Leak Paths Research Articles

Comparative study of tube to tubesheet welding qualification on heat exchanger

Plant turnarounds are one of the most critical events in the Saudi Arabian oil and gas industries. Shutdowns have the potential to affect the plant’s cash flow and objectives either in a constructive nor a bad way. A heat exchanger is an essential equipment for some procedures in the oil and gas refineries plant and it will decide the plant’s shutdown span. In a heat exchanger, welded tube to tubesheet joint is a basic factor which requires a different qualification. In this paper, tube to tubesheet welding for various material combination (carbon steel-stainless steel, stainless steel-stainless steel, and alloy steel-alloy steel) is qualified. According to American Society of Mechanical Engineers (ASME) Section IX QW-193 mock up qualification is done and QW-193.1 to QW-193.1.3 (visual test, liquid penetrant test and macro etching test) likewise is followed and lab tested for acceptance criteria. Ten model welds are needed to determine the eligibility of each procedure according to the required standard and totally 60 models (every 20) tried to rate this 3 procedure qualification record and welder qualification test record (PQR / WQTR). Qualification of the sheet from tube to tube follows all needed variables referred to in QW-288. This study proposes the improved minimum and maximum value for current (I), voltage (V), and travel speed (mm/min) required for above said materials qualifications. Further discourse is made over the heat input (KJ/mm), after effects of minimum leak path all things considered (Macro etching test), and classifies the range qualified against utilized base material for better understanding according to QW-288 essential variables.

Testing and Evaluation Method Investigates Cement Expansion in Cased-Hole Wells

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 208679, “Cement Expansion in Cased-Hole Environments: A Novel Laboratory Testing and Evaluation Method With Successful Field Implementation,” by Vasilii Sukhachev, Amir Salehpour, SPE, and Ilshat Akhmetzianov, SPE, Schlumberger, et al. The paper has not been peer reviewed. Preventing bulk shrinkage of cement in cased-hole wells is important when dealing with the risk of sustained casing pressure (SCP). Current studies based on membrane tests suggest that, even with a high-percentage content of expanding agents, cement systems show little or no expansion in dry media. An alternative laboratory method for measuring cement expansion, followed by a successful field application, was used to evaluate the total bulk volume change of the cement based on curing in a dry environment and in water. Optimal cement-slurry properties were obtained. The developed cement system, which was successfully implemented in the field, provided long-term zonal isolation with no indication of SCP to date. Introduction For a major operator in the Caspian region, drilling development wells required passing through shallow overpressured permeable zones containing water and biogenic gas. The optimal casing design used to minimize the likelihood and severity of SCP consisted of isolating the shallow-soil zone by use of a cemented 24-in. liner followed by a 20-in. surface casing cemented using a two-stage method (Fig. 1). During the first stage, the cement was pumped inside of the 24-in. liner. Then, an external casing packer placed in the 20-in. casing was inflated against the 24-in. liner before performing the second-stage cement job. The second-stage cementing operation brought the cement above the top of 24-in. liner and provided sufficient length to isolate against the shallowest distinct permeable zone. Cement is considered a primary wellbore barrier element. However, the cement placed between the 24-in. liner and the 20-in. casing and in the annulus of the 30-in. conductor and 20-in. casing is subject to shrinkage. This shrinkage leaves a potential leak path all the way to the wellhead on an offshore platform. The cement systems chosen consisted of the 1.92-SG (16-lbm/gal) slurry for the second stage cementing of the 20-in. casing because this stage was the system most susceptible to shrinkage. This slurry was placed in the annular space of the 20-in. casing and both the 24-in. liner and the 30-in. conductor. The curing temperatures and pressures in the field are 32°C and 6.74 MPa at the bottom of the well and 28°C and 2.48 MPa at the top of the cement. The top of the cement will be in direct contact with the spacer, a fluid with 89% water content; however, the remainder of the cement column will not have access to excess water during curing. If acceptable results could be achieved with the 1.92-SG cement system, then the same blend could be used for the 24-in. liner tail and the first-stage tail slurry of the 20-in. casing. This method is a possible solution to obtain a more-robust system that prevents the risk of SCP.

