A modeling approach for analysis of coupled multiphase fluid flow, heat transfer, and deformation in fractured porous rock

This paper presents the methodology in which two computer codes—TOUGH2 and FLAC3D—are linked and jointly executed for coupled thermal–hydrologic–mechanical (THM) analysis of multiphase fluid flow, heat transfer, and deformation in fractured and porous rock. TOUGH2 is a well-established code for geohydrological analysis with multiphase, multicomponent fluid flow and heat transport, while FLAC3D is a widely used commercial code that is designed for rock and soil mechanics with thermomechanical and hydromechanical interactions. In this study, the codes are sequentially executed and linked through external coupling modules: one that dictates changes in effective stress as a function of multi-phase pore pressure and thermal expansion, and one that corrects porosity, permeability, and capillary pressure for changes in stress. The capability of a linked TOUGH-FLAC simulator is demonstrated on two complex coupled problems related to injection and storage of carbon dioxide in aquifers and to disposal of nuclear waste in unsaturated fractured porous media.     

A fully coupled thermo-hydro-mechanical model for unsaturated porous media

In examining potential host rocks for such purposes as the disposal of high-level radioactive wastes,it is important to understand the coupled thermo-hydro-mechanical（THM） behavior of a porous medium.A rigorous and fully unified coupled thermo-hydro-mechanical model for unsaturated porous media is required to simulate the complex coupling mechanisms involved.Based on modified Darcy＇s and Fourier＇s laws,equations of mechanical equilibrium,mass conservation and energy conservation are derived by introducing void ratio and volumetric liquid water content into the model.The newly derived model takes into account the effects of temperature on the dynamic viscosity of liquid water and void ratio,the influence of liquid flow on temperature gradient（thermo-osmosis）,the influence on mass and heat conservation equations,and the influence of heat flow on water pressure gradient and thermal convection.The new coupled THM constitutive model is constructed by a finite element program and is used to simulate the coupled behavior of a tunnel during excavation,ventilation and concrete lining stages.Oil and gas engineering,underground disposal of nuclear waste and tunnel engineering may be benefited from the development of the new model.     

非饱和土孔隙气、水、汽、热耦合运动之模拟(英文)

旨在介绍一个描述非饱和土孔隙气、水、汽、热耦合运动的理论模型。该模型假定孔隙气和孔隙水运动分别遵循达西定律 ,而影响水蒸汽运动的两种主要因素分别是分子扩散和孔隙气运动 ,其中受分子扩散影响的孔隙水蒸汽运动可用Fick定律描述。热转移则主要包括了三种形式 ,即传导、对流和汽化潜热。根据有限单元法 ,编制了一个三维的计算机程序用以模拟非饱和土孔隙气、水、汽、热的耦合运动。通过数值分析与干沙试验结果之比较 ,验证了文中之理论模型和计算机程序的可靠性     

模拟热–水–应力耦合作用的三维节理单元及其数值分析

对于具有相同厚度的空间8节点等参数节理单元,考虑温度和孔隙水压力的影响,将其的应力–位移关系加以拓展,建立了相应的应力平衡方程、水连续性方程、能量守恒方程,使之可用于分析饱和–非饱和岩体中存在不连续面时的热–水–应力耦合问题,并结合实体单元,研制出相应的三维有限元程序。针对一个假定的核废物地质处置模型,通过有、无节理的热–水–应力耦合过程数值分析,显示与无节理的情况相比,由于节理的存在使得近场岩体中的温度、应力及水流速度的分布与数值都发生了明显的变化,特别是节理附近的岩体中有某种水流"冷却"作用和一定程度的应力集中现象,其它部位岩体的应力也有所增加。     

Coupled hydro-mechanical model for fractured rock masses using the discontinuous deformation analysis

A coupling analysis model is proposed to study the hydro-mechanical response of the fluid flow in fractured rock mass with the method of discontinuous deformation analysis (DDA). The DDA coupled hydro-mechanical model is interpreted in details by expressing the fracture fluid flow equations, the coupling process and the global coupled equations. For the mechanical response, the hydraulic pressure is determined first, followed by the coupled motion equations expressed under the DDA framework, to study the interaction between the fluid flow along the fractures and the movement of the rock blocks. In the fluid flow analysis, the cubic law is applied to study the steady flow along the fractures using the finite difference method (FDM). A real case of cavern excavation is analyzed by the proposed DDA coupled 2D hydro-mechanical model, to study the influence of fluid flow on the rock cavern stability during the excavation phase. The results show that the DDA coupled hydro-mechanical model is suitable for the stability and seepage analysis of practical engineering problems.     

