Rheological numerical simulation for thermo-hydro-mechanical coupling analysis for rock mass

Under the environment of seepage field, stress field and temperature field interaction and influence, the three fields will not only produce coupling effect, but also have deformation with time due to the rheological behavior of rock mass. In the paper, based on the fundamental theories of rock mass coupling theory and rheological mechanics, the rheological model for fully coupled thermo-hydro-mechanical analysis for rock mass was set up, and the corresponding constitutive relationship, the conservation equation of mass and the conservation equation of energy were given, and the finite element formulas were derived for coupling analysis of rock mass. During establishing governing equations, rock mass was assumed approximately as macro-equivalent continuum medium. The obtained rheological numerical model for fully coupled thermo-hydro-mechanical analysis can be used for analyzing and predicting the long-term stability of underground caverns and slope engineering under the condition of thermo-hydro-mechanical coupling with rheological deformation.

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.

Coupled thermo-hydro-mechanical simulation of CO2 enhanced gas recovery with an extended equation of state module for TOUGH2MP-FLAC3D

As one of the most important ways to reduce the greenhouse gas emission,carbon dioxide（CO2）enhanced gas recovery（CO2-EGR） is attractive since the gas recovery can be enhanced simultaneously with CO2sequestration.Based on the existing equation of state（EOS） module of TOUGH2 MP,extEOS7C is developed to calculate the phase partition of H2O-CO2-CH4-NaCl mixtures accurately with consideration of dissolved NaCI and brine properties at high pressure and temperature conditions.Verifications show that it can be applied up to the pressure of 100 MPa and temperature of 150℃.The module was implemented in the linked simulator TOUGH2MP-FLAC3 D for the coupled hydro-mechanical simulations.A simplified three-dimensional（3D）1/4 model（2.2 km×1 km×1 km） which consists of the whole reservoir,caprock and baserock was generated based on the geological conditions of a gas field in the North German Basin.The simulation results show that,under an injection rate of 200,000 t/yr and production rate of 200,000 sm3/d,CO2breakthrough occurred in the case with the initial reservoir pressure of 5 MPa but did not occur in the case of 42 MPa.Under low pressure conditions,the pressure driven horizontal transport is the dominant process;while under high pressure conditions,the density driven vertical flow is dominant.Under the considered conditions,the CO2-EGR caused only small pressure changes.The largest pore pressure increase（2 MPa） and uplift（7 mm） occurred at the caprock bottom induced by only CO2injection.The caprock had still the primary stress state and its integrity was not affected.The formation water salinity and temperature variations of ±20℃ had small influences on the CO2-EGR process.In order to slow down the breakthrough,it is suggested that CO2-EGR should be carried out before the reservoir pressure drops below the critical pressure of CO2.

Comparison of numerical codes for coupled thermo-hydro-mechanical simulations of fractured media

Geo-energy and geo-engineering applications,such as improved oil recovery(IOR),geologic carbon storage,and enhanced geothermal systems(EGSs),involve coupled thermo-hydro-mechanical(THM)processes that result from fluid injection and production.In some cases,reservoirs are highly fractured and the geomechanical response is controlled by fractures.Therefore,fractures should explicitly be included into numerical models to realistically simulate the THM responses of the subsurface.In this study,we perform coupled THM numerical simulations of water injection into naturally fractured reservoirs(NFRs)using CODE_BRIGHT and TOUGH-UDEC codes.CODE_BRIGHT is a finite element method(FEM)code that performs fully coupled THM analysis in geological media and TOUGH-UDEC sequentially solves coupled THM processes by combining a finite volume method(FVM)code that solves nonisothermal multiphase flow(TOUGH2)with a distinct element method(DEM)code that solves the mechanical problem(UDEC).First,we validate the two codes against a semi-analytical solution for water injection into a single deformable fracture considering variable permeability based on the cubic law.Then,we compare simulation results of the two codes in an idealized conceptual model that includes one horizontal fracture and in a more realistic model with multiple fractures.Each code models fractures differently.UDEC calculates fracture deformation from the fracture normal and shear stiffnesses,while CODE_BRIGHT treats fractures as equivalent porous media and uses the equivalent Young’s modulus and Poisson’s ratio of the fracture.Finally,we obtain comparable results of pressure,temperature,stress and displacement distributions and evolutions for the single horizontal fracture model.Despite some similarities,the two codes provide increasingly different results as model complexity increases.These differences highlight the challenging task of accurately modeling coupled THM processes in fractured media given their high nonlinearity.

