Annular Gas Flow Research Articles

Numerical analysis of arc physics and energy transfer under circumferential gas constraint effect

Energy dispersion and easy instability associated with arcs, the traditional metal processing heat source, restrict its development and application in the modern high-precision welding and additive manufacturing. This work aims to study the characteristics of a novel arc under circumferential gas constraint effect which can improve the arc stability and processing accuracy. A magnetohydrodynamics model was established to calculate the arc physics and heat transfer, considering the electron emission characteristics of tungsten and the effect of metal vapor. It indicates that the annular flexible constrained gas flow can significantly increase the physical characteristics such as arc temperature, plasma flow velocity and current density compared with traditional free arc which is named GTA. The arc heat generation increasement of the novel arc makes the heat transfer from arc to base metal higher than that of GTA. The additional annular gas flow inhibits the diffusion of metal vapor, stabilizing the arc and protecting the tungsten electrode from contamination. However, as the arc length decreases, eddy of arc plasma flow appears inside the arc, which can cause metal vapor to contaminate the tungsten. It's found that this novel gas flow constrained arc needs to be applied with a small current when welding with small arc length to avoid the eddy. Finally, the accuracy of the calculation model is verified by experimental detection of arc temperature.

An experimental study of volcanic tremor driven by magma wagging

SUMMARYProtracted episodes of 0.5–7 Hz pre-eruptive volcanic tremor (PVT) are common at active stratovolcanoes. Reliable links to processes related to magma movement consequently enable a potential to use properties of PVT as diagnostic eruptive precursors. A challenging feature of PVT is that generic spectral and amplitude properties of the signal evolve similarly, independent of widely varying volcano structures and conduit geometries on which most physical models rely. The ‘magma wagging’ model introduced in Jellinek & Bercovici (2011) and extended by Bercovici et al. (2013), Liao et al. and Liao & Bercovici (2018) makes progress because it depends on magma dynamics that are only weakly sensitive to volcano architecture: The flow of gas through a permeable foamy annulus of gas bubbles excites, modulates and maintains a wagging oscillation of a central magma column rising in an erupting conduit. ‘Magma wagging’ and resulting PVT are driven through an energy transfer from a ‘Bernoulli mode’ related to azimuthal variations in annular gas flow speeds. Consistent with observations, spectral and amplitude properties of PVT are predicted to evolve before an eruption as the width of the annulus decreases with increased gas fluxes. To confirm this critical Bernoulli-to-wagging energy transfer we use extensive experiments and restricted numerical simulations on wagging oscillations excited on analogue viscoelastic columns by annular air flows. We also explore sensitivities of the spatial and temporal characters of wagging to asymmetric annular air flows that are intractable in the existing magma wagging model and expected to occur in nature with spatial variations in annulus permeability. From high-resolution time-series of linear and orbital displacements of analogue column tops and time-series of axial deflections and accelerations of the column centre line, we characterize the excitation, evolution, and steady-state oscillations in unprecedented detail over a broad range of conditions. We show that the Bernoulli mode corresponds to the timescale for the buildup of axial elastic bending stresses in response to pressure variations related to air flows over the heights of columns. We identify three distinct wagging modes: (i) rotational (cf. Liao et al. 2018); (ii) mixed-mode and (iii) chaotic. Rotational modes are favoured for symmetric, high intensity forcing and a maximal delivery of mechanical energy to the fundamental magma wagging mode. Mixed-mode oscillations regimes are favoured for a symmetric, intermediate intensity forcing. Chaotic modes, involving the least efficient delivery of energy to the fundamental mode, occur for asymmetric forcing and where the intensity of imposed airflow is low. Numerical simulations also show that where forcing frequencies are comparable to a natural mode of free oscillation, power delivered by peripheral air flows is concentrated at the lowest frequency fundamental mode generally and spread among higher frequency natural modes where air pressure and column elastic forces are comparable. Our combined experimental and numerical results make qualitative predictions for the evolution of the character of volcanic tremor and its expression in seismic or infrasound arrays during natural events that is testable in field-based studies of PVT and syn-eruptive volcanic tremor.

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.

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%.

