Experimental and advanced computational modelling study of downhole elastomer seal assemblies

Elastomers seals are widely used in various drilling, completion, and production equipment. One such equipment is liner hanger which has become integral part of modern well designs. Failure of liner hanger seal assembly can compromise well integrity, and lead to severe health, safety, and environmental consequences. Concerns regarding reliability of elastomer seals in liner hanger assemblies have been raised by the regulators as well as industry. This dissertation work provides detailed investigation of design, and failure of downhole elastomer seal assemblies using experimentally supported advanced computational modeling techniques. This work is partially supported by Bureau of Safety and Environmental Enforcement (BSEE) and it is set in the context of liner hanger assemblies. However, major outcomes of this research also applies to other downhole seal assemblies. Specific objectives of this dissertation are - (i) investigate performance of liner hanger seal assembly under various design, operational, and failure scenarios, (ii) develop operating envelops and identify critical parameters influencing performance of the elastomer seal assembly, (iii) develop a modelling tool for predicting leakage through elastomer seal interface considering surface characteristics, (iv) generate guidelines for design and qualification of elastomer seals and provide regulatory recommendations. Novel technical aspects of this research work are – (i) studying material behavior of different elastomer material (NBR, EPDM, FKM, FEPM, FFKM, PTFE) under normal and downhole conditions, (ii) using the elastomer material data in true-scale finite element (FEA) models to evaluate equipment level performance of seal, (iii) scaled laboratory tests and analytical calculations to validate FEA models, and (iv) development of a leakage modelling tool that can predict leakage rates as a function of surface topography of seal interface and operating conditions. Results from this dissertation indicate that type and design of seal equipment determines which elastomer properties need to be qualified. Hardness and elastic modulus alone may not be good predictors of fitness-for-service of seal assembly. For example, performance of expandable liner hanger seal assembly primarily depends on seal dimensions and elastomer shear modulus while performance of conventional liner hanger seal assembly mainly depends on elastomer bulk modulus. Selection of appropriate elastomer material for a certain application depends not only on chemical environment and temperature but also on assembly design, operational constraints, and thermal changes. Comparative evaluation demonstrated that conventional liner hanger seal assembly outperforms expandable liner hanger seal assembly in terms of contact pressure generated per unit energization but it is more prone to failure than expandable assembly. Contact pressure at seal-pipe interface, as predicted by macro-scale FEA models, does not accurately indicate fluid pressure that can be effectively sealed. Leakage modelling studies demonstrated that surface characteristics of elastomer and fluid properties determines the contact pressure needed to achieve complete sealability. Leakage modelling approach developed in this work can be an invaluable tool in seal design workflow for determining target seal energization needed for complete sealability

AN INVESTIGATION OF DISTORTION-INDUCED FATIGUE CRACKS IN STEEL HIGHWAY BRIDGES: FROM CRACK DETECTION TO FATIGUE RETROFIT

Distortion-induced fatigue (DIF) cracking is the primary maintenance and structural safety concern in steel bridges built before the 1980s in the United States, requiring frequent inspections and costly repairs. Human visual inspections to characterize fatigue cracks have many drawbacks, including inconsistencies in identification, significant time and monetary costs, and safety risks posed to the lives of both inspectors and the traveling public due to lane closures. On the other hand, many retrofits aimed at mitigating the effects of DIF and stopping fatigue crack growth have been analyzed both experimentally and computationally; however, most of them require removal of the concrete deck, which disrupts the traveling public.In this dissertation, a holistic study is presented aimed at advancing the state-of-the-art in advancing detection, characterization, and repair of distortion-induced fatigue in steel bridge structures. A digital image correlation (DIC) based methodology is proposed for detecting and monitoring fatigue cracks in steel bridges. Also, a numerical study of three types of finite element (FE) analysis computational measures (Hot Spot Stress (HSS) analysis, J-integrals, and Stress Intensity Factors (K)) was performed to evaluate their predictive capabilities for characterizing DIF cracking propensity. Moreover, an analytical study was conducted to assess the effectiveness of CFRP applied over cracked steel plates, and to determine the influence of various modeling parameters on the analytical solution. Finally, a novel Carbon Fiber Reinforced Polymer (CFRP)-Steel retrofit is proposed to repair complex patterns of DIF cracks in steel bridge web gaps without the need for bolting to the flange or concrete deck removal. The DIC-based methodology was developed from in-plane compact fracture specimens then evaluated through a half-scale steel girder to cross-frame connection experimental test. The laboratory test results verified that the proposed DIC methodology could robustly quantify the fatigue crack length. The FE results of the three computational measures (HSS, K, and J) showed good alignment with experimental observations, and they were capable of predicting and characterizing DIF cracking propensity. The FE results of strengthening cracked steel plates with CFRP showed effectiveness in reducing maximum principal stress and stress intensity factor at the crack tips and in reducing crack propagation rate. The experimental results of the novel CFRP-Steel retrofit showed that the retrofit was effective in preventing DIF crack initiation and propagation

