1,721,071 research outputs found
Uniqueness conditions in a hyperbolic model for oil recovery by steamdrive
Get PDFBruining, J.; Duijn, C.J. van. (1999). Uniqueness conditions in a hyperbolic model for oil recovery by steamdrive. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/3433
The impact of business ownership change on employee relations:Buy-outs in the UK and the Netherlands
No full textImmiscible and Miscible Gas-Oil Gravity Drainage in Naturally Fractured Reservoirs
No full textIn the phase of declining oilfields and at a time when recovering hydrocarbons has becoming more difficult, effective techniques are the key to extract more oil from mature fields. The difficulty of developing effective techniques is often the real obstacle when dealing with heterogeneous formations such as naturally fractured reservoirs, which are among the most challenging class of hydrocarbon formations. Because of the large degree of heterogeneity, in terms of reservoir properties, the production methods from naturally fractured reservoirs differ from the production methods from conventional reservoirs. To identify an efficient and optimum strategy for oil production from naturally fractured reservoirs, fluid displacement mechanisms at reservoir conditions need to be understood. Gas injection into the conventional reservoirs has been already common practice for years. However, gas injection into naturally fractured reservoirs is still controversial. This is mainly due to the existence of highly permeable fracture systems in fractured reservoirs. These highly permeable and interconnected fracture networks offer easy-flow pathways to the injected gas. As a result, the injected gas is usually produced without sufficient contact with the oil in the matrix, which is left behind and unproduced. Therefore, it is of practical importance to examine the efficiency of a gas injection process in naturally fractured reservoirs to achieve a reliable and effective production process. This thesis is a collection of experimental and theoretical work on the efficiency of gas-oil gravity drainage process in naturally fractured reservoirs. Gas injection experiments were performed using a modified experimental set-up to simulate the gravity-drainage process in naturally fractured reservoirs. In this thesis, the possibilities and limitations of improved oil recovery from fractured reservoirs by gas injection are investigated.Geoscience and EngineeringCivil Engineering and Geoscience
Particle enhanced foam flow in porous media near the critical micelle concentration
No full textThis thesis was performed in the framework of ErasmusMundus EU-INDIA scholarship programme. The main goal is to elucidate particle enhanced foam flow (surfactant water and nitrogen gas) in porous media near the critical micelle concentration. The thesis is divided in four parts: in the first part the modeling of foam flow is investigated, in the second part variables affecting the steady state pressure drop during foam flow are discussed, in the third part the stability of ash particles in the bulk dispersion is tested and in the final part, the effect of the particles on foam flow stability in porous media is experimentally studied. During the period of the study, the set ups for fluid flow and laser scattering of liquids are built and calibrated. We measured pressure drop histories before and after injection of ash particle dispersions with nitrogen gas (N2) across the measurement points in unconsolidated sand packs (1860 and 130 Darcy) and a Bentheimer sand stone core (3 Darcy). This was carried out for various surfactant concentrations (0.0375, 0.075 and 0.15 w/w%), for various gas and surfactant solution velocities (0.27-3.97 m/day), for two salinities (0, 0.5M NaCl) and for two pH values (6.5, 3.0). We used a mathematical formulation with a bubble population function by history matching the experiments. The two-phase flow model that leads to four equations, viz., a pressure equation, a water saturation equation, a bubble density equation and a surfactant transport-adsorption equation can describe the pressure drop during the foam flow experiments. Within the model, the rate of change of bubble density during the transient state can be equated to the bubble density generation function plus the terms accounting for the bubble transport, i.e., by convection and diffusion divided by the porosity saturation product. The effect of the variables (e.g. permeability) on the foam flow is studied by using symbolic regression. We applied a Monte Carlo method (Bootstrap) to calculate the parametric uncertainties. The data driven model obtained without prior knowledge of an underlying physical process can elucidate the general behavior and hierarchy of the variables affecting the steady state pressure drop. The statistical model gives the variable spaces for which more experiments are needed. The trends obtained from the subset of data cannot be derived from the complete data set purely on statistical grounds. To use ash particles in foamflow through porous media we measured their colloidal stability when surfactant is present or absent in the dispersion. We measured properties of dispersions, viz., zeta potential, sedimentation-coagulation behavior, light absorption and particle size for pH values ranging from3 to 11. For the optimal stability of an ash dispersion, we recommend an alkaline mediumwhen surfactant is absent and an acidic medium, when surfactant is present. An ash particle dispersion alone with nitrogen gas cannot generate foamin porous media. The flow of ash particles with foamin porous media (Bentheimer and sandpack) is related to the colloidal stability of the ash dispersion. We observed tiny change in the permeability of the porous media after foam flow experiments with ash particles.Geoscience & EngineeringCivil Engineering and Geoscience
Successful management buyouts: Are they really more entrepreneurial?
