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Numerical manifold method modeling of coupled processes in fractured geological media at multiple scales

Journal of Rock Mechanics and Geotechnical Engineering 2020年第4期12卷 667-681页

作者： Mengsu Hu jonny rutqvist Lawrence Berkeley National Laboratory BerkeleyCA94720USA

The greatest challenges of rigorously modeling coupled hydro-mechanical(HM)processes in fractured geological media at different scales are associated with computational geometry.These challenges include dynamic shearing and opening of intersecting fractures at discrete fracture scales as a result of coupled processes,and contact alteration along rough fracture surfaces that triggers structural and physical changes of fractures at micro-asperity scale.In this paper,these challenges are tackled by developing a comprehensive modeling approach for coupled processes in fractured geological media based on numerical manifold method(NMM)at multiple scales.Based on their distinct geometric features,fractures are categorized into three different scales:dominant fracture,discrete fracture,and discontinuum asperity scales.Here the scale is relative,that of the fracture relative to that of the research interest or domain.Different geometric representations of fractures at different scales are used,and different governing equations and constitutive relationships are applied.For dominant fractures,a finite thickness zone model is developed to treat a fracture as a porous nonlinear domain.Nonlinear fracture mechanical behavior is accurately modeled with an implicit approach based on strain energy.For discrete fractures,a zero-dimensional model was developed for analyzing fluid flow and mechanics in fractures that are geometrically treated as boundaries of the rock matrix.With the zero-dimensional model,these fractures can be modeled with arbitrary orientations and intersections.They can be fluid conduits or seals,and can be open,bonded or sliding.For the discontinuum asperity scale,the geometry of rough fracture surfaces is explicitly represented and contacts involving dynamic alteration of contacts among asperities are rigorously calculated.Using this approach,fracture alteration caused by deformation,re-arrangement and sliding of rough surfaces can be captured.Our comprehensive model is able to handle the computational challen

关键词： Dominant fractures Discrete fractures Discontinuum asperity scale Coupled processes Numerical manifold method(NMM)

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Influence of hysteretic stress path behavior on seal integrity during gas storage operation in a depleted reservoir

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Journal of Rock Mechanics and Geotechnical Engineering 2020年第4期12卷 886-899页

作者： Pierre Jeanne Yingqi Zhang jonny rutqvist Lawrence Berkeley National Laboratory Energy Geosciences DivisionBerkeleyCA94720USA

In this study,we numerically investigate the influence of hysteretic stress path behavior on the seal integrity during underground gas storage operations in a depleted reservoir.Our study area is the Honor Rancho Underground Storage Facility in Los Angeles County(California,USA),which was converted into an underground gas storage facility in 1975 after 20 years of oil and gas production.In our simulations,the geomechanical behavior of the sand reservoir is modeled using two models:(1)a linear elastic model(non-hysteretic stress path)that does not take into consideration irreversible deformation,and(2)a plastic cap mechanical model which considers changes in rock elastic properties due to irreversible deformations caused by plastic reservoir compaction(hysteretic stress path).It shows that the irreversible compaction of the geological layer over geologic time and during the reservoir depletion can have important consequences on stress tensor orientation and magnitude.Ignoring depletion-induced irreversible compaction can lead to an over-estimation of the calculation of the maximum working reservoir pressure.Moreover,this irreversible compaction may bring the nearby faults closer to reactivation.However,regardless of the two models applied,the geomechanical analysis shows that for the estimated stress conditions applied in this study,the Honor Rancho Underground Storage Facility is being safely operated at pressures much below what would be required to compromise the seal integrity.

关键词： Gas storage Stress path Geomechanical simulation Caprock integrity Fault stability Modified cam-clay model Honor rancho underground storage facility

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Interactive roles of geometrical distribution and geomechanical deformation of fracture networks in fluid flow through fractured geological media

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Journal of Rock Mechanics and Geotechnical Engineering 2020年第4期12卷 780-792页

作者： Qinghua Lei Xiaoguang Wang Ki-Bok Min jonny rutqvist Department of Earth Sciences ETH ZürichZürichSwitzerland Laboratoire HydroSciences Montpellier Universite de MontpellierMontpellierFrance Department of Energy Systems Engineering Seoul National UniversitySeoulRepublic of Korea Energy Geosciences Division Lawrence Berkeley National LaboratoryBerkeleyUSA

