Annular Fluid Research Articles

Modeling analysis of the temperature profile and trapped annular pressure induced by thermal-expanded liquid in a deep gas well

The trapped annular pressure (TAP) caused by thermal expansion is one of the serious challenges for the safe production of a deep gas well. Therefore, this article proposes a model to calculate the temperature profile of the deep gas well based on the heat transfer process and the gas properties. With the help of the temperature model, the TAP in the tubing–casing annulus is analyzed according to the annular fluid distribution and the volume consistence law. The results indicate that the temperature inside the tubing string decreases faster under higher bottom hole pressure. When the tubing–casing annulus is totally filled with the annular protection liquid, the TAP continues increasing with the production rate. Considering the high production rate, the TAP is inevitable and high enough to damage the integrity of the deep gas well. The nitrogen gas mitigates the TAP by reducing the annular liquid volume and providing the extra space to accommodate the thermal-expanded annular liquid. A good mitigation performance can be achieved no matter how large the production rate is. The mitigation performance can be divided into the fast-decreasing stage, the efficient control stage, and the stable stage. These three stages occur as the nitrogen gas column length increases. The compression of the nitrogen gas volume plays a major role in the fast decrease stage while the reduction of the annular liquid plays a major role in the stable stage. For the best cost-effectiveness, the nitrogen gas column is recommended in the efficient control stage and should not exceed 15%.

Modal added-mass matrix of an elongated flexible cylinder immersed in a narrow annular fluid, considering various boundary conditions. New theoretical results and numerical validation

This paper considers the fluid–structure interaction problem of two coaxial cylinders separated by a thin layer of fluid. The flexible inner cylinder is imposed a small amplitude harmonic displacement corresponding to a dry vibration mode of an Euler–Bernoulli beam, while the external cylinder is rigid. A new theoretical formulation based on the assumption of a narrow fluid annulus is derived to estimate the modal added-mass matrix of the vibrating cylinder. This formulation accounts for the finite length of the flexible cylinder, clearly highlights the effect of the aspect ratio of the vibrating cylinder on the structure of the added-mass matrix, and covers all types of classical boundary conditions in the same theory and can easily be implemented in any numerical computing environment. The diagonal coefficients of the added-mass matrix are shown to increase with the confinement, with the aspect ratio of the flexible cylinder, and are sensitive to the wave-number of the vibration mode. Also importantly, we show that the dry vibration modes generate off-diagonal coefficients that vanish for an infinitely long cylinder. Our theoretical observations are corroborated by an extensive set of CFD numerical simulations, covering all types of classical boundary conditions, different confinement configurations, and different aspect ratios of the vibrating cylinder. The results obtained are presented in graphical form, which can be directly applied in engineering applications.

Predicting the annular pressure behavior during water injection in offshore wells with a transient, multiphysics model

Predicting the pressure behavior of annular fluids during production and injection operations in oil wells became a major research topic in the exploration of deep-water reservoirs. Although pseudo-steady-state models can predict annular pressure buildup in most production scenarios, more robust transient approaches are required in injection operations due to the short time scales involved. This paper presents a new transient thermo-structural model that calculates temperature and pressure profiles during a recycled saltwater injection operation in a well with three concentric annuli. The model was compared with field data acquired uninterruptedly for a seven-day period containing two consecutive well shutdowns. The maximum relative deviation between the numerical results and the field down-hole pressure was lower than 4%. Absolute down-hole temperature deviations were smaller than 5 °C. Furthermore, simulations based on a newly proposed dimensionless inter-annular pressure difference parameter facilitated identifying the risk of thermally-induced casing collapse due to contrasting rates of pressure change between neighboring annuli. The effectiveness of mitigating such risks using rupture disks was evaluated numerically, and the most favorable scenario consisted of rupture disks installed in the two casings connecting the three annuli.

Field-controlled flow and shape of a magnetorheological fluid annulus.

We investigate the behavior of a magnetorheological (MR) fluid annulus, bounded by a nonmagnetic fluid and confined in a Hele-Shaw cell, under the simultaneous effect of in-plane, external radial and azimuthal magnetic fields. A second-order mode-coupling theory is used to study the early nonlinear stage of the pattern-forming dynamics. We examine changes in the morphology of the MR fluid annular structure as a function of its magnetic-field-tunable rheological properties, as well as the combined magnetic field's intensities, and thickness of the ring. Our weakly nonlinear perturbative results show that, depending on the system control parameters, the MR fluid annulus adopts various stationary shapes. These equilibrium annular structures present slightly bent, asymmetric fingered protrusions which may emerge on the inner, outer, or even on both boundaries of the magnetic fluid ring. On top of these morphological changes, we find that the resulting permanent shape patterns rotate with a well defined angular velocity. We focus on analyzing how the overall shape of the fingered patterns, in particular their sharpness and asymmetric form, as well as the number of resulting fingers are impacted by the magnetic-field-dependent yield stress of the MR fluid annulus. The influence of the magnetically controlled rheological properties of the MR fluid on the angular velocity of the rotating annulus is also scrutinized.