Get PDFThe paper explores the impact of entrepreneurial management dimensions on post-MBO financial performance. We use Stevenson’s conceptualization of entrepreneurship (1983), empirically validated by Brown, Davidsson and Wiklund (2001), positing that entrepreneurial companies will be involved in recognizing and exploiting opportunity, regardless of the resources controlled. From the literature we hypothesize positive effects of entrepreneurial management dimensions on post-MBO financial performance. We find that successful buyout managers cannot be classified as entrepreneurs on all entrepreneurial dimensions. Instead they ambidextrously combine the pursuit of valuable opportunities with the exploitation and control of their resources. Implications for theory and managerial practice are discussed.Entrepreneurial Management;Financial Performance;Management Buyouts
Measurements of capillary pressure and electric permittivity of gas-water systems in porous media at elevated pressures: Application to geological storage of CO2 in aquifers and wetting behavior in coal
No full textSequestration of CO2 in aquifers and coal layers is a promising technique to reduce greenhouse gas emissions. Considering the reservoir properties, e.g. wettability, heterogeneity and the caprocks sealing capacity, the capillary pressure is an important measure to evaluate the efficiency, the success and the safety of storage applications. In this research, the capillary pressure behavior was investigated for the CO2-water system in quartz and coal. Measurements were conducted at pressures and temperatures, ranging from ambient to reservoir conditions, where CO2 is present as supercritical fluid. Furthermore, the relation between capillary pressure hysteresis and interfacial area was investigated, measuring the capillary pressure and the electric permittivity as a function of frequency, simultaneously. The results from the quartz samples, showed a dependence of the capillary pressure on the CO2 pressure. Moreover, only dissolution rate effects for gaseous CO2 in the water were observed. Significant capillary pressure fluctuations and negative values during imbibition were observed at near supercritical conditions. From the coal experiments it was observed that with increasing CO2 pressures the wettability of medium rank coal altered from water-wet to CO2-wet. High rank coal was CO2-wet during primary imbibition experiments in the entire pressure range. The relation between capillary pressure and interfacial area has been investigated by measuring the capillary pressure and the electric permittivity at 100 kHz as function of the water saturation. The permittivity data showed hysteresis between drainage and imbibition. Furthermore, non-monotonic behavior was observed which was attributed to polarization of the gas-water and water-solid interfaces. The permittivity hysteresis is provoked by the different phase distributions and geometries. From these results it was concluded that the capillary pressure is a unique function of the permittivity and the water saturation.Geoscience & EngineeringCivil Engineering and Geoscience
Enhanced transport phenomena in CO2 sequestration and CO2 EOR
No full textThe results of this thesis give insight into the (mass)-transfer during flow of gases, especially CO2, in various gas-liquid systems. A number of experiments was performed to investigate the transport phenomena through interfaces with and without surfactant monolayers. The observed phenomena have been incorporated into physical models to predict the fate of CO2 overlaying a bulk liquid or liquid saturated porous media. Moreover, dynamics of (CO2)-foam flow in oil-free and oil-saturated porous media was studied using X-ray tomography.GeotechnologyCivil Engineering and Geoscience
Quantitative characterization of solute transport processes in the laboratory using electrical resistivity tomography
No full textThe shallow subsurface is an important zone from a social, economical, and environmental point of view. The increased use of the shallow subsurface together with the call for its protection and sustainable exploitation have increased the need for tools to monitor and characterize the subsurface, as well as for an improved understanding of the hydrogeological processes taking place in this zone. The limitations of traditional point sampling techniques have led to the field of hydrogeophysics, in which geophysical methods are employed to characterize the subsurface at a relatively high spatial resolution but with minimal disturbance. A common geophysical method used for hydrogeological characterization is electrical resistivity tomography (ERT). ERT uses the injection of electrical currents and the measurement of the resulting voltages to obtain a spatial distribution of the electrical conductivity, which is related, among other things, to volumetric moisture content and ionic strength of the pore water. Although ERT is an established tool for qualitative visualization of hydrogeological processes in the subsurface, its quantitative application is relatively new and constitutes an active area of research. The main aim of this study is to explore the quantitative characterization of solute transport processes in porous media in the laboratory using ERT. For this purpose a flexible, high-speed, and modular ERT measurement system was developed in collaboration with the Electronic and Mechanical Support Division. The new system can perform measurements with a much higher temporal resolution than commercially available field equipment. The ERT system uses a pulsed direct current source and eight channels for simultaneous voltage measurement, and can be connected to a total of 128 electrodes. An error analysis for the ERT system shows that the measurement noise of a transfer resistance measurement (the ratio of injected current and measured voltage) in the used laboratory setup is described by a Gaussian probability distribution with e ? N(e ?,se ). The standard deviation of the measurement errors (se ) can be estimated from repeatability measurements, while the mean (e ?, representing systematic errors) can be estimated from reciprocity measurements. The