Search

• One of the few papers on foam-liquid displacements using a Hele-Shaw cell. • 2D micromodel pattern is printed from an actual rock using 3D printing technology. • The results showed that secondary foam injection is the most efficient technique. • The effectiveness of foam is due to both emulsification and displacement effects. • Surfactant concentration in foams should be optimized to account for retention. Gas flooding is one of the most effective enhanced oil recovery (EOR) techniques. However, mobility control of the injected gas has always been an issue, especially for heterogeneous and fractured reservoirs. There is still a need to understand the phase interference (relative Permeability) and oil production from such heterogeneous and fractured systems. This study investigates the displacement of oil in a fractured system, conducted in a Hele-Shaw set up with a pattern of obstructions or impermeable regions. This 2D pattern is printed from an actual rock cross-section, obtained from high-resolution micro-computed tomography. Different displacement fluids were utilized, including water, gas, and foam in secondary and tertiary injection modes. Relative permeability curves were generated, which depict the interference between the phases flowing through the Hele-Shaw cell with tortuosity. The results showed that secondary foam injection is the most efficient technique as opposed to the other methods. Also, N 2 -foam achieves better recovery than air-foam, which is due to the solubility of N 2 gas in the aqueous phase. This study provides more insight into foam flooding when high surfactant concentration is used. Under this condition, the effectiveness of foam injection compared to conventional water and gas injections is due to both emulsification and foam displacement effects. [ABSTRACT FROM AUTHOR]

The effect of salinity of injected water and reservoir water in water alternating gas injection in one of an Iranian reservoir was investigated. Usually in simulation works water salinity of reservoir or salinity of injected water may be ignored. Without applying salinity effect, it would cause estimated overall oil recovery to be different with real. Salinity of water phase would increase the viscosity of injected water also obviously the mobility ratio of oil. Because of this improvement in water mobility, more oil would be displaced toward production wells. The problem of increasing salinity of injected water is that too much salt may harm pump equipments. Over time, salt deposits in pumping devices may cause reduction in pumps efficiency and corrosion of pipes, thus injecting salty water would cause corrosion problems and pump failures. Since the salinity of the reservoir water in the study was not available so it was chosen in two modes: reservoir water without salt (Cs = 0) and with concentration of 140 kg/m3(Cs = 140). In this work 0.8 reservoir pore volume WAG was injected. Salinity of injected water was varied from zero to 4,800 kg/m3; by increasing salinity of injected water, because of mobility ratio increment between water and oil, production efficiency would be increased. The economic dimension of this view must be examined. By considering reservoir water salinity, recovery would be increased in WAG injection. [ABSTRACT FROM AUTHOR]

Abstract: Permeability decline during waterflooding by varying water composition, in particular with low salinity or high pH water, has been observed in numerous laboratory studies. This has been explained by the lifting, migration and subsequent plugging of pores by fine particles. Recently, mathematical models have been presented to investigate the concept of using this permeability decline for mobility control during a waterflood. Now, these models need to be tested against observations during a core flood test. This paper presents a systematic laboratory study to investigate the underlying physics mechanisms for improved oil recovery as a consequence of injecting low-salinity water. Three sister plugs of Berea sandstone were used in the experiments. The first plug was subjected to single-phase waterflood for permeability measurements with varying salinities from 4 (high-salinity) to 0 (low-salinity) g/L NaCl. Core permeability decreased from 495 to 60md, confirming the effect of changing water composition on permeability. The second plug saturated with high-salinity water was subjected first to primary oil flood (using Soltrol) to the connate water saturation and then to a benchmark waterflood using the same water. The oil recovery was noted and the core was restored to the connate water by a secondary oil flood. Finally, low-salinity waterflood was carried out and oil recovery was recorded. Experimental observations were interpreted using a numerical model. In order to check the reproducibility of the observations, the same experimental procedure was applied on the third plug. Results confirmed the reproducibility of the observations. Significant decrease in water relative permeability by approximately 50% and some decrease in residual oil saturation by about 5% were observed during the low-salinity waterflood in comparison with the high-salinity waterflood. Treatment of the low-salinity coreflood data by a numerical model reveals the decrease in water relative permeability with increasing water saturation at high water saturations. This observation is explained by the expansion of rock surface exposed to low-salinity water during the increase of water saturation. The laboratory data matched by the numerical model shows a high surface exponent value (n A =30), which is explained by a sharp surface area rise at high water saturations. The abnormal behavior of water relative permeability in response to low-salinity waterflood has resulted from matching water permeability increase at low water saturations and decrease at high saturations. [Copyright &y& Elsevier]

