First-principles molecular-dynamics simulations of a hydrous silica melt: Structural properties and hydrogen diffusion mechanism

We use {\it ab initio} molecular dynamics simulations to study a sample of liquid silica containing 3.84 wt.% H$_2$O.We find that, for temperatures of 3000 K and 3500 K,water is almost exclusively dissolved as hydroxyl groups, the silica network is partially broken and static and dynamical properties of the silica network change considerably upon the addition of water.Water molecules or free O-H groups occur only at the highest temperature but are not stable and disintegrate rapidly.Structural properties of this system are compared to those of pure silica and sodium tetrasilicate melts at equivalent temperatures. These comparisons confirm the picture of a partially broken tetrahedral network in the hydrous liquid and suggest that the structure of the matrix is as much changed by the addition of water than it is by the addition of the same amount (in mole %) of sodium oxide. On larger length scales, correlations are qualitatively similar but seem to be more pronounced in the hydrous silica liquid. Finally, we study the diffusion mechanisms of the hydrogen atoms in the melt. It turns out that HOSi$_2$ triclusters and SiO dangling bonds play a decisive role as intermediate states for the hydrogen diffusion.Comment: 25 pages, 18 figures. submitte

Electronic redistribution around oxygen atoms in silicate melts by ab initio molecular dynamics simulation

The structure around oxygen atoms of four silicate liquids (silica, rhyolite, a model basalt and enstatite) is evaluated by ab initio molecular dynamics simulation. Thanks to the use of maximally localized Wannier orbitals to represent the electronic ground state of the simulated system, one is able to quantify the redistribution of electronic density around oxygen atoms as a function of the cationic environment and melt composition. It is shown that the structure of the melt in the immediate vicinity of the oxygen atoms modulates the distribution of the Wannier orbitals associated with oxygen atoms. In particular the evaluation of the distances between the oxygen-core and the orbital Wannier centers and their evolution with the nature of the cation indicates that the Al-O bond in silicate melts is certainly less covalent than the Si-O bond while for the series Mg-O, Ca-O, Na-O and K-O the covalent character of the M-O bond diminishes rapidly to the benefit of the ionic character. Furthermore it is found that the distribution of the oxygen dipole moment coming from the electronic polarization is only weakly dependent on the melt composition, a finding which could explain why some empirical force fields can exhibit a high degree of transferability with melt composition.Comment: 27 pages, 7 figures. To be published in Journal of Non-Crystalline Solid

A coupled model for the prediction of fluid and reservoir properties from minifrac tests.

A computer program was developed for the calculation of all parameters mentioned above and also to automatically curve-fit the main output and diagnostic plots of the model.New equations for the analysis of fracturing fluid spurt loss and subsequent calculation of fracturing fluid and reservoir properties based on spurt loss, if identified, are introduced.The present study presents a coupled model for the determination of fluid and reservoir properties, such as fracturing fluid efficiency, leak-off coefficient, fracture half-length, fracture width, spurt-loss coefficient, reservoir permeability and filter-cake resistance. In essence this model can be used for the analysis and interpretation of the fracturing fluid leak-off characteristics as well as the filter-cake reservoir flow based on pressure decline data from a minifrac test.Many models, describing the mathematics and application of a mini-fracture have been developed. However, a significant number of assumptions limit their applicability, and consequently often times they can not be used successfully for the delineation of fluid and reservoir properties necessary for the calibration of the actual fracturing operation.In addition, a new pressure derivative equation was derived and incorporated into the model. This is used as an alternate tool for the case in which classic minifrac analysis techniques fails to produce reasonable means of interpretation, which if used would result in unreliable fluid and reservoir properties sought from such a test.Also, sensitivity analysis is performed, to enhance its applicability by indicating which parameters are playing a major role in the interpretation of a calibration treatment.A step by step procedure is included and application of the model is demonstrated on real field data obtained from minifrac tests performed on several oil wells. The fluid and reservoir properties determined with this model, are verified by comparison with results obtained from post-fracture tests (i.e. build up, history matching).One of the most significant contributors to the oil and gas industry as a primary means of well production increase, is the hydraulic fracturing treatment. In essence, more then a million of such treatment operations have been conducted on over 44% of the drilled wells. Nowadays, the costs of these operations, applied for the stimulation of oil and gas wells, have significantly increased, along with the increase of treatment size, pump rates and pressures, varying from fifteen thousands to more then one million dollars per operation. Consequently, a correct calibration of such operations is very important, and to ensure this, mini-fracture treatments are used

Theoretical and experimental possibilities to set up some sensors systems involved in active safety process

Abstract The safety systems present in the construction of modern motor vehicles fully benefit from all development research efforts in the field of autonomous motor vehicles. In this paper, we have analyzed, theoretically and experimentally, for three distinct types of passenger cars, the following active safety systems: 1. Autonomous Emergency Braking system, designed as an automatic braking process of the vehicle, in response to the detection of a probable collision, to reduce the speed of the vehicle; 2. Forward Collision Warning, implemented as an audiovisual warning that is present on board a motor vehicle, to alert the driver; 3. Dynamic Brake support, which further amplifies the braking force to detect the imminent collision to achieve a higher deceleration than in the situation of a normal speed reduction. As a following were study three states such as: Car-to-Car Rear Stationary as a possible collision in which a vehicle travels forwards towards another stationary vehicle and the front structure of the vehicle strikes the rear structure of the other, Car-to-Car Rear Moving as a probable collision in which a vehicle travels forwards towards another vehicle that is travelling at constant speed and the front structure of the vehicle strikes the rear structure of the other and possible collisions named Car-to-Car Rear Braking in which a vehicle travels forwards towards another vehicle that is travelling at constant speed and then decelerates, and the frontal structure of the vehicle strikes the rear structure of the other. The three situations have been examined, experimentally and theoretic, in order to obtain important information about the configurations of sensor systems that compose safety systems. The work allowed to obtain configuration information, experimentally confirmed, with an impact on the improvement of future autonomous vehicles system.</jats:p

Visitor Management, a Tool for Sustainable Tourism Development in Protected Areas

The article analyzes visitor management techniques as a way to develop sustainable tourism in protected areas. Visitor management is an important tool in recreational and protected areas, as increasing use levels can negatively impact the quality of recreational experience as well as natural resources. To meet the requirements of both nature and visitors, a prudent and careful management is necessary. In order to manage protected areas within acceptable ecological and social carrying capacities, tourism planners need to monitor visitor numbers, leisure activities and behaviour and understand expectations and motivations