Bruno Figueiredo

Review: The state-of-art of sparse channel models and their applicability to performance assessment of radioactive waste repositories in fractured crystalline formations

Laboratory and field experiments done on fractured rock show that flow and solute transport often occur along flow channels. ‘Sparse channels’ refers to the case where these channels are characterised by flow in long flow paths separated from each other by large spacings relative to the size of flow domain. A literature study is presented that brings together information useful to assess whether a sparse-channel network concept is an appropriate representation of the flow system in tight fractured rock of low transmissivity, such as that around a nuclear waste repository in deep crystalline rocks. A number of observations are made in this review. First, conventional fracture network models may lead to inaccurate results for flow and solute transport in tight fractured rocks. Secondly, a flow dimension of 1, as determined by the analysis of pressure data in well testing, may be indicative of channelised flow, but such interpretation is not unique or definitive. Thirdly, in sparse channels, the percolation may be more influenced by the fracture shape than the fracture size and orientation but further studies are needed. Fourthly, the migration of radionuclides from a waste canister in a repository to the biosphere may be strongly influenced by the type of model used (e.g. discrete fracture network, channel model). Fifthly, the determination of appropriateness of representing an in situ flow system by a sparse-channel network model needs parameters usually neglected in site characterisation, such as the density of channels or fracture intersections.RésuméDes expériences réalisées en laboratoire et sur le terrain sur des roches fracturées montrent que l’écoulement et le transport de soluté se produisent souvent le long de canaux d’écoulement. Le terme de ‘Canaux clairsemés’ fait référence au cas où ces canaux sont caractérisés par un écoulement dans des longues voies d’écoulement séparés les uns des autres par de grandes distances par rapport à la taille du domaine d’écoulement. Une étude bibliographique est présentée, apportant des informations nécessaires pour évaluer si le concept de réseau de canaux clairsemés est une représentation appropriée pour le système d’écoulement dans une roche fracturée fermée de faible transmissivité, telle que la roche environnant le dépôt de déchets nucléaires dans des roches cristallines profondes. Un nombre d’observations sont faites dans cette revue bibliographique. Premièrement, les modèles de réseau de fractures classiques peuvent conduire à des résultats erronés pour l’écoulement et le transport de soluté dans les roches fracturées fermées. Deuxièmement, une dimension d’écoulement de 1, telle que déterminée par l’analyse des données de pressions dans les tests en forage, peut être indicative d’un écoulement canalisé, mais cette interprétation n’est pas unique ou définitive. Troisièmement, dans des canaux dispersés, la percolation peut être davantage influencée par la forme de la fracture que la taille de la fracture et son orientation, mais d’autres études sont nécessaires. Quatrièmement, la migration des radionucléides à partir d’un casier de déchets dans un dépôt vers la biosphère peut être fortement influencée par le type de modèle utilisé (par ex. réseau de fractures discrètes, modèle à canaux). Cinquièmement, la détermination de la pertinence de la représentation d’un système d’écoulement in situ par un modèle de réseau de canaux clairsemés nécessite des paramètres qui sont habituellement négligés lors de la caractérisation du site, tel que la densité des canaux ou des intersections de fractures.ResumenLos experimentos de laboratorio y de campo realizados en roca fracturada muestran que el flujo y transporte de solutos se producen a menudo a lo largo de los canales de flujo. “Canales dispersivos” se refieren al caso en que estos canales se caracterizan por un flujo de largas trayectorias separadas entre sí por grandes distancias en relación con el tamaño del dominio de flujo. Se presenta un estudio de la literatura que reúne información útil para evaluar si el concepto de red de canales dispersivos es una representación adecuada del sistema de flujo en una roca de apretada fracturación de baja transmisividad, como la que rodea a un repositorio de residuos nucleares en rocas cristalinas profundas. Una serie de observaciones se hacen en esta revisión. En primer lugar, los modelos de redes de fractura convencionales pueden conducir a resultados inexactos de flujo y transporte de solutos en rocas de una apretada fracturación. En segundo lugar, una dimensión de flujo de 1, como se determina por el análisis de los datos de presión en pruebas de pozos, puede ser indicativo de flujo canalizado, pero esta interpretación no es única o definitiva. En tercer lugar, en los canales dispersivos, la percolación puede estar más influenciada por la forma de la fractura que por el tamaño y la orientación de fractura, pero son necesarios más estudios. En cuarto lugar, la migración de radionucleidos de un depósito de residuos a la biosfera puede estar fuertemente influenciada por el tipo de modelo utilizado (por ejemplo, la red de fracturas discretas, modelo de canal). En quinto lugar, la determinación de la conveniencia de que representa un sistema de flujo en situ por un modelo de red dispersiva de canales necesita parámetros generalmente desatendidos en la caracterización del sitio, tales como la densidad de canales o intersecciones de fracturas.摘要对断裂岩进行的室内室外实验显示,水流和溶质运移经常沿水流通道出现。“稀疏通道”指的就是,在这些通道内水流在相对于水流范围有很大间隔并且相互隔绝的很长水流通道内流动。这展示了文献研究结果,把有用的信息整合在一起,用于评价稀疏通道网络概念是否能适当展示透水性低的密封断裂岩诸如深层结晶岩核废料储存地的水流系统。在本评论中进行了一些观察性研究。第一,常规断裂网络模型可能导致密封断裂岩石中水流和溶质运移的结果不精确。第二,由井实验压力数据分析结果确定的水流维度1可能是指示通道化的水流,但这样的解译不是唯一的或者不是决定性的。第三,在稀疏通道中,渗透受到断裂形状的影响可能比受到断裂大小和方向的影响要大,但需要进一步的研究。第四,放射性核素从储存地废料罐迁移到生物圈可能极大地受到所使用的模型类型影响(例如,离散断裂网络,通道模型)。第五,由稀疏通道网络模型确定代表原地水流系统的适宜性需要通常原地描述中忽略的参数,如通道或断裂交叉点的密度。ResumoExperimentos laboratoriais e de campo feitos em rochas fraturadas demonstram que o escoamento e o transporte de solutos geralmente ocorrem ao longo de canais de fluxo. ‘Canais esparsos’ se refere ao caso onde esses canais são caracterizados pelo escoamento em longos trajetos de fluxo, separados entre si por grandes espaçamentos, relativos ao tamanho do domínio de fluxo. Um estudo bibliográfico é apresentado, trazendo informações úteis para avaliar se um conceito de rede de canais esparsos é uma representação adequada do sistema de fluxo em rochas fraturadas estreitas e de baixa transmissividade, como os que cercam um depósito de resíduos nucleares em rochas cristalinas profundas. Numeroras observações são feitas nessa revisão. Primeiro, modelos de rede fraturada convencionais podem levar a resultados imprecisos para escoamento e transporte de solutos em rochas fraturadas estreitas. Segundo, uma dimensão de fluxo de 1, como a determinada pela análise de dados de pressão em poços de teste, pode ser indicativo de fluxo canalizado, mas essa interpretação não é exclusiva ou definitiva. Terceiro, em canais esparsos, a percolação pode ser mais influenciada pela forma da fratura do que pelo tamanho da fratura e orientação, mas estudos adicionais são necessários. Quarto, a migração de radionucleídios do contêiner de resíduos em um repositório para a biosfera pode ser fortemente influenciada pelo tipo de modelo utilizado (p.ex. rede de fraturas discretas, modelo de canal). Quinto, a determinação da adequabilidade em representar sistemas e escoamento in situ por modelos de rede de canais esparsos necessita de parâmetros geralmente negligenciados na caracterização do local, como a densidade de canais ou intersecção de fraturas.