Convective heat transfer of water flow in intersected rock fractures for enhanced geothermal extraction

Numerous intersected rock fractures constitute the fracture network in enhanced geothermal systems.The complicated convective heat transfer behavior in intersected fractures is critical to the heat recovery in fractured geothermal reservoirs.A series of three-dimensional intersected fracture models is constructed to perform the flow-through heat transfer simulations.The geometry effects of dead-end fractures(DEFs)on the heat transfer are evaluated in terms of intersected angles,apertures,lengths,and the connectivity.The results indicate that annular streamlines appear in the rough DEF and cause an ellipse distribution of the cold front.Compared to plate DEFs,the fluid flow in the rough DEF enhances the heat transfer.Both the increment of outlet water temperatureΔToutand the ratio of heat production Qrpresent the largest at the intersected angle of 90°while decline with the decrease of the intersected angle between the main flow fracture(MFF)and the DEFs.The extension of the length of intersected DEFs is beneficial to heat production while enhancing its aperture is not needed.Solely increasing the number of intersected DEFs induces a little increase of heat extraction,and more significant heat production can be obtained through connecting these DEFs with the MFF forming the flow network.     

Assessment of green roof thermal behavior: A coupled heat and mass transfer model

Green roofs have a positive effect on the energy performance of buildings, providing a cooling effect in summer, along with a more efficient harnessing of the solar radiation due to the reflective properties found inside the foliage. For assessing these effects, the thermodynamic model was developed as well as the thermo-physical properties of the green roof components were characterized. Its typologies and vegetation styles should also be studied. The proposed model is based on energy balance equations expressed for foliage and soil media. In this study, the influence of the mass transfer in the thermal properties and evapotranspiration were taken into account. We then added the water balance equation into our model and performed a numerical simulation. By assuming the outdoor conditions, the roof support temperature and the drainage water as inputs, the model evaluates the temperatures evolution at foliage and soil ground levels. A parametric study was performed using the proposed model to classify green roofs depending on the considered climate condition. Comparisons were undertaken with a roof slab concrete model; a significant difference (of up to 30 °C) in temperature between the outer surfaces of the two roofs was noticed in summer. The model was experimentally validated according to green roof platform, which was elaborated. The mass transfer effect in the subtract was very effective in reducing the model errors. Simulation results show that the use of vegetation in the roof building improves not only thermal comfort conditions, but the energy performance of a building.     

A new estimation method and an anisotropy index for the deformation modulus of jointed rock masses

The deformation modulus of a rock mass is an important parameter to describe its mechanical behavior.In this study,an analytical method is developed to determine the deformation modulus of jointed rock masses,which considers the mechanical properties of intact rocks and joints based on the superposition principle.Due to incorporating the variations in the orientations and sizes of joint sets,the proposed method is applicable to the rock mass with persistent and parallel joints as well as that with nonpersistent and nonparallel joints.In addition,an anisotropy index AIdmfor the deformation modulus is defined to quantitatively describe the anisotropy of rock masses.The range of AIdmis from 0 to 1,and the more anisotropic the rock mass is,the larger the value of AIdmwill be.To evaluate the proposed method,20 groups of numerical experiments are conducted with the universal distinct element code(UDEC).For each experimental group,the deformation modulus in 24 directions are obtained by UDEC(numerical value)and the proposed method(predicted value),and then the mean error rates are calculated.Note that the mean error rate is the mean value of the error rates of the deformation modulus in 24 directions,where for each direction,the error rate is equal to the ratio of numerical value minus predicted value to the numerical value.The results show that(i)for different experimental groups,the mean error rates vary between 5.06%and 22.03%;(ii)the error rates for the discrete fracture networks(DFNs)with two sets of joints are at the same level as those with one set of joints;and(iii)therefore,the proposed method for estimating the deformation modulus of jointed rock masses is valid.     