On the Degradation of Lubricating Grease

A comprehensive literature review on physical and chemical degradation monitoring and life estimation models for lubricating greases is presented in chapter one. Degradation mechanisms for lubricating grease are categorized and described, and an extensive survey of the available empirical and analytical grease life estimation models including degradation monitoring standards and methods are presented. In chapter two, irreversible thermodynamic theory is employed to study the mechanical degradation of lubricating grease. A correlation between the mechanical degradation and entropy generation is established and the results are verified experimentally using a rheometer, a journal bearing test rig, and a modified grease worker machine. It is shown that the degradation rate is linearly related to the entropy generation, and that it can be used for estimation of the mechanically degraded grease life. In chapter three, a model is presented that uses the principles of irreversible thermodynamics to predict the life of a lubricating grease undergoing mechanical shearing action. Here we restrict our attention to operating temperatures far below the initial activation energy needed to initiate chemical degradation or base oil evaporation. Thus, mechanical degradation is the dominant degradation process. The predictions of the model are validated using the experimental results obtained by testing three greases subjected to different shear rates and temperatures. In chapter four, mechanical life of grease in an elastohydrodynamic (EHL) line contact between two steel rollers is studied. Grease traction curves are measured and reported in different conditions. Three successive lubricating phases of “Fully grease covered rollers”, “Slippage and grease separation” and “Formation of liquid lubricant reservoir” are observed and their behaviors are examined. The traction of the grease is monitored during a long term mechanical degradation process. Our mechanical life prediction model is applied to the lubricating grease at the contact. In chapter five, chemical degradation is studied from an energy point of view. A theory is introduced based on acquired experimental results, and is verified using a roller tester rig. The theory is used to estimate the chemical life of a grease at different temperatures. Summary and conclusions are given in Chapter six along with recommendations for future studies

Structure, Rheology and Optical Properties of Plasmonic Fluids

Fluids with tunable optical and rheological properties are of fundamental and practical interest. They can be easily processed to manufacture thin films and interfaces for applications such as molecular detection and light trapping in photovoltaics. Cationic surfactants such as cetyl-trimethylammonium bromide have the ability to self assemble with metallic nanoparticles to form a corona or a double-layer vesicular structure. These structures upon further interaction with wormlike micelle fragments are hypothesized to form micelle-nanoparticle elastic networks. In this dissertation, solution phase self-assembly is utilized to uniformly distribute various metallic nanoparticles to produce stable multicomponent plasmonic fluids with remarkable color uniformity. The optical properties of the fluids can be robustly tuned by varying the species, concentration, size and/or shape of the nanoparticles. Multicomponent plasmonic fluids capable of broadband absorption of visible light are produced via the self-assembly route. Small angle X-ray scattering and rheological studies suggest that the nanoparticles are incorporated into the wormlike micelle network to form a more compact double network. These fluids exhibit rich rheological behavior depending on the nanoparticle concentration and the salt to surfactant molar ratio. Specifically, non-monotonic dependence of zero shear viscosity on nanoparticle concentration, rheopexy, shear thickening, shear banding and shear thinning are observed. The fluids exhibit enhanced viscoelasticity upon the addition of more nanoparticles. The mechanical, rheological and optical properties of plasmonic fluids greatly depend upon the temperature due to the structural changes of the micellar solutions. The application of plasmonic fluids to efficient light trapping in photovoltaic cells, plasmon-enhanced microalgal growth and optofluidic devices have been designed and demonstrated in this dissertation

