In most of free vortex wake models (FVWMs), the induced velocity is computed by Biot-Savart law. But the details of velocity calculation are still incomplete in their self-integrated loss of adjacent segment's influence. Curved filament correction has already been studied to recover the FVWM in helicopter problems. In this work, an extended FVWM with the correction is developed intended to improve aerodynamic predictions of wind turbines. Numerical simulations are performed on ring vortices and practical modeling of flow state of both fixed and floating wind turbines. It has been shown that the newly-designed technique may generate higher fidelity.
Among multiple modeling methods in aerodynamics of wind turbines, vortex lattice method (VLM) with straight line segmentation have been commonly used. The trailing filaments generated by the blades are assumed to convect freely with material lines of concentrated vorticity in potential flow. Such force free motion is governed by the vortex transportation equation. The governing equation is a partial differential equation which can be solved by various numerical approximation with high-order accuracy in both time and space domain.
It has been studied that for the straight-line segmentation, the approximation of induced velocity is relatively accurate with respect to corresponding theoretical result with the exclusion of self-induced velocity. It means that the collocation points lie in nowhere in vicinity to the discrete vortex segments (Gupta and Leishman, 2005). When it comes to the case that collocation points are extremely close to the discrete segments, the self-induced velocities tend to be infinite. The solutions for this singularity can be eliminated by “cutoff’ process (Bhagwat and Leishman, 2001) and vortex core models (Leishman,2006). These solutions are initially introduced by core regularization to eliminate singularity of the collocation points or simply fulfill the physical mechanism. However, techniques with these processes are incomplete because they fail to add up the total induced velocity.
Summary With the continuous development of full tensor gradiometer (FTG) measurement techniques, the precision and amount of geophysical data is increasing. Consequently, the time-consuming problem in three dimensional ( 3D) density inversion of high-precision FTG data needs to be considered. Preconditioned conjugate gradient (PCG) algorithm is an efficient way in fast solving linear inversion equations, which can reduce the number of inverse iterations. In our study, an improved preconditioned conjugate gradient algorithm is proposed by combining the symmetric successive over-relaxation (SSOR) technique and the incomplete Choleksy decomposition conjugate gradient algorithm (ICCG). Since preparing preconditioner requires extra time, a parallel implement based on GPU is proposed.
The aerodynamic and hydrodynamic performance of floating offshore wind turbines interact more than that of traditional fixed ones under the influence of structure oscillation and unsteady environments. The details of the aerodynamic performance still remain to be discussed and are challenging to predict accurately. Here included in this paper, the aerodynamic performances of floating offshore wind turbines affected by the hydrodynamic terms in simulation are studied and discussed for more accurate simulation and prediction results. The quasi-steady BEM theory is chosen as the key theory to discuss the effect on the aerodynamic performance of floating offshore wind turbines by their hydrodynamic terms. A series of different formulas for characterizing the calculation of local velocity in the axial and tangential direction are tested to summarize the inherent law of quasi-steady BEM theory. The formulas derived from low-frequency motion seem more reasonable than the classic ones in the simulation based on the quasi-steady assumption.
Wind energy has been occupying an important position of renewable energy around the world. Meanwhile, offshore wind energy was experiencing sustainable growth with an average annual increasing rate of 30% during the last five years. Now, most offshore wind turbines have been installed in shallow waters (less than 50 m). Compared with shallow waters, deep waters may supply much more sites for wind turbines and greater resource with stronger and more consistent wind, also with less turbulence intensity (Jonkman, 2007). However, fixed offshore wind turbines applied in shallow waters may not be suitable for deep waters out of economic consideration. Inspired by the development progression from fixed substructures to floating ones in offshore petroleum industry, floating offshore wind turbines were developed and expected as the future of offshore wind energy.
Although floating offshore wind turbines are evolved from fixed offshore wind turbines, their dynamic performance is quite different from that of fixed offshore wind turbines. In addition to the structure oscillation of tower and blades, also found in fixed offshore wind turbines, floating offshore wind turbines always run with irregular motion of floating substructures attributing to wind, waves and currents. As a result, the aerodynamic and hydrodynamic performance of floating offshore wind turbines interact more than that of fixed offshore wind turbines. On one hand, the motion of a floating substructure induced by waves and currents will change the relative wind velocity on the rotor which affects its aerodynamic performance. On the other hand, the thrust and torque from a rotor induced by wind will influence the motion of the floating substructure which affects its hydrodynamic performance. The interaction brings challenges to the study of floating offshore wind turbines, especially in simulation and prediction of their aerodynamic performance.
