Auli Niemi

Hydraulic characterization and upscaling of fracture networks based on multiple-scale well test data

Hydraulic properties and upscaling characteristics of low-permeability fractured rock are analyzed based on systematic well test data from three different measurement scales. First, tests are simulated in a large number of geological fracture network realizations, and the acceptable fracture transmissivity distribution parameters that produce the observed statistics of the two smallest measurement scales, i.e., 2-m and 10-m scales, are defined. Instead of a single value, a range of acceptable parameter values can be found to produce the observed result. Second, upscaling simulations are carried out with the calibrated networks. These indicate that the investigated system cannot be properly modeled by means of a continuum tensor presentation but would better be represented by means of “equivalent fracture” statistics. Third, the conductive characteristics of the calibrated 30-m network blocks are compared to well test results from the same scale. The results from this preliminary analysis indicate that one-dimensional borehole observations interpreted with standard continuum-based methods may considerably underestimate the three-dimensional conductive characteristics of heterogeneous, noncontinuum fractured media.

A study of flow-wetted surface area in a single fracture as a function of its hydraulic conductivity distribution

The contact area between flowing water and rock-the flow-wetted surface (FWS)-is a main factor controlling the rock-matrix diffusion and sorption of flowing solute in a rock fracture. Flow channeli ...

A Regional-Scale Particle-Tracking Method for Nonstationary Fractured Media

A regional scale transport model is introduced that is applicable to non-stationary and statistically inhomogeneous fractured media, provided that hydraulic flow, but not necessarily solute transport, can be approximated by equivalent continuum properties at some block scale. Upscaled flow and transport block properties are transferred from multiple fracture network realizations to a regional model with grid elements of equal size to that found valid for continuum approximation of flow. In the large-scale model, flow is solved in a stochastic continuum framework, whereas the transport calculations employ a random walk procedure. Block-wise transit times are sampled from distributions linked to each block-conductivity based on its underlying fracture network. To account for channeled transport larger than the block scale, several alternatives in sampling algorithm are introduced and compared. The most reasonable alternative incorporates a spatial persistence length in sampling the particle transit times; this tracer transport persistence length is related to interblock channeling, and is quantified by the number N of blocks. The approach is demonstrated for a set of field data, and the obtained regional-scale particle breakthroughs are analyzed. These are fitted to the one-dimensional advective-dispersive equation to determine an effective macroscale dispersion coefficient. An interesting finding is that this macroscale dispersion coefficient is found to be a linear function of the transport persistence, N, with a slope equal to a representative mean block-scale dispersion coefficient and a constant that incorporates background dispersion arising from the regional heterogeneous conductivity field.

Effects of single-fracture aperture statistics on entrapment, dissolution and source depletion behavior of dense non-aqueous phase liquids.

Understanding of the entrapment and dissolution behavior of dense non-aqueous phase liquids (DNAPLs) in single fractures is important for modeling contaminant flux generation from fractured sites. Here a systematic numerical study is presented to investigate the effect of fracture aperture statistics on DNAPL migration, entrapment and dissolution within individual, variable-aperture fractures. Both fractures with open and closed bottom boundaries were considered. For the simulation a continuum-based two-phase model was used with a capillary pressure function which calculates the entry pressure based on the local aperture. Prior to application the model was compared against the invasion percolation approach and found more suitable for the present study, in particular as it allows a more versatile presentation of boundary conditions. The results showed that increasing aperture standard deviation and/or decreasing correlation length lead to larger amounts of entrapped DNAPL (due to the fact that larger standard deviation produces more distinct contrast between small and large aperture regions and the fact that longer correlation length provides more possible channels through the fracture) as well as larger maximum and average sizes of DNAPL blobs, and subsequently lead to longer times for complete dissolution. To understand the relationship between the solute flux and the remaining mass, a simplified source depletion function which links the outflow concentration to the DNAPL saturation was found adequate to describe the dissolution process for the case where the bottom boundary is open for DNAPL migration and thus the DNAPL does not accumulate to form a pool. The parameters in this function were not very sensitive to variations in correlation length but were sensitive to aperture standard deviation. The same average entrapped DNAPL saturation produced considerably smaller solute concentrations in cases with larger aperture variability due to the larger average size of DNAPL blobs (i.e., smaller contact area for DNAPL dissolution). Boundary conditions had a significant impact on DNAPL entrapment and dissolution. A closed boundary at the bottom led to DNAPL pooling (i.e., large continuous blobs) which causes significant tailing in the dissolution breakthrough curve due to water bypassing.