Stability of Thermocapillary Flow in High-Prandtl-Number Liquid Bridges Exposed to a Coaxial Gas Stream

The stability of thermocapillary flow in a liquid bridge made from 5cSt silicone oil (Pr = 68) is computed numerically. To improve the numerical model as compared to the standard approach, we consider the flow in the liquid bridge fully coupled to the flow in the ambient gas, temperature-dependent material parameters, and a dynamically deforming liquid–gas interface. We address the full two-phase-flow problem of interest for the space experiment JEREMI and investigate the effect of a steady axisymmetric coaxial gas flow which is imposed at the inlet of the annular gap between the liquid bridge and the outer confining cylinder. Under zero-gravity the flow is primarily driven by the imposed temperature gradient with viscous stresses from the gas phase being small. However, the heat transfer between liquid and the gas, and thus the temperature fields are strongly affected by the forced flow in the gas phase. As a result the stability of the steady axisymmetric flow depends sensitively on the flow direction and the temperature of the gas. If the temperature of the gas is identical to that of the support rod of the liquid bridge a gas stream opposing the thermocapillary stresses strongly destabilizes the basic flow. In a co-flow configuration the basic state is stabilized. Curves of neutral Reynolds numbers as functions of the strength of the annular gas flow are discussed for two aspect ratios of the liquid bridge.

A model for calculating bottom-hole pressure from simple surface data in pumped wells

Bottom-hole flowing pressure and pump intake pressure (PIP) are important parameters to optimize the performance of oil wells. In recent years, downhole sensors are becoming widely used in electrical submersible pump systems to measure these pressures. However, it is still rare to use downhole sensors in sucker rod and progressive cavity pumped wells. In this study, two correlations were developed to calculate bottom-hole flowing pressure and PIP from readily available field data. The two correlations do not require measurements of buildup tests, but they rely on measuring the dynamic fluid level and estimating fluid gradient correction factor using either (1) the tubing gas flow rate or (2) the annular gas flow rate. Then, the PIP is calculated by the summation of either (1) the tubing pressure plus the tubing gaseous liquid column pressure or (2) the casing pressure, annular gas column pressure and the annular gaseous liquid column pressure. The correlations were developed using 419 field data points (389 points for training and 30 for testing) of wide range for each input parameter. Using the training data, the mean absolute percentage deviation (MAPD) between the calculated and measured PIPs is 25% and 20% for the first and second correlations, respectively. However, the testing data showed MAPD of 33% and 12% for the first and second correlations, respectively. The accuracy of these correlations is significantly higher than that of the previously available methods, and the correlations require simpler input. Such study is an original contribution to calculate the PIP with improved accuracy and without downhole pressures sensors.

Research on coherent jet oxygen lance in BOF steelmaking process

The properties of conventional supersonic oxygen jet and coherent jet were simulated by Fluent in this paper. Their velocity field and jet radius were analyzed comparatively. The attenuation of gas jet was slowed down by the coherent jet oxygen lance. Besides, the effects of annular flow rate on the central oxygen jet velocity were also researched. It is showed that the core of the jet is gradually prolonged with the increase of annular gas flow rate. Based on the simulations, the designed coherent jet oxygen lance was experimented in 35 ton converter. It is found that the end-point phosphorus content is decreased from 0.024 to 0.016%, and dephosphorization rate is increased significantly, the average consumption of steel material is reduced by 3.4 kg/t, and iron-loss of slag is also reduced, which is favorable to improve oxygen utilization and metallic yield. This research provides good foundation for coherent jet technology promotion and application in Basic Oxygen Furnace (BOF).

The Effect of Droplet Sizes on Overspray in Aerosol‐Jet Printing

One of the most important concerns of aerosol‐jet based printing (AJP) is the substantial amount of overspray (OS) that can accompany printed traces. In this study, the authors develop a full, three‐dimensional (3‐D) computational fluid dynamics (CFD) model of the aerosol carrier gas flow (CGF), that is, confined by an annular sheath gas flow (ShGF) during the AJP process. The authors then use this model to pinpoint the fundamental fluid mechanics principles that control the OS in a standard AJP process as a function of the droplet size distribution and the ShGF rate. In their model, the authors consider the trajectories of various sized, monodisperse droplets in an ink‐stream in order to unravel the manner in which the OS is dictated by the intricate interplay between drop size and both gas flow (CGF and ShGF) rates. Their results explain several experimental results, namely 1) that there is an abundance of smaller sized drops in the OS region at low ShGF rates; 2) that the OS first reduces and then increases as the ShGF rate increases; and 3) that there is no longer a prevalence of smaller particles in the OS region at larger ShGF rates. The authors also discuss the hitherto unaddressed issues related to how 1) the large impact velocity of the ink droplets at the substrate surface (and the resultant Weber number) and 2) the misalignment between the CGF and the cylindrical axis of the ShGF annulus appear to affect OS. The authors anticipate that their analysis will provide an important understanding in the optimization of operating parameters for AJP with respect to OS that can result in an overall improvement in printing resolution.

