then lessens the compatibility of the experimental data to a theory of permeability test method. The investigation of the effect of temperature variation in the surrounding environment on permeability test data is therefore made (Bernabé 1987, Takahashi & Xue 1989, Nakano et al. 1991), and the methods for controlling temperature during permeability test are devised (Sutherland & Cave 1980, Trimmer 1982, Bernabé 1986, Zhang etal. 2000, Nara etal. 2011, Kato et al. 2013). The influence of temperature variation in surrounding environment on permeability test data is apparent qualitatively, but does not become clear quantitatively.

ABSTRACT: Hydraulic-mechanical (HM) processes including fracture propagation in geomaterials are important factors for many geo-environmental issues. A coupled HM simulation tool, TOUGH-RBSN, has been developed to investigate permeability change of porous medium due to mechanical initial compaction and postfailure deformation. TOUGH2 is a widely used simulator of subsurface multiphase flow, and RBSN is a type of mechanical lattice model based on the Rigid-Body-Spring Network concept. In this paper, an implementation of the HM coupled module is first described for stress induced porosity changes which affect the conditions of rock permeability. Thereafter, the basic capabilities of the modeling approach are demonstrated through the transient pulse tests during triaxial compression loading. In most respects, the simulation results meet expectations: permeability decreases due to compaction before the onset of microcracking, and increases during progressive failure process. The visualization of interior damage development is facilitated by a 3D Voronoi-based discretization technique. 1 INTRODUCTION Hydraulic-mechanical (HM) processes including fracture propagation in geomaterials are important factors for many geo-environmental issues related to nuclear waste disposal, geologic carbon sequestration, and hydraulic fracturing. For example, crack opening or fault reactivation induced by stress perturbation of rock may significantly change local hydraulic conditions (e.g. permeability), which potentially reduces confinement capability of a radioactive waste repository. Proper modeling of such coupled processes is critical for any performance assessment of geological engineering practices. Various numerical models have been developed to study coupled HM processes of geomaterials (Yuan & Harrison 2006, Tan et al. 2014). Bruno (1994) used a discrete element model to simulate various biaxial loading conditions and associated permeability changes. Lu et al. (2013) studied the fracture-fluid interaction effects using the finite element model, in which damage and flow coupling is based on the macroscopic elastic modulus tensor. Since geomaterials and fracture processes are three-dimensional, it is important to consider HM processes in spatial domain. However, numerical models related to the coupled HM processes is mostly conducted within a 2D framework. Proper 3D modeling capabilities are needed to study the mechanism of HM processes through comparison with laboratory testings.

ABSTRACT: In this paper, effect of matrix permeability and fracture density on flow pattern and overall permeability is presented. According to the literature, dual fracture method (DFM) is an efficient numerical method for investigating the flow in dual porous mediums. In this method, matrix permeability is presented with polygons called voronoi elements (VE) and discrete fracture network (DFN) is used to show fracture system. It is expected that a permeable matrix has to lead to higher overall permeability. Effects of fracture density and aperture patterns on overall permeability and representative elementary volume (REV) have been studied as well. Using a computer code based upon Monte Carlo simulation technique, DFNs with different aperture patterns and densities were generated with regard to real field data. All these models were numerically analyzed via computer code using Distinct Element Method (DEM). The results show that permeability increases with fracture density and with adding permeable matrix to the model, huge increases in overall permeability occur in most cases. Also it was revealed that there is a relationship between overall permeability and DFN aperture pattern. With increasing the density of fractures, REV decreased as well. 1 INTRODUCTION Understanding flow process in naturally fractured dual porous media is of interest in environmental engineering applications, in geohydrology or in oil reservoirs engineering, when porous material is made of rocks, which is crossed by networks of fractures. Recently, fractured rocks attracted the attention in connection with the problem of geological isolation of radioactive waste. Dual porous media often exhibit a variety of heterogeneities, such as fractures, fissures, cracks, and macro-pores or inter-aggregate pores. The granular rock mass consists of solid rock, cut by a network of fractures. Water flows unevenly through an intricate network of paths formed by fracture intersections and micro-channels between matrix blocks or grains (Jourde et al. 2002). Recently, permeability of the matrix is becoming progressively more important. Previously, due to computing limitations, it was accepted that permeability of the matrix is so low and it can be neglected. But with advances in earth sciences for the past few years, precision and reliability in predicting the overall permeability became more important in earth projects and matrix permeability is an important factor for a precise calculating and predicting of the overall permeability. There are some methods to study a Dual Permeability Medium. In 1993, Zimmerman (Zimmermann et al. 1993) created a numerical model for dual porosity mediums. In the same year a simple but useful numerical method was developed to estimate the overall permeability in a Dual Porosity-Dual Permeability medium by Priest. Bai expanded an analytical solution for flow in oil reservoirs in 1994. Different types of fracture geometry such as connected and unconnected fracture systems in a Dual Permeability Medium were investigated by Odling (Odling 1997).

