Дисертації з теми "Fractured chalk"

This thesis is concerned with the mass compressibility of fractured chalk and its influence on the settlement of shallow foundations. A review of the literature reveals nineteen case records of load-settlement behaviour from relatively small diameter « 1m) plate loading tests but only six welldocumented case records of the behaviour of shallow foundations on chalk. The plate loading tests indicate that highly fractured near-surface chalk undergoes yield at relatively low stresses (200 - 400kPa) resulting in a significant reduction in stiffness. This behaviour contrasts with that observed in other rock types with similar discontinuity patterns. For chalk it has only been observed in one case record for a full-scale foundation. Little is understood about the mechanisms causing yield. At the time of starting this research, based largely upon the experience ..gained from in-situ loading tests carried out at Mundford, Norfolk (Ward et aI., 1968), it was known that factors such as fracture spacing and aperture played an important role in controlling the load-settlement behaviour of shallow foundations. Little attention was paid to the large variation in intact properties displayed by the chalk. In this research nine 1.8m diameter plate loading tests have been carried out by the author on chalks with different intact mechanical properties and similar discontinuity patterns. These data are used to evaluate other in-situ tests (such as SPT, surface-wave geophysics and visual assessment) as means of providing parameters for the prediction of foundation settlement. The results of this research indicate that fractured near-surface chalk undergoes yield within the range of stresses likely to be imposed by shallow foundations and that the pre-yield stiffness of the rock mass is controlled to a large extent by the looseness of the fracture-block system, which in tum appears to be associated with the intact mechanical properties of the rock. The post yield-stiffness of the rock mass is generally about one tenth of the pre-yield stiffness and is relatively insensitive to the rock material properties.