Final Design Report: Polymer Fatigue Characterization Test Method

Polylogix is a team dedicated to the design, build, and testing of a fatigue machine to simulate cyclic loading on a biomedical polymer. This project is sponsored by Endologix, Inc. to provide test data characterizing mechanical material properties of various formulations of polymer used in abdominal aortic aneurysm surgeries. With this project goal, the machine must be able to test the polymer at body conditions; these include a testing temperature of 37°C and a cycling frequency ranging from 1 Hz to 10 Hz. This report proposes the following solution to this design challenge: an AC motor-driven mechanism utilizing a planetary gearbox and pulley system to reduce the speed of the motor to those necessary to achieve testing frequencies of 1 Hz, 2 Hz, 5 Hz, 8 Hz, and 10 Hz. From the drive mechanism, a shaft-mounted cam translates the rotational motion to linear motion through a cam follower. This cam follower carries a load cell and test specimen grip fixtures which clamp onto the specimen to apply cyclic loading. To vary the percent elongation applied to the test specimen, the cam has been designed to be interchangeable to accomplish a range of elongations: 10%, 20%, 30%, 40%, and 50%

Dynamics and viscoelastic properties of Hydrogen-bonding telechelic associating polymers

Supramolecular polymers (also termed as associating polymers), which are connected by non-covalent interactions between polymer chains, have become an increasingly important class of polymers and gained tremendous interest in the last few decades. The co-existence of reversible secondary interactions and covalent bonding makes supramolecular polymers promising candidates for functional materials. Immense effort has been put on the development of chemical structure design, while the understanding of their physical properties is rather limited, especially in the melt state. In this dissertation, we studied the dynamics and viscoelastic properties of H-bonded telechelic associating polymers by tuning the association strength, main chain length, flexibility and polarity. A systematical analysis was conducted by employing a combination of experimental techniques: dielectric spectroscopy, differential scanning calorimetry, rheology and small angle X-ray spectroscopy. We demonstrated that hydrogen-bonding has a strong influence on both segmental and slower dynamics in the polydimethylsiloxane (PDMS) and poly(propylene glycol) (PPG) systems with low molecular weights. The supramolecular association of hydroxyl-terminated PDMS chains leads to the emergence in dielectric and mechanical relaxation spectra of the so-called Debye process traditionally observed in monohydroxy alcohols. Then we investigated telechelic associating PMDS with different hydrogen bonding end groups, e.g. NH2, NHCO-COOH (amide-acid groups). Remarkably, a single species of end group forms two qualitatively different types of associates in PDMS-NHCO-COOH: transient bonds which allow stress release by a bond-partner exchange mechanism, and effectively permanent bonds formed by a phase segregated fraction of end groups which are stable on the timescale of the transient mechanism. In the following work, we studied telechelic PDMS and PPG with three types of H-bonding end-groups possessing different interaction strengths and a non-H-bonding end-group as reference were compared. Unraveling the mechanisms of many molecular processes and structure-dynamics-property relationship in supramolecular polymers is of great importance for both fundamental studies and industrial applications. Findings in this work suggested that the backbone length, flexibility and polarity, the strength and lifetime of the associating groups, and the ratio of characteristic time scales between backbone and chain ends should be considered in the design of associating polymers to achieve the desired properties

