جستجوی مقالات مرتبط با کلیدواژه « discrete fracture network » در نشریات گروه « مهندسی معدن »

Rock mass consists of intact rocks and discontinuities such as fractures, which have a significant impact on the mechanical and hydraulic properties of the rock mass. For example, in sensitive analyzes such as tunnel stability simulation. Therefore, it is very important to determine the exact parameters of stone jointing, such as orientation and length. Using Discrete Fracture Network is one of the common methods to simulate jointed rock. Since the construction of the fracture network is done using statistical distribution functions, the uncertainty in the construction of the fracture network has always been one of the great challenges of engineers. In this study, the fracture network of of Emamzadeh Hashem tunnel was constructed by a code developed in MATLAB and 3DEC software, and the influence of statistical distribution functions on the uncertainty in the construction of the fracture network was investigated. The results indicate that a negative exponential distribution can lead to relatively large errors in constructing the fracture network, especially when used to generate the fracture dip direction.. Also, the results of the parametric study showed that the use of statistical distribution functions that have the variance of the data in their PDF (Probability Distribution Function) can increase the accuracy in the production of fracture parameters such as slope, slope direction and effect length, and To reduce the uncertainty in the production of the fracture network.

The interpretation of the pressure data of naturally fractured reservoirs (NFRs) has particular significance. The most famous theory for analyzing the pressure data of NFRs is the dual-porosity model presented by Warren and Root. Recent studies have shown that the dual-porosity model may not be appropriate for interpreting well test from all NFRs because this model has limits. In this study, the pressure transient response of naturally fractured reservoirs was investigated by using numerical simulation without considering analytical and semi-analytical methods. To this end, a set of models including connected and disconnected fracture networks was simulated in the numerical simulator. The Warren and Root well-testing signature was observed in all simulations but it was highly evident for a well was located in the matrix and was negligible for a well that was intersected by fractures. The results of the simulation for the well that was intersected by fractures showed the bilinear flow regime with the slope of 1/4. The period of this flow regime increased in the unconnected fracture network and changed to the linear flow regime with a slope of 1/2 in two cases: firstly, by increasing fracture permeability in the connected fracture network, secondly, in the small-unconnected fractures. Moreover, the sensitivity analysis was performed on the well location in the connected fracture network. This research showed that by decreasing the distance between well and fracture network, the transition period becomes deeper and appears earlier.

Summary:

Almost all of the proposed models of Discrete Fracture Networks (DFN) embedded within rock masses are discontinuities with zero tension strength. While, potential discontinuities and weak surfaces such as rock bridges, veinlet, and schistose surfaces are a candidate for breakage under stress and have also a significant effect on rock mass strength. Simultaneously with geometrical parameters, this geomechanical heterogeneous nature of fractures is crucial for understanding rock mass behavior and characteristics. This paper focuses on the probabilistic effect of potential discontinuity properties on the rock mass strength using Potential Discrete fracture networks (PDFN)-bonded block models (BBM) framework. The cohesive crack model is used to define block contact behavior. A comparison of the results indicates the importance of the proposed model for the assessment of realistic rock mass strength instead of traditional DFN.

Analysis of discrete fracture network (DFN) employing the complex nature of fracture patterns plays an important role in understanding the micro and macromechanical behavior of rock mass. Rock mass behavior analyses have traditionally been undertaken for discontinuity with zero tension strength along DFN to calculate mechanical properties. In point of this view, such methods regardless of geometrical parameters generally assume the same possibility of failure for all fractures in the rock mass. The distribution of the DFN strength parameter in Potential DFN can result in important changes in the strength of rock mass and remains a challenge in rock engineering design. The term of PDFN is used for those fracture network with tension and shear strength. Detailed analysis of the employed PDFN which elaborately highlights the role of the distribution of input strength property of each fracture that controls the real strength of rock mass has been presented in this research.

