Annular Gas Research Articles

Study of the dynamics for gas accumulation in the annulus of production wells

Accumulation of associated petroleum gas in the annulus is one of the negative factors that impede the intensification of mechanized oil production. An increase in annular gas pressure causes growth of bottomhole pressure, a decrease in back pressure to the formation and the inflow of formation fluid. In addition, accumulation of gas in the annulus leads to displacement and a decrease in the liquid level above the submersible pump. Insufficient level of the pump submersion (rod or electric submersible) causes a number of complications in the operation of mechanized production units associated with overheating of the elements in pumping units. Therefore, the development of technologies for optimizing the gas pressure in the annulus is relevant. Method for calculating the intensity of gas pressure increase in the annulus of production wells operated by submersible pumps has been developed. Analytical dependence for calculating the time interval of gas accumulation in the annulus, during which the dynamic level decreases to the pump intake, is obtained. This value can be used to estimate the frequency of gas withdrawal from the annulus using compressors. It has been found that the rate of increase in annular gas pressure in time increases non-linearly with a rise in the gas-oil ratio and a decrease in water cut, and also linearly increases with a rise in liquid flow rate. Influence of the operating (gas-oil ratio) and technological (value of the gas pressure maintained in the annulus) factors on the flow rate of the suspended reciprocating compressor driven by the beam engine, designed for forced withdrawal and redirection of the annular gas into the flow line of the well is analyzed.

Modeling and Analysis of Sustained Annular Pressure and Gas Accumulation Caused by Tubing Integrity Failure in the Production Process of Deep Natural Gas Wells

Abstract Sustained annular pressure caused by tubing leakage seriously threatens the safe production of deep gas wells. Therefore, it is necessary to fully understand the characteristics of sustained annular pressure and find potential methods to reduce risk. However, most models are about annular pressure caused by thermal expansion and cement integrity failure. Therefore, this paper establishes a model based on the energy conservation law, gas pressure–volume–temperature (PVT) properties, and volume consistency law, to study sustained annular pressure caused by tubing leakage. The results indicate that the pressure and gas volume in tubing-casing annulus increase simultaneously and gradually slow down after tubing leakage happens. The decrease of bottom-hole pressure can reduce the risk of annular pressure, which can be realized by a downhole choke. Other measures can also control the rising speed of annular pressure, including enhancement of production rate, increase of the initial length of the gas column and annular liquid with high compressibility, but the impact of formation energy and annular gas volume should be considered. Sustained annular pressure caused by shallow leaking point has a faster rising speed, longer rising period, and higher pressure value. Large leaking point leads to a remarkable increase of leaking rate and pressure rising speed. The integrity of the upper tubing string should be strengthened.

An Accelerating Sweep Frequency Excitation Method for the Rotordynamic Coefficients Identification of Annular Gas Seals Based on Computational Fluid Dynamics

Abstract This article proposes a rotordynamic identification method using the accelerating sweep frequency excitation method. The computational fluid dynamics transient solution combined with the moving grid method is utilized to obtain the transient flow field of the seal excited by the whirling rotor with an accelerating frequency. Rotordynamic coefficients at swept frequencies are obtained by analyzing the transient response force acting on the rotor. Rotordynamic coefficients of three published experimental seals including a labyrinth seal, a fully partitioned pocket damper seal and a honeycomb seal are identified to validate the proposed method. The results show that the predicted rotordynamic coefficients are all well agreement with the trend of experimental data. Compared with the existing numerical models based on transient solutions, the central processing unit consumption of the proposed method is substantially reduced by 98% when achieving the same frequency resolution. In addition, the impact of the exciting acceleration on the identification accuracy is also illustrated.