Bonding formation and gas absorption using Au/Pt/Ti layers for vacuum packaging

In this study, we developed a metal multilayer that can provide hermetic sealing after degassing the assemblies and absorbing the residual gases in the package. A package without a leak path was obtained by the direct bonding of the Au/Pt/Ti layers. After packaging, annealing at 450 °C caused thermal diffusion of the Ti underlayer atoms to the inner surface, which led to absorption of the residual gas molecules. These results indicated that a wafer coated with a Au/Pt/Ti layer can provide hermetic sealing and absorb residual gases, which can simplify vacuum packaging processes in the electronics industry.

Analysis of Tube-to-Tubesheet Welding in Carbon Steel Heat Exchangers of a Double Plate Header Box.

The rear wall of the header box serves as a tubesheet in heat exchangers of double plate header box. Tube-to-tubesheet welding must be performed using orbital Gas Tungsten Arc Welding (GTAW) with a head extension, which is passed through the corresponding hole in the front wall (plugsheet) of the header box, where the welding machine is supported. In this project, the effect of parallelism deviations between the plugsheet and the tubesheet of carbon steel header box is analyzed to evaluate its influence on the quality of the tube-to-tubesheet welding. Welded tube (SA-210 Gr. A1) to tubesheet (SA-516 Gr. 70) coupons are manufactured simulating the parallelism deviations previously analyzed in two double plate header boxes of air-cooled heat exchangers using two different preheating temperatures. Macrographic analysis is performed in order to evaluate the weld penetration (minimum leak path) and length of the weld leg in tube-to-tubesheet joints. The results obtained show important variations in those parameters when the parallelism deviations are equal to or greater than −1 mm over the theoretical distance as well as when the distance approaches +1 mm or more. Finally, the incorporation of dimensional controls prior to the welding process is discussed and the implementation of improvements in orbital GTAW equipment is recommended as an optimal solution for this kind of heat exchangers.

Experimental investigation on sodium leak behaviour through a pinhole

Sodium is most preferred coolant in Fast Breeder Reactors (FBR) due to its favourable neutronic and excellent heat transfer properties. However, when exposed to atmosphere, sodium reacts with oxygen and moisture resulting in sodium fire leading to rise in ambient temperature and release of harmful sodium oxide aerosols. Owing to this, prevention and early detection of sodium leak is of highest priority in the design of sodium systems. In all sodium systems, wire type leak detectors are provided for early detection of sodium leak. However, sodium leak from small defects such as hairline crack and pinholes may lead to oxidation of sodium and deposition of reaction products plugging the leak path. Such leaks may go undetected, cause local corrosion of structural material and may lead to sudden failure. Hence, investigation on sodium leaks from small openings is vital for sodium safety. Series of experiments were carried out to understand such sodium leak scenario through a small aperture simulating a pinhole defect. Sodium plugging phenomenon was studied at various oxygen concentrations from 0.5% to 3% in the leak zone, as a function of driving pressure. Conditions for unplugging were also established by varying sodium temperature and driving pressure, for plugged cases.

Leakages Through Radial Cracks in Cement Sheaths: Effect of Geometry, Viscosity, and Aperture

Abstract Annular cement sheath is considered to be one of the most important barrier elements in the well, both during production and after well abandonment. It is however well-known that mechanical damage to the cement sheath might result in leakage pathways, such as microannuli and radial cracks, and thus loss of zonal isolation. In this paper, we have studied the effect of geometry, aperture, and viscosity on the resulting pressure-driven flow through real radial cracks in cement sheaths using computational fluid dynamics (CFD) simulations. Real radial cracks were created by downscaled laboratory pressure cycling experiments and the resulting geometries were mapped by X-ray computed tomography (CT). This gave a unique 3D volume of the degraded cement sheaths which provides detailed information about the morphology, such as the irregular apertures and roughness, as well as locations of the radial cracks. In this study, we have used five experimentally created geometries, varying from barely connected to fully connected and almost uniform cracks. Additionally, theoretical uniform models with homogeneous aperture and a smooth surface were created for comparison. The simulations were performed by importing the experimentally created leak paths into a CFD simulation software, making it possible to determine the actual flowrate as a function of pressure drop. Methane gas, water, and oil were used as model fluids. The simulation results show that fluid flow through real cracks in cement sheath is complex with torturous paths, especially around bottlenecks and narrow sections. Additionally, the results show that flow of both methane gas- and water are not linear and hence does not follow Darcy's law. This illustrates that simple models are not able to fully describe fluid flow through such complex geometries.