Simulation of the primary breakup of a high-viscosity liquid jet by a coaxial annular gas flow

The conversion of low-grade fossil and biogenic energy resources (petcoke, biomass) to a synthesis gas in a high pressure entrained flow gasification process opens a wide spectrum for high efficient energy conversion processes. The synthesis gas can be used for production of methane (SNG), liquid fuels (BtL, CtL) or as fuel for operation of a gas turbine in a combined cycle power plant (IGCC). The production of a tar free high quality syngas is a challenging objective especially due to the fact that typical liquid or suspension fuels for entrained flow gasifiers feature viscosities up to 1000 mPas. Fuel droplet conversion at typical entrained flow gasification conditions is characterized by heat up, evaporation and subsequent degradation of the vapour phase. To guarantee a high fuel conversion rate in the gasifier an efficient atomization of the fuel is required. Mainly twin-fluid burner nozzles are used for atomization of those typically high viscous fuels. The present study is focused on the assessment of the accuracy of CFD computations for the primary breakup of high-viscosity liquids using an external mixing twin fluid nozzle. In a first step experiments were performed with a Newtonian glycerol-water-mixture featuring a liquid viscosity of 400 mPas. Jet breakup was investigated using a high speed camera as well as PIV and LDA-System for a detailed investigation of the flow field. In a second step the experimental results serve as reference data to assess the accuracy of CFD computations. Compressible large eddy simulations (LES) were performed to capture the morphology of the primary breakup as well as the important flow field characteristics. A Volume of Fluid (VOF) approach was used to track the unsteady evolution and breakup of the liquid jet. Comparison of experimental and numerical results showed good agreement with respect to breakup frequency, velocity fields and morphology.

Technology Focus: Well Integrity (January 2015)

Technology Focus Cementing operations represent one of the more critical aspects concerning well integrity. Despite the large amount of research and the huge number of operations throughout the years, the industry still faces well-integrity problems during and after cementing operations. These occurrences have been experienced by the petroleum industry worldwide and can happen at any moment of the well life cycle, from the drilling phase to abandonment after the well’s production life. Here, I have categorized these well-integrity issues according to the moment at which they manifest: during the cement displacement inside the well (minutes); after the cement placement (hours to days), and after the cement has been cured (months to years). The first category may result in serious well-control problems, including blowouts (the Macondo blowout was related to cementing operations). According to a survey published by the former Minerals Management Service, during the period between 1992 and 2006, the leading factor contributing to blowouts in the US offshore area was cementing (18 out of 54 blowouts reported). These well-control problems are normally caused by improper design of the cementing operation in terms of hydrostatic pressure of the cement slurries or preflushes, failure when mixing the cement slurry to achieve its planned density, lost circulation during the cement displacement, or failure of casing float equipment. In my opinion, well-control courses must dedicate more time to discussing well-control problems during cementing operations. The second category encompasses those issues normally associated with the loss of hydrostatic pressure of the cement slurry during its initial hydration period and consequent gas flow inside the annulus. This condition also can cause well-control problems, pressure buildup in the annulus between casing strings, and zonal-isolation problems later on. The industry has several ways to mitigate these occurrences that include, but are not limited to, the use of gas-block additives, the design of cement slurries with short transition times, the application of surface pressure on the annulus, the use of external casing packers, and the implementation of multistage cementing operations. Despite the application of these mitigation measures, annular gas flow after cementing is still a recurrent problem, and remedial solutions are normally expensive and difficult. The last category refers to long-term problems normally caused by poor cementing practices, with detrimental effects on zonal isolation. Inadequate drilling-mud removal during the cement-slurry displacement in the annulus, insufficient cement height that may lead to casing leakage and corrosion problems, and the channels that result from the gas flow previously mentioned are some of the factors that can contribute to long-term well-integrity failures. Again, remedial solutions are normally expensive. JPT Recommended additional reading at OnePetro: www.onepetro.org. IPTC 16592 Mitigation of Deepwater Shallow Hazards With Simplified Cement System: South China Sea Case Study by Todd B. Ellis, Baker Hughes, et al. OTC 24369 Well-Integrity Analysis Applied to Workover Prediction by T.C. da Fonseca, Petrobras, et al. SPE/IADC 163474 Automated Alarms for Smart Flowback Fingerprinting and Early Kick Detection by Tarab H. Ali, Baker Hughes, et al.