Tertiary recovery or Improved Oil Recovery (IOR) methods are key processes to replace or upgrade reserves, which can be economically recovered, beyond conventional methods. Therefore, the application of IOR methods offers opportunities to increase the hydrocarbon reserves that have been produced in addition to those coming from exploration and reservoir appraisal. The purpose of this thesis is to combine experiments, computations, and theory to make fundamental advances in our ability to predict transport phenomena as well as the IOR potential involved in tertiary CO2 injection at the lab scale in a matrix fracture system. This is done by using rock and fluid samples similar to one of the chalk fractured reservoirs in the North Sea. The work involves a review of key physical mechanisms and calculation methods for the modelling of fluid flow in fractured reservoirs. The main matrix fracture fluid exchange mechanisms described are gravity drainage, capillary imbibition and molecular diffusion. Also described are the estimation of the recovery performance for a single block and a stack of blocks surrounded by gas. The effect of interfacial tension on the ultimate recovery has been discussed and the definition of the minimum miscibility pressure for single porosity and dual porosity system is described. Numerical modelling of gravity drainage for a matrix blocks surrounded by gas has been described. Numerical estimation of gas-oil gravity drainage by reducing the number of grid blocks in vertical direction in a draining matrix column is common practice in order to reduce the simulation time. However this can lead to systematic numerical errors and consequently underestimation of the recovery. In order to minimize the underestimation of the reservoir performance, a set of pseudo functions needs to be developed that not only satisfy the actual responses in the fine grid simulation but also reduce the simulation time. The effectiveness and the accuracy of such pseudo functions are extensively discussed and the different simulation models have been run to quantify the underestimation of recovery by coarse griding in the numerical modelling of gravity drainage. The importance of the molecular diffusion to recover oil from a high fracture intensity system is described as well as the basic concept for calculating the molecular diffusion based on the Fick’s second law. Corresponding laboratory methods for the estimation and measurement of the oil and gas diffusion coefficients are reviewed. The effect of molecular diffusion on the interfacial tension and eventually on the gas-oil capillary pressure is presented. A compositional study of a non-equilibrium gas injection process such as CO2 requires an equation of state (EOS) model which can predict the CO2/oil phase behaviour. In order to make such EOS model, a set of pVT experiments using fluids involved in the core flooding has been performed and finally the EOS models were tuned against experimental pVT data. The necessary steps to perform pVT experiments including making live reservoir oil, constant composition expansion, single flash, viscosity measurements and CO2-oil swelling are described. Gas injection is known to have a significant potential for high ultimate recovery in many oil fractured reservoirs with tall matrix blocks. The high ultimate recovery in these reservoirs could be due to the effectiveness of the gravity drainage mechanism. Fractured chalk reservoirs in the North Sea have a very high porosity (up to 45%), and low matrix permeability (3-4 mD) with small matrix block size. In order to quantify the dominant transport mechanisms and potential of Improved Oil Recovery (IOR) in the case of CO2 injection in the North Sea chalk fractured reservoirs, CO2 injection experiments at reservoir conditions have to be performed in the laboratory. The feasibility of such laboratory experiments initially has been verified by performing compositional simulation. In these simulations by varying the experimental parameters, such as core height and fracture size, the optimum matrix and fracture geometry were designed and the summary of the task is presented in Paper 1- Appendix A. CO2 injection experiments under reservoir conditions in the presence of different water saturation at reservoir conditions have been carried out. A unique technique has been developed for saturating the matrix system with reservoir fluids. This method ensures a homogeneous fluid composition within the pore system before the fracture system is initialized with the CO2. A complete description of, rock and fluids, experimental procedure and experimental results is given in Chapters 3, 4 and Papers 2 and 3 in Appendices B, C. In order to investigate the effect of temperature on the oil recovery mechanism, CO2 injection experiments were carried out at initial reservoir temperature (130 ºC) and a low temperature 60 ºC which representing the water flooded parts in the reservoir. The effect of initial water saturation also was investigated at reservoir temperature 130 ºC by performing two experiments with different initial water saturation. Results from these experiments show a high potential for oil recovery in all experiments. In the high temperature experiments, the produced oil had a variable composition during CO2 injection, while at the low temperature condition, the produced oil initially had a constant composition and then it started to change. Different behaviour of produced oil composition in the high and low temperature might be due to dominant of diffusion mechanism in the high temperature experiments. In the low temperature (60 ºC) experiment, at the early stage of CO2 injection the produced oil had constant composition for a short period of time and then it changed to variable composition similar to the high temperature case. This behaviour maybe is due to high solubility of CO2 into oil and consequently more oil swelling than the high temperature condition. In order to quantify the above mechanisms, several attempts have been done to history match the experiments by using compositional simulator. But in all cases, it was not possible to history match the experiments. The weakness of the simulator was due to the improper formulation which was used for calculating the cross phase diffusion between the oil and gas phase in the matrix and fracture system. The details of simulation work as well as the cross phase diffusion issue are discussed in Chapter 5 and Paper 2 in Appendix B.