Effects of gas permeation on the sealing performance of PEMFC stacks

The sealing performance of gaskets is critical to the safety of PEMFC stacks. Leak through the interface between gasket and BPP or MEA frame has been widely investigated, while gas permeation through gaskets is ignored in most studies. In this work, helium mass spectrometry leak measurement and finite element calculation were used to investigate the leaking and sealing mechanisms. It shows that interfacial leak and gas permeation of gaskets could be distinguished by their difference in terms of steady-state flow rate and stabilization process. Gas permeation is the main leak path of gaskets. The permeation flow rate is non-negligible for the safety of fuel cell stacks and needs to be considered when designing gaskets.

Towards understanding the effect of leak in Spiking Neural Networks

Spiking Neural Networks (SNNs) are being explored to emulate the astounding capabilities of human brain that can learn to perform robust and efficient computations with noisy spikes. A variety of spiking neuron models have been proposed to resemble biological neuronal functionalities. The simplest and most commonly used among these SNNs are leaky-integrate-and-fire (LIF), which contain a leak path in their membrane potential and integrate-and-fire (IF), where the leakage path is absent. While the LIF models have been argued as more bio-plausible, a comparative analysis between models with and without leak from a purely computational point of view demands attention, which we try to address in this paper. Our results reveal that LIF model provides improved robustness and better generalization compared to IF. Frequency domain analysis demonstrates that leak aids in eliminating high-frequency components from the input, thus enhancing noise-robustness of SNNs. Additionally, we compare the sparsity of computation between these models. In general, for the same input, the LIF model would be expected to achieve higher sparsity compared to IF due to the layer-wise decay of spikes caused by membrane potential leak with time. However, contrary to this expectation, we observe that leak decreases the sparsity of computation. Therefore, there exists a trade-off between robustness and energy-efficiency in SNNs which can be optimized through suitable choice of amount of leak in the models.

Noninterference Enforcement via Supervisory Control in Bounded Petri Nets

Security of distributed control systems is affected by the presence of information leaks, which permit to external intruders to infer the state of the system itself. Noninterference deals with the absence of such leak paths in a dynamic system modeled as discrete-event system. For a system that presents information leaks, a supervisory control strategy to enforce noninterference is proposed in this article. The approach is based on the solution of optimization problems on a Petri net model of the system under investigation. The effectiveness of the proposed enforcement strategy is shown by means of illustrative examples.

Guidelines to minimize the occurrence of mega-wedge falls in rock caverns

Over the last decade, large rock caverns have been developed for storing vast volumes of hydrocarbons and in particular crude oil. In this context, large rock caverns have light support made of grouted steel or fibreglass rock bolts. Reinforcing cables are strictly prohibited to avoid leak paths for the product. Therefore, the risk of mega-wedge occurrence is high since the size of the potential mega-wedges is proportional to the cavern size. Some geometrical situations are more detrimental than others and the possibility of a local or extensive collapse be very high, including in rock masses which can be ranked as good and above. Such an apparent paradox could have been detrimental to the concept of large caverns because mega-wedges are extremely difficult to stabilize once discovered because rock bolts are generally too short to stabilize them. Anticipation is therefore the key. First, we clarify the term mega-wedge and then, analyse the various possibilities of occurrence in large caverns, as well as classical tunnels. Guidelines are proposed to identify whether the conditions are met for experiencing mega-wedge failure at hand. Two main geological structures, shear fractures and smooth-persistent-planar-spaced (SPPS) joints, are favourable for mega-wedge formation. These two fracture types are analysed and criteria are given to ensure a quick and efficient determination procedure. The field approach is synthesised by a decision chart, to be used at site, during the excavation works.