Annular Pressure Reduction During Primary Cementing

Annular pressure reduction during cementing is a major factor causing annular gas flow. It has been widely accepted and proven experimentally that the pressure reduction phenomenon results from the shear stress opposing downward motion of slurry undergoing volume reduction. The models that have been proposed to describe this process are based on the gel strength and shear stress developments in time and ignore system compressibility. They explain the pressure reduction process observed in the lab where compressibility of the system is very small. However, the models cannot explain the pressure reduction patterns observed on the field where compressibility is significant and the time-dependent effects of cement slurry volume loss significantly contributes to the process. The paper presents a mathematical model combining the effects of gel strength, volume reduction, and compressibility of cement slurry to describe pressure loss in the annular cement column. Results from the model, shown in the paper, compare very well with the data from the laboratory and field tests. Also, the simulated results explain discrepancies between the pressure loss patterns observed in the lab and field tests.

Sedimentation of nanoparticles in the field of centrifugal forces

The coefficients of mechanical resistance to motion of carbon nanoparticles (graphenes, nanotubes, and fullerenes) are theoretically determined. The average velocity of their motion in an annular gas flow rotating at a constant angular velocity is calculated. The results are obtained within the ideal gas model.

Behaviour of an air-assisted jet submitted to a transverse high-frequency acoustic field

Acoustic instabilities with frequencies roughly higher than 1 kHz remain among the most harmful instabilities, able to drastically affect the operation of engines and even leading to the destruction of the combustion chamber. By coupling with resonant transverse modes of the chamber, these pressure fluctuations can lead to a large increase of heat transfer fluctuations, as soon as fluctuations are in phase. To control engine stability, the mechanisms leading to the modulation of the local instantaneous rate of heat release must be understood. The commonly developed global approaches cannot identify the dominant mechanism(s) through which the acoustic oscillation modulates the local instantaneous rate of heat release. Local approaches are being developed based on processes that could be affected by acoustic perturbations. Liquid atomization is one of these processes. In the present paper, the effect of transverse acoustic perturbations on a coaxial air-assisted jet is studied experimentally. Here, five breakup regimes have been identified according to the flow conditions, in the absence of acoustics. The liquid jet is placed either at a pressure anti-node or at a velocity anti-node of an acoustic field. Acoustic levels up to 165 dB are produced. At a pressure anti-node, breakup of the liquid jet is affected by acoustics only if it is assisted by the coaxial gas flow. Effects on the liquid core are mainly due to the unsteady modulation of the annular gas flow induced by the acoustic waves when the mean dynamic pressure of the gas flow is lower than the acoustic pressure amplitude. At a velocity anti-node, local nonlinear radiation pressure effects lead to the flattening of the jet into a liquid sheet. A new criterion, based on an acoustic radiation Bond number, is proposed to predict jet flattening. Once the sheet is formed, it is rapidly atomized by three main phenomena: intrinsic sheet instabilities, Faraday instability and membrane breakup. Globally, this process promotes atomization. The spray is also spatially organized under these conditions: large liquid clusters and droplets with a low ejection velocity can be brought back to the velocity anti-node plane, under the action of the resulting radiation force. These results suggest that in rocket engines, because of the large number of injectors, a spatial redistribution of the spray could occur and lead to inhomogeneous combustion producing high-frequency combustion instabilities.

Convergent/divergent segmented exhaust nozzle

A segmented exhaust nozzle for attenuating noise from a turbofan jet engine without adversely impacting the operability or operability limit related performance of the engine. The exhaust nozzle includes spaced apart fan nozzle inner and outer walls which form an annular exhaust gas flow path therebetween. The fan nozzle outer wall is segmented at the downstream end. The outer wall curves inwardly towards the inner wall and then turns back away from the inner wall to form an arcuate protrusion that extends into the exhaust gas flow path forming an aerodynamic throat. Through the segmented portion of the nozzle, the outer wall then continues to curve away from the inner wall before again curving back towards the inner wall at a nozzle exit station. The nozzle exit effective area is approximately equal in cross sectional area to a conventional exhaust nozzle exit area. The inwardly curving and then segmented outwardly curving portion of the exhaust nozzle forms a geometric influction that serves to reduce noise without negatively affecting engine operability or operability limit related performance.