ABSTRACT: When a tunnel is excavated, pore water inside the rock starts evaporating from tunnel surface. On the other hand, pore water is supplied from the behind of tunnel wall. The effect of water supply on drying and wetting- induced deformation behaviors was investigated experimentally so as to simulate the evaporative condition of tunnel surface. Shirahama sandstone is used as a representative rock sample with mechanical isotropy to facilitate the interpretation of the transient behaviors in strain. As a result, the deformation accompanied with drying and wetting cycles is greater closer to the evaporating surface, depending on the water contents at each position of the sample. In the environmental condition to be adopted in this report, the relative humidity change in atmosphere induces deformation at least 7.5 cm below the evaporating surface. In case of no water supply condition, the larger deformation is observed because of the large water loss. Shirahama sandstone shows the isotropic and momentary deformation in response to relative humidity change even at the center of the sample in the quasi-static state. It implies that moisture distribution inside the sample will be quickly formed in response to atmospheric humidity change. 1 INTRODUCTION The researches on nuclear waste disposal is ongoing over the world. When tunnels are excavated in deep underground, the excavation disturbed/damaged zone (EDZ) will be developed due to the stress redistribution. The formation of EDZ results in increase in permeability of the rock around the tunnel. Simultaneously, ventilation of air must be done in order to keep the working environment safe and adequate. On the other hand, ventilation is concerned to create the unsaturated zone around the tunnels and to enlarge the EDZ. As with other porous media, the rock is shrinking with decreasing moisture content. Large shrinkage may cause the formation of desiccation cracks in the rock. Such defects should enhance the permeability of the rock around the tunnel. Therefore the drying-induced deformation behaviors of representative sedimentary rocks should be evaluated adequately and are accumulated in our laboratory (ex. Osada 2014). The literature reviews in this topic are also provided in Osada (2014).

ABSTRACT: A pilot scale research was carried out to verify the feasibility of compressed air storage in lined rock cavern. Two caverns with a diameter of 6m were excavated at a depth of 100 m below the surface ground in a mine. This paper presents assessment technologies of geomechanical characteristics and performance of a compressed air energy storage system in lined rock cavern. The thermo-hydro-mechanical analysis using the coupled TOUGH-FLAC has been conducted to simulate the performance of the CAES pilot, to assess geomechanical stability, and to predict air leakage of lined rock cavern system under the long-term operation condition. Multi-channel borehole tests has been done to measure air and water permeability of the system, and also a borehole seismic exploration has been performed to characterize the excavation damaged zone surrounding the storage caverns. 1 INTRODUCTION Compressed air energy storage (CAES) in rock is considered as one of the effective technologies for large amount of electric energy storage. In the case of CAES in underground, lining of rock caverns is essential to prevent air leakage Construction of lined rock cavern (LRC) is more expensive than salt domes. However, CAES system in LRC has some advantages in the respect of the site selection and the liaison between production and demand. Moreover, they can reduce an additional expense due to construction of storage space at depth since lined system allows air tightness and mechanical stability (Song et al. 2012). Air tightness and mechanical stability are the key factors in underground storage of compressed air. These depend on the various design factors such as the shape and volume of storage caverns, the depth, operation pressure, water table, rock strength and permeability. A pilot scale study on underground CAES has been carried out. The purpose of this project is to develop core technologies for CAES in lined rock cavern. A pilot plant is designed for 1 MW-scale LRC system and constructed at about 100m depth of an on-going mine. The R&D program of the project includes the key topics such as investigation of feasibility of the concept, specification of optimum lining components for a given pilot scale, development of field-scale test apparatuses of permeability and excavation damaged zone (EDZ) of lining and rock mass, assessment of air tightness of LRC system, analysis of long-term stability, assessment of geomechanical and thermodynamic performance of CAES in LRC.