The Machar Cretaceous chalk oilfield, Central North Sea, is a structural trap of chalk folded above a diaper of Zechstein salt, and formed a regional leakoff point for overpressure from Jurassic sandstones deeper in the basin. Chalk was deposited as pelagic sediment, but re-deposition by gravity flows improves reservoir quality. Diagenesis of the Machar chalk matrix occurs during burial to 1Km. Stylolites form in the chalk at depths below 600m and cause cementation of the reservoir through pressure dissolution and precipitation in a closed system. During diagenesis of the chalk matrix the underlying salt diaper evolves and as a result four fracture sets begin to form in the chalk reservoir. Hydrocarbon charge at mid-Miocene halted matrix cementation resulting in an exceptionally porous (30%mD) reservoir. The growth and evolution of the diaper is marked by the formation of healed fractures within the chalk reservoir. Fracture filling calcite records the evolution of the Tor Formation reservoir from a closed system, rock-dominated environment to an open diagenetic system. Four different fracture types have been discovered within the Tor formation on the Machar field. Of these, the first to form was Fracture Type 1, relating to bedding stylolites formed through minor extension at the crest of the reservoir as the salt attempts to remain buoyant. This fracture is filled with Calcite 1. δ¹³C values within this calcite (+ 2.0 to + 2.3‰ PDB) are similar to the values measured in the matrix chalk (+ 1.5 to + 3‰ PDB), suggesting that like the matrix cement this formed in a rock-dominated system. ⁸⁷Sr/⁸⁶Sr (0.7078 to 70787) values within Calcite 1,also similar to the matrix values (0.70770-0.70791) supports this theory. Negative δ¹⁸O (-5 to -7‰ PDB) values measured in Calcite 1 are explained by increased burial temperature during precipitation. Fracture type 2, the second fracture to form, is filled with Calcite 2; δ¹³C values (+ 3.6 to + 5.9‰ PDB), are more positive than the matrix values (+ 1.5 to + 3.0‰ PDB), and mark the opening of the reservoir to an external fluid. Fracture type 3 is filled with Calcite 3, and forms as Palaeogene sediments begin to down build around the evolving salt diaper. Calcite 3 precipitated from an external fluid that is restricted in its migration between tectonic stylolites. Calcite 3 is replaced by saddle dolomite, celestite, barite and fluorite.

Tertiary recovery or Improved Oil Recovery (IOR) methods are key processes to replace or upgrade reserves, which can be economically recovered, beyond conventional methods. Therefore, the application of IOR methods offers opportunities to increase the hydrocarbon reserves that have been produced in addition to those coming from exploration and reservoir appraisal.

The purpose of this thesis is to combine experiments, computations, and theory to make fundamental advances in our ability to predict transport phenomena as well as the IOR potential involved in tertiary CO₂ injection at the lab scale in a matrix fracture system. This is done by using rock and fluid samples similar to one of the chalk fractured reservoirs in the North Sea.

The work involves a review of key physical mechanisms and calculation methods for the modelling of fluid flow in fractured reservoirs. The main matrix fracture fluid exchange mechanisms described are gravity drainage, capillary imbibition and molecular diffusion. Also described are the estimation of the recovery performance for a single block and a stack of blocks surrounded by gas. The effect of interfacial tension on the ultimate recovery has been discussed and the definition of the minimum miscibility pressure for single porosity and dual porosity system is described.

Numerical modelling of gravity drainage for a matrix blocks surrounded by gas has been described. Numerical estimation of gas-oil gravity drainage by reducing the number of grid blocks in vertical direction in a draining matrix column is common practice in order to reduce the simulation time. However this can lead to systematic numerical errors and consequently underestimation of the recovery.

In order to minimize the underestimation of the reservoir performance, a set of pseudo functions needs to be developed that not only satisfy the actual responses in the fine grid simulation but also reduce the simulation time. The effectiveness and the accuracy of such pseudo functions are extensively discussed and the different simulation models have been run to quantify the underestimation of recovery by coarse griding in the numerical modelling of gravity drainage.

The importance of the molecular diffusion to recover oil from a high fracture intensity system is described as well as the basic concept for calculating the molecular diffusion based on the Fick’s second law. Corresponding laboratory methods for the estimation and measurement of the oil and gas diffusion coefficients are reviewed. The effect of molecular diffusion on the interfacial tension and eventually on the gas-oil capillary pressure is presented.

A compositional study of a non-equilibrium gas injection process such as CO₂ requires an equation of state (EOS) model which can predict the CO₂/oil phase behaviour. In order to make such EOS model, a set of pVT experiments using fluids involved in the core flooding has been performed and finally the EOS models were tuned against experimental pVT data. The necessary steps to perform pVT experiments including making live reservoir oil, constant composition expansion, single flash, viscosity measurements and CO₂-oil swelling are described.

Gas injection is known to have a significant potential for high ultimate recovery in many oil fractured reservoirs with tall matrix blocks. The high ultimate recovery in these reservoirs could be due to the effectiveness of the gravity drainage mechanism.