Modeling Oil Flows in Engine Sumps: Drop Dynamics and Wall Impact Simulation

A computational preliminary design tool has been developed to help simulate drop-related processes that take place in an oil sump of a gas turbine engine accounting for drop motion, deformation, breakup, and drop∕wall interactions including wall film impact and potential splashing. Aerodynamic interactions with the gas phase are considered using an exact solution of the Navier–Stokes equations to approximate the annular gas flow. Detailed results for the baseline case that attempts to replicate the conditions found in a typical oil sump of a turbofan engine are presented. In addition, the results of more general parametric studies utilizing a simplified geometry that investigated the effects of changing various parameters are discussed.

EFFECT OF PARTICLE SIZE DISTRIBUTION ON LOCAL VOIDAGE IN A BENCH-SCALE CONICAL FLUIDIZED BED DRYER

ABSTRACT The effect of particle size distribution (PSD) on local voidage has been investigated in a conical fluidized bed containing dried placebo pharmaceutical granule. For each of the five PSDs examined, the static bed height was varied between 0.12 and 0.17 m and the superficial gas velocity was varied between 0.05 and 0.75 m/s. The local voidage was measured using a twin-plane electrical capacitance tomography (ECT) system. A wide PSD containing 12 wt% solids with a diameter of 2 mm or larger resulted in two different types of gas flow: an annular gas flow up to a gas velocity of 0.50 m/s and a centrally concentrated gas flow above 0.50 m/s. The mixtures containing less coarse material exhibited a centrally concentrated gas flow surrounded by a dense phase at the walls of the bed over the entire range of gas velocities and bed heights examined. Consideration of previous work by other researchers suggests that the behavior of the wide PSD mixture is due to segregation at the lower velocities. The local voidage was sensitive to small changes in static bed height. For the wide PSD mixture at a fixed gas velocity, the gas tended to spread more uniformly over the bed cross-section as static bed height increased. The opposite was true of the other mixtures, i.e., the gas flow became more centralized with increasing bed height.

Hydrodynamic characteristics of a two-dimensional jetting fluidized bed with binary mixtures

A cold model of a two-dimensional jetting fluidized bed with a cross section 300 mm×50 mm was employed to study the jet penetration depth and flow regime transitions. Four types of particles and four kinds of binary mixtures were used as bed materials in the test. In addition, a new multi-channel pitot tube system was developed to investigate the penetration depth. Experimental results indicate that the penetration depth decreases when the annular gas flow rate is increased from 1.0 Q mf to 2.5 Q mf, while it remains constant for annular gas flow rate ranging from 2.5 Q mf to 3.0 Q mf (annular gas flow rate including fluidization air from the plenum chamber and the V type gas distributor). A new correlation is presented for predicting the penetration depth for different annular gas flow rate. The effects of particulate density, particulate diameter, jet velocity, static bed height, nozzle diameter, and annular gas flow rate on flow regime transitions, i.e. jet, transition flow regime and spout, were systematically examined. A correlation to predict flow regime transitions was also developed, which was in good agreement with the experimental data from the present study and other references.

Experiments on conduit flow and eruption behavior of basaltic volcanic eruptions

Multiphase flow in basaltic volcanic conduits is investigated using analog experiments and theoretical approaches. Depending on gas supply, large gas bubbles (gas slugs) may rise through basaltic magma in regimes of distinct fluid‐dynamical behavior: ascent of single slugs, supplied slugs fed from the gas source during ascent, and periodic slug flow. An annular flow regime commences at the highest gas supply rates. A first set of experiments demonstrates that the growth of gas slugs due to hydrostatic decompression does not affect their ascent velocity and that excess pressure in the slugs remain negligible. The applicability of theoretical formulae describing slug ascent velocity as a function of liquid and conduit properties is evaluated in a second set of experiments. A third set of experiments with continuous gas supply into a cylindrical conduit are scaled to basaltic conditions over Morton, Eotvös, Reynolds, and Froude numbers. Gas flow rate and liquid viscosity are varied over the whole range of flow regimes to observe flow dynamics and to measure gas and liquid eruption rates. Foam generation by slug bursting at the surface and partial slug disruption by wake turbulence can modify the bubble content and size distribution of the magma. At the transition from slug to annular flow, when the liquid bridges between the gas slugs disappear, pressure at the conduit entrance drops by ∼60% from the hydrostatic value to the dynamic‐flow resistance of the annular flow, which may trigger further degassing in a stored magma to maintain the annular flow regime until the gas supply is exhausted and the eruption ends abruptly. Magma discharge may also terminate when magma ascent is hindered by wall friction in long volcanic conduits and the annular gas flow erodes all magma from the conduit. Supplied slugs are found to reach much higher rise velocities than unsupplied slugs and to collapse to turbulent annular flow upon bursting at the surface. A fourth set of experiments uses a conduit partially blocked by built‐in obstacles providing traps for gas pockets. Once gas pockets are filled, rising gas slugs deform but remain intact as they move around obstacles without coalescence or significant velocity changes. Bursting of bubbles coalescing with trapped gas pockets causes pressure signals at least 3 orders of magnitude more powerful than gas pocket oscillation induced by passing liquid. Our experiments suggest a refined classification of Strombolian and Hawaiian eruptions according to time‐dependant behavior into sporadically pulsating lava fountains (driven by stochastic rise of single slugs), periodically pulsating lava fountains (resulting from slug flow), and quasi‐steady lava fountains (oscillating at the frequency of annular‐flow turbulence).