Abstract: Introducing engineered nanoparticles into an aquifer or reservoir can potentially increase the storage efficiency and mitigate the risk of leakage of stored CO2. We have measured flow pattern and pressure drop during core floods in which high pressure liquid CO2 or a CO2 analogue fluid displaces an aqueous suspension of nanoparticles. The displacement front is more spatially uniform and travels more slowly compared to a control displacement with no in- situ nanoparticles. Pressure measurements are consistent with generation of a viscous phase such as an emulsion during the displacement. These observations suggest that a nanoparticle stabilized emulsion is formed during the displacement which acts to suppress the viscous instability. [Copyright &y& Elsevier]

ABSTRACT According to the conventional mobility control requirement, the displacing phase mobility (e.g. polymer) should be equal to or less than the sum of the mobilities of several displaced phases (e.g. water and oil). When the oil mobility is much lower than the water mobility, the sum of the displaced phase mobilities will be almost the same as the water mobility. Then, according to the requirement, the displacing polymer mobility is almost the same as the water mobility. As a result, the polymer solution will flow preferentially along water channels, leaving oil undisplaced. In this paper, we propose that the displacing phase mobility (polymer) should be equal to or less than the lowest mobility of displaced phases (generally, oil phase mobility) multiplied by its normalized saturation. This mobility requirement is validated by extensive simulation results at different conditions. It is also justified by field practices. Some published experimental data are analyzed to justify the proposed requirement. The proposed mobility requirement provides a criterion for mobility control design in multiphase displacement processes in porous media. © 2012 Curtin University of Technology and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Modeling the flow of emulsion in porous media is extremely challenging due to the complex nature of the associated flows and multiscale phenomena. At the pore scale, the dispersed phase size can be of the same order of magnitude of the pore length scale and therefore effective viscosity models do not apply. A physically meaningful macroscopic flow model must incorporate the transport of the dispersed phase through the porous material and the changes on flow resistance due to drop deformation as it flows through pore throats. In this work, we present a dynamic capillary network model that uses experimentally determined pore-level constitutive relationships between flow rate and pressure drop in constricted capillaries to obtain representative transient macroscopic flow behavior emerging from microscopic emulsion flow at the pore level. A parametric analysis is conducted to study the effect of dispersed phase droplet size and capillary number on the flow response to both emulsion and alternating water/emulsion flooding in porous media. The results clearly show that emulsion flooding changes the continuous-phase mobility and consequently flow paths through the porous media, and how the intensity of mobility control can be tuned by the emulsion characteristics. [ABSTRACT FROM AUTHOR]

Power saving is one of the key issues in Mobile Ad-Hoc Networks (MANETs). It can be realized in Medium Access Control (MAC) layer and network layer. However, previous attentions were mainly paid to MAC layer or network layer with the aim of improving the channel utilization by adopting variable-range transmission power control. In this paper we focus on the power saving in both MAC layer and network layer, and propose a Power Adjusting Algorithm (PAA). In the presence of host's mobility, PAA is designed to conserve energy by adjusting the transmission power to maintain the route's connectivity and restarting the route discovery periodically to find a new route with better energy efficiency dynamically. After analyzing the operations of PAA, we find that the length of route discovery restarting period is a critical argument which will affect power saving, and an energy consumption model is abstracted to find the optimal value of the restarting period by analyzing the energy consumption of this algorithm. PAA can handle the mobility of MANET by adjusting the transmission power and in the meantime save energy by restarting route discovery periodically to balance the energy consumption on route discovery and packet delivering. Simulation results show that, PAA saves nearly 40% energy compared with Dynamic Source Routing protocol when the maximum speed of mobile hosts is larger than 8 m/s. [ABSTRACT FROM AUTHOR]

Abstract: The use of the population-balance approach is extended to include the description of foam transport in fractures. We study numerically foam flow resistance as a function of gas and liquid velocities and the degree of fracture heterogeneity. The mechanisms of foam flow resistance, generation, and coalescence are similar among fractured and unfractured media but the macroscopic expression of non-linear foam physics is different. The possibility of continuously varying gas–liquid curvature in response to capillary pressure and the relationship of curvature to foam apparent viscosity, however, results in different pressure drop behavior in fractures as gas flow rate and foam quality are varied. The full physics foam model is modified to incorporate this relationship in the simulations. The foam flow resistance parameter is then calculated locally. The simulated fracture models include one- and two-dimensional heterogeneous and a radial homogeneous model. Model predictions compare favorably to experimental literature data. [Copyright &y& Elsevier]