Chapter 7: Continuum Modeling of Hydraulic Fracturing in Complex Fractured Rock Masses.

Abstract In this chapter we present and demonstrate the use of a continuous approach to model hydromechanics and hydraulic fracturing of complex fractured rock masses. This fracture continuum modeling approach is developed and applied within the framework of the FLAC3D geomechanical code, or the TOUGH-FLAC coupled multiphase flow and geomechanical simulator. In the fracture continuum approach, discrete fractures are mapped onto a fine numerical grid of continuum elements and we modify the properties of those continuum elements adequately and as accurately as possible to represent the hydraulic and mechanical behavior of the fractures. Moreover, for modeling failure in tension and shear, including hydraulic fracturing, an elastic–brittle stress–strain relation is used, with the degradation of the mechanical properties of continuum elements undergoing failure by tension or shear. The use of the fracture continuum method is verified and demonstrated for modeling hydromechanical behavior complex fractured rock, for modeling fracture propagation across geological features, and for modeling of a classical hydraulic fracturing stress measurement operation from a vertical well.

Continuum Modeling of Hydraulic Fracturing in Complex Fractured Rock Masses

Abstract In this chapter we present and demonstrate the use of a continuous approach to model hydromechanics and hydraulic fracturing of complex fractured rock masses. This fracture continuum modeling approach is developed and applied within the framework of the FLAC3D geomechanical code, or the TOUGH-FLAC coupled multiphase flow and geomechanical simulator. In the fracture continuum approach, discrete fractures are mapped onto a fine numerical grid of continuum elements and we modify the properties of those continuum elements adequately and as accurately as possible to represent the hydraulic and mechanical behavior of the fractures. Moreover, for modeling failure in tension and shear, including hydraulic fracturing, an elastic–brittle stress–strain relation is used, with the degradation of the mechanical properties of continuum elements undergoing failure by tension or shear. The use of the fracture continuum method is verified and demonstrated for modeling hydromechanical behavior complex fractured rock, for modeling fracture propagation across geological features, and for modeling of a classical hydraulic fracturing stress measurement operation from a vertical well.

Mathematical Modeling: Approaches for Model Solution

The governing equations and mathematical models describing CO2 spreading and trapping in saline aquifers and the related hydro-mechanical and chemical processes were described in Chapt. 3. In this chapter, the focus is on methods for solving the relevant equations. The chapter gives an overview of the different approaches, from high-fidelity full-physics numerical models to more simplified analytical and semi-analytical solutions . Specific issues such as modeling coupled thermo-hydro-mechanical-chemical processes and modeling of small-scale processes , such as convective mixing and viscous fingering , are also addressed. Finally, illustrative examples of modeling real systems, with different types of modeling approaches, are presented.