Modeling of nonaqueous phase liquid (NAPL) migration in heterogeneous saturated media: Effects of hysteresis and fluid immobility in constitutive relations

The confidence in model predictions for nonaqueous phase liquid (NAPL) transport in stochastically heterogeneous systems is limited. The fundamental approaches as well as the constitutive models ha ...

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.

Deep hydrogeology: a discussion of issues and research needs

In this essay, “deep hydrogeology” is somewhat arbitrarily defined as hydrogeology in the subsurface deeper than 1 km, below which the effect of residual permeability at high stresses becomes evident (Neuzil 2003; Rutqvist and Stephansson 2003; Liu et al. 2009). Studies have shown that meteoric fluids are present in the earth’s crust from land surface to at least a depth of 10–15 km (Kozlowsky 1987; Taylor Jr 1990; Zharikov et al. 2003; Ge et al. 2003). At such depths, interaction with surface water and surface events over time periods of 100 or 1,000 years may be minimal, except in areas of very deep mining activities or where deep convection is enhanced by active magmatism. Deep drilling to several kilometers in depth is often done for petroleum and geothermal reservoir exploration and exploitation. The focus of such activities is reservoir identification, capacity evaluation, and fluid and heat extractability. However, it is largely an open area of research to understand the state, structure and evolution of deep hydrogeology over time scales of tens of thousands of years or more, especially in areas lacking petroleum and geothermal resources. Interest in attaining such an understanding has emerged from the need for long-term predictions related to nuclear waste disposal and from recognition of the role that hydrogeology may play in seismicity, orogenesis and various geological processes, as well as in global fluid and chemical cycles. A number of wide-ranging questions may be asked regarding deep hydrogeology, several of which are as follows: What are the current and past states of fluid pressure, temperature and chemical composition in deep formations? How does fluid transport mass and heat? What are the fluid sources and driving mechanisms? What are the magnitude and distribution of porosity and permeability? What are the occurrence and characteristics of large-scale flow, including thermally and chemically driven convection systems? What is the nature of local anomalous fluid pressures and what are their implications? The purpose of this essay is to discuss key issues and research needs in deep hydrogeology. It is based on a workshop on the subject held at Uppsala University in Sweden, with participants from 11 countries, including the USA, Russia, Japan and a number of European countries (Tsang et al. 2012). The following discussion will be divided into sections on permeability structures, driving forces, coupled processes, borehole testing and data analysis, followed by a few concluding remarks.

Dissolution of dense non-aqueous phase liquids in vertical fractures : Effect of finger residuals and dead-end pools

Understanding the dissolution behavior of dense non-aqueous phase liquids (DNAPLs) in rock fractures under different entrapment conditions is important for remediation activities and any related predictive modeling. This study investigates DNAPL dissolution in variable aperture fractures under two important entrapment configurations, namely, entrapped residual blobs from gravity fingering and pooling in a dead-end fracture. We performed a physical dissolution experiment of residual DNAPL blobs in a vertical analog fracture using light transmission techniques. A high-resolution mechanistic (physically-based) numerical model has been developed which is shown to excellently reproduce the experimentally observed DNAPL dissolution. We subsequently applied the model to simulate dissolution of the residual blobs under different water flushing velocities. The simulated relationship between the Sherwood number Sh and Peclet number Pe could be well fitted with a simple power-law function (Sh=1.43Pe⁰·⁴³). To investigate mass transfer from dead-end pools, another type of trapping in rock fractures, entrapment and dissolution of DNAPL in a vertical dead-end fracture was simulated. As the entrapped pool dissolves, the depth of the interface between the DNAPL and the flowing water increases linearly with decreasing DNAPL saturation. The interfacial area remains more or less constant as DNAPL saturation decreases, unlike in the case of residual DNAPL blobs. The decreasing depth of the contact interface changes the flow field and causes decreasing water flow velocity above the top of the DNAPL pool, suggesting the dependence of the mass transfer rate on the depth of the interface, or alternatively, the remaining mass percentage in the fracture. Simulation results show that the resultant Sherwood number Sh is significantly smaller than in the case of residual blobs for any given Peclet number, indicating slower mass transfer. The results also show that the Sh can be well fitted with a power-law function of Pe and remaining mass percentage. The obtained relationships of dimensionless groups concerning the mass transfer characteristics at the level of individual fractures can be further used in predictive modeling of dissolution at a larger (fracture network) scale.