Fractured chalk reservoirs in the North Sea have a very high porosity (up to 45%), and low matrix permeability (3-4 mD) with small matrix block size. In order to quantify the dominant transport mechanisms and potential of Improved Oil Recovery (IOR) in the case of CO₂ injection in the North Sea chalk fractured reservoirs, CO₂ injection experiments at reservoir conditions have to be performed in the laboratory.

The feasibility of such laboratory experiments initially has been verified by performing compositional simulation. In these simulations by varying the experimental parameters, such as core height and fracture size, the optimum matrix and fracture geometry were designed and the summary of the task is presented in Paper 1- Appendix A.

CO₂ injection experiments under reservoir conditions in the presence of different water saturation at reservoir conditions have been carried out. A unique technique has been developed for saturating the matrix system with reservoir fluids. This method ensures a homogeneous fluid composition within the pore system before the fracture system is initialized with the CO₂.

A complete description of, rock and fluids, experimental procedure and experimental results is given in Chapters 3, 4 and Papers 2 and 3 in Appendices B, C. In order to investigate the effect of temperature on the oil recovery mechanism, CO2 injection experiments were carried out at initial reservoir temperature (130 ºC) and a low temperature 60 ºC which representing the water flooded parts in the reservoir. The effect of initial water saturation also was investigated at reservoir temperature 130 ºC by performing two experiments with different initial water saturation.

Results from these experiments show a high potential for oil recovery in all experiments. In the high temperature experiments, the produced oil had a variable composition during CO₂ injection, while at the low temperature condition, the produced oil initially had a constant composition and then it started to change. Different behaviour of produced oil composition in the high and low temperature might be due to dominant of diffusion mechanism in the high temperature experiments.

In the low temperature (60 ºC) experiment, at the early stage of CO₂ injection the produced oil had constant composition for a short period of time and then it changed to variable composition similar to the high temperature case. This behaviour maybe is due to high solubility of CO₂ into oil and consequently more oil swelling than the high temperature condition.

In order to quantify the above mechanisms, several attempts have been done to history match the experiments by using compositional simulator. But in all cases, it was not possible to history match the experiments.

The weakness of the simulator was due to the improper formulation which was used for calculating the cross phase diffusion between the oil and gas phase in the matrix and fracture system. The details of simulation work as well as the cross phase diffusion issue are discussed in Chapter 5 and Paper 2 in Appendix B.

Using laboratory, numerical and field experiments this study investigated whether borehole measurements of self-potential (SP) can be used to monitor seawater intrusion into the fractured UK Chalk aquifer. The SP, a natural voltage, arises in water saturated fractured porous media due to gradients in pressure (electrokinetic (EK) potential) and concentration (exclusion-diffusion (EED) potential), both features of seawater intrusion. An electrode array was installed in a monitoring borehole c.1.7 km from the coast, in Saltdean, East Sussex, and c.1.3 km from an active abstraction borehole. Head fluctuations in the monitoring borehole were controlled by tidal processes and seasonal changes in inland head. SP monitoring over 1.5 years revealed tidal SP signals. The fluctuations (c.600 μV) were two orders of magnitude larger than those observed at an inland site in the same aquifer, near Reading in Berkshire. Numerical simulation, supported by laboratory measurements, of the coupled hydrodynamic and electrical processes in the coastal aquifer suggested that the EK potential generated by tidal processes was one order of magnitude too small to be responsible for the tidal SP fluctuations. Instead, SP was caused by the EED potential that arose due to the concentration gradient between groundwater and seawater across the saline front (i.e. the 1000 mg/l isoline) some distance from the borehole. The saline front moved through a fracture at the base of the borehole in response to tides. A vertical SP gradient (c.0.22 mV/m), only present in the coastal borehole, was also observed. Modelling suggested that the gradient was due to the close proximity of the saline front (c.4 m) below the borehole and was caused by the EED potential. In August 2013 and 2014, tides and a decline in inland head caused saline water to enter the borehole. Fluid electrical conductivity logging showed that entry was via the fracture. Prior to each occurrence of saline breakthrough, an increase in the SP of c.300 μV was observed, commencing c.7 days before saline water was detected in the borehole. Although this study focused on a monitoring borehole, SP arrays could be installed in abstraction boreholes. The results suggest that SP monitoring may be used to provide early warning of saline water breakthrough, allowing for improved management of groundwater resources in coastal aquifers.