Leakage and Rotordynamic Characteristics for Three Types of Annular Gas Seals Operating in Supercritical CO2 Turbomachinery

Abstract The balance piston seal in multiple-stage centrifugal compressors and axial turbines sustains the largest pressure drop through the machines and therefore plays an important role in successful full load operation at high rotational speed. This is especially true for power dense turbomachines in supercritical CO2 power cycles that generate or expend higher fluid pressures (above the critical value 7.3 MPa) and density (close to water 1000 kg/m3), because the fluid forces generated by the balance piston seals are directly proportional to the fluid density and the pressure drop across the seal. This paper presents a comprehensive assessment and comparison on the leakage and rotordynamic performance of three types of annular gas seals for application in a 14 MW supercritical CO2 turbine. These three seals represent the main seal types used in high-speed rotating machines at the balance piston location in efforts to limit internal leakage flow and achieve rotordynamic stability, including a labyrinth seal (LABY), a fully partitioned pocket damper seal (FPDS), and a hole-pattern damper seal (HPS). These three seals were designed to have the same sealing clearance and similar axial lengths. To enhance the seal net damping capability at high inlet preswirl condition, a straight swirl brake was also designed and employed at seal entrance for each type seal to reduce the seal inlet preswirl velocity. Numerical results of leakage flow rates, rotordynamic force coefficients, cavity dynamic pressure, and swirl velocity developments were analyzed and compared for three seal designs at high positive inlet preswirl (in the direction of shaft rotation), using a proposed transient computational fluid dynamic (CFD)-based perturbation method based on the multiple-frequency elliptical-orbit rotor whirling model and the mesh deformation technique. To take into account of real gas effect with high accuracy, a table look-up procedure based on the National Institute of Standards and Technology reference fluid properties database was implemented, using an in-house code, for the fluid properties of CO2 in both supercritical and subcritical conditions. Results show that the inlet swirl brake can significantly reduce the preswirl velocity at seal entrance, lowering the effective damping crossover frequency fco (or even fco = 0) to maximize the full operational frequency range of the machines. In stability analysis phase of a MW-scale supercritical CO2 turbine/compressor, the seal stiffness effects on the rotor mode shape must be evaluated carefully, where the seal stiffness is sufficiently large (comparable to the bearing stiffness). From a rotordynamic viewpoint, the HPS seal with entrance swirl brake is a better seal concept for the balance piston seal in supercritical CO2 turbomachinery, which possesses the largest positive effective damping throughout the entire subsynchronous frequency range.

Research and Design of Automatic Chip Removal Model Based on Cutting Machine

Aiming at the widespread environmental pollution problem of cutting machine tools, an automatic chip removal model platform is designed based on solid works. The model platform has the functions of automatic collection of waste chips and automatic chip removal through the air guide groove. A new type of cutting seam is designed to prevent the scraps from being taken out by the cutter, and remove the scraps adhered on the cutter due to high temperature. The inner loop gas path is designed, which is composed of two air inlets and 180 degree evenly distributed air outlets, forming a unique annular gas path. The gas path relies on the annular gas path, which blows away the waste chips during the cutting work of the machine tool, and enters the inner cavity of the machine tool through the new cutting seam. The automatic chip removal model platform is based on the chip removal of the machine tool itself, which can improve the chip removal efficiency of the waste and reduce the scratches on the surface of the material.

The break phenomenon of self-pulsation for liquid-centered swirl coaxial injectors

Self-pulsation boundaries that spray switches between stable and self-pulsated behaviors are experimentally obtained under water and air flow conditions for a liquid-centered swirl coaxial injector configured with different recess lengths. The underlying mechanisms responsible for self-pulsation are investigated comprehensively. Results show that for the non-recessed injector, self-pulsation only occurs in a relatively narrower region, associated with weaker spray oscillation with the amplitude less than 2 mm2/Hz correspondingly. In particular, for the injector with larger recess length, a break region is generated with increasing the momentum flux ratio. It is indicated that spray transforms from the self-pulsated behavior to the stable behavior, and develops into self-pulsated behavior once again. The break phenomenon of self-pulsation reveals a destabilizing/stabilizing effect created by the violent gas-liquid flow dynamics. Self-pulsation occurring on either side of the break region is found to be caused by different mechanisms. As such, self-pulsation of the liquid-centered swirl coaxial injector is classified into two types. The first type occurs in the injectors configured with relatively small recess length, or with large recess length before the occurrence of the break phenomenon. This self-pulsation is caused by the periodic blockage of the conical liquid sheet. Whereas, the second type is attributed to the periodic squeezing influences of annular gas on the conical liquid sheet, corresponding to the self-pulsation occurring in the injectors with larger recess length and after the disappearance of the break phenomenon.