Heat Transport in Unsaturated Zone Thermal Energy Storage – Analysis with Two-Phase and Single-Phase Models

We analyze a field experiment where ambient air is injected into the soil during the summer and extracted again during the winter. A multiphase model accounting for the conductive transport as well as the convective transport with the moving liquid and gas phases is used along with a more simplified single-phase model where the convective transport is due to the gas alone. The latter model also accounts for subzero wintertime temperatures. The multiphase model captures well both the seasonal variations and the actual test sequence, the main calibration being in the adjustment of medium permeabilities based on the observed pressure responses. The effect of the injection pump on the temperature and humidity of the injection air needs to be known accurately. Taking into account the humidity of the injection air explicitly instead of using humidity-corrected enthalpy values also has an effect. The effect of various humidity and specific enthalpy assumptions is of the order of 1–1.5°C, while ignoring the wintertime subzero temperatures has an effect of 1–2°C. These differences are of the same order of magnitude as the heterogeneity-introduced differences in field data. Using the simplified single-phase model typically appears to cause a difference of 1–2°C, but can yield an even higher deviation of the order of 3–4°C.

Understanding the effect of single-fracture heterogeneity from single-well injection-withdrawal (SWIW) tests

The single-well injection-withdrawal (SWIW) tracer test is a method used to estimate the tracer retardation properties of a fracture or fracture zone. The effects of single-fracture aperture heterogeneity on SWIW-test tracer breakthrough curves are examined by numerical modelling. The effects of the matrix diffusion and sorption are accounted for by using a particle tracking method through the addition of a time delay added to the advective transport time. For a given diffusion and sorption property (Pm) value and for a heterogeneous fracture, the peak concentration is larger compared to a homogeneous fracture. The cumulative breakthrough curve for a heterogeneous fracture is similar to that for a homogeneous fracture and a less sorptive/diffusive tracer. It is demonstrated that the fracture area that meets the flowing water, the specific flow-wetted surface (sFWS) of the fracture, can be determined by matching the observed breakthrough curve for a heterogeneous fracture to that for a homogeneous fracture with an equivalent property parameter. SWIW tests are also simulated with a regional pressure gradient present. The results point to the possibility of distinguishing the effect of the regional pressure gradient from that of diffusion through the use of multiple tracers with different Pm values.RésuméLe test de traçage par injection-pompage en puits unique est une méthode utilisée pour estimer le retard d’un traceur, caractéristique d’une fracture ou d’une zone de fractures. Les effets de l’hétérogénéité liée à une fracture ouverte isolée sur les courbes de réponse du traceur, dans le test de d’injection-pompage en puits unique, sont étudiés par modélisation numérique. Les effets de la diffusion matricielle et de la sorption sont pris en compte en utilisant une méthode de traçage de la particule passant par l’addition d’un temps de retard et du temps de transfert advectif. Pour une valeur donnée (Pm) du paramètre de diffusion et de sorption, le pic de concentration est plus marqué pour une fracture irrégulière que pour une fracture régulière. La courbe de réponse cumulative du signal pour une fracture irrégulière est la même que pour une fracture régulière et un traceur moins sorbé et moins diffusé. On démontre que la surface de fractures en contact avec