In this study,the combined effects of geometrical distribution and geomechanical deformation of fracture networks on fluid flow through fractured geological media are investigated numerically.We consider a finite-sized model domain in which the geometry of fracture systems follows a power-law length scaling.The geomechanical response of the fractured rock is simulated using a hybrid finitediscrete element model,which can capture the deformation of intact rocks,the interaction of matrix blocks,the displacement of discrete fractures and the propagation of new cracks.Under far-field stress loading,the locally variable stress distribution in the fractured rock leads to a stress-dependent variable aperture field controlled by compression-induced closure and shear-induced dilatancy of rough fractures.The equivalent permeability of the deformed fractured rock is calculated by solving for the fracture-matrix flow considering the cubic relationship between fracture aperture and flow rate at each local fracture segment.We report that the geometrical connectivity of fracture networks plays a critical role in the hydromechanical processes in fractured rocks.A well-connected fracture system under a high stress ratio condition exhibits intense frictional sliding and large fracture dilation/opening,leading to greater rock mass permeability.However,a disconnected fracture network accommodates much less fracture shearing and opening,and has much lower bulk permeability.We further propose an analytical solution for the relationship between the equivalent permeability of fractured rocks and the connectivity metric(i.***.percolation parameter)of fracture networks,which yields an excellent match to the numerical results.We infer that fluid flow through a well-connected system is governed by traversing channels(forming an“in parallel”architecture)and thus equivalent permeability is sensitive to stress loading(due to stress-dependent fracture permeability),whilst fluid flow through a disconnected system is more ruled by matrix(linking is

关键词： Fracture Stress Aperture Connectivity Permeability

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Mesh generation and optimization from digital rock fractures based on neural style transfer

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Journal of Rock Mechanics and Geotechnical Engineering 2021年第4期13卷 912-919页

作者： Mengsu Hu jonny rutqvist Carl I.Steefel Energy Geosciences Division Lawrence Berkeley National LaboratoryBerkeleyCA94720USA

The complex geometric features of subsurface fractures at different scales makes mesh generation challenging and/or expensive.In this paper,we make use of neural style transfer(NST),a machine learning technique,to generate mesh from rock fracture images.In this new approach,we use digital rock fractures at multiple scales that represent’content’and define uniformly shaped and sized triangles to represent’style’.The 19-layer convolutional neural network(CNN)learns the content from the rock image,including lower-level features(such as edges and corners)and higher-level features(such as rock,fractures,or other mineral fillings),and learns the style from the triangular grids.By optimizing the cost function to achieve approximation to represent both the content and the style,numerical meshes can be generated and optimized.We utilize the NST to generate meshes for rough fractures with asperities formed in rock,a network of fractures embedded in rock,and a sand aggregate with multiple grains.Based on the examples,we show that this new NST technique can make mesh generation and optimization much more efficient by achieving a good balance between the density of the mesh and the presentation of the geometric features.Finally,we discuss future applications of this approach and perspectives of applying machine learning to bridge the gaps between numerical modeling and experiments.

关键词： Convolutional neural network(CNN) Neural style transfer(NST) Digital rock Discrete fractures Discontinuum asperities Grain aggregates Mesh generation and optimization

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Linked multicontinuum and crack tensor approach for modeling of coupled geomechanics, fluid flow and transport in fractured rock

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Journal of Rock Mechanics and Geotechnical Engineering 2013年第1期5卷 18-31页

作者： jonny rutqvist Colin Leung Andrew Hoch Yuan Wang Zhen Wang Lawrence Berkeley National Laboratory (LBNL) Imperial College Serco Energy Hohai University Tongji University

In this paper, we present a linked multicontinuum and crack tensor approach for modeling of coupled geomechanics, fluid flow, and solute transport in fractured rock. We used the crack tensor approach to calculate effective block-scale properties, including anisotropic permeability and elastic tensors, as well as multicontinuum properties relevant to fracture-matrix interactions and matrix diffusion. In the mod- eling, we considered stress dependent properties, through stress-induced changes in fracture apertures, to update permeability and elastic tensors. We evaluated the effectiveness and accuracy of our multicon- tinuum approach by comparing our modeling results with that of three independent discrete fracture network （DFN） models. In two of the three alternative DFN models, solute transport was simulated by particle tracking, an approach very different from the standard solute transport used in our multicon- tinuum modeling. We compared the results for flow and solute transport through a 20 m x 20 m model domain of fractured rock, including detailed comparison of total flow rate, its distribution, and solute breakthrough curves. In our modeling, we divided the 20 m ~ 20 m model domain into regular blocks, or continuum elements. We selected a model discretization of 40 x 40 elements （having a side length of 0.5 m） that resulted in a fluid-flow rate equivalent to that of the DFN models. Our simulation results were in reasonably good agreement with the alternative DFN models, for both advective dominated transport （under high hydraulic gradient） and matrix-diffusion retarded transport （under low hydraulic gradient）. However, we found pronounced numerical dispersion when using larger grid blocks, a problem that could be remediated by the use of a finer numerical grid resolution, while maintaining a larger grid for evalua- tion of equivalent properties, i.e. a property grid overlapping the numerical grid. Finally, we encountered some difficulties in using our approach when element sizes were so small that only one