A geo-engineering classification for rocks and rock masses

In this article, an attempt is made to assess the reliability of predicting the uniaxial compressive strength and the corresponding modulus of a rock mass by current approaches. These two basic engineering properties, when estimated from rock mass rating (RMR), Q and geological strength index (GSI), indicate hardly any change in the modulus ratio with the change in the quality of the rock mass from very good to very poor. However, the modulus ratio obtained from the relations involving the joint factor, J_f, indicate a definite decrease in the modulus ratio with a decrease in the quality of the rock mass. The strength and modulus in the unconfined and confined states, the modulus ratio and failure strain in the unconfined case were linked to J_f in earlier publications based on a large experimental database. Some of these relations were adopted to verify the response of jointed test specimens, the response of the rock mass during excavations for mining and civil underground chambers, in establishing ground reaction curves including the extent of the broken zone, and the bearing capacity of shallow foundations.The joint factor is now linked to RMR, Q and GSI. The prediction of compressive strength and modulus of the rock mass appears to be more suitable. For classifying the rock, based on these properties, the Deere and Miller engineering classification, applicable to intact rocks, has been suitably modified and adopted. The results of different modes of failure of jointed specimens establish definite trends of changes in the modulus ratio originating from the intact rock value on the modified Deere and Miller plot. A geo-engineering classification is evolved by considering strength, modulus, quantifiable weathering index and lithological aspects of the rock.     

A two-port envelope model for building heat transfer

A zone envelope thermal model is derived from first principles, based on the exact two-port solution to the heat conduction equation. A single set of matrices, one for each frequency of interest, is used to describe the thermal behaviour of the total envelope. The set of matrices must be computed once for a particular zone and cooling loads for various conditions, or the passive response, can then be obtained. The method is based on the same principles underlying the CTF method. However, the transfer function is calculated for the whole envelope by combining the two-port matrices of the individual walls, after simplification of the room interior heat transfer network. The conduction transfer coefficients of the individual walls forming the zone are not required. Except for the simplification of interior radiation, the method is completely exact and easy to implement. It is also numerically efficient. For load prediction, an implementation in the frequency domain, based on the prime factor FFT algorithm is convenient. For time domain simulation with active control, the envelope conduction coefficients are easily obtained by transforming to the Z-domain. This represents a modern solution of the envelope conduction problem, which is convenient to implement in modern object oriented computer programming languages.     

Three-dimensional nonlinear analysis for the coupled problem of the heat transfer of the surrounding rock and the heat convection between the air and the surrounding rock in cold-region tunnel

According to the basic theories of heat transfer, geocryology and fluid mechanics, taking the coupled problem of the heat transfer of the rock surrounding the tunnel and the heat convective between the air in the tunnel and the rock surrounding the tunnel into account, three-dimensional calculating model of the coupled problem are presented. The finite element formulae of this problem are obtained by Galerkin’s method, and the computer program of the finite element is compiled. Using the program, three-dimensional nonlinear analyses for the coupled problem of the heat transfer of the rock surrounding the tunnel and the heat convective between the air in the tunnel and the rock surrounding Fenghuo mountain tunnel on the Qinghai–Tibet Railway are made. The agreement between the calculated results and the in-situ observed data is seen to be very good. The calculated results illustrate that the freezing–thawing situation of the rock surrounding the tunnel can correctly be predicted even if the air temperature distribution along the tunnel is unknown. In thus way, the large cost of in-situ observation for the air temperature in the tunnel can be saved.     

Failure mechanism and control technology of large deformation for Muzhailing Tunnel in stratified rock masses

Bulletin of Engineering Geology and the Environment - Deep tunnels in soft rock with high in situ stresses always exhibit large deformations. A case study in the Muzhailing Tunnel was carried out...     

A new energy-absorbing bolt for rock support in high stress rock masses

An energy-absorbing rock support device, called a D bolt, has been recently developed to counteract both burst-prone and squeezing rock conditions that occur during underground excavation. The bolt is a smooth steel bar with a number of anchors along its length. The anchors are firmly fixed within a borehole using either cement grout or resin, while the smooth sections of the bolt between the anchors may freely deform in response to rock dilation. Failure of one section does not affect the reinforcement performance of the other sections. The bolt is designed to fully use both the strength and the deformation capacity of the bolt material along the entire length. The bolt has large load-bearing and deformation capacities. Static pull tests and dynamic drop tests show that the bolt length elongates by 14–20% at a load level equal to the strength of the bolt material, thereby absorbing a large amount of energy. The impact average load of a 20 mm D bolt is 200–230 kN, with only a small portion of the load transferred to the bolt plate. The cumulative dynamic energy absorption of the bolt is measured to be 47 kJ/m. D bolts were tested in three deep mines. Filed measurements show that D bolts are loaded less than rebar bolts. This paper presents the layout and principle of the D bolt, and corresponding results from static, dynamic, and field tests.     