Methodology and Approaches:

In the BBM-DEM, the Voronoi tessellation scheme is employed and the material is simulated as assemblies of several particles bonded together at their contact areas. In order to define contact law of BBM, a cohesive crack model (CCM) is implemented in the UDEC. The calibration process is carried out to obtain contact properties. DFN is written in c++, in which the probability distribution of cohesion, friction angle, and tension strength is considered. The sensitivities of the rock mass strength calculated using SRM e.g., UDEC-DFN to the variability of the input parameter are investigated. A discussion of results is then made base on a reference simulation performed without a distribution method. 4 different P21 are investigated.

The cohesive crack model is implemented in UDEC to define contact law among generated BBM through the Voronoi tessellation technique. In order to assess the effect of the distribution of mechanical parameters in PDFN on the strength of rock mass, DFN is written and this feature is added. The results indicate the importance of the proposed model for the assessment of realistic rock mass strength in engineering applications.

Wellbore instability and drilling fluid loss in fracture formation is one of the main issues in deep drilling. In order to determine an efficient drilling methodology it is necessary to investigate the effect of fracture on instability and fluid loss mechanism. In this article in order to evaluation of the vertical wellbore stability and fluid loss in fracture formation, three dimensional simulation of of a wellbore in the Persian Gulf was carried out using Discrete Fracture Network (DFN) and Distinct Element Method (DEM). Validation of the model and stability analysis of wellbore was carried out using maximum allowed movement, normalized yield zone radius criteria and caliper log data. The initial analysis of the model showed that the wellbore is in an unstable state for kakhdumi formation. In order to investigate the hydromechanical mechanism in fracture formation, drilling fluid was injected by rate of 25 BPH and viscosity of 1.08 cP to the wellbore. Slip in fractures, shear displacement and the volume of fluid loss was determined as main parameters for wellbore stability analysis. The effect of in-situ stresses ratio (σ_H/σ_h ) on instability mechanism and fluid loss was carried out based on six different scenarios for in-situ stresses ratio. By increasing in-situ stresses ratio and in an anisotropic (σ_H/σ_h =2) satat, slips and shear displacement along the discontinuity increased. In this case, for 25 BPH drilling fluid flow ratio the fluid pressure decrease along the discontinuities. The parametric study for five different fluid flow ratio showed that in (σ_H/σ_h =1.06) the fluid expansion in fracture increased. Moreover, tension failure and shear displacement decreased in low fluid flow ratio. In 5 BPH fluid flow ratio, the fluid pressure in fractures decreased compared with higher fluid flow ratio. This is because of less shear displacement and fluid expansion along fracture in lower fluid flow ratio.

One of the most important issues about the underground spaces is to calculate the amount of water seepage into them. Realistic estimation of seepage is a very influential factor in the development of an operational project. The main cause of the seepage is hydraulic conductivity. One of the most important ways of estimating the amount of water seepage into the underground spaces is the Lugeon and Lofran field tests. These tests are performed in rocky and soil environments, respectively. In this research, the effective factors on the amount of hydraulic conductivity changes have been investigated by using a discrete fracture network (DFN). Finally, a model has been developed to help determine the hydraulic conductivity of the fractured rock mass. Since work in a discrete environment software is complex in terms of software, it is very important to obtain a simple model. The number of fractures and dimensions of the block model are two important factors in the speed of modeling. Finally, the filter was intended to remove fractures smaller than 10 meters. Also, the dimension of the block model is at least 7 meters (with these dimensions, the answers independent of the size of the fracture block). Finally, the answers were verified with the lugeon test. The result is a high adaptation between experimental data and calculated data.