ERT setup (ERT measurement system, electrodes, and experimental tank) has been used to perform a set of relatively simple, well-defined, static experiments. For these experiments the experimental tank was filled with an aqueous solution into which a solid plastic cylinder was placed. The collected ERT measurements have been used in uncoupled as well as coupled inverse approaches to estimate parameters such as the location and radius of the plastic cylinder. The statistical inversion algorithm DREAM(ZS), based on the Markov Chain Monte Carlo method, was used to estimate the model parameters. The reconstructed electrical conductivity images required in the uncoupled inversion were obtained using an algorithm based on Occam’s inversion. The model parameters estimated with the uncoupled inverse approach are corrupted due to the regularization bias introduced into the reconstructed electrical conductivity image. This bias particularly affects the estimates of the plastic cylinder’s electrical conductivity and radius. The poor performance of the uncoupled inverse approach is mainly caused by the use of a smoothness prior in the image reconstruction. Obviously, such a prior is not in agreement with the sharp conductivity contrast between the plastic cylinder and the aqueous solution. Conversely, the model parameters estimated in the coupled inverse approach are in close agreement with the measured values. The residual between the measured and simulated transfer resistances in the coupled inversion suggest that the model errors are larger than the measurement noise. The model errors are mainly attributed to errors in electrode position or shape and errors in the shape of the experimental tank. Even though the model errors are statistically significant, the total data errors are too small to significantly affect the estimated parameters values. The results show that when accurate forward models are used in a coupled inverse approach, ERT measurements can provide accurate and precise parameter estimates. Subsequently, the feasibility of ERT to quantitatively characterize solute transport processes in our laboratory setup was explored. For this purpose, a straightforward solute transport experiment was performed, consisting of a series of three single-step tracer injections into a saturated homogeneous sand column. The use of ERT to quantify processes in porous media introduces additional uncertainty since a petrophysical relationship is required to relate the bulk electrical conductivity modelled with ERT to the properties of the porous medium. In addition, the solute transport model is subject to uncertainty in porous medium parameters as well as petrophysical parameters, uncertainty in boundary and initial conditions, and model structural errors (e.g. caused by assuming a homogeneous porous medium while in reality the medium is heterogeneous). The reconstructed electrical conductivity images for the tracer injection experiments clearly were affected by the spatially variable resolution of the smoothness-constrained image reconstruction as well as by inversion artefacts. The applied smoothing primarily caused overestimation of the dispersion of the tracer front. The reconstructed electrical conductivities could be improved by the use of a process-based prior which includes specific information about the solute transport process in the regularized inversion. However, the success of this prior is dependent on the accuracy of the solute transport model and petrophysical relationship used to generate the set of feasible electrical conductivity models. The ERT measurements collected during the tracer injections were also used to estimate the parameters of a one-dimensional solute transport model in an uncoupled as well as a coupled inverse approach. In both inverse approaches, the solute transport parameters could be estimated to a high precision. Moreover, the estimated values were physically realistic and agreed relatively well with the expected values. Surprisingly, the results obtained in the uncoupled and coupled inversions were comparable, despite the fact that the reconstructed electrical conductivities were clearly affected by regularization. An explanation for this observation was found in the variable sizes of the elements used in the ERT mesh. The smaller elements near the electrodes, where ERT spatial resolution was high, relatively contributed more to the objective function in the uncoupled inversion than the larger elements towards the centre of the domain, where ERT spatial resolution was low. As a result, the uncoupled inversion was biased towards areas with good data quality. Although the solute transport parameters estimated with the coupled inversion are physically realistic and agree relatively well with the expected values, the (mis)fit between the observed and simulated transfer resistances suggests that significant model errors in the solute transport and/or petrophysical model must have been present. These model errors likely are the result of heterogeneities in the solute transport caused by small-scale preferential flow (cm or less). Possible causes for small-scale preferential flow are unsaturated conditions in the sand column, the periodic invasive sampling of water at the sampling ports, or non-uniform tracer injection at the sand column’s bottom boundary. Overall, the results presented in this thesis show that ERT can be used to quantitatively characterize a solute transport process in a laboratory sand column, given that a coupled inverse framework is used and the forward models are accurate enough representations of reality. In general, the success of quantitative characterization using ERT measurements is highly dependent on the information available in addition to the ERT measurements, such as information about the dominant flow and transport processes, porous medium properties, heterogeneities, and initial and boundary conditions. When little or no information is available, ERT is best used in a standard image reconstruction using a smoothness prior or other general prior. Although the resulting images will only provide qualitative information, this information could still be