Impact of dynamic modelling of the flame subgrid scale wrinkling in large-Eddy simulation of light-round in an annular combustor

Ignition of annular gas turbine combustors, also called light-round, is a challenging problem for aeronautic reliability and safety. Large Eddy Simulation (LES), based on massively parallel computations, has been already assessed as a reliable tool to analyse and predict such a phenomenon. The present study focuses on the effect of the flame subgrid scale wrinkling factor entering the combustion model. Large Eddy Simulations of the annular MICCA-Spray laboratory-scale chamber are performed with constant and dynamic strategies for the wrinkling parameter and are compared against experimental data. A bi-modal distribution of the model exponent during light-round is obtained with a different behaviour between the flames stabilized around each injector, and the flame fronts propagating in a circumferential direction. The impact of such locally defined and time-dependent evolution is analysed by studying several quantities associated to the flame propagation mechanisms. It is shown that the dynamic procedure entails a reduction of the wrinkling factor that is not balanced by an increase of the resolved flame surface. After considering the exhaust of burnt gases that affects the ignition in such a semi-confined system, the dynamic formulation leads to a moderate slowing down of the flame. Surprisingly, the sounder dynamic model does not yield better predictions of the experimental data, revealing the need to address other modelling improvements in the future. However, this does not temper the need for a dynamic combustion model in numerical simulation of light-round.

Impingement-Induced Stability Analysis of Intake Manifold Air Jets Using High Speed PIV and POD

The flow stability induced by the impingement of gas jets exiting from the intake manifold affects the in-cylinder flow characteristics of internal combustion engine. Using high-speed planar particle image velocimetry (PIV) with proper orthogonal decomposition (POD) analysis, an investigation was conducted to reveal the spatio-temporal characteristics of annular gas jets impinging in a region between the exits of two intake valves. Unique flow behaviors are identified where strong initial interaction of impinging jets appears near the valve exit as a result of fierce flow vorticity competition in both clockwise and counter-clockwise directions. This mixing zone exhibits strong fluctuations in the angle of the merged gas jet. Flow vorticity and merged jet angle are highly correlated with each other, and the quasi-periodical behavior of the jet impingement is linked to the kinetic energy dissipation. In addition, using POD, the underlying flow structures show large-scale rotating structures with translation which are responsible for the quasi-periodical behavior. In summary, three types of flow stability can be identified resulting from different levels of induced impingement: one-way interaction of single jet flow case, two-way interaction of dual impinging jets with equal flow magnitude, and transitional one-way interaction of dual impinging jets with unequal flow magnitude.

Numerical investigation on spray self-pulsation characteristics of a liquid-centered swirl coaxial injector

Self-pulsation characteristics of a liquid-centered swirl coaxial (LCSC) injector are investigated numerically based on the coupled volume of fluid VOF and Level Set (CLSVOF) model. Good agreements of simulation results and experiment data, in aspect for self-pulsation frequency and spray angle, are generally achieved. On the basis of the flow dynamics of the injector, mechanism of spay self-pulsation in recessed LCSC injector is proposed. The self-pulsation arises from the periodic blocking of annular gas by the liquid sheet. Self-pulsated “Christmas tree” spray patterns result from the periodic vortex rings generating and shedding around the liquid sheet. Theoretical analysis demonstrates that the self-pulsation frequency is almost the same as the frequency of the liquid sheet impacting the recess chamber wall, and the model may be able to predict the self-pulsation frequency. Pressure oscillation inside the air-core is closely related to the primary breakup of the liquid sheet, and the frequency is consistent with the self-pulsation frequency. Effects of the gas-liquid mass flow rate ratio (GLR) on the self-pulsation frequency and pressure oscillation frequency inside the air-core are negligible. The self-pulsation intensity peaks under conditions with the critical mixing flow pattern, while, the pressure oscillation intensity inside the air-core increases with the elevating of GLR. It seems that the spray self-pulsation could improve the spray angle.

