Leakage Mitigation Research Articles

NBI Spectral Leakage Mitigation Based on Compressed Sensing in OFDM Systems

The performance of an orthogonal frequency division multiplexing system is degraded in the presence of narrowband interference (NBI). This degradation arises from spectral leakage of the interference power during demodulation, corrupting many subcarriers. There have been many attempts to mitigate the NBI, but most schemes need some prior information about the NBI or resources in frequency or time domain. Recently, compressive sensing (CS) theory has been introduced, and some researches show that CS theory can be applied to estimate the NBI effectively. However, they still need resources such as frequency domain known elements or time domain interference free region. Also, CS recovery algorithms require high computational complexity. In this paper, we propose an widely usable NBI estimation and mitigation scheme using CS theory without any requirement. We verified that the proposed scheme can sufficiently mitigate spectral leakage of the NBI. Then, we substantially reduce its computational complexity by introducing a neighborhood searching method by dominant samples based on sampling theory.

Computational Study of Surface Signature of Anthropogenic CO2 at a Potential Carbon Sequestration Site, San Juan Basin

Abstract Injection of anthropogenic carbon dioxide into geologic formations is a technology that can be deployed in the relatively short term in order to avoid potential harm to the environment caused by excess CO2 in the atmosphere. Success of sequestering CO2 in underground reservoirs is strongly dependent on the prevention of leakage back into the atmosphere and the ability to mitigate should significant leakage occur. Both detection of leakage and reliable risk mitigation plans require a robust monitoring system. The space and time span of CO2 sequestration projects is large, which results in trade-offs between cost and robustness of monitoring. In order to make a cost-effective decision without compromising monitoring effectiveness, knowledge of CO2 transport in the vadose zone and seepage mechanisms into the atmosphere is essential. This study focuses on the simulations of hypothetical CO2 leakage into the ∼100 m thick vadose zone at an actual site in the San Juan Basin, the United States. Hypotheti...

Study of Mineral Trapping of CO2 and Seal Leakage Mitigation

Abstract Mineral trapping of carbon dioxide is the safest and the most permanent sequestration mechanism. Brine composition, brine pH, system temperature and pressure have been reported to have significant effects on mineral trapping of CO2 in brines. It has also been reported that iron cation in brine could cause pH instability that may lead to lower carbonation conversion rate. This study aims to investigate whether ferric or ferrous iron caused this pH instability. pH stability studies and high temperature/high pressure carbonation experiments (150 °C,100 bar) with the addition of 1.0 M KOH were conducted using different synthetic brines with Fe3+, Fe2+, and without Fe. It can be concluded from this study that ferrous iron causes pH instability. However, ferrous iron might promote carbonate precipitation in non-pure CO2 streams in iron oxyhydroxide-containing saline aquifers as seal leakage mitigation.

Wellbore leakage mitigation using engineered biomineralization

Abstract Research on microbially induced calcite precipitation (MICP) is reported. MICP may serve to reduce near-wellbore permeability, reduce CO 2 – related corrosion, and lower the risk of unwanted migration of CO 2 or other fluids. MICP research on the lab scale has demonstrated the ability to seal sandstone cores using injection strategies engineered to control precipitation. Experimentation was also aimed at transitioning MICP strategies for field implementation. MICP was evaluated in the field in a hydraulically fractured sandstone formation at a Walker County, Alabama well. The field experiment resulted in greatly reduced injectivity indicating that the fractured formation was plugged after MICP treatment.

Tip Clearance Investigation of a Ducted Fan Used in VTOL Unmanned Aerial Vehicles—Part II: Novel Treatments Via Computational Design and Their Experimental Verification

Ducted fan based vertical lift systems are excellent candidates to be in the group of the next generation vertical lift vehicles, with many potential applications in general aviation and military missions. Although ducted fans provide high performance in many “vertical take-off and landing” (VTOL) applications, there are still unresolved problems associated with these systems. Fan rotor tip leakage flow adversely affects the general aerodynamic performance of ducted fan VTOL unmanned aerial vehicles (UAVs). The current study utilized a three-dimensional Reynolds-averaged Navier–Stokes (RANS) based computational fluid dynamics (CFD) model of ducted fan for the development and design analysis of novel tip treatments. Various tip leakage mitigation schemes were introduced by varying the chordwise location and the width of the extension in the circumferential direction. Reduced tip clearance related flow interactions were essential in improving the energy efficiency and range of ducted fan based vehicles. Full and inclined pressure side tip squealers were also designed. Squealer tips were effective in changing the overall trajectory of the tip vortex to a higher path in radial direction. The interaction of rotor blades and tip vortex was effectively reduced and the aerodynamic performance of the rotor blades was improved. The overall aerodynamic gain was a measurable reduction in leakage mass flow rate. This leakage reduction increased thrust significantly. Experimental measurements indicated that full and inclined pressure side tip squealers increased thrust obtained in hover condition by 9.1% and 9.6%, respectively. A reduction or elimination of the momentum deficit in tip vortices is essential to reduce the adverse performance effects originating from the unsteady and highly turbulent tip leakage flows rotating against a stationary casing. The novel tip treatments developed throughout this study are highly effective in reducing the adverse performance effects of ducted fan systems developed for VTOL UAVs.