Shrinkage of Oil Well Cement Slurries

Abstract Gas leakage into and through the cemented annulus in oil and gas wells is a safety problem. The conditions for gas migration develop when the hydrostatic pressure of the hydrating cement slurry column slowly declines and finally falls below the pore pressure of a gas bearing formation. This pressure decline is mainly caused by chemical shrinkage of the cement. A low shrinkage will reduce the pressure decline and hence the risk of gas migration. A new and simple method for measurement of total chemical shrinkage at high temperatures has been developed. It has proved to be reliable and sensitive for monitoring the cement hydration process. The equipment was used to show that there is a marked difference in shrinkage behaviour for the tested slurries up to 180 ° C, probably due to a temperature effect on the cement hydration chemistry. The results confirmed the correlation between total shrinkage and cement content, and thus it follows that using more extender will reduce the likelihood of gas migration. Introduction Cement slurries are used in oil and gas wells for cementing the steel casing to the wellbore and thus sealing the rock formations from the well. Gas leakage into and through the cemented annulus is still a problem, in particular where shallow gas sands are penetrated and in deep gas wells, i.e., under high temperature and high pressure (HTHP) conditions. The leakage may manifest itself either as vertical communication between permeable gas reservoirs or migration all the way to the surface. Gas pressure in excess of the cement hydrostatic slurry pressure is the driving force behind gas migration. Reasons and mechanisms of gas migration are discussed by Levine et al.,(1) Sabins et al.(2), and Cheung and Beirute(3). Annular gas flow may be initiated when the hydrostatic pressure of the cement slurry declines and falls below the pore pressure of a gas bearing formation due to the combined effect of shrinkage, fluid loss to the porous well bore and gel strength build up. Normally a slurry volume reduction would be compensated for by contraction and downward movement of fluids from above, thus maintaining its hydrostatic head. However, during the setting process the cement slurry develops a gel structure causing it to stick to the wall thus hindering compensation of the shrinkage. The second requirement for gas migration is that the cement must enable gas flow into the cement either by permeability, channels, microannuli or microfractures before the slurry has reached a sufficient strength. Thus, the gas migration phenomenon is thought to be initiated during the transition state, from being fluid to becoming hard, i.e., between initial and final set of the cement. A low shrinkage is preferable because the resulting hydrostatic pressure decline will be lower than for a slurry with high shrinkage, i.e., pressure equilibrium between gas and slurry column is reached at a later point of time. The chemical shrinkage may be divided into two parts: external shrinkage and total shrinkage.

Interrelationship Between Critical Cement Properties and Volume Changes During Cement Setting

Summary This paper surveys cementing terminology for clarifying cement setting and its capability to control interzonal fluid flow. It also demonstrates the results of several test procedures designed to measure two or more cement properties simultaneously under conditions that simulate job applications. These procedures test the validity of theories and procedures for controlling annular gas flow, cement bonding, interzonal channeling, and intrazone isolation. From these results, guidelines may be developed for using laboratory data to determine times for temperature logs, perforating, and stimulation treatments. Existing laboratory equipment was modified and new equipment was designed to measure the interrelationships between static-gel-strength development, volume reductions, hydration temperatures, permeability changes, net plastic-state shrinkage, and compressive strength. Test results indicated that (1) hydration volume reduction (HVR) during the transition period is relatively small and shows a general correlation to unit volume cement content; (2) temperature increases from hydration show a rough correlation to static-gel-strength development; (3) permeability during static-gel-strength development decreases rapidly and shows a definite correlation to fluid-loss test values; (4) plastic-state shrinkage is only a very small part of the total HVR; (5) a general correlation exists between compressive strength, HVR, and heat of hydration; (6) the HVR for an expansive cement was greater than that of a nonexpansive cement, but its plastic-state shrinkage was less.