This paper proffers some guidelines for using a surfactant-polymer (SP) flooding operation so that the feasibility and profit can be optimized. Many successful projects have shown that surfactant-polymer flooding is an effective means to improve oil recovery after or during water-flooding. Here, the guidelines for evaluating whether a surfactant-polymer flood is suitable for a given field are developed and the procedure for a full implementation is summarized. These guidelines cover primary screening, laboratory measurements, reservoir simulation, economic analysis, pilot testing, and field application. Detailed methods also are presented for the optimization of chemicals (including compatibility tests between chemicals and reservoir fluids and rock), well pattern and spacing preparation, and slug and injection process design. The correlations between the above phases of operation are also emphasized. The work offers reservoir engineers a reference for decision-making in SP flooding.
Oil-production from Enhanced Oil Recovery projects continues to supply an increasing percentage of the world's oil. Taber, et al., (1997) estimated that 3% of the worldwide oil production came from EOR. That number has continued to increase and in the future it is expected that EOR will eventually produce the majority of the world's oil. Thus choosing the most efficient recovery method becomes increasingly important to petroleum engineers. Using a surfactant-polymer solution as an injectant is a technique to enhance oil recovery from a water-flooded reservoir by improving both displacement and sweep efficiencies. Polymer functions work by adding certain concentrations of water-soluble polymers to injection water to increase the injectant viscosity; surfactant functions work by adding certain concentrations of surfactants to injection water to reduce the interfacial tension (IFT) between the displacing and displaced phases. The polymer makes the displacing phase more closely match the viscosity of the in situ oil and thus achieves a more favorable mobility ratio (Hornof, et al. 1983; Needham and Doe 1987; Maitin, 1992; Melo, et al., 2005). Surfactant-polymer flooding reduces the interfacial tension between the trapped crude oil and its associated brine. Flow of the surfactant formulation through the reservoir allows the oil droplets trapped in small pore spaces to deform and become mobile oil. When the interfacial tension is reduced the capillary forces are reduced and the oil is allowed to flow. Once they are released these oil droplets then coalesce and form a new flowing oil bank. The more viscous polymer solution is used behind the surfactant to keep the surfactant and oil bank moving toward the producing wells. Surfactant-polymer processes result in a final oil saturation much lower than that produced by water-flooding and the more efficient reservoir sweep reduces the amount of injection fluid needed to recover a given amount of oil (Beghelli, et al., 1989; Roshanfekr, et al., 2008). A number of reviews on the application and benefits of surfactant-polymer flooding exist. Over the past thirty years, quite a few of surfactant-polymer flooding projects have been conducted on modest scales in China, especially at China's largest field, Daqing (Liu, et al. 2004).
The industrial experience at Daqing showed that chemical flooding significantly could enhance the oil recovery. Meanwhile, the fluid behaviors in the reservoirs became more complex due to the injection of chemicals. The dynamic connectivity between injection and production wells formed by water flood has been changed significantly due to the participation of chemical solutions. In this research, an improved grey relation analysis method (IGRA) is introduced to predict the dynamic connectivity between injection and production wells and to estimate the effects of connectivity changes. Plenty of sensitivities in chemical floods are considered in this new method so that the influence of errors caused by a poor indices choice in the variable process description can be minimized. An application was carried out in a polymer flooding pilot of the Xingshugang Unit, Daqing. By comparing the results of both the water and chemical flooding prediction results with IGRA, the dynamic connectivity changes have been demonstrated. Tracer tests have been conducted for both water and chemical flood. The interpretations of the tracer tests have also proven the accuracy of predictions. The results of chemical flooding prediction have been used to adjust the injection and production rate for individuals.
Chen, Guo (Daqing Oilfield Co. Ltd.) | Li, Ye (Daqing Research Institute) | Wang, Jinmei (Daqing Research Institute) | Ma, Moran (Daqing Research Institute) | Lu, Kewei (Daqing Oilfield Co. Ltd.) | Jin, Guangzhu (Daqing Oilfield Co. Ltd.) | Sun, Hongli (Daqing Research Institute)
Daqing Oilfield has conducted various complex chemical improved oil recovery methods in both pilot tests and field applications such as alkali-surfactant-polymer (ASP) flooding, ASP+Foam flooding and multi-polymer flooding, in the meantime, some new process mechanism for various chemical flooding techniques has been rediscovered, for example, polymer elasticity could increase micro-scale oil displacement efficiency, making the existing chemical flooding simulators insufficient to handle these complex chemical flooding process mechanism. In this paper we develop an applied chemical flooding simulator to have capability to simulate various complex chemical flooding processes in Daqing Oilfield. In comparison with existing chemical simulators, the newly incorporated features include the chemical synergistic effect among surfactant, alkali and oil in ASP flooding with ultra-low surfactant concentration, increasing micro-scale oil displacement efficiency by polymer elasticity, multi-polymer commixing flooding process, foam behavior dependent on the reservoir pore structure and the ratio of gas to liquid in foam system, relative permeability that is a function of polymer elasticity and capillary number. The simulator models the other main physicochemical phenomena in chemical flooding process such as convection, dispersion, diffusion, adsorption, aqueous reactions,in-situ generation of surfactant from acidic crude oil, etc.