Characterization and monitoring of GOES-16 ABI stray light and comparison with Himawari-8 AHI and GOES-17 ABI

The Advanced Baseline Imager (ABI) aboard Geostationary Operational Environmental Satellite (GOES)-16 and -17 satellites represent the next-generation geostationary multispectral imaging instrument. Since GOES-16 ABI imagery data became available, stray light was observed in ABI visible, near-infrared (VNIR), and 3.9 μm, i.e., CH07, channels. A stray-light characterization scheme was developed to quantitatively monitor stray-light variation in ABI imagery. The stray-light analysis is focused on ABI CH07, whose nighttime radiometric performance being impacted by stray light is of main concern. It is found that the stray light in the ABI imagery occurs over ∼3 months around spring and fall equinox each year. The maximum stray light of GOES-16 ABI CH07 is ∼0.65 K at 300 K scene in zone of normal performance (ZONP), i.e., region with relative solar angle >7.5 deg, which is within the radiometric requirement of 1 K. The analysis of Himawari-8 Advanced Himawari Imager (AHI) CH07 data indicates that its maximum stray light is ∼3.35 K at 300 K scene in ZONP, much higher than that of GOES-16. This confirms the effectiveness of reducing the major stray-light leaking path in ABI as a result of lessons learned from Himawari-8 AHI. The magnitude of GOES-17 ABI CH07 stray light is shown to be ∼0.45 K in ZONP, slightly lower than GOES-16. The analysis of AHI 3.9 and 6.2 μm channel stray-light radiance ratio is shown to match the ratio of solar irradiance spectrum, which suggests that the stray-light issue is due to direct leakage of solar radiation through the instrument. Further characterization and monitoring of GOES-16 ABI VNIR channel stray light also help understand the solar origin of ABI stray light. This paper also investigated cases of strong atmospheric refraction-induced stray-light contamination onto ABI detectors during solar eclipse seasons and presents scheme to reduce such potentially harmful contamination.

A New Integrated Modeling Approach with Case Studies for Gas Transmission of Container Closure Headspace.

This article presents a new theoretical integrated modeling approach for calculating container closure integrity (CCI) that concurrently accounts for both diffusion and mass/volumetric flow in real time; practical case studies are also presented. For pharmaceutical, biological, cell, and gene therapies, container closure systems (CCSs) must ensure drug sterility and stability by safeguarding against microbial contamination and gaseous ingress (e.g., oxygen, carbon dioxide, moisture) according to product requirements. In addition to the testing approach for evaluating CCI performance, a modeling approach can be an important part of a CCI control strategy. Modeling is a powerful tool that provides information in situations where testing is not feasible, technically impossible, too time-consuming, or too expensive. Previously published models have lacked a systematic approach or the versatility needed to coherently and concurrently integrate both diffusion and effusion to solve problems arising in field applications. The new integrated modeling approach described in this article applies a robust numerical method to real-world applications. The model is based on the law of conservation and continuity for molecular flow, Fick's law of diffusion, and the Darcy-Weisbach theory of frictional mass/volumetric flow. This new integrated modeling approach handles time-dependent diffusion and effusion by combining diffusion and mass/volumetric flow seamlessly in real time. For a CCS under vacuum filled with nitrogen, this new modeling approach is able to reveal that oxygen ingress into the CCS through a leak path will enter in two phases, starting with effusion and continuously followed by diffusion in a seamless transition. Our integrated modeling approach is able to calculate and capture the exact timing of the phase transition point, providing unique understanding of complicated CCS problems. Using the finite difference method, all modeling results are numerically solved from the governing equations along with initial and boundary conditions for each individual case. The modeling results were precise and consistent with previously published testing results. This new integrated modeling approach displayed its capability and versatility to handle complicated leakage scenarios in practical applications. As a part of CCI control strategy, the modeling approach is a powerful tool for evaluating leaks, gauging their leak sizes, determining whether the CCS conforms to product requirements, and making informed decisions accordingly. Although additional studies are to be carried out to fully develop the potential of this model, the applications hold great promise and in addition provide insight into CCI and may also provide a solid foundation for CCI testing method development and validation for CCI performance.