THM Coupled Modeling in Near Field of an Assumed HLW Deep Geological Disposal Repository

One of the most suitable ways under study for the disposal of high-level radioactive waste (HLW) is isolation in deep geological repositories. It is very important to research the thermo-hydro- mechanical (THM) coupled processes associated with an HLW disposal repository. Non-linear coupled equations, which are used to describe the THM coupled process and are suited to saturated-unsaturated porous media, are presented in this paper. A numerical method to solve these equations is put forward, and a finite element code is developed. This code is suited to the plane strain or axis-symmetry problem. Then this code is used to simulate the THM coupled process in the near field of an ideal disposal repository. The temperature vs. time, hydraulic head vs. time and stress vs. time results show that, in this assumed condition, the impact of temperature is very long (over 10 000 a) and the impact of the water head is short (about 90 d). Since the stress is induced by temperature and hydraulic head in this condition, the impact time of stress is the same as that of temperature. The results show that THM coupled processes are very important in the safety analysis of an HLW deep geological disposal repository.

FEM analyses for influences of pressure solution on thermo-hydro-mechanical coupling in porous rock mass

The model of pressure solution for granular aggregate was introduced into the FEM code for analysis of thermo-hydro- mechanical （T-H-M） coupling in porous medium. Aiming at a hypothetical nuclear waste repository in an unsaturated quartz rock mass, two computation conditions were designed： 1） the porosity and the permeability of rock mass are fimctions of pressure solution; 2） the porosity and the permeability are constants. Then the corresponding numerical simulations for a disposal period of 4 a were carried out, and the states of temperatures, porosities and permeabilities, pore pressures, flow velocities and stresses in the rock mass were investigated. The results show that at the end of the calculation in Case 1, pressure solution makes the porosities and the permeabilities decrease to 10%-45% and 0.05%-1.4% of their initial values, respectively. Under the action of the release heat of nuclear waste, the negative pore pressures both in Case 1 and Case 2 are 1.2-1.4 and 1.01-l.06 times of the initial values, respectively. So, the former represents an obvious effect of pressure solution. The magnitudes and distributions of stresses within the rock mass in the two calculation cases are the same.

Numerical analysis of thermal impact on hydro-mechanical properties of clay

As is known, high-level radioactive waste （HLW） is commonly heat-emitting. Heat output from HLWwilldissipate through the surrounding rocks and induce complex thermo-hydro-mechanical-chemical（THMC） processes. In highly consolidated clayey rocks, thermal effects are particularly significantbecause of their very low permeability and water-saturated state. Thermal impact on the integrity of thegeological barriers is of most importance with regard to the long-term safety of repositories. This studyfocuses on numerical analysis of thermal effects on hydro-mechanical properties of clayey rock using acoupled thermo-mechanical multiphase flow （TH2M） model which is implemented in the finite elementprogramme OpenGeoSys （OGS）. The material properties of the numerical model are characterised by atransversal isotropic elastic model based on Hooke＇s law, a non-isothermal multiphase flow model basedon van Genuchten function and Darcy＇s law, and a transversal isotropic heat transport model based onFourier＇s law. In the numerical approaches, special attention has been paid to the thermal expansion ofthree different phases： gas, fluid and solid, which could induce changes in pore pressure and porosity.Furthermore, the strong swelling and shrinkage behaviours of clayey material are also considered in thepresent model. The model has been applied to simulate a laboratory heating experiment on claystone.The numerical model gives a satisfactory representation of the observed material behaviour in thelaboratory experiment. The comparison of the calculated results with the laboratory findings verifies thatthe simulation with the present numerical model could provide a deeper understanding of the observedeffects. 2014 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. All rights reserved.