Abstract: Carbon dioxide (CO2) flooding is a conventional process in which the CO2 is injected into the oil reservoir to increase the quantity of extracting oil. This process also controls the amount of released CO2 as a greenhouse gas in the atmosphere which is known as CO2 sequestration process. However, the mobility of the CO2 inside the hydrocarbon reservoir is higher than the crude oil and always viscous fingering and gravity override problems occur during a CO2 injection. The most common method to overcome these problems is to trap the gas bubbles in the liquid phase in the form of aqueous foam prior to CO2 injection. Although, the aqueous foams are not thermodynamically stable, special care should be considered to ensure bulk foam preparation and stability. Selection of a proper foaming agent from a large number of available surfactants is the main step in the bulk foam preparation. To meet this purpose, many chemical and crude oil based surfactants have been reported but most of them are not sustainable and have disposal problems. The objective of this experimental study is to employ Lignosulfonate and Alkyl Polyglucosides (APGs) as two sustainable foaming agents for the bulk foam stability investigations and foam flooding performance in porous media. In the initial part, the bulk foam stability results showed that APGs provided more stable foams in comparison with Lignosulfonate in all surfactant concentrations. In the second part, the results indicated that the bulk foam stability measurements provide a good indication of foam mobility in porous media. The foaming agent’s concentration which provided the maximum foam stability also gave the highest value of mobility reduction in porous media. [Copyright &y& Elsevier]

The flow properties of complex fluids through porous media give rise to multiphase flow displacement mechanisms that operate at different scales, from pore-level to Darcy scale. Experiments have shown that injection of oil-in-water emulsions can be used as an effective enhanced-oil recovery (EOR) method, leading to substantial increase in the volume of oil recovered. Pore-scale flow visualization as well as core flooding results available in the literature have demonstrated that the enhanced recovery factor is regulated by the capillary number of the flow. However, the mechanisms by which additional oil is displaced during emulsion injection are still not clear. In this work, we carried out two different experiments to evaluate the effect of emulsion flooding both at pore and macro scales. Visualization of the flow through sand packed between transparent plexiglass parallel plates shows that emulsion flooding improves the pore-level displacement efficiency, leading to lower residual oil saturation. Oil recovery results during emulsion flooding in tertiary mode (after waterflooding) in parallel sandstone cores with very different absolute permeability values prove that emulsion flooding also leads to enhancement of conformance or volumetric sweep efficiency. Combined, the results presented here show that injection of emulsion offers multiscale mechanisms resulting from capillary-driven mobility control. [ABSTRACT FROM AUTHOR]

This work concerns the experimental investigation of surfactant alternating CO2 injection in carbonate rocks. The core samples provided from a low-temperature fractured light oil reservoir, located in southwest Iran. The experiments were designed to observe the effect of CO2–foam injection on gas mobility and oil recovery at different surfactant concentrations. The core samples were initially saturated with synthetic/field brine, 5,000 ppm, and then flooded with live oil to reach connate water saturation at reservoir condition, 115°F and 1,700 psia. The commercial surfactant used was sodium lauryl sulfate as an anionic surfactant. The results of this work, along with field-scale simulation and/or economic considerations, could be helpful in making reliable decisions about optimum condition of foam-assisted water-alternating-gas (FAWAG) processes. Core flooding results demonstrated that macroscopic sweep efficiency increased due to foam generation inside the core. In addition, it led to an increase of between 5 and 12% in the recovery factor in comparison to a water-alternating-gas (WAG) process. Furthermore, the value of the critical micellar concentration for the surfactant–oil system was about 2,000 ppm. After primary and secondary recovery processes, there is a large amount of trapped oil in the reservoir. Extensive research has been directed toward enhancing the recovery of this oil, but limited success has been achieved. FAWAG injection appears to have more applicability to recover the trapped resources. However, little attention has been paid to experimental investigation of FAWAG processes in low-temperature oil reservoirs. [ABSTRACT FROM AUTHOR]

