Comparison of the Maximal Spatial Throughput of Aloha and CSMA in Wireless Ad-Hoc Networks

International audienceIn this paper we compare the spatial throughput of Aloha and Carrier Sense Multiple Access (CSMA) in Wireless multihop Ad-Hoc Networks. In other words we evaluate the gain offered by carrier sensing (CSMA) over the pure statiscal collision avoidance which is the basis of Aloha. We use a Signal-to-Interference-and-Noise Ratio (SINR) model where a transmission is assumed to be successful when the SINR is larger than a given threshold. Regarding channel conditions, we consider both standard Rayleigh and negligible fading. For slotted and non-slotted Aloha, we use analytical models as well as simulations to study the density of successful transmissions in the network. As it is very difficult to build precise models for CSMA, we use only simulations to compute the performances of this protocol. We compare the two Aloha versions and CSMA on a fair basis, i.e. when they are optimized to maximize the density of successful transmissions. For slotted Aloha, the key optimization parameter is the medium access probability, for non-slotted Aloha we tune the mean back-off time, whereas for CSMA it is the carrier sense threshold that is adjusted. Our study shows that CSMA always outperforms slotted Aloha, which in turn outperforms its non-slotted version

Modeling the flow of non-Newtonian fluids in packed beds at the pore scale

Flow and transport in porous media are important in many science and engineering applications such as composite materials, subsurface water contamination, packed-bed reactors, and enhanced oil recovery. The general approach to modeling such processes is at the continuum scale. Semi-empirical expressions, such as Darcy\u27s law, are substituted for velocity in the continuity equation, which is then coupled with a momentum, mass, and energy balance. While a continuum approach is acceptable in some cases, additional modeling is required for certain non-linear flows, such as multi-phase flows, inertial flows, non-Newtonian flows, and reactive flows. Pore-scale modeling is a first-principles approach to modeling flow and transport in porous media. In this work, network models that are physically representative of specific unconsolidated media are created. The networks can be used to model a wide range of flows, but the focus here is on polymers and suspensions that exhibit non-Newtonian behavior. The network models are used to model steady flow as well as displacement by less viscous fluids. The transient displacement is used to investigate important viscous fingering patterns. While simple boundary conditions are typically imposed in network modeling (e.g. a pressure gradient in one dimension), a more general approach has been developed where boundary conditions are also imposed by direct coupling to an adjacent continuum region. Important qualitative and quantitative results are obtained from the network model for non-Newtonian fluids. Preferential flow pathways form in the network due to the inherent heterogeneity and interconnectivity in porous media. Quantitative results of Darcy velocity versus applied pressure gradient show different behavior than semi-empirical models (analogous to Darcy\u27s law) for non-Newtonian fluids. The transient displacement patterns for non-Newtonian fluids are also different than for Newtonian fluids. If the fluid exhibits a yield stress, a steady state is reached in which some of the original non-Newtonian fluid is left trapped in the network. The displacement patterns are affected by the boundary conditions, which can be determined from direct coupling to a continuum region

Drilling Fluid Additives for Wellbore Strengthening and Reservoir Protection

The objective of the research was to optimise drilling fluid additives for wellbore strengthening, preventing lost circulation and avoiding drilling fluid induced formation damage. Industry standard testing, such as HTHP fluid loss tests following API 13B, yield limited insight into important areas such as wellbore strengthening and formation damage. Therefore, new testing methodologies were developed and evaluated. These provided new insight into important areas for designing and evaluating drilling fluids and drilling fluid additives for wellbore strengthening and reservoir protection. Key conclusions were that exposing particles to mechanical wear significantly impacted the relative performance of materials used for preventative treatments. Oil-based fluids were found to create a high-degree of internal formation plugging, whereas water-based fluids more predominantly isolated the wellbore pressure from the pore-pressure though an external filter-cake. Inclusion of cellulose based fibres where the D90 value value ⪞ 3/2 the median pore size was shown to reduce internal plugging and reduce formation damage, in both water-based fluids and oil-based fluids. Particle degradation studies showed that CaCO3 degraded rapidly for particles > 23 μm and that the most wear resistant particles were selected cellulose-based materials. Combinations of fine CaCO3 and slightly coarser cellulose mixtures were found to be effective for creating low-permeability filter-cakes and preventing formation damage. For preventative treatment in drilling conditions with large differences between the matrix pore-size and the aperture of natural or induced fractures, a dual mode particle size distribution was found to be effective in both laboratory studies and field applications. In such situations, the fine mode of the PSD provided low filter-cake permeabilities when the particles followed an Andreasen distribution with a packing factor of around 0.08-0.10. Natural and induced fractures were most effectively sealed when granular cellulose particles made up the coarse mode of the PSD and these particles were sized similar to or slightly larger than the fracture aperture