关键词： Fractured rockModelingFluid flowStressMulticontinuumCrack tensor

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Effects of in situ stress measurement uncertainties on assessment of predicted seismic activity and risk associated with a hypothetical industrial-scale geologic CO_2 sequestration operation

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Journal of Rock Mechanics and Geotechnical Engineering 2016年第6期8卷 873-885页

作者： Pierre Jeanne jonny rutqvist Haruko M.Wainwright William Foxall Corinne Bachmann Quanlin Zhou Antonio Pio Rinaldi Jens Birkholzer Energy Geosciences Division Lawrence Berkeley National Laboratory Swiss Seismological Service Swiss Federal Institute of Technology(ETHZ)

Carbon capture and storage(CCS) in geologic formations has been recognized as a promising option for reducing carbon dioxide(CO) emissions from large stationary sources.However,the pressure buildup inside the storage formation can potentially induce slip along preexisting faults,which could lead to felt seismic ground motion and also provide pathways for brine/COleakage into shallow drinking water aquifers.To assess the geomechanical stability of faults,it is of crucial importance to know the in situ state of stress.In situ stress measurements can provide some information on the stresses acting on faults but with considerable uncertainties.In this paper,we investigate how such uncertainties,as defined by the variation of stress measurements obtained within the study area,could influence the assessment of the geomechanical stability of faults and the characteristics of potential injection-induced seismic events.Our modeling study is based on a hypothetical industrial-scale carbon sequestration project assumed to be located in the Southern San Joaquin Basin in California,USA.We assess the stability on the major(25 km long) fault that bounds the sequestration site and is subjected to significant reservoir pressure changes as a result of 50 years of COinjection.We present a series of geomechanical simulations in which the resolved stresses on the fault were varied over ranges of values corresponding to various stress measurements performed around the study area.The simulation results are analyzed by a statistical approach.Our main results are that the variations in resolved stresses as defined by the range of stress measurements had a negligible effect on the prediction of the seismic risk(maximum magnitude),but an important effect on the timing,the seismicity rate(number of seismic events) and the location of seismic activity.

关键词： Carbon dioxide(CO_2) sequestration Geomechanical simulations Induced seismicity Uncertainty on in situ stress

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Oda裂隙张量的DC模型在裂隙岩体渗流模拟中的应用

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力学季刊 2011年第4期32卷 486-492页

作者：王臻 jonny rutqvist 戴瑛同济大学航空航天与力学学院 Earth Sciences Division Lawrence Berkeley National LaboratoryUniversity of CaliforniaBerkeleyCalifornia 94720USA

针对裂隙岩体渗流非均质各向异性的特点,我们用基于离散连续模型(discrete continuum model,简称DC模型)的方法,对裂隙岩体渗流和溶质运移进行了数值模拟。在此工作中我们采用Oda裂隙张量理论计算各个单元的渗透张量,组成整个渗流区域... 详细信息

针对裂隙岩体渗流非均质各向异性的特点,我们用基于离散连续模型(discrete continuum model,简称DC模型)的方法,对裂隙岩体渗流和溶质运移进行了数值模拟。在此工作中我们采用Oda裂隙张量理论计算各个单元的渗透张量,组成整个渗流区域的渗透张量场;并采用具有"旋转不变性"特征的九点差分格式,来减少网格离散方式对数值模拟结果的影响;并对一benchmark test model(BTM)模型进行了数值模拟。模拟结果与BTM结果很吻合。

关键词：裂隙岩体渗流离散连续模型裂隙张量理论九点差分格式

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Fault activation and induced seismicity in geological carbon storage--Lessons learned from recent modeling studies

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Journal of Rock Mechanics and Geotechnical Engineering 2016年第6期8卷 789-804页

作者： jonny rutqvist Antonio P. Rinaldi Frederic Cappa Pierre Jeanne Alberto Mazzoldi Luca Urpi Yves Guglielmi Victor Vilarrasa Energy Geosciences Division Lawrence Berkeley National Laboratory (LBNL) Berkeley USA Swiss Seismological Service Swiss Federal Institute of Technology (ETHZ) Zürich Switzerland GeoAzur University of Nice Sophia-Antipolis Cote Azur Observatory Nice France UMSNH-IIM Instituto de Investigaciones en Ciencias de la Tierra Morelia Mexico Institute of Environmental Assessment and Water Research (IDAEA) Spanish Council for Scientific Research (CSIC) Barcelona Spain