beneficial for the construction of an appropriate hydrogeological model. Once a hydrogeological model is constructed, its unknown parameters could be estimated using the ERT measurements in a coupled inverse approach. In a controlled laboratory experiment additional information is readily available, which increases the feasibility of ERT for quantitative characterization. Additional information may come from imposed boundary and initial conditions and the experimental design, as well as from independent measurements of the soil’s properties (e.g., porosity, water content) and petrophysical parameters. Alternatively, additional information may come from other geophysical datatypes as well as hydrological measurements. Since the success of quantitative characterization of hydrogeological processes using ERT is highly dependent on the information available in addition to the ERT measurements, future research should focus on the integration of multiple types of geophysical data (e.g., electrical resistivity tomography, ground-penetrating radar, induced potential, self potential, electromagnetics, time-domain reflectometry) and local hydrological data (e.g., concentration, hydraulic head, flowrates).Geoscience & EngineeringCivil Engineering and Geoscience
Accurate Sorption Measurements on Coal and Activated Carbon using Accurate Equations of State
No full textSince industrial revolution, due to the increasing demand of energy, anthropogenic emissions in the atmosphere are constantly growing. The International Energy Agency (IEA) predicted a 57% increase of energy demand from 2004 to 2030 (IEA, 2004) of which, 85% consists of fossil fuels. Actions need to be taken in order to mitigate CO2 emissions generated through human activities. In the last twenty years CO2 emissions became a political and industrial priority in many governmental and commercial institutions; Carbon Capture and Storage (CCS) is one of the main options but it is a temporary solution. CCS is based on capturing CO2 from large point sources. The gas is transported via pipelines and injected in deep underground formations, such as depleted gas and oil fields, deep saline aquifers and unminable coal seams. The last method is the option that will be treated in this thesis. Coal can be favorable if CO2 replaces coal gas that mostly consists of methane (CH4). This is called CO2- Enhanced Coal Bed Methane (CO2-ECBM) production. The feasibility and economical viability of CO2-ECBM depend on geological factors such as heterogeneities of coals and their pore systems. This is why the development and implementation of reservoir simulators for ECBM production and CO2 storage require detailed and reliable information on the physical and chemical processes that are initiated by injection of gases in the coal layers. The most important processes to deal with are sorption behavior, of the coal competitive sorption of the different gases presents, multi-phase transport, permeability behavior and initial amount and compositions of the gas injected. In this study the main objective is to get a better understanding of the coalwater- gases system. The activities involved experimental work and theory development to acquire data and theory for field scale modeling. Thinking in terms of real case scenarios we consider the use of an impure CO2 stream, i.e., impurities in the CO2, either flue gas components or water, and their effect on coal behavior. The activities in this research involve the experimental results of sorption on dry and wet coal and the sorption of flue gas type of gas mixtures measured with a manometric set up at 318 K and up to 160 bar. A new aspect of the thesis is to focus on the different ways to obtain sufficiently accurate Equations of State (EoS) to be used in a manometric set up. The experimental results allowed us to interpret and test different models concerning sorption and thermodynamic behavior of gases. The results in general show that sorption and desorption of CH4 and N2 on coal are fully reversible, meanwhile this is not happening for CO2. The equilibration time for the CO2 sorption on coal is much larger than for N2 and CH4. An increase in temperature is negatively affecting the sorption capacity of the coal. The swelling induced by CO2 injection on coal is a fully reversible phenomenon and it is positively related to the sorption. The sorption of CO2 on wet coal is inhibited by the presence of water. The density of the CO2-H2O gas phase in the temperature and pressure range of the study can be calculated using the Span and Wagner EoS for pure CO2. The CO2 dissolved in water, assuming that water in its sorbed phase behaves as in its free phase, can be described by a Peng-Robinson-Stryjek-Vera EoS that is optimized for the CO2-water system. The adequacy of an EoS to predict the density of a mixture can be tested by using a combination of the manometric set up with a density meter. A Helium mixture containing 1% O2 and 1% NO2 can be described with the Mc Carty EoS for pure Helium. In this case, the maximum relative difference from the experimentally determined density is of 6·10?3. A CO2 mixture containing 1% O2, 1% He and 1% NO2 cannot be described accurately with any of the existing EoS. Results concerning the excess sorption isotherm are influenced by the choice of a specific EoS. The maximum of the excess sorption can vary 25.88%, depending on which EoS is used for the calculations. A combination of different EoS for different pressure ranges gives an accurate result, with a maximum relative difference from the experimentally determined density of 0.05. In this case the maximum excess sorption gives a value of 7.81 mol/kg, which is in agreement with the literature concerning pure CO2 sorption on activated carbon. In the desorption process from activated carbon with the two mixtures mentioned previously, the mass spectrometer measurements show that for the specified P,T range, no reactions occur between the gas and the activated carbon. The results of this research give an alternative direction with respect to the use of impure CO2 in ECBM.GeotechnologyCivil Engineering and Geoscience
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