Abstract: Permeability decline during corefloods with varying water composition, especially with low salinity water, has been widely reported in the literature. It has often been explained by the lifting, migration and subsequent plugging of pores by fine particles, which has been observed in numerous core flood tests with altered water composition (salinity, pH) and temperature. This effect can be considered to provide a relatively simple method for mobility control during waterflooding. In previous research, the Dietz model for waterflooding in a layer-cake reservoir with a constant injection and production rate was combined with a particle detachment model to investigate the effect of fines migration and induced permeability decline on reservoir sweep efficiency. In this work, the analytical model was extended to waterflooding with a given pressure drop between injection and production wells. The modelling showed that permeability decline in the water swept zone, caused by the alteration of the injected water composition and induced fines migration, may be able to improve waterflood performance by delaying water breakthrough and reducing the water cut. [Copyright &y& Elsevier]

Microemulsions have recently made advances in enhanced oil recovery processes in which chemicals, especially surfactants, are used to recover the oil from natural oil reservoirs. This technique relies on the knowledge of interfacial properties among oil, water, and solid rock reservoirs in the occasional presence of natural gas under extreme conditions. Surfactant-based chemical systems have been reported in many academic studies and their technological implementation is a potential candidate in enhanced oil recovery (EOR) activities. For instance, it was determined that a mobilized buffer (polymer) with viscosity either equal to or greater than the mobilized oil enhanced the recovery efficiency considerably. However, EOR based on chemicals like alkaline-surfactant-polymer (ASP) is a complex technology requiring a high level of expertise for its industrial implementation. The surfactant-polymer interaction is a rapidly growing research area for efficient oil recovery by improving slug integrity, adsorption, and mobility control. This review article evaluates the injecting fluid system to highlight some recent advances in the use of chemicals in EOR, especially with microemulsions. It further reveals the current status and future outlook for EOR technology in oil fields and describes the opportunities for strategic utilities and load growth in petroleum industry. [ABSTRACT FROM AUTHOR]

Several researchers have proposed that mobility control mechanisms can positively contribute to oil recovery in the case of emulsions generated in Enhanced-Oil Recovery (EOR) operations. Chemical EOR techniques that use alkaline components or/and surfactants are known to produce undesirable emulsions that create operational problems and are difficult to break. Other water-based methods have been less studied in this sense. EOR processes such as polymer flooding and LoSal™ injection require adjustments of water chemistry, mainly by lowering the ionic strength of the solution or by decreasing hardness. The decreased ionic strength of EOR solutions can give rise to more stable water-in-oil emulsions, which are speculated to improve mobility ratio between the injectant and the displaced oil. The first step toward understanding the connection between the emulsions and EOR mechanisms is to show that EOR conditions, such as salinity and hardness requirements, among others, are conducive to stabilizing emulsions. In order to do this, adequate stability proxies are required. This paper reviews commonly used emulsion stability proxies and explains the advantages and disadvantage of methods reviewed. This paper also reviews aqueous-based EOR processes with focus on heavy oil to contextualize in-situ emulsion stabilization conditions. This context sets the basis for comparison of emulsion stability proxies. [ABSTRACT FROM AUTHOR]

Abstract: We evaluated the rheological characteristics of ι- and κ-Carrageenan in aqueous solutions. Viscosities strongly increased with increasing polymer concentration or salinity. Monovalent Na+ cations were more effective in increasing viscosity than divalent Ca2+ cations. A more complex brine containing Na+, Mg2+ and Ca2+ cations also showed high viscosities at a high salinity. We observed shear thinning behavior for these polysaccharide solutions. We explain these rheological phenomena with molecular processes, specifically the conformation change of the ι-Carrageenan from random coil to double helix. In the context of enhanced oil recovery, ι-Carrageenan solutions can reach much higher viscosities at high salinities than standard polyacrylamide solutions. Moreover, Carrageenans are renewable, nontoxic, green substances. [ABSTRACT FROM AUTHOR]