Evaluation of exit pattern factors of an annular aero gas turbine combustor at altitude off-design conditions

Abstract This paper presents the experimental results of an annular combustor. A full scale full annular combustion chamber is tested at ground test stand simulating altitude off-design operating conditions. Traversing thermocouple rakes are used to measure temperatures over the entire annulus at combustor exit. The radial temperature profile is found to be a strong function of operating parameters while the circumferential pattern factor is within the design goal. Increase in reference Mach number at these off-design conditions has caused the redial temperature profiles to deviate and increase from the intended profile. These results will be used for benchmarking the computational model. The computational model will be used for detail study of the temperature non-uniformity over the entire flight envelope and also for modification of combustor liner for performance improvement.

Evaluation of exit pattern factors of an annular aero gas turbine combustor at altitude off-design conditions

Abstract This paper presents the experimental results of an annular combustor. A full scale full annular combustion chamber is tested at ground test stand simulating altitude off-design operating conditions. Traversing thermocouple rakes are used to measure temperatures over the entire annulus at combustor exit. The radial temperature profile is found to be a strong function of operating parameters while the circumferential pattern factor is within the design goal. Increase in reference Mach number at these off-design conditions has caused the redial temperature profiles to deviate and increase from the intended profile. These results will be used for benchmarking the computational model. The computational model will be used for detail study of the temperature non-uniformity over the entire flight envelope and also for modification of combustor liner for performance improvement.

Disintegration co-flowing gas-liquid jet coupled with forced perturbation

In this study, numerical simulation is employed to diagnose the behavior of round liquid jet under forced perturbation with the low-velocity coaxial gas flow. A sinusoidal longitudinal perturbation with high amplitude and infinite frequency is imposed on the inlet of the liquid jet. An annular gas with lower velocity flows coaxially. The volume of fluid (VOF) approach is used to capture the liquid-gas interface. Various circumstances are considered such as properties of the liquid, the ratio of gas velocity to liquid velocity, and the properties of perturbation. Different cap formations, ligament breakup, and the droplet generation processes are depicted. The results indicate that the slower gas flow can affect the liquid jet breakup behavior and droplet generation. Also, liquid properties can considerably influence the cap formation and morphology of breakup. This investigation is validated by other numerical and experimental research in the basic conditions, while co-flowing gas-liquid jet considered in the present study is completely novel.

Annular Gas and Drillstring’s Transverse Amplitude

In drilling construction, drill string’s transverse vibration greatly improves the drill risk and complexity. Drill string is in buckling will lead it in transverse vibration, at the same time, when gas invasion appears into annular, drill string vibration will be intensified, which will create severe fatigue breakdown to drill string and improve the drill complexity. This paper has established a two-dimensional fluid-structure coupling model by CFD software to research the effect annular gas on drill string’s transverse vibration and explore the function relation between gas volume and drill string transverse amplitude. The modelling is established in static vertical interval, 1m drill string, and set the gas column length range from 88.9mm to 226.7mm to explore the law of the annular air in different well depth and transverse vibration of drill string. It can be used to predict the maximum amplitude which created by gas flowing out, so as to judge whether drill string will touch the well wall and vibrate.

BEHAVIOR OF CO-FLOW JETS COUPLED WITH A LONGITUDINAL PERTURBATION

In this study, a numerical simulation is used to show the physics of a coupled forced perturbation and liquid jet in the presence of a low-velocity coaxial gas flow. The volume of fluid (VOF) approach is employed to capture the liquid–gas interface. A sinusoidal velocity with a finite frequency and amplitude is applied at the liquid jet inlet to define a forced perturbation. An annular gas flow with a velocity lower than that of the liquid jet is imposed to examine its influence on the liquid jet breakup mechanism. The annular gas flow is modulated by the sinusoidal velocity inlet, and the effect of this flow on the liquid jet breakup mechanism is analyzed. Different ratios of the gas velocity to liquid velocity are studied. The results indicate that the low-velocity gas flow can considerably affect the behavior of the liquid jet, and this effect becomes more significant as the amplitude of the forced perturbation increases.