Use of existing wells for a geochemical assessment of groundwater near a CCS site: A case study in Japan

One of the public concerns associated with CO2 geological storage is possibility of the contamination of portable water by CO2 injection. Major negative impacts on portable aquifers are considered increase of heavy metal concentration associated with acidification of groundwater by buoyant CO2 and increase of salinity by intruding saltwater (brackish water to brine) from deeper storage reservoir and adjacent aquifers. In order for CO2 and/or saltwater to enter shallow groundwater, there must be a leak path from deep underground. It is considered that such concerns can be reduced by deepening the understanding of the underground structure. In this study, we examine the existing wells to make a conceptual model of groundwater system around Tomakomai area, where off-shore CO2 storage has been demonstrated in Japan. Our results suggested that CO2 and saltwater in the reservoir are less likely to leak unless a new path toward the surface is created.

Identification of the piston machine combustion chamber tightness

PurposeThe lack of integrity of the piston machine combustion chamber manifests itself in leakages of the working fluid between the piston and the cylinder liner, at valves mounted in the cylinder head and between the head and the liner. An untight combustion chamber leads to decreased power output or efficiency of the engine, while leaks of a fluid may cause damage to many components of the chamber. The actual value of working chamber leak is a desired and essential piece of information for planning operations of a given machine.Design/methodology/approachThis research paper describes causes and mechanisms of leakage from the working chamber of internal combustion engines. Besides, the paper outlines presently used methods and means of leak identification and states that their further development and improvements are needed. New methods and their applicability are presented.FindingsThe methods of leak identification have been divided into diagnostic and non-working machine leak identification methods. The need has been justified for the identification of leakage from the combustion chamber of a non-working machine and for using the leakage measure as the value of the cross-sectional area of the equivalent leak, defined as the sum of cross-section areas of all leaking paths. The analysis of possible developments of tightness assessment methods referring to the combustion chamber of a non-working machine consisted in modelling subsequent combustion chamber leaks as gas-filled tank leak, leak from another element of gas-filled tank and as a regulator of gas flow through a nozzle.Originality/valueA measurement system was built allowing the measurement of pressure drop in a tank with the connected engine combustion chamber, which indicated the usefulness of the system for leakage measurement in units as defined in applicable standards. A pneumatic sensor was built for measuring the cross-sectional area of the equivalent leak of the combustion chamber connected to the sensor where the chamber functioned as a regulator of gas flow through the sensor nozzle. It has been shown that the sensor can be calibrated by means of reference leaks implemented as nozzles of specific diameters and lengths. The schematic diagram of a system for measuring the combustion chamber leakage and a diagram of a sensor for measuring the cross-sectional area of the equivalent leak of the combustion chamber leakage are presented. The results are given of tightness tests of a small one-cylinder combustion engine conducted by means of the set up measurement system and a pre-prototype pneumatic sensor. The two solutions proved to be practically useful.

Gas leak detection in galvanised steel pipe with internal flow noise using convolutional neural network

Galvanised Steel Pipe (GSP) is the most common gas pipeline in populated areas. Existing leak detection research aimed at welded steel pipe is not suitable for GSP system due to their differences in line pressure, connection method, and leak path. This paper presents a gas leak detection method for galvanised steel pipe based on acoustic emission. An experimental setup composed of eight segments is designed to quantitatively simulate gas leak in GSP network considering flow-induced noise. The experiments verify that internal flow noise demonstrates similarity to leak-induced signals and thus interferes with leak detection based on shallow machine learning approaches. Convolutional Neural Network (CNN) is therefore introduced to solve the problem. Different network architectures are investigated and evaluated. Two types of inputs are discussed, namely time-domain signal and time-frequency distribution. Leak detection result show that the proposed method is robust to internal flow noise. When leak rate is greater than 0.03 L/s, the best model achieves overall accuracy more than 93 % in both the test set and the cross-validation set. The model performances indicate that traditional frequency analysis is ineffective to improve the flow-noise robustness of the CNN-based leak detector.