Thermo-hydro-mechanical modeling of fault discontinuities using zero-thickness interface element

In this paper,a coupled thermo-hydro-mechanical(THM)simulation in a faulted deformable porous medium is presented.This model involves solving the mass conservation,linear momentum balance,and energy balance equations which are derived from the Biot’s consolidation theory.Fluid pore pressure,solid displacement,and temperature are chosen as initial variables in these equations,and the finite element method in combination with the interface element is used for spatial discretization of continuous and discontinuities(fault)parts of the medium to solve the equations.The main purpose of this study is providing precise formulations,applicability,and ability of the triple-node zero-thickness interface element in THM modeling of faults.It should be noted that the system of equations is solved using a computer code written in Matlab program.In order to verify the developed method,simulations of index problems such as Mandel’s problem,and coupled modeling of a faulted porous medium and a faulted aquifer are presented.The modeling results obtained from the developed method show a very good agreement with those by other modeling methods,which indicates its accuracy.

Parametric study of thermo-hydro-mechanical response of claystone with consideration of steel corrosion

In this paper,the thermo-hydro-mechanical(THM)response of claystone is studied via a series of parametric studies,considering the evolution of mechanical properties and deformation behavior of corroded steel.The numerical simulations are performed by using a coupled THM finite element code and two different constitutive models:a visco-elastoplastic model for geological formation and a von Mises type model for steel liner.The mechanical properties and deformation behavior of corroded steel are described in a conceptual model.Finally,a disposal tunnel supported by a steel liner is studied and a series of parametric studies is defined to demonstrate the corrosion effects of steel liner on the THM response of the claystone.The comparison of different numerical calculations exhibits that the volumetric expansion related to corrosion products has an important impact on the stress and displacement fields in the claystone surrounding the disposal tunnel.However,the evolutions of temperature and liquid pressure in the claystone are essentially controlled by its THM properties and independent of the steel corrosion.

Simulations of THM processes in buffer-rock barriers of high-level waste disposal in an argillaceous formation

The main objective of this paper is to investigate and analyse the thermo-hydro-mechanical（THM） coupling phenomena and their influences on the repository safety.In this paper,the high-level waste（HLW） disposal concept in drifts in clay formation with backfilled bentonite buffer is represented numerically using the CODE BRIGHT developed by the Technical University of Catalonia in Barcelona.The parameters of clay and bentonite used in the simulation are determined by laboratory and in situ experiments.The calculation results are presented to show the hydro-mechanical（HM） processes during the operation phase and the THM processes in the after-closure phase.According to the simulation results,the most probable critical processes for the disposal project have been represented and analyzed.The work also provides an input for additional development regarding the design,assessment and validation of the HLW disposal concept.

TOUGHREACT搭接FLAC^(3D)算法

多孔介质THM多场耦合分析在诸如核废料地质储存、干热岩和CO2地质埋藏等地质科学领域广泛应用,鉴于其计算的复杂性往往需要借助数值手段。基于角点-中心点网格转换以及三维三点拉格朗日插值方法,通过编制程序将TOUGHREACT和FLAC3D搭接,实现了多相多组分流体-温度-岩石力学的耦合计算分析。为了验证程序的效率及适用性,构建了Salah场地深部咸水层CO2储存数值模型,模拟CO2连续注入3a的地下流体运移和储、盖层的岩石力学变化,并将地表位移与inSAR实测值进行了对比分析。以此为基础进一步模拟了中国西部某盆地CO2地质储存三维筒注模型。结果表明:搭接程序能平滑连接TOUGHREACT和FLAC3D,其考虑岩石力学影响下的注入压力和地表位移等成果符合理论及实际监测规律,同时得到岩石孔隙率、渗透率随CO2注入时间增加而变大的结论。

核废料地质贮存介质黏土岩的三维各向异性热-水-力耦合数值模拟

为了解黏土岩在放射性废料长期贮存中的热-水-力耦合过程,结合MontTerri核废料贮存地下岩石试验工程中黏土岩各种物理量的各向异性特点,应用多孔介质力学耦合理论研究了该黏土岩在加热和冷却全过程中由于热荷载引起的耦合效应场。研究过程考虑温度升高引起的孔隙水黏滞性改变对渗透系数的影响。研究结果表明,岩体力学参数、水力学参数和热传导参数的各向异性特性是影响岩体的温度场、孔隙压力场和应力场分布的最主要因素。各向异性耦合模型与各向同性耦合模型的数值模拟对比研究结果表明:各向异性模型数值结果能更加客观地反映该地下岩石试验工程中黏土岩在受热状态下的热-水-力耦合效应;同时,也表明岩体在加热过程中一直处于受压状态,而在冷却过程中局部会出现拉应力,从而有可能导致拉裂缝的产生。