Proper generalized decomposition for solving coupled heat and moisture transfer

This paper proposes a reduced order model to simulate heat and moisture behaviour of material based on proper general decomposition (PGD). This innovative method is an a priori model reduction method. It proposes an alternative way for computing solutions of the problem by considering a separated representation of the solution. PGD offers an interesting reduction of numerical cost. In this paper, the PGD solution is first compared with a finite element solution and the commercial validated model Delphin in an 1D case. The results show that the PGD resolution techniques enable the field of interest to be represented with accuracy, with a relative error rate of less than 0.1%. The study remains in the hygroscopic range of the material. As the numerical gain of the method becomes interesting when the space dimension increases, this resolution strategy was then used on a 2D multi-layered test case. The dynamics and amplitude of hygrothermal fields are perfectly represented by the PGD solution. Temperature and vapour pressure modelled with PGD can be used for post-processing and analysing the behaviour of an assembly.     

Modeling coupled heat transfer and air flow in a partitioned building with a zonal model: Application to the winter thermal comfort

The assessment of building thermal comfort quality in the Mediterranean context necessitates detailed information concerning local air speed and temperature inside the space. We have extended the three-dimensional zonal model ZAER (Zonal AERial model) to enable predictions of air flow pattern and thermal distributions between and within rooms. Numerical simulations from the new program have been compared with data obtained from measurements on the experimental cell Minibat (CETHIL, INSA Lyon laboratory) and with the prediction of another zonal model as well as a computational fluid dynamics (CFD) tool. The comparison indicates that this new program is an effective model for predicting air flow and temperature distribution in a partitioned building. By coupling ZAER with a thermal comfort model, we study the influence of a passive solar component belonging to a south-oriented room upon the winter thermal comfort of an unconditioned Tunisian dwelling. The obtained results show that this simulation tool has the potential to describe realistically the thermal comfort within a dwelling, and that a Trombe wall can be a useful heating component to improve thermal winter comfort in the Tunisian context, even in another room.     

Shear strength criteria for rock,rock joints,rockfill and rock masses: Problems and some solutions

Although many intact rock types can be very strong, a critical confining pressure can eventually be reached in triaxial testing, such that the Mohr shear strength envelope becomes horizontal. This critical state has recently been better defined, and correct curvature or correct deviation from linear Mohr–Coulomb (M-C) has finally been found. Standard shear testing procedures for rock joints, using multiple testing of the same sample, in case of insufficient samples, can be shown to exaggerate apparent cohesion. Even rough joints do not have any cohesion, but instead have very high friction angles at low stress, due to strong dilation. Rock masses, implying problems of large-scale interaction with engineering structures, may have both cohesive and frictional strength components. However, it is not correct to add these, following linear M-C or nonlinear Hoek–Brown (H-B) standard routines. Cohesion is broken at small strain, while friction is mobilized at larger strain and remains to the end of the shear deformation. The criterion ‘c then σ_n tan φ’ should replace ‘c plus σ_ntan φ’ for improved fit to reality. Transformation of principal stresses to a shear plane seems to ignore mobilized dilation, and caused great experimental difficulties until understood. There seems to be plenty of room for continued research, so that errors of judgement of the last 50 years can be corrected.     

A three-dimensional numerical model for thermohydromechanical deformation with hysteresis in a fractured rock mass

A three-dimensional finite-element solution to the problem of coupled thermohydromechanical deformation, groundwater flow, and heat transport in deformable fractured porous media is presented in this paper. The governing equations are based on Biot’s consolidation theory for poroelastic materials, extended to the non-isothermal environment. The normal and lateral deformations in joints are simulated by a new joint element. The new joint element is based on the Bandis–Barton models and is capable of simulating normal and lateral deformations with dilatancy, contractancy, and hysteresis due to irrecoverable damages/rubblization. A three-dimensional finite-element code Model Of Transport In Fractured porous media (MOTIF) has been developed based on the theoretical framework presented herein. Verification results with experimental data and analytical solutions are presented in this paper. An application example with flow of fluid through a non-isothermally deforming joint is also presented. Results indicate that non-isothermal deformation could play a major role in the transport of fluid and water-borne substances in fractured rocks.     

冻结土壤、水水平换热管耦合传热分析

以冻结土壤、水-水平换热管之间耦合传热作用为研究对象,对冻结条件下非饱和土壤连续性方程和热量迁移方程,采用有限容积数值方法进行离散求解,给出了部分温度场、冻结率分布图,并分析了耦合传热原因。分析结果表明：冻结土壤温度场是大致与地表平行的水平线,固液相变区高峰值发生在有回水管的上部,低值发生在无水平换热管的地方,水平换热管使冻结锋面向地表方向偏移,水平换热管进回水位置布置方式不同影响冻结土壤相变区域的大小与位置,回水管在单侧和中间布置是较优的布置方式。