In the light of current concerns related to induced seismicity associated with geological carbon sequestration(GCS),this paper summarizes lessons learned from recent modeling studies on fault activation,induced seismicity,and potential for leakage associated with deep underground carbon dioxide(CO2) injection.Model simulations demonstrate that seismic events large enough to be felt by humans require brittle fault properties and continuous fault permeability allowing pressure to be distributed over a large fault patch to be ruptured at once.Heterogeneous fault properties,which are commonly encountered in faults intersecting multilayered shale/sandstone sequences,effectively reduce the likelihood of inducing felt seismicity and also effectively impede upward CO2leakage.A number of simulations show that even a sizable seismic event that could be felt may not be capable of opening a new flow path across the entire thickness of an overlying caprock and it is very unlikely to cross a system of multiple overlying caprock units.Site-specific model simulations of the In Salah CO2storage demonstration site showed that deep fractured zone responses and associated microseismicity occurred in the brittle fractured sandstone reservoir,but at a very substantial reservoir overpressure close to the magnitude of the least principal stress.We conclude by emphasizing the importance of site investigation to characterize rock properties and if at all possible to avoid brittle rock such as proximity of crystalline basement or sites in hard and brittle sedimentary sequences that are more prone to injection-induced seismicity and permanent damage.

关键词： Carbon dioxide(CO_2) injection Fault rupture Induced seismicity Ground motion Leakage Modeling

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裂隙煤岩各向异性渗透率模型和煤层注气THM耦合行为

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煤炭学报 2017年第S2期42卷 407-416页

作者：马天然刘卫群 jonny rutqvist 张宏学赵晓东中国矿业大学深部岩土力学与地下工程国家重点实验室江苏徐州221116 劳伦斯伯克利国家实验室地球科学部美国伯克利94720 安徽理工大学理学院安徽淮南232001

CO2煤层地质封存可以减少温室气体排放,同时可提高煤层气的采收率。注气开采过程涉及到温度场(T),多相多成分流场(H)和应力场(M)之间的相互耦合。煤层割理裂隙渗透率是影响CO2地质封存和煤层气开采率的重要参数。煤岩渗透性的关键性因... 详细信息

CO2煤层地质封存可以减少温室气体排放,同时可提高煤层气的采收率。注气开采过程涉及到温度场(T),多相多成分流场(H)和应力场(M)之间的相互耦合。煤层割理裂隙渗透率是影响CO2地质封存和煤层气开采率的重要参数。煤岩渗透性的关键性因素裂隙张开度同时受控于法向应力和剪胀效应。考虑基质和割理的共同作用,提出基于组合裂隙三向平板简化的各向异性渗透率模型。在此基础上,建立了注入CO2提高煤层气采收率的三维数值模型并利用耦合软件TOUGH-FLAC进行求解。模拟结果表明,气体注进采出的孔压作用会引起煤层膨胀或收缩。与孔压,吸附应力/应变以及温度相比,剪胀对裂隙渗透率的影响不明显。在注采过程中,渗透率表现出明显的各向异性,注入井附近的异向渗透率甚至可达30倍差异。此外,注气初期应适当控压,井口附近的高压损伤将带来不必要的裂隙气体泄漏,导致注气失败。

关键词：各向异性裂隙渗流 CO2-ECBM 温度-流场-应力(THM) TOUGH-FLAC模拟器

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Impact of stress on solute transport in a fracture network: A comparison study

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Journal of Rock Mechanics and Geotechnical Engineering 2013年第2期5卷 110-123页

作者： Zhihong Zhao jonny rutqvist Colin Leung Milan Hokr Quansheng Liu Ivars Neretnieks Andrew Hoch Jirí Havlíek Yuan Wang Zhen Wang Yuexiu Wu Robert Zimmerman Royal Institute of Technology StockholmSweden Lawrence Berkeley National Laboratory BerkeleyUSA Imperial College LondonUK Technical University of Liberec Czech Republic Institute of Rock and Soil Mechanics Chinese Academy of Sciences Wuhan University Serco TAS DidcotUK Hohai University Tongji University

This paper compares numerical modeling of the effect of stress on solute transport （advection and matrix diffusion） in fractured rocks in which fracture apertures are correlated with fracture lengths. It is mainly motivated by the performance and safety assessments of underground radioactive waste repositories. Five research teams used different approaches to model stress/deformation, flow and transport pro- cesses, based on either discrete fracture network or equivalent continuum models. The simulation results derived by various teams generally demonstrated that rock stresses could significantly influence solute transport processes through stress-induced changes in fracture apertures and associated changes in per- meability. Reasonably good agreement was achieved regarding advection and matrix diffusion given the same fracture network, while some observed discrepancies could be explained by different mechanical or transport modeling approaches.

关键词： Fractured rocks Solute transport Discrete element method Equivalent continuum Dual-porosity model Particle tracking method

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