We study the problem of a mobile target (the mouse) trying to evade detection by one or more mobile sensors (we call such a sensor a cat) in a closed network area. We view our problem as a game between two players: the mouse, and the collection of cats forming a single (meta-)player. The game ends when the mouse falls within the sensing range of one or more cats. A cat tries to determine its optimal strategy to minimize the worst case expected detection time of the mouse. The mouse tries to determine an optimal counter movement strategy to maximize the expected detection time. We divide the problem into two cases based on the relative sensing capabilities of the cats and the mouse. When the mouse has a sensing range smaller than or equal to the cats', we develop a dynamic programming solution for the mouse's optimal strategy, assuming high level information about the cats' movement model. We discuss how the cats' chosen movement model will affect its presence matrix in the network, and hence its payoff in the game. When the mouse has a larger sensing range than the cats, we show how the mouse can determine its optimal movement strategy based on local observations of the cats' movements. We further present a coordination protocol for the cats to collaboratively catch the mouse by: 1) forming opportunistically a cohort to limit the mouse's degree of freedom in escaping detection; and 2) minimizing the overlap in the spatial coverage of the cohort's members. Extensive experimental results verify and illustrate the analytical results, and evaluate the game's payoffs as a function of several important system parameters. [ABSTRACT FROM AUTHOR]

Recent work in wireless sensor networks, or simply called WSNs, has drawn attention to the mobility capability of each node. In Stojmenovic and Lin (IEEE Trans Parallel Distrib Syst 12: 1023–1032, ), it is proved that the optimal positions of the relay nodes along a single active flow must lie entirely on the line between the source and destination with each node spaced evenly along such a line. Based on this, we propose two practical solutions to control the relay nodes in WSNs to approach their optimal positions in the local relative coordinate system. One uses one-hop neighbor information and the other one uses two-hop neighbor information. Basically, each relay node will approach the midpoint on the line composed of neighbors. For the latter control scheme, we also discuss its different implementation with outdated two-hop neighbor information (lagged by one-round neighbor information exchange and update). This is an improvement since given nodes only reuse the two-hop neighbor information previously saved at its one-hop neighbors and does not require any extra neighbor information collection. All the new methods prevent oscillations by demanding minimal moving distance per round ( MDPR), otherwise the node does not move. Unlike the one presented in Goldenberg et al. (Proceedings of the 5th ACM International Symposium on Mobile Ad Hoc Networking and Computing (Mobihoc’04), pp 163–174 ) using only one-hop neighbor information, our methods will converge more quickly. The experimental results show a substantial improvement on the speed of achieving the optimal configuration and the total moving distance of nodes. [ABSTRACT FROM AUTHOR]

Abstract: We introduce a capability-based access control model integrated into a linguistic formalism for modeling network aware systems and applications. Our access control model enables specification and dynamic modification of policies for controlling process activities (mobility of code and access to resources). We exploit a combination of static and dynamic checking and of in-lined reference monitoring to guarantee absence of run-time errors due to lack of capabilities. We illustrate the usefulness of our framework by using it for implementing a simplified but realistic scenario. Finally, we show how the model can be easily tailored for dealing with different forms of capability acquisition and loss, thus enabling different possible variations of access control policies. [Copyright &y& Elsevier]

Abstract: The flow of oil-in-water emulsions through quartz micro-capillary tubes was analyzed experimentally. The capillaries were used as models of connecting pore-throats between adjacent pore body pairs in high-permeability media. Pressure drop between the inlet and outlet ends of the capillary was recorded as a function of time, for several values of the volumetric flow rate. Several distinct emulsions were prepared using synthetic oils in deionized water, stabilized by a surfactant (Triton X-100). Two oils of different viscosity values were used to prepare the emulsions, while two distinct drop size distributions were obtained by varying the mixing procedure. The average oil drop size varied from smaller to larger than the neck radius. The results are presented in terms of the extra-pressure drop due to the presence of the dispersed phase, i.e. the difference between the measured pressure drop and the one necessary to drive the continuous phase alone at the same flow rate. For emulsions with drops smaller than the capillary throat diameter, the extra-pressure drop does not vary with capillary number and it is a function of the viscosity ratio, dispersed phase concentration and drop size distribution. For emulsions with drops larger than the constriction, the large oil drops may partially block the capillary, leading to a high extra pressure difference at low capillary numbers. Changes in the local fluid mobility by means of pore-throat blockage may help to explain the additional oil recovery observed in laboratory experiments and the sparse data on field trials. [Copyright &y& Elsevier]