The 29th Aerospace Mechanisms Symposium

The proceedings of the 29th Aerospace Mechanisms Symposium, which was hosted by NASA Johnson Space Center and held at the South Shore Harbour Conference Facility on May 17-19, 1995, are reported. Technological areas covered include actuators, aerospace mechanism applications for ground support equipment, lubricants, pointing mechanisms joints, bearings, release devices, booms, robotic mechanisms, and other mechanisms for spacecraft

Electrospun Nanofiber Yarns for Nanofluidic Applications

This dissertation is centered on the development and characterization of electrospun nanofiber probes. These probes are envisioned to act like sponges, drawing up fluids from microcapillaries, small organisms, and, ideally, from a single cell. Thus, the probe performance significantly depends on the materials ability to readily absorb liquids. Electrospun nanofibers gained much attention in recent decades, and have been applied in biomedical, textile, filtration, and military applications. However, most nanofibers are produced in the form of randomly deposited non-woven fiber mats. Recently, different electrospinning setups have been proposed to control alignment of electrospun nanofibers. However, reproducibility of the mechanical and transport properties of electrospun nanofiber yarns is difficult to achieve. Before this study, there were no reports demonstrating that the electrospun yarns have reproducible transport and mechanical properties. For the probe applications, one needs to have yarns with identical characteristics. The absorption properties of probes are of the main concern. These challenges are addressed in this thesis, and the experimental protocol and characterization methods are developed to study electrospun nanofiber yarns

Coupled Simulation Of Hydraulic Fracturing, Production, And Refracturing For Unconventional Reservoirs

Horizontal well drilling and multi-stage hydraulic fracturing are two key techniques for the development of unconventional reservoir. However, the production from tight formation is associate with fast depletion of reservoir. When oil price is low, drilling new horizontal wells is not profitable. Creating secondary fractures from existing hydraulic fractured wells, i.e., refracture is an alternative method to increase stimulated reservoir volume (SRV) and gain additional production from existing hydraulic fractured wells. To optimize refracturing well selection and operation, it’s of economic importance to acquire knowledge from initial hydraulic fracturing operation, production history, and refracturing design perspectives. This initiated the idea of this research to develop an integrated hydraulic fracturing, production, and refracturing model. This research work mainly comprises of three sections. In the first section, hydraulic fracturing models were built using XSite software, a lattice-based simulator, to analyze the effect of changing rock properties and in-situ stresses on fracture propagation in a layered reservoir. The challenge was to quantify degree of fracture containment using the hydraulic fracturing simulator. To overcome this fracture aperture contours were obtained to quantify fracture containment with two proposed penetration parameters. The modeling results suggest that brittle rocks favor vertical migration of hydraulic fracture, while increasing minimum horizontal stress tends to inhibit vertical growth of hydraulic fracture and lead to containment at layer interface. In the Second part of this study, an innovative integrated multi-stage hydraulic fracturing and production model was built for a shale gas reservoir. The challenge was to utilize distributed fracture data presented from the lattice-based hydraulic fracturing simulator for history matching in the reservoir simulator. To identify fracture geometry, a moving tip clustering and linear regression clustering algorithms were developed to discretize distributed fracture data points using multiple crack segments. The former algorithm is prone to capture fracture with microcracks that contribute to SRV, thus contributing to higher simulated production. The latter algorithm mainly captures the major fracture path without consideration of microcracks. The modeling results also suggest that gas slippage, matrix shrinkage, and fracture closure play important roles in shale gas production. In the third section, an innovative hydraulic fracturing, production, refracturing, and post-refracturing production model was developed. The challenge in this part was to simulate refracture propagation based on existing fracture geometry and pore pressure distribution with higher accuracy and efficiency. A model was built by simulating the fracture and refracture propagation in XSite and modeling reservoir depletion and post refracturing reservoir depletion in the continuum mechanism based simulator. The results suggest the propagation of refractures is driven by proppant and depletion induced stress shadow and contributes to larger SRV and higher hydrocarbon production. The proposed algorithms and integrated models can potentially be applied in the field for better refracturing design to enhance ultimate recovery of oil and gas