压力溶解和自由面溶解/沉淀对双重孔隙岩体中热-水-应力耦合影响的有限元分析

引入裂隙开度的压力溶解和自由面溶解/沉淀模型,针对一个假设的位于饱和双重孔隙-裂隙岩体中的高放废物地质处置库,拟定3种计算工况:(1)裂隙开度是压力溶解和自由面溶解/沉淀的函数;(2)裂隙开度仅随压力溶解而变化(这2种工况中基岩的孔隙率亦是应力的函数);(3)裂隙开度和基岩的孔隙率均为常数,进行热-水-应力耦合的二维有限元分析,考察岩体中的温度、裂隙开度及渗透系数、孔隙水压力、地下水流速和应力的变化、分布情况。结果表明:自由面溶解/沉淀引起的裂隙开张及渗透系数增量的绝对值要明显大于压力溶解产生的裂隙闭合及渗透系数减量的绝对值,而压力溶解对裂隙的开度及其渗透系数的影响较小;同时计入压力溶解和自由面溶解/沉淀相比于仅考虑压力溶解,同时计入压力溶解和自由面溶解/沉淀的裂隙开度及其渗透系数分别约为仅考虑压力溶解时的1.5和7.0倍;在释热温度场的作用下,计算域中的裂隙水压力随时间先上升再下降,但变化幅度不大;3种工况下岩体中的应力量值及分布差别很小。

塑性扩容梯度对孔隙介质岩体中T-H-M耦合作用的二维有限元分析

由双重孔隙-裂隙介质热-水-应力耦合模型退化为单一孔隙介质模型,将其与岩体扩容梯度引入笔者所研制的二维有限元程序中,使用Mohr-Coulomb准则,计入塑性扩容对岩体孔隙率及渗透系数的影响,针对一个假设的实验室尺度且位于饱和孔隙介质岩体中的高放废物地质处置库模型,拟定不同扩容梯度值的5种工况,进行4年处置时段的数值模拟,考察了岩体中的温度、正应力、塑性区、孔隙率及渗透系数、孔隙水压力和地下水流速的变化、分布情况。结果主要显示,相比于不考虑扩容梯度的工况,考虑扩容梯度工况的正应力、孔隙率及渗透系数、孔隙水压力和地下水流速等的分布与塑性区的分布有明显的对应关系,呈现了某种"剪切带效应";正应力量值、塑性区面积、孔隙率及渗透系数、孔隙水压力、地下水流速等均随所取扩容梯度值的变大而增加。

FEM analyses for influences of stress corrosion and pressure solution on THM coupling in dual-porosity rock mass

With the models of stress corrosion and pressure solution, by Yasuhara et al., two computation conditions were designed for a hypothetical nuclear waste repository in an unsaturated dual-porosity rock mass: (i) the fracture apertures are changed with the stress corrosion and pressure solution (the porosity of rock matrix is also a function of stress); (ii) the fracture apertures and the porosity of rock matrix are constants. Then the corresponding two-dimensional FEM analyses for the coupled thermohydro-mechanical processes were carried out, and the states of temperatures, rates and magnitudes of aperture closure, pore pressures, flow velocities and stresses in the rock mass were investigated. The results show the followings: The aperture closure rates caused by stress corrosion are almost six orders higher than those caused by pressure solution, and the two kinds of closure rates climb up, then decline, furthermore tend towards stability; the fracture apertures decrease from the original value and tend towards the residual value, and the contact-area ratios of asperities increase also from the original value and tend towards the nominal value; when the effects of stress corrosion and pressure solution are considered, the negative fracture pressures in near field rise very highly; the differences between the magnitudes and distributions of stresses within the rock mass in two calculation cases are very small.