Hydro-dynamically controlled alteration of fractured Portland cements flowed by CO2-rich brine

Deficient sealing in the well cement plugs and annulus due to mechanical fracturing is an important risk of CO2 leakage from the reservoir to others permeable layers and the surface. Such situation was reproduced at laboratory scale in order to determine the hydro-chemical control on the fracture permeability. Specifically, we investigated the effect of CO2 rich-brine flowing through fractured Portland cements at T=60°C and P=10MPa and variable flow rates. We showed that carbonation process is dominant and induces permeability decrease and leakage mitigation for extremely low flow rate whereas for high flow rate injection the permeability remains constant due to the precipitation of a low density secondary Si-rich phase which maintains the initial fracture aperture. At intermediate flow rate the hydraulic aperture can increase due to the densification of the material triggered by the net precipitation of low porosity calcite. These results emphasize that more complex behaviors than those considered from batch experiments may take place in the vicinity of flowing fractures. Specifically, the redissolution of the neoformed calcite as well as the development of amorphous phases, both controlled by the CO2-rich brine renewing rate in the fracture may prevent the healing fracture.

Abandoned well CO2 leakage mitigation using biologically induced mineralization: current progress and future directions

AbstractMethods of mitigating leakage or re‐plugging abandoned wells before exposure to CO2are of high potential interest to prevent leakage of CO2 injected for geologic carbon sequestration in depleted oil and gas reservoirs where large numbers of abandoned wells are often present. While CO2resistant cements and ultrafine cements are being developed, technologies that can be delivered via low viscosity fluids could have significant advantages including the ability to plug small aperture leaks such as fractures or delamination interfaces. Additionally there is the potential to plug rock formation pore space around the wellbore in particularly problematic situations. We are carrying out research on the use of microbial biofilms capable of inducing the precipitation of crystalline calcium carbonate using the process of ureolysis. This method has the potential to reduce well bore permeability, coat cement to reduce CO2–related corrosion, and lower the risk of unwanted upward CO2 migration. In this spotlight, we highlight research currently underway at the Center for Biofilm Engineering (CBE) at Montana State University (MSU) in the area of ureolytic biomineralization sealing for reducing CO2 leakage risk. This research program combines two novel core testing systems and a 3‐dimensional simulation model to investigate biomineralization under both radial and axial flow conditions and at temperatures and pressures which permit CO2 to exist in the supercritical state.This combination of modelling and experimentation is ultimately aimed at developing and verifying biomineralization sealing technologies and strategies which can successfully be applied at the field scale for carbon capture and geological storage (CCGS) projects. © 2013 Society of Chemical Industry and John Wiley & Sons, Ltd

Numerical Investigation of Microbially Induced Calcite Precipitation as a Leakage Mitigation Technology

In this work we investigate the performance of a numerical model for microbially induced calcite precipitation predicting the reduction of permeability over time due to biomineralization under reservoir pressure in sandstone cores. Although the model was previously validated with experiments in sand columns under atmospheric pressure, its calculations are consistent with experimental data towards the final half of the experiment, if the implemented porosity permeability relation is fitted to the final experimental porosity and permeability. The initial reduction of permeability is underestimated by the model, indicating an inconsistency between model and experiment. Calculated and measured porosity are nearly identical.

Axial Flow Fan Tip Leakage Flow Control Using Tip Platform Extensions

Performance of an axial flow fan unit is closely related to its tip leakage mass flow rate and level of tip/casing interactions. The present experimental study uses a stereoscopic particle image velocimeter to quantify the three dimensional mean flow observed near the blade tip, just downstream of a ducted fan unit. After a comprehensive description of the exit flow from the baseline fan, a number of novel tip treatments based on custom designed pressure side extensions are introduced. Various tip leakage mitigation schemes are introduced by varying the chordwise location and the width of the extension in the circumferential direction. The current study shows a proper selection of the pressure side bump location and width are the two critical parameters influencing the success of each tip leakage mitigation approach. Significant gains in the axial mean velocity component are observed when a proper pressure side tip extension is used. It is also observed that a proper tip leakage mitigation scheme significantly reduces the tangential velocity component near the tip of the axial fan blade. Reduced tip clearance related flow interactions are essential in improving the energy efficiency of ducted fan systems. A reduction or elimination of the momentum deficit in tip vortices is also essential to reduce the adverse performance effects originating from the unsteady and highly turbulent tip leakage flows rotating against a stationary casing.