The solving scheme for the simulator is sequential solution method, deriving pressure equation, saturation equation and concentration equation from mass conservation equation, and then solving the pressure equation implicitly, followed by obtaining the implicit solution of the saturation equation through Newtonian method. For concentration equation, an operator splitting method is employed to split it into a diffusion equation and convection equation, obtaining implicit solution by alternately solving the diffusion and convection equation. The solution procedure for diffusion equation is ADI. The convection equation is discretized using an implicit single point upstream method, by taking advantage of the reservoir flow potential field, to obtain an implicit non-iteration solution algorithm.
The developed model is a 3-D 3-phase (oil, water and gas) and multi-component chemical flooding simulator, having ability to simulate single polymer flooding, multi-polymer flooding, ASP flooding, and ASP+Foam flooding. It has been successfully applied in various chemical flooding simulations. This paper also presents some cases for its application in Daqing Oilfield.
Li, Ye (the University of British Columbia) | Nabavi, Yasser (the University of British Columbia) | Alidadi, Mahmoud (the University of British Columbia) | Klaptocz, Voytek R. (the University of British Columbia) | Rawlings, G. William (the University of British Columbia) | Calisal, Sander M. (the University of British Columbia)
Although significant similarities exist between wind and tidal current driven turbines, new challenges and design opportunities unique to the operation in an ocean environment require deeper research and investigation. In this paper, the recent numerical analysis of a vertical axis tidal current turbine done at UBC is summarized along with a brief description of the experimental work currently underway. The numerical work is conducted using a commercial Computational Fluid Dynamics (CFD) package, FLUENT, as well as potential flow analytical methods. This study compares both numerical approaches and provides some insight into the recommended design modifications that will increase turbine efficiency due to be implemented in upcoming experimental testing. INTRODUCTION Traditionally, tidal power is associated with head driven turbines integrated into a barrage structure. This arrangement was typical of installations in 1960’s and 1970’s and some examples include the Annapolis project in Canada as well as the La Rance, France installation. These barrage type installations, however, have been found to pose a large ecological footprint. In recent years, the focus has shifted towards a more environmentally benign method of harnessing tidal energy by developing devices capable of capturing the kinetic energy from tidal current flows similarly to the approach taken by wind turbines. Since the design principle is almost the same as wind turbines, tidal current turbines typically come in two different configurations: horizontal axis and vertical axis as shown in Figure 1. By benefiting from extensive research and development in the wind industry, horizontal axis tidal current turbine has reached the pre-commercialization stage a few years ago (Frankael 2002). However, the vertical axis tidal turbine is still under development. Although patented over eighty years ago (Darriues 1931), the vertical axis turbine did not see extensive investigation until the 1970’s when Canadian National Research Council (South and Rangi, 1972, Templin 1974, etc)
Liu, Yuling (Daqing Oilfield Co. Ltd.) | Liu, He (Daqing Oilfield Co. Ltd.) | Zhou, Wanfu (Production Engineering & Research Institute) | Yang, Baoquan (Production Engineering & Research Institute) | Wang, Qingguo (Production Engineering & Research Institute) | Li, Ye (E & D Research Institute of Daqing Oilfield Company)
In the Daqing oilfield, acid stimulation has become a main method used to increase water volume injected into the low permeability sandstone reservoirs, but there are still three problems. First, much of the acid flows into the high permeability layer leaving the low permeability layer untreated due to the permeability difference. Second, clay particles migrate in the low permeability layer, which results in high-speed acid reflux and causes new damage to the formations. Finally, operation costs increase because the spent acid needs to be flushed out of the hole and safely treated.
This paper analyzes the properties of lithology, block-up, and interlayer differences for the thin interbeds, thus showing the newly developed separate zone and zero discharge acidizing techniques.
Through separate zone mathematical models and solid and liquid diverting agents, acid can more effectively enter the low permeability layer achieving separate zone acidizing. The two kinds of diverting agents can be solely or jointly applied in terms of permeability differences. When an agent (solid agent or liquid agent) plugs up a layer on the maximum degree, the injection pressure is at or above 2900 psi, and the plugging ratio exceeds 90%. Diverting agents will also dissolve completely in 24 hours without any damage to the formations.
In addition, a new complex acid system has been developed. The HF-HNO3 acid system mixed with good-performance complexing agents can more effectively inhibit precipitation. Therefore, acid reflux is not necessary after acidizing and â€œzero dischargeâ€? is achieved. Laboratory experiments showed that natural core permeability increased by 240%, and the resulting acid reaction did not cause secondary precipitation until pH values were above 7.
In the Daqing oilfield, tapping the potential of the thin and poor layers in a reservoir has become an important method in the stabilization of oil production. In order to continue developing the Daqing oilfield, the acidizing technique needs to be applied to numerous water injection wells because of low permeability, high clay mineral content, and severe pollution in the thin and poor layers.