ABSTRACT: The effective pressure law for the permeability of sandstone is experimentally studied by conducting Darcy's law permeability test under various confining pressure (from 14 to 58 MPa) and pore pressure (from 8 to 23 MPa). Supercritical CO 2 as a pore fluid has been adopted instead of water for carbon capture and storage (CCS) study. The effective pressure law for permeability is associated with curved iso-permeability lines. We can express the effective pressure as P eff = P c — χ(P c ,P p )P p , where the coefficient χ corresponds to the slope of iso-permeability line according to the confining(P c ) and pore(P p ) pressure. It was confirmed that the coefficient is not a constant because of the nonlinearity of iso-permeability line. Cycling test of the permeability has been conducted for analyzing the phenomenon using a modified factorial method. The influence factor and interaction between the pore and confining pressure were analyzed. 1 INTRODUCTION In CO 2 storage project, CO 2 permeability of reservoir rock for a given in-situ stress and pore pressure is very important to monitor and predict diffusion range of the CO 2 . The permeability of gas is different from the permeability of liquid such as distilled water or brine. The permeability of supercritical CO 2 (hereafter scCO 2 ) should be examined considering the mixed phases and therefore mixed properties of gas and liquid. For the laboratory test, pore pressure of rock specimens generated by CO 2 injection must be 7.3 MPa or higher to keep CO 2 in supercritical state. Confining pressure on the specimens should be greater than the pore pressure. The influence of external (confining) and internal (pore) pressures on rock permeability has been reported by many researchers. There are, however, no cases where scCO 2 , has been used as injection fluid. It is well known that the permeability of porous media is a pressure-dependent property. Therefore, an effective pressure law affects the permeability. Because K (permeability) depends on P c (confining pressure) and P p (pore pressure), we can replace the two-variable function by a single variable one, P eff (P c , P p ), according to the effective pressure law: (equation)(1)

Abstract The fully-coupled thermo-hydro-mechanical (THM) model presented in Haxaire et al. (2013) has been implemented in the finite element code PLAXIS 2D. An application to radioactive waste storage is conducted and analysed. It is inspired by the OPERA research programme, in the Netherlands (Verhoef & Schröder 2011). The study consists of a simplified scenario of galleries in a layer of Boom Clay at a depth of 500 m. The formulation and the implementation is stable and provides qualitative results in accordance with expectation. Further studies, in particular validations based on in situ experiments, will have to be conducted to demonstrate the ability of the model to be simulate the complex coupled THM phenomena inherent to radioactive waste storage. 1 Introduction The model presented in Haxaire et al. (2013) has been implemented and released in the finite element code PLAXIS 2D (Plaxis 2015a). The code offers a full coupling between the thermal, hydraulic and mechanical phenomena, for unsaturated porous media. The implementation supports plane strain and axisymmetric problems. The calculations can be steady state, transient, semi- and fully-coupled. The pore fluid has one constituent, water, and allows phase change: the three available states are ice, liquid and vapour. The time integration scheme is fully implicit to ensure robustness and stability. Transient and steady-state analyses can be performed, with automatic time stepping procedures for the transient analyses to provide efficient analysis times. Downscaling of the time step is triggered if the convergence criteria is exceeded and upscaling is used as the calculation becomes more and more stable. To evaluate the performance of the model and its implementation, a case study is presented. It is based on the research work conducted at TU Delft in the framework of the OPERA research programme (Arnold et al. 2015). The OPERA project is a five year project running from 2011 - 2016, where a substantial research programme is being carried out to lead towards the development of a generic safety case (Verhoef & Schröder 2011). The work carried out by TU Delft for the OPERA project initially assessed the thermal behaviour of the proposed repository and found that due to the long term (100 year) proposed interim storage the yielded temperatures were unlikely to exceed proposed limits. The THM processes close to the proposed repository were not assessed. In this paper, only the ability of the developed code to simulate the THM processes close to the proposed repository is illustrated. The complexity of the interaction between the THM phenomena, the near and far field scale difference, in addition to the large time scale, pushes the code to its boundaries, making thus an ideal application to test and benchmark the implementation.