Summary:This study has been performed with the aim of investigation of dynamic loading effect on permeability of rock mass. The method is 2D numerical modeling that because of discontinuous nature of rock mass and also existence of fracture networks in it, the modeling has been carried out by Discrete Fracture Network-Discrete Element Method (DFN-DEM) conflation approach. Results show that dynamic loading changed the transmissivity of fractures and consequently increased the permeability of fractured rock mass.
Dynamic loading is a phenomenon that may be applied to rock mass in nature and even leads to changes in some of its geo-mechanical properties such as permeability. The variation in the amount of fluid flow from which the predicted value in a sensitive project such as underground power stations, hydrocarbon fluid flow in its reservoirs and repositories of buried of nuclear waste can cause damages and demolitions. Hence, investigation of dynamic loading effects on the permeability of rock mass is important. In previous studies, some research has been accomplished in the field of rock mass hydromechanics and interaction between static stress and fluid flow in rock mass. However, the lack of evaluation of stress form (static or dynamic) on permeability of rock mass has been felt.
Methodology and Approaches: In this study hydro-mechanical numerical modeling has been carried out under static and dynamic stress conditions. All of geometrical and mechanical properties of the models have been for Sellafield site in Cambria, England. As previously mentioned, modeling has been performed by DFN-DEM conflation approach using UDEC. In order to realize results of the method, data from a real earthquake have been utilized as a dynamic boundary conditions. Modeling in this study has been conducted in two groups. Group 1 contains models which have been placed under fluid flow without dynamic loading (in static conditions). In the group 2, the same models have been put under dynamic loading and then under fluid flow conditions.
Results and The results show that in contrary with the previous consideration, at least dynamic loading changes the transmissivity of fractures and therefore violates the permeability of fractured rock masses. Despite the fact that in our case study, fracture stiffness is relatively high, calculated permeability of rock mass is greater by 26% at dynamic loading compared with the static loading condition. The major reason is that dynamic loading has caused successive moving the blocks and possible changes in their positions relative to the previous state.

One of the most critical factors for estimation of cavability in prefeasibility stage in caving extraction methods is evaluation of In situ fragmentation. Poor estimation of this variable can lead to production and processing problems, or in the worst scenario, failure of the project. In recent years with help of new stochastic discontinuity geometrical modeling, joints those are the main factor for in situ fragmentation, modeled more in tune with reality. Often in nature, discontinuity persistence especially in joints is not infinite. This has a great impact on the cavability of ore bodies. In a new recent stochastic geometrical modeling that is named discrete fracture network (DFN), discontinuities modeled finite that is more in tune with reality. Mathematica Software is selected for writing 2dimensional discrete fracture network code (DFN2D), because of its good graphics and calculations abilities. This code (DFN2D) has good different abilities like producing of numerical and graphical outputs, making finite and infinite joints models, defining of complete blocks with computation of their areas from joints intersections, consideration of joints hierarchies, production of suitable outputs for numerical codes like UDEC & PFC2D and statistics outputs of any model geometrical parameters. With processing of information from joint study in a copper mine and fitness of well probability density functions for any geometrical parameters of joint sets, 2D geometrical modeling with DFN2D was made. Statistics outputs from model and joint intensity and density had a good fitness with reality (input data). In situ fragmentation computed from area of complete blocks also rate of blocky area is defined.

Evaluating the effects of sulfuric acid solution used in leaching of copper metal on hydro- mechanical behavior of rock mass، and also understanding of chemo- hydro- mechanical coupling processes related to this phenomenon are the main objectives of this research work. Biotechnology is regarded as one of the most promising and certainly the most revolutionary solution to the defects of pyrometallurgy. It holds the promise of dramatically reducing the capital cost of the projects. It also offers the opportunity to reduce the environmental pollution. A set of laboratory experiments were implemented for evaluating the influences of sulfuric acid with different pH values; 1، 3 and 5 on the properties of three types of Andesite rock joints in heap leaching site of DAREZAR copper mine. Joint roughness coefficient، joint shear stiffness، joint aperture، and shear strength of degraded rock joints in different sulfuric acid concentrations were measured. Using a Discrete Fracture Network-Distinct Element Method (DFN-DEM) approach، the effect of chemical solution on permeability of the fractured rock mass is evaluated. The numerical modeling results show that with decreasing pH value of copper metal leaching solution، the coefficients of equivalent permeability tensor increases. The effect of ratio of horizontal to vertical stresses (k0) on hydraulic properties of Andesite rock mess is also negligible.