Gas Labyrinth Seals: Improved Prediction of Leakage in Gas Labyrinth Seals Using an Updated Kinetic Energy Carry-Over Coefficient

Abstract Though simple and fast, bulk-flow models (BFMs) for gas labyrinth seals (LSs) often predict the mass flow inaccurately. The BFM models rely on classical Neumann's equation model to characterize the flow through a labyrinth tooth. Presently, a computational fluid dynamics (CFD) analysis quantifies the effects of tip clearance (Cr) and operating conditions on the prediction of LS mass flow, and then derives an updated kinetic energy carry-over coefficient (μ1i) to improve the accuracy of Neumann's leakage equation. μ1i is a function of the seal tip clearance (Cr), the tooth pitch, and the total teeth number; but it does not depend on the seal supply or discharge pressures. The analysis selects a 14-teeth on stator LS (length/diameter = L/D = 0.29) with clearance Cr = (1/733)D and operating at nominal supply (Pin) and discharge (Pout) pressures equal to 73 bar and 51 bar, respectively, and at a rotor speed of 12 krpm (surface speed = 138 m/s). The CFD produces flow fields for LSs with a clearance varying from 80% to 200% of the nominal Cr, a gas supply pressure from 60 bar to 100 bar, and with various discharge pressures giving a pressure ratio (PR = Pout/Pin) ranging from 0.40 to 0.85. The numerous predictions deliver the mass flow as well as the bulk-flow velocities and cavity pressures within the seals. The kinetic energy carry-over coefficient (μ1i) increases with respect to the seal radial clearance (Cr). μ1i shows a parabolic correlation with PR; at first, μ1i increases with a rise in PR from a low value; and then, a further increase in PR leads to a decrease in μ1i. The coefficient μ1i is only sensitive to the PR and not to the magnitude of either the supply or discharge pressures. Lastly, for use with Neumann's leakage model, the CFD predictions produce an updated μ1i, a function of the seal geometry and the PR condition. Integration of the new μ1i correlation into a BFM code improves its accuracy to predict LS mass flow rate, a 19% difference against test data reduces to within 6%. A TOS LS tested by Ertas et al. (2012, Rotordynamic Force Coefficients for Three Types of Annular Gas Seals With Inlet Preswirl and High Differential Pressure Ratio,” ASME J. Eng. Gas Turbine Power, 134(4), p. 4250301) serves to further validate the accuracy of the modified leakage model.

Towards system pressure scaling of gas assisted coaxial burner nozzles – An empirical model

The present study investigates the influence of system pressure, gas velocity, and annular gas gap width on the resulting droplet size. Three external-mixing twin-fluid atomizers are operated at a constant liquid mass flow. The nozzle geometry is kept similar, except that the annular gas gap width is changed. At every system pressure level (1 – 21 bar), three different gas velocities were investigated by changing the gas mass flow. High-speed camera images are used for observation of primary breakup and discussed with regard to local measurements of droplet size performed by a phase Doppler anemometer. The gas momentum flux as well as the gas momentum flow were applied to describe the atomization process under varying operating conditions. Finally, an empirical model is derived, enabling the system pressure scaling of external-mixing twin-fluid atomizers for the range of gas momentum flow under investigation.

Influence of Gas Solubility on Zero Gas Flow Characteristics in MPD Operation

In this paper the authors have put forward the concept of annular zero gas flow and developed the residual gas model, considering the parameters of gas solubility, slippage, and the oil-based drilling fluid volume coefficient [1-3]. The calculation results show that in the process of continuous gas influx at the bottom of the well, the phenomenon of annular zero gas flow can be defined in terms of gas solubility [4-6]. The increase in the back pressure, the decrease in the gas invasion rate, and the decrease in the oil-based ratio can cause an increase in the length of the annular zero gas flow well section. When the back pressure increases from 0.1 to 4.0 MPa, the length of the annular zero gas flow section increases from 1000 to 2700 m. If the zero gas flow is not taken into consideration, the maximum error of the annular void ratio is 21.22% [7-10]. When the CO2, gas invades the bottom of the well, the oil-based ratio increases from 0.1 to 5, the length of the annular zero gas flow section increases from 1500 to 2250 m, and the maximum error of the annular void ratio is 26.57%.