There has recently been a steady increase in the number of licenses granted for the abstraction of water from the Chalk aquifer beneath London to supply "open loop" geothermal systems (Environment Agency, 2007). However, there has been little research conducted on how the water re-injected by these systems, which often differs in temperature by as much as 10°C, will interact with the fractured Chalk aquifer in both the short and long term. An analytical solution developed by Bodvarsson (1989) was used to show that, for most configurations of a geothermal system, thermal transport would be governed by fractures. It was then proved that the United States Geological Survey SUTRA code could be used to construct a more detailed model of the aquifer. A thermal test was devised to collect hydrogeological and thermal data. This test, along with conventional site investigation techniques, was used at a site in central London. A detailed numerical model of the geothermal system and the aquifer was then constructed in SUTRA. The results showed that the fracture zones found during testing would affect the system performance. Building on these results a procedure was developed for designers, to ensure such systems function in an appropriate way.

Pour faire face aux problèmes émergents de pollution et de dégradation de la qualité des eaux, il est nécessaire de maîtriser le fonctionnement hydrogéologique des roches réceptrices de polluants. Cela implique de définir la vulnérabilité des aquifères et d'optimiser la modélisation des phénomènes de rétention et des mécanismes de transport des particules dans les roches. Dans les aquifères karstiques, les fractures servent de voies préférentielles pour les particules, permettant ainsi leur transport rapide. Le transport des particules et de la matière dissoute dans les fractures est régi par l'advection et la dispersion qui sont influencées par plusieurs facteurs. L'objectif de cette étude est de contribuer à une meilleure compréhension des mécanismes de transport des particules solides et des matières dissoutes dans les fractures et des différents facteurs qui influencent ces mécanismes. A cette fin, un programme expérimental a été développé pour comprendre l'influence de la vitesse d'écoulement, de l'ouverture de la fracture, de l'orientation de la fracture et de la force ionique sur le transport des particules de kaolinite et du traceur dissous (fluorescéine) dans des échantillons de craie fracturée. Un modèle numérique a été développé sur la base de l'équation d'advection-dispersion, afin de déterminer les paramètres de transport et de comprendre en profondeur les interactions entre les particules et la surface de la fracture dans différentes conditions.Les résultats de cette étude ont révélé que l'effet hydrodynamique est significatif quelle que soit l'orientation de la fracture, la récupération des particules augmentant avec la vitesse d'écoulement. Les particules de kaolinite sont transportées plus rapidement que la fluorescéine en raison de l'effet d'exclusion de taille et du coefficient de dispersion plus élevé de la fluorescéine. Le coefficient d’attachement augmente avec la vitesse d'écoulement pour toutes les orientations de fracture et est indépendant de l'ouverture de la fracture. Inversement, le coefficient de détachement, qui est négligeable pour les petites vitesses d'écoulement, est plus important dans les petites ouvertures en raison d'une contrainte de cisaillement plus élevée. Les résultats ont montré, aussi, que l'orientation des fractures affecte de manière significative le transport des particules de kaolinite mais a un impact négligeable sur la fluorescéine. L'augmentation de l'orientation verticale des fractures améliore la récupération et la dispersion des particules, tandis que le coefficient d'attachement diminue. Enfin, les résultats de l'effet de la force ionique indiquent qu'une force ionique plus élevée augmente la rétention des particules et diminue le taux de récupération. Le coefficient d'attachement présente une augmentation linéaire et le coefficient de détachement suit une évolution exponentielle avec l'augmentation de la force ionique.Les résultats de l'étude soulignent l'importance de prendre en compte des vitesses d'écoulement élevées dans l'étude des effets hydrodynamiques, de l'ouverture de la fracture et de l'effet de la force ionique pour comprendre les mécanismes de transport des particules de taille micrométrique dans les fractures de la craie. Elle contribue également à faire progresser la compréhension de l'effet de l'orientation des fractures sur le transport des particules. Cette compréhension est essentielle pour évaluer les risques pour les ressources en eaux souterraines et pour faire progresser les mesures de protection de l'environnement