Compared with partially hydrolyzed polyacylamide (HPAM) and glycerinum, the unique characteristics of associative polymers (AP-H1 and AP-H2) to generate the flow resistance, control the adverse mobility, especially regulate the macro-/micro- heterogeniety were systematically studied through different laboratory measurements in one-dimensional sand-packed models and two-dimensional visual models. And the feasibility to greatly enhance oil recovery by associative polymer combination was verified based on the special characteristics. The results show that, AP-H1 and AP-H2 not only could keep a relatively good injectivity and penetration behavior at a lower permeability of 314 × 10−3 μm2, but also could establish the resistance factors that were orders of magnitude higher than HPAM (2000 mg/L) at similar even lower apparent viscosity or lower concentration. At similar viscosity, the oil (2122 mPa·s) displacement by AP-H1 (1200 mg/L) could achieve a better mobility control effect and obtain greater oil recovery than that of HPAM to displace the oil of 303.6 mPa·s. Under different macro-/micro- planar and longitudinal heterogeneous conditions of a permeability contrast of 2–10 times, when displacing oil of different viscosity (15–2122 mPa·s), AP-H1 could achieve significant mobilization and greater sweep to low permeability zones and obtain a greater EOR value, even could lead to the fluid diversion of the post-water injection to a certain extent. In addition, AP-H1 could further obviously activate the most of the cluster-like residual oil in both outer large and inner small channels over the glycerol and HPAM, namely present a good ability to regulate the micro-heterogeneity. More importantly, at the permeability contrast of 10 times, the slug combination of associative polymers (AP-H2 + AP-H1) not only could ensure greater sweep to the high-permeability layer, but also could achieve the maximum impact on the low-permeability layer, which obtained significantly higher oil recovery than single polymer slug or "associative polymer + HPAM" combination slug. Finally, it can be indicated that associative polymers have great feasibility to significantly enhance oil recovery in heterogeneous reservoirs based on the reasonable design of molecular structure and slug combination of associative polymers. The findings of this study can help for better understanding of the mechanism of mobility control and heterogeneity regulation of associative polymer, as well as the oil displacement effect by slug combination of different associative polymers, and also can provide a new design idea for the efficient development of heterogeneous reservoirs. Based on the reasonable design of molecular structure and slug combination of associative polymers, associative polymers not only can significantly enlarge macroscopic weep through blocking high-permeability zone, but also can noticeably enhance microscopic sweep to the low-permeability zone. Therefore, associative polymers have great feasibility to significantly enhance oil recovery in heterogenous reservoirs. Specifically, compared with partially hydrolyzed polyacylamide (HPAM) and glycerinum, the unique characteristics of associative polymers (AP-H1 and AP-H2) to generate the flow resistance, control the adverse mobility, especially regulate the macro-/micro- heterogeniety were systematically studied through different laboratory measurements in one-dimensional sand-packed models and two-dimensional visual models. And the feasibility to greatly enhance oil recovery by associative polymer combination was verified based on the special characteristics. The results show that, AP-H1 and AP-H2 not only could keep a relatively good injectivity and penetration behavior at a lower permeability of 314 × 10−3 μm2, but also could establish the resistance factors that were orders of magnitude higher than HPAM (2000 mg/L) at similar even lower apparent viscosity or lower concentration. At similar viscosity, the oil (2122 mPa s) displacement by AP-H1 (1200 mg/L) could achieve a better mobility control effect and obtain greater oil recovery than that of HPAM to displace the oil of 303.6 mPa·s. Under different macro-/micro- planar and longitudinal heterogeneous conditions of a permeability contrast of 2–10 times, when displacing oil of different viscosity (15–2122 mPa·s), AP-H1 could achieve significant mobilization and greater sweep to low permeability zones and obtain a greater EOR value, even could lead to the fluid diversion of the post-water injection to a certain extent. In addition, AP-H1 could further obviously activate the most of the cluster-like residual oil in both outer large and inner small channels over the glycerol and HPAM, namely present a good ability to regulate the micro-heterogeneity. More importantly, at the permeability contrast of 10 times, the slug combination of associative polymers (AP-H2 + AP-H1) not only could ensure greater sweep to the high-permeability layer, but also could achieve the maximum impact on the low-permeability layer, which obtained significantly higher oil recovery than single polymer slug or "associative polymer + HPAM" combination slug. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper presents an algorithm for positioning circles in a given region to maximise the covered area. Our algorithm has applications in wireless networks, such as positioning a given number of mobile stations in a given region, one goal of which is to cover the largest area possible. Although the evaluation of the function value, i.e., the total covered area, is difficult, we bypass this difficulty by calculating the gradient of the total covered area directly. As long as nodes continuously move in directions that guarantee increasing coverage, a configuration of node positions corresponding to a maximal covered area can eventually be identified. [ABSTRACT FROM AUTHOR]