Abstract This paper presents a grain-based discrete element model for simulating the quasi-brittle failure of rocks and associated permeability evolution under hydro-mechanical loading. In this approach, the development of crack along grain boundaries controls the degree of damage and the associated permeability alteration in material. As a result, the overall permeability of the model, which is a function of degree of micro-cracking and crack connectivity, can be calculated at each stress state. Micro-mechanical parameters of the model is calibrated to Lac du Bonnet granite such that the model reproduces the physics similar to that of the rock during compression and tension. The numerical experimentations demonstrate the capability of DEM-Voronoi model to mimic the pre- and post-failure response of materials. The calibrated model accurately predicts, in a quantitative sense, the macroscopic properties of granite such as elastic properties, damage thresholds, peak strengths, triaxial strength envelope, and hydraulic conductivity of granite at critical damage thresholds. 1 Introduction It is well-documented in geomechanics-related literature that the permeability of the rock is highly stress-dependent (e.g. Souley et al. 2001). This stress-dependency is due to the fact that when rock is subjected to mechanical loading, the rock microstructure can close, open, extend and induce new cracks, which in turn change the mechanical and hydraulic properties of rock. As a result, the stress-dependent permeability is a function of two fundamental mechanisms: microcracking-induced permeability change which is related to growth of the micro-cracks, and stress-induced pore volume change in which the pore volume is a function of interaction between fluid pressure and mechanical stress. In the former case, damage-modified permeability evolution relates to heterogeneous nature of rock material and presence of internal micro-structure with different sizes and properties. Thus, when the rock is subjected to a compressive load, local tensile stress concentration induced on these internal micro-structures initiates a complex process of fracture propagation in rock. Generation of these crack corridors (channels) modifies the overall permeability of the rock by the extent that is dependent on the opening, density, connectivity of such cracks.

Abstract In the paper, two modeling strategies for the computation of the effective permeability of heterogeneous, fractured rocks utilizing homogenization over a representative elementary volume (REV) are proposed and compared. One method is based on a new continuum micromechanics model. Here, the REV represents distributed fractures idealized as penny shaped inclusions in a porous matrix. The effective properties computed by this model are anisotropic and depend on the intrinsic properties of the porous matrix and the topology and density of the fractures. We propose a novel Cascade Continuum Micromechanics model (CCM), which is able to predict a fracture percolation threshold for a particular fracture density as a function of the topology of the fractures. The second modeling strategy is based on an Extended/Generalized Finite Element model (XFEM-GFEM) recently developed for numerical simulations of hydraulic fracturing in deep geothermal reservoirs. The predictions for the effective permeability from both models are compared for a REV containing distributed fractures with different aspect ratios and crack densities. 1 Introduction The effective transport property of heterogeneous rocks with diffusively distributed fractures at various scales (from microcracks in the sub-mm to macroscopic fractures in the m range) is strongly influenced by their distribution, orientation and their interactions with the porous matrix material. Reliable estimates for effective transport properties of fractured rocks are relevant in various projects in subsurface engineering, such as the construction of tunneling or caverns, the exploitation of oil and gas and geothermal energy reservoirs or the installation of underground storage systems (see, e.g. Tiab & Donaldson (2012), Economides & Nolte (2000) and references therein). In this paper, we propose two methods to determine the effective permeability of heterogeneous rocks. The first strategy is based on continuum micromechanics. Based on a representative elementary volume element (REV), representing distributed fractures idealized as penny shaped inclusions in a porous matrix, the effective anisotropic permeability is predicted. To investigate the percolation probability of the fracture network, we use a novel Cascade Continuum Micromechanics model (CCM) (Timothy & Meschke 2013). The model predicts a fracture percolation threshold for a particular fracture density as a function of the topology of the fractures. This provides initial estimates for the connectivity characteristics of the fracture system and allows to characterize the interrelations between diffusivity, permeability and the anisotropic stiffness. The second strategy uses computational homogenization based on a discrete representation of distributed cracks using a novel Extended/Generalized Finite Element model (Meschke & Leonhart 2015) recently proposed for numerical simulations of hydraulic fracturing in deep geothermal reservoirs. Both modeling strategies are investigated and compared by means of different configurations of fractured rocks.