In order to face the emerging problems of pollution and deterioration in water quality, it is necessary to master the hydrogeological functioning of pollutant-receiving rocks. This involves, defining the vulnerability of aquifers and optimising the modelling of the retention phenomena and transport mechanism of particles in rocks. In karstic aquifers, fractures serve as preferential pathways for particles, thus allowing their rapid transport. The transport of particles and dissolved matter in fractures is governed by advection and dispersion which are influenced by several factors. The objective of this study is to contribute to a better understanding of the mechanisms of transport of solid particles and dissolved matter in fractures and the different factors influencing these mechanisms. For this purpose, an experimental program was developed to understand the influence of flow velocity, fracture aperture, fracture orientation and ionic strength on the transport of kaolinite particles and fluorescein dissolved tracer in fractured chalk samples. In addition, a numerical model was developed based on the Advection-Dispersion equation, to determine the transport parameters and deeply understand the particle-fracture surface interactions under different conditions. The results of this study revealed that the hydrodynamic effect is significant regardless of fracture orientation, with particle recovery increasing as flow velocity increases. Kaolinite particles travel faster than fluorescein due to the size exclusion effect and the higher dispersion coefficient of fluorescein. The attachment coefficient increases with flow velocity for all fracture orientations and is independent of fracture aperture. Conversely, the detachment coefficient, which is negligible for small flow velocities, is greater in smaller apertures due to higher shear stress. The findings showed that fracture orientation significantly affects the transport of kaolinite particles but has a negligible impact on fluorescein as a dissolved tracer. Increasing the fracture orientation vertically enhances particle recovery and dispersion, while the attachment coefficient decreases. The effect of ionic strength indicate that higher ionic strength increases particle retention and decreases the recovery rate. With the attachment coefficient exhibiting a linear increase and the detachment coefficient follows an exponential trend with increasing ionic strength.The study findings highlight the importance of considering high flow velocities in studying the hydrodynamic effect, fracture aperture, and IS effect in understanding micron-sized particle transport mechanisms in chalk fractures. It also contributes to the advancement of understanding the effect of fracture orientation on the transport of particles by using experimental methods. These understandings are essential for assessing risks to groundwater resources and advancing environmental protection measures

Acid fracture conductivity and the effect of key variables in the etching process during acid fracturing can be assessed at the laboratory scale. This is accomplished by using an experimental apparatus that simulates acid injection fluxes comparable to those in actual acid fracture treatments. After acid etching, fracture conductivity is measured at different closure stresses. This research work presents a systematic study to investigate the effect of temperature, rock-acid contact time and initial condition of the fracture surfaces on acid fracture conductivity in the Austin Chalk formation. While temperature and rock-acid contact are variables normally studied in fracture conductivity tests, the effect of the initial condition of the fracture surface has not been extensively investigated. The experimental results showed that there is no significant difference in acid fracture conductivity at high closure stress using smooth or rough fracture surfaces. In addition, we analyzed the mechanisms of acid etching and resulting conductivity creation in the two types of fracture surfaces studied by using surface profiles. For smooth surfaces, the mechanism of conductivity creation seems connected to uneven etching of the rock and roughness generation. For rough surfaces, acid conductivity is related to smoothing and deepening of the initial features on the sample surface than by creating more roughness. Finally, we compared the experimental results with Nirode-Kruk correlation for acid fracture conductivity.