Sand production: A smart control framework for risk mitigation

Due to the current global oil price, the sand production is considered undesirable product and the control of sand production is considered as one of the main concerns of production engineers. It can damage downhole, subsea equipments and surface production facilities, also increasing the risk of catastrophic failure. As a result of that it costs the producers multiple millions of dollars each year. Therefore, there are many different approaches of sand control designed for different reservoir conditions. Selecting an appropriate technique for preventing formation sand production depends on different reservoir parameters. Therefore, choosing the best sand control method is the result of systematic study. In this paper the sand production factors and their effects are presented where the emphasis is given towards the sand prediction to determine the probability of producing sand from the reservoir, followed by the correct prevention implementation of sand control method. The combination of these two is presented as a smart control framework that can be applied for sand production management

Analysis of Dynamic Channel Bonding in Dense Networks of WLANs

Dynamic Channel Bonding (DCB) allows for the dynamic selection and use of multiple contiguous basic channels in Wireless Local Area Networks (WLANs). A WLAN operating under DCB can enjoy a larger bandwidth, when available, and therefore achieve a higher throughput. However, the use of larger bandwidths also increases the contention with adjacent WLANs, which can result in longer delays in accessing the channel and consequently, a lower throughput. In this paper, a scenario consisting of multiple WLANs using DCB and operating within carrier-sensing range of one another is considered. An analytical framework for evaluating the performance of such networks is presented. The analysis is carried out using a Markov chain model that characterizes the interactions between adjacent WLANs with overlapping channels. An algorithm is proposed for systematically constructing the Markov chain corresponding to any given scenario. The analytical model is then used to highlight and explain the key properties that differentiate DCB networks of WLANs from those operating on a single shared channel. Furthermore, the analysis is applied to networks of IEEE 802.11ac WLANs operating under DCB-which do not fully comply with some of the simplifying assumptions in our analysis-to show that the analytical model can give accurate results in more realistic scenarios

Application of new types of screens to solve the problem of sand production in oil fields

During the drilling, completion and production of the wells sand production is one of the common operational problems encountered throughout producing petroleum from unconsolidated reservoirs. It has negative effect on the economy and production and in severe cases, it may lead to abandonment of the well. Furthermore, large amount of sand production can lead to a variety of complications, technical problems, such as erosion of casing. Sand screen is a mechanical means of controlling sand production and has been considered as one of the effective and widely applied sand control method in petroleum industry over the years. Nowadays, most of the treatments are sand screen which is reliable, cost effective, long term and common sand controlling technique. In order to achieve an effective gravel packed completion sand screens are the most important part of completion design. All the above mentioned is discussed in the first part of the research report. Additionally, as background information, sand production and causes, confirmation and monitoring of sand production and sand control methods are presented in the first part as well. The second part of the report emphasizes main points while applying the sand screen methods to the sand producing reservoirs. In Azerbaijan, a number of wells with producing sand are completed with putting sand screens. The operation and other factors are researched through the real field example – West Absheron Field

Delay Performance and Mixing Times in Random-Access Networks

We explore the achievable delay performance in wireless random-access networks. While relatively simple and inherently distributed in nature, suitably designed queue-based random-access schemes provide the striking capability to match the optimal throughput performance of centralized scheduling mechanisms in a wide range of scenarios. The specific type of activation rules for which throughput optimality has been established, may however yield excessive queues and delays. Motivated by that issue, we examine whether the poor delay performance is inherent to the basic operation of these schemes, or caused by the specific kind of activation rules. We derive delay lower bounds for queue-based activation rules, which offer fundamental insight in the cause of the excessive delays. For fixed activation rates we obtain lower bounds indicating that delays and mixing times can grow dramatically with the load in certain topologies as well