高放废物工程屏障的热–水–力耦合作用分析(英文)

采用多相多类方法,提出热–水–力耦合作用分析的综合模型。分析考虑多孔介质的三相(固、液、气)和三类(矿物、水分、空气)模型,该模型是根据控制方程、本构关系和平衡条件来建立的;并提出用于模拟高放废物地下处置体系附近场地性状的热–水–力耦合分析通用程序。热–水–力耦合分析可将工程屏障中不同过程间发生的主要相互作用,以及附近岩石的散热和水化现象都考虑进去。通过试验结果的验证,得出一些关于热–水–力方面的重要结论。

Numerical simulation of seasonal snow in Tianshan Mountains

Snowfall in the Tianshan Mountains in China is frequent during winter;thus,avalanches have become a severe issue in snow-covered areas.Accumulation and metamorphosis,as well as hydrothermal exchanges with the environment,considerably affect the stability of snow on slopes.Therefore,a hydrothermal model of snow cover and its underlying surfaces must be developed on the basis of meteorological data to predict and help manage avalanches.This study adopted the conceptual model of snow as a porous medium and quantitatively analysed its internal physical processes on the basis of the thermal exchanges amongst its components.The effects of local meteorological factors on snow structure and the redistribution of energy and mass inside the snow cover in the Tianshan Mountains were simulated.Simulation results showed that deformation as a result of overlying snow and sublimation of snow cover at the bottom is the main cause of density variation in the vertical profile of snow cover.Temperature drives water movement in snow.The low-density area of the bottom snow is the result of temperature gradient.The simulation results of the long-term snow internal mass distribution obtained by the method established in this study are highly consistent with the actual observed trend of variation.Such consistency indicates an accurate simulation of the physical characteristics of snow cover in small and microscale metamorphism in the Tianshan Mountains during the stable period.

脆性岩石热–水–力耦合断裂的断口分析

利用自行设计的热–水–力(THM)耦合断裂试验和扫描电镜试验,研究脆性岩石THM耦合断裂的宏微观特征;通过有限元法和新型应力强度因子比断裂准则,判断THM耦合断裂模式,揭示出THM耦合断裂机制。结果表明:红砂岩试件具有3种THM耦合宏观断裂轨迹和对应的3种THM耦合微观断裂特征,即低温、低水压、高围压条件下的断裂轨迹为横向断裂,微观上表现为穿晶的剪切断裂;高温、高水压、低围压条件下的断裂轨迹为纵向断裂,微观上表现为沿晶的拉伸断裂;中等温度、中等水压、中等围压条件的断裂轨迹为双向断裂,微观上表现为含沿晶和穿晶的拉剪复合型断裂。基于有限元法和新型最大应力强度因子比断裂准则判断出的红砂岩试件THM耦合断裂模式与基于宏微观断口分析得到的THM耦合断裂机制完全一致。

水–热–力三场耦合分析平台的升级与验证

水–热–力三场耦合研究是岩土工程领域的前沿课题,在冻土路基工程、核废料处置和地热开发等方面均有着广阔的应用前景。通过对原分析平台3G2001的深入剖析,指出该分析平台存在的不足,对分析平台解法块部分子程序和等效刚度矩阵主元太小等程序内部缺陷进行修正,并对分析平台进行升级。最后借助于E.Penner和T.Ueda的单向冻胀试验,验证升级后的平台的可靠性和精度。研究表明:3G2012分析结果与试验实测数据及M.Shen和B.Ladanyi的分析结果均吻合较好,符合冻胀规律和工程经验,特别是对冻胀变形的模拟较M.Shen和B.Ladanyi等的计算结果更接近实测值,能够推广应用于解决实际岩土工程问题。

Dynamic response of bilayered saturated porous media based on fractional thermoelastic theory

Considering the thermal contact resistance and elastic wave impedance at the interface,in this paper we theoretically investigate the thermo-hydro-mechanical(THM)coupling dynamic response of bilayered saturated porous media.Fractional thermoelastic theory is applied to porous media with imperfect thermal and mechanical contact.The analytical solutions of the dynamic response of the bilayered saturated porous media are obtained in frequency domain.Furthermore,the effects of fractional derivative parameters and thermal contact resistance on the dynamic response of such media are systematically discussed.Results show that the effects of fractional derivative parameters on the dynamic response of bilayered saturated porous media are related to the thermal contact resistance at the interface.With increasing thermal contact resistance,the displacement,pore water pressure,and stress decrease gradually.