Characteristics of sustained annular pressure and fluid distribution in high pressure and high temperature gas wells considering multiple leakage of tubing string

Tubing leakage is one of the main reasons to cause annular pressure in HPHT gas wells. This kind of annular pressure is called “sustained” because it is long-lasting and can not be eliminated by releasing. Sustained annular pressure is adverse to well barrier reliability, gas well management and repair. Even fire hazard and tubing fracture may happen. Therefore, it is necessary to study related characteristics of sustained annular pressure and fluid distribution, thus reducing potential risk. However, nearly all available research concentrates on cement integrity failure. So this paper develops a model of sustained annular pressure caused by multiple tubing leakage points. The wellbore temperature and pressure are calculated first as the gas leakage power, which considers the impact of pressure and temperature on gas properties. And then, tubing leakage point is classified as gas-leak and gas-liquid-leak. Fluid Leakage rate is calculated. Finally an equation is proposed to calculate the annular fluid distribution and annular pressure. To solve this model, the coupling relationship between leakage rate, sustained annular pressure and fluid distribution is analyzed. And then the well space and leakage time are divided into short segment. Two typical situations are selected as example. One is multiple gas leakage points. The other is combination of gas-leak and gas-liquid-leak points. By the proposed model, characteristics are analyzed, including annular pressure, pressure balance position, annular gas volume and liquid level. Also, sensitivity of factors is evaluated, including leakage point position, size, production rate and bottom pressure. The results indicate that rising process of sustained annular pressure can be divided into three zones (a, b, c). Annular pressure should be released to zone c when acoustic log is applied to locate leakage points. Method based on U-principle is not suitable to locate multiple tubing leakage points. Both pressure and liquid level should be monitored to evaluate tubing integrity. For multiple gas-leak points, position and size of leakage point have impact on maximum annular pressure and final liquid level. When gas-liquid-leak point exists, sustained annular pressure and liquid level continue changing, so annular fluid distribution and its change law can help to judge the existence of gas-liquid-leak points. Increase of production rate and decrease of bottom pressure can be considered as potential measures to reduce the rising rate and maximum sustained annular pressure under both two situations. This paper can help to diagnose tubing integrity by the characteristics of annular pressure and fluid distribution. Acoustic log is recommended to locate tubing leakage point. Potential measures are discussed to reduce maximum annular pressure. The proposed model can also be applied to calculate maximum annular gas volume and gas leakage rate when parameters of tubing leakage points are known, thus helping to assess potential risk.

Improving the rotordynamic stability of short labyrinth seals using positive preswirl

Introducing a negative preswirl at the upstream of annular gas seals has been considered as an effective way to improve the system stability. This paper demonstrates a stability enhancement approach for a short labyrinth seal using positive preswirls. The static and dynamic characteristics of the labyrinth seal with various blade numbers (5, 10, 15), inlet preswirl ratios (–0.3, –0.15, 0, 0.15, 0.3) were studied. Results show that the inlet preswirl ratio has a dramatic effect on the circumferential location of the high-pressure spot for each seal cavity, particularly for the first cavity. The inlet preswirl ratio has opposite effects on the system stability due to the difference of high-pressure spot locations between the first cavity and the others. An increasing positive inlet preswirl could improve the system stability for the labyrinth seal with fewer blades (e.g. 5 blades). Its characteristics is mainly dominated by the first seal cavity. For the labyrinth seal with 10 blades, the system characteristics shows slight dependency on the inlet preswirl ratio. For the labyrinth seal with more blades (e.g. 15 blades), the negative inlet preswirl still increases the system stability, which agrees with the conventional conclusion. The paper provides a deeper understanding on the stability improvement of the labyrinth seal.

Gasdynamic and Acoustic Characteristics of a Subsonic Jet-Edge Rod Generator of Acoustic Radiation

The design of a subsonic jet-edge rod generator of the Hartmann-type oscillator and a scheme of measuring its acoustic parameters are presented. In this generator, unlike the Hartmann oscillator, not a cylindrical gas jet but an annular gas jet, moving along the thin cylindrical rod positioned at the axis of the generator nozzle, is used. A parametric investigation of the gasdynamic and acoustic processes arising in the tubular cavity of such a generator as a result of the interaction of a subsonic gas jet with this cavity has been performed with consideration for the nonstationary structure of the gas flow in the generator cavity. Recommendations on the choice of the design of a jet-edge rod generator of acoustic radiation with regard for its practical application are given.