Gas Absorption in Package Using Au/Pt/Ti Bonding Layer

Vacuum packaging is required for the successful long-term operation of various kinds of micro-electromechanical system (MEMS) devices. The following points are important to complete and maintain encapsulated internal vacuum conditions: Degassing MEMS assemblies before packagingPackaging MEMS devices without leakageAbsorbing residual gases after packaging Our research group has developed a simplified MEMS packaging process using a Au/Ti layer (from top to bottom) as a bonding and gettering layer[1]. The assemblies were firstly metalized with thin Au/Ti films. The Au surfaces formed direct bonding without leakage even at room temperature. Moreover, when the packaged structure was annealed at 200 °C, the Ti atoms diffused to the inner surface of the package and absorbed residual gas molecules. This approach allows the simplification of the MEMS packaging process as the bonding and gettering layers can be simultaneously fabricated.However, we found that the Au/Ti film cannot form bonding after degas annealing at 200 °C because Ti oxides were formed on the surface. The annealing at 200 °C effectively removes the water molecules physi and chemi-sorbed on the MEMS assemblies, which deteriorate the packaged environment. The purpose of this study is to develop a metal multilayer that enables both the packaging after degassing at 200 °C and the absorption of residual gas at higher temperatures. The diffusion of Ti atoms can be controlled by fabricating a Pt barrier layer between the Au and Ti layers. A thick Pt layer enables bonding after degassing by preventing the diffusion of Ti during degassing. However, the gas absorption process requires a moderate thickness of the Pt layer because the Ti atoms should reach the surface at high temperatures. In this study, the packaging using thin Au/Pt/Ti multilayers were performed for the advanced MEMS packaging process.For the fabrication of packaged environments, a Si substrate with cavities was bonded with a flat Si substrate using the Au/Pt/Ti layers, as illustrated in Fig. 1. They were thermally oxidized before the bonding procedures. Firstly, the substrates were metalized with Au/Pt/Ti layers of 3/5/40 nm in thickness. The substrates were annealed at 200 °C under vacuum conditions for degassing. Immediately after degassing, the Au surfaces were brought in contact and pressed for the bond formation. Figure 2 shows the mapping of the interfacial gaps observed using a scanning acoustic microscope. Ultrasonic waves reflected in the bright areas, indicating the presence of unbonded areas at the interface. The bright square-shaped regions were due to the packaged areas. While particles on the surface were observed as tiny round voids, there was no leak path for the packages.For the absorption of residual gas molecules, the bonded specimens were annealed at 450 °C for 180 min. Figure 3 depicts the X-ray photoelectron spectra obtained from the inner surface of the annealed package. While the Ti atoms were absent on the surface before the high-temperature annealing step, Ti compounds were present after the high-temperature annealing. This is indicated that the Ti diffused through the Au and Pt layers and reacted with the residual gas molecules on the surface.These results suggested that the 3/5/40-nm-thick Au/Pt/Ti layers can form direct bonding after degassing and also absorb residual gas molecules in the package. We expect that this packaging technique would contribute to future MEMS manufacturing because high-vacuum sealing can be obtained by the simple metal multilayer structure.[1]. Y. Kurashima et al., 45th International Conference on Micro and Nano Engineering, 2019. Figure 1

Simulation of fluid flow through real microannuli geometries

Debonding of well cement towards casing or rock, i.e. microannuli formation, is a typical failure mode of cement barriers. Consequently, such microannuli may act as leak paths during leakages of downhole fluids such as hydrocarbons or CO2. Current leakage models usually assume that microannuli have uniform geometries with homogeneous apertures, which results in linear Darcy flow. However, several recent studies have shown that microannuli do not have uniform geometries and subsequently, fluid flow through real microannuli is complex and non-linear.This paper presents results of CFD simulations of fluid flow through real microannuli geometries, where the microannuli were experimentally created and visualized in 3D by X-ray computed tomography (CT). Simulations were performed with three different fluids of varying viscosity and density – methane gas, liquid water, and liquid oil. The results visualize how the non-uniform flow path geometry and surface roughness of real microannuli create tortuous flow patterns, especially locally around bottlenecks and other heterogeneities. It is also seen that these complex flow patterns result in non-linear dependencies between flow rate and pressure difference for the less viscous fluids, i.e. gas and water, whereas the more viscous oil display linear flow.