l’eau libre ou surface mouillée spécifique de la fracture, peut être déterminée par ajustement des courbes de réponse du signal observées pour une fracture irrégulière et une fracture régulière de propriétés paramétrées équivalentes. Les tests d’injection-pompage en puits unique sont également simulés avec le gradient de pression régional. Les résultats soulignent la possibilité de distinguer l’effet du gradient de pression régional de l’effet de la diffusion, grâce à l’utilisation de traceurs multiples à valeurs (Pm) différentes.ResumenUna prueba de trazador en un pozo único de extracción e inyección (SWIW) es un método usado para estimar las propiedades del retardo de un trazador de una fractura o zona de fractura. Los efectos de la heterogeneidad de la apertura de una fractura única sobre las curvas de ruptura de las pruebas de trazador SWIW se examinan por modelación numérica. Los efectos de la matriz de difusión y sorción son considerados mediante el uso del método de seguimiento de partículas a través de la adición de un tiempo de retardo añadido al tiempo de transporte advectivo. Para un valor dado de la propiedad de difusión y sorción (Pm) y para una fractura heterogénea, el pico de la concentración es mayor en comparación con una fractura homogénea. La curva acumulativa de ruptura para una fractura heterogénea es similar a la de una fractura homogénea y un trazador menos difusivo/sortivo. Se demuestra que en el área de fractura que se encuentra con el agua que fluye, el flujo de la superficie específica mojada de la fractura (sFWS), puede determinarse comparando la curva observada de ruptura para una fractura heterogénea con respecto a una fractura homogénea con un parámetro de propiedad equivalente. Las pruebas SWIW también se simulan con un gradiente de presión regional presente. Los resultados apuntan a la posibilidad de distinguir el efecto del gradiente de presión regional de la difusión a través del uso de trazadores múltiples con diferentes valores de Pm.摘要单井回灌-抽水 (SWIW) 示踪试验是一种确定裂隙或裂隙带中滞留示踪特性的方法。在SWIW试验中单裂隙非均质缝隙沿曲线踪迹由数值模型模拟。这类网状扩散和吸附作用采用质点示踪方法通过延时增加对流传导时间来说明。对于给定的扩散和吸附特征值(Pm)和非均质裂隙,浓度峰值要比均质裂隙大。穿透曲线在非均质裂隙中与均质裂隙类似,并且较少吸附/扩散。这显示出在裂隙地区遇到水流,裂隙表面被水流湿润(sFWS),可以由非均质裂隙中观测穿透曲线确定,而在均质裂隙中则由特征参数确定。SWIW试验还用于模拟区域压力梯度。通过用不同的Pm值的多种示踪,其结果指出各种可能有关由扩散产生的区域压力梯度作用的差别。ResumoO ensaio de traçador em furo único de injeção e extração (SWIW – single-well injection-withdrawal) é um método usado para estimar as propriedades de retardamento do traçador numa fratura ou zona fraturada. Os efeitos da heterogeneidade da abertura de uma fratura única nas curvas de variação da concentração do traçador ao longo do tempo dos ensaios de injeção e extração em furo único são examinados por modelação numérica. Os efeitos da difusão e sorção da matriz são contabilizados mediante o uso de um método de rastreamento de partículas, através da adição de um atraso ao tempo de transporte advectivo. Para um dado valor das propriedades de difusão e sorção (Pm) e para uma fratura heterogénea, o pico da concentração é maior quando comparado com o de uma fratura homogénea. A curva cumulativa de variação da concentração ao longo do tempo para uma fratura heterogénea é semelhante à duma fratura homogénea e um traçador menos sortivo/difusivo. Demostra-se que a área de fratura que contacta com o fluxo de água, a superfície de fluxo húmida específica da fratura, pode ser determinada combinando a curva de variação da concentração ao longo do tempo observada para uma fratura heterogénea com a duma fratura homogénea com um parâmetro de propriedade equivalente. Os testes SWIW são também simulados na presença de um gradiente de pressão regional. Os resultados apontam para a possibilidade de distinguir o efeito do gradiente de pressão regional do efeito da difusão através do uso de múltiplos traçadores com diferentes valores de Pm.