Experimental study on the integrity of casing-cement sheath in shale gas wells under pressure and temperature cycle loading

During large-scale multistage hydraulic fracturing in shale-gas wells, the cement sheath loses its integrity easily because of repeated temperature and pressure variations in the wellbore. This is likely to form a potential leak path, subsequently leading to the loss of zonal isolation and sustained casing pressure. To prevent such barrier failures, it is crucial to investigate and understand the failure mechanism of cement sheath. In this study, an experimental apparatus and evaluation method of the sealing integrity and mechanical integrity for casing-cement sheath under temperatures and pressures cycle loading have been presented in this paper by which sixteen groups of full-scale integrity tests of general and high-strength cement sheath have been conducted under temperature ranging from 30 to 150 °C and pressure ranging from 0 to 70 MPa cycling loading. The anti-channeling/sealing performance and interface mechanical behavior of casing-cement sheath are measured and characterized respectively under different loading-unloading methods. The cycle numbers required for sealing integrity failure of cement sheath under different cyclic temperatures and pressures have been obtained. The interface mechanical properties (cementing force, friction force, shearing strength and lateral bonding strength) of cement sheath have been obtained, and effects of alternating temperatures and pressures on those mechanical properties and late integrity have been analyzed. Research results can provide a very important reference for the fracturing design of shale-gas wells.

Probability of Uncontrolled External Leakage During the Production Phase of a Subsea Well

Abstract This work presents a methodology for evaluating the uncontrolled external leakage probability of a subsea well during the production phase. Based on a barrier diagram, an algorithm for possible leak path identification is proposed, considering different operation modes: gas lift operation, free-flowing, or well closed at the subsea Christmas tree. Considering the equivalency between these paths and the minimum cut sets from a fault tree modeling, the uncontrolled external leakage probability is calculated using the upper bound approximation. The effect of common cause failures is considered for the failure mode fail-to-close-valve. The instantaneous availability function of each component is considered. Non-repairable, repairable, and periodically tested items are used. Probability distribution parameters are estimated in order to make a case study. The failure rate functions determined are used as input for the proposed model, regarding the following failure modes: fail-to-close, external-leakage, and internal-leakage at the closed position. Finally, failure probability results and sensitivity analysis are demonstrated for a base case study. Parameters like time between tests, inspections, and component reliability are varied in order to identify the impact on the uncontrolled external leakage probability. The main objective of the proposed methodology is to support decision-making on the well integrity management system during the production phase of a subsea well. To this end, actual and reliable input data should be considered.

Autonomous and intrinsic self-healing Al2O3 membrane employing highly-wetting and CO2-selective molten salts

High temperature ceramic membranes often suffer catastrophic failure and loss of selectivity due to the initiation and propagation of cracks, leading to the creation of leak paths for undesired permeants. Cracking occurs due to the brittle nature of ceramics and e.g. materials expansivity mismatch or stresses accumulated at sealing points. One way to overcome this problem is to produce a self-healing membrane, where the healing phase restores perm-selectivity to the membrane following cracking. Here we show that the molten salt in a supported molten-salt membrane for CO2 permeation can be used to self-heal a working membrane in operation, all the while being the selective phase for permeation. By designing a membrane with a sacrificial crack-inducing agent (removed to create a clear leak-path) we show in a single experiment how a permeating membrane can crack and leak, then autonomously self-heal via an intrinsic mechanism; controlled spreading of the highly-wetting and CO2 selective molten salt. No external triggers or self-healing stimuli are required, indicating that highly-wetting and selective liquids can be used to increase membrane lifetime by self-healing, reducing capital and operating costs.