Protection of downhole pumping equipment in case of high sand production

Most of oil reservoirs in Kazakhstan are currently at the final phase of mining, the productive strata locate in weakly cemented sedimentary rocks, and sand production is one of the main complication factors in oil recovery. This study aimed to find the main causes of failure of deep-well pumping equipment in oil reservoirs with high sand production and to elaborate practical guidance for a soil user to reduce the adverse effect of such failures. The research procedure included the statistical analysis and systematization of information on equipment failures in Buzachi-North oil field, as well as the numerical modeling of influence exerted by well geometry on wear of oil well tubing and rods in the conditions of sand production using bundled software PC-PUMP. Based on the research findings, some practical recommendations were proposed to optimize well operation in the conditions of high sand production, which allowed expanding repair interval. The recommendations included the method earlier unused in oil production, namely, protection of deepwell pumping equipment from sand plugging with the help of sand valves meant for the periodic (as sand accumulates in oil well tubing OWT, or during well shuts-in) removal of sand-and-liquid column from OWT to annulus. This reduces probability of deep-well pump rotor jamming and preserves feasibility of the system restart. Inclusion of the sand valve in the deep-well pump assembly eliminates additional fitting of downhole pumping equipment with standard back and drain valve as the sand valving performs their functions. Moreover, the sand valving enables different operations connected with pumping of process fluids in annulus at disengaged pump. The proposed design also allows communication between OWT and annulus for complete drain of liquid from OWT by means of pneumatic effect from ground surface (well mouth) du ring maintenance and complete overhauls in wells. The use of such multi-function valves is a promising trend to enhance efficiency of processes in borehole mining of fluid minerals. It allows simplifying deep-well pump hookup due to reduced number of various-purpose valves and owing to multitask of implemented operations, as well as eliminates engagement of supplemental special machinery and human resources.

Prediction and Analysis of Sustained Annular Pressure of Gas Wells Considering the Influence of Production System and Liquid Thermodynamic Properties

Abstract Sustained annular pressure seriously affects the safety of wellbores. However, most models take the annulus liquid thermodynamic parameters and tubing pressure as constant values. Therefore, the prediction model of wellbore temperature and pressure field was first established based on the basic theory of heat transfer to provide basic parameters for the follow-up sustained annular pressure calculation. Second, based on fluid mechanics theory, a prediction model of sustained annular pressure was developed considering the influence of tubing pressure fluctuation and thermostatic coupling on the thermodynamic parameters of annulus fluid. Then, the iterative method was used to solve the annular pressure. Combined with the field monitoring data of a high-pressure gas well, the model’s accuracy was verified. Finally, a parameter sensitivity analysis was performed, and annular pressure regulation steps were determined. The obtained results revealed that the developed prediction model met engineering accuracy requirements. In a high-pressure gas well, after repeated cycles of shutting in and opening the well for 19 times, gas column height was equal to the leakage point position. Gas column height, leakage point depth, shut-in well annulus temperature, and liquid density were found to affect the change speed and stable value of the annular pressures of shut-in and open wells. The sensitivities of leakage point depth and shut-in annulus temperature to annular pressure regulation were the highest and lowest, respectively. Research results helped us reasonably and quickly manage and control sustained annular pressure gas wells.

Vibration-assisted annular fluid displacement for rig-less well abandonment operations

Through-tubing abandonment is the operation where the production tubing is cut or punched and cemented inside the production casing. This can establish a full cross-sectional barrier or isolation in wells where the cement behind the production casing provides zonal isolation. Furthermore, isolation in the form of a through-tubing abandonment plug can be placed from a light-well intervention vessel, which reduces the risk and carbon footprint compared to conventional abandonment designs that rely on a full mobile offshore drilling unit. A potential challenge involved in placing the through-tubing abandonment plug is the risk of incomplete fluid displacement from the annulus behind the tubing: The tubing eccentricity may be considerable where the barrier should be placed, and the original packer fluid may be contaminated resulting in unfavorable displacement conditions. Furthermore, the tubing cannot be reciprocated or rotated to compensate for these conditions.Agitator tools that generate lateral and axial vibrations in the tubing for mitigating potentially adverse displacement conditions have recently become available to the industry. We study potential effects of such tools by performing annular displacement experiments using a novel test rig that enables lateral movement of the inner tubing. The original fluid to be displaced was viscosified with a polymer additive to generate apparent yield stress behavior and to thereby develop challenging displacement conditions, particularly in the eccentric annulus. Experimental results and supporting numerical simulations show that lateral motion of the tubing is effective in displacing the yield stress fluid from the narrow side of the eccentric annulus, and to oppose the tendency toward stratification in both concentric and eccentric annuli. The experiments further show that vibration is effective in improving the displacements at inclinations ranging from vertical to 80° from the vertical, and that high frequency vibrations are more effective than lower frequency vibrations at the same amplitude. Results from a mechanical vibration analysis suggests the vibration amplitudes increase with increasing flow rates, and that potentially beneficial vibrations can propagate over axial lengths that are comparable or longer than minimum barrier length requirements.

Study on borehole temperature distribution when the well-kick and the well-leakage occurs simultaneously during geothermal well drilling

When the well-leakage or the well-kick occurs during drilling geothermal wells, the borehole temperature is redistributed. It causes the rheology and density of the mud to change, which in turn affects the borehole pressure control, borehole stability and cementing quality. In order to understand the distribution law of borehole temperature under complex working conditions in geothermal well drilling, this paper establishes a borehole temperature distribution model when well-leakage and well-kick occur simultaneously. The borehole temperature distribution law when well-leakage and well-kick occur simultaneously is obtained. When both the well-kick and the well-leakage exist in the drilling process, the temperature of the borehole and the formation fluctuates near the location where the well-kick and well-leakage occurs. Under the ordinary cycle condition, the highest temperature of annular drilling fluid is located at 5325 m; When the well-leakage occurs at the bottomhole and the well-kick occurs at a specific location in the borehole, the highest temperature of annular drilling fluid is at 5250 m; When the well-kick takes place at the bottomhole and the well-leakage occurs at a specific location in the borehole, the highest temperature of annular drilling fluid at 5286 m place. Compared with the ordinary cycle condition, when the well-leakage occurs at the bottom of the well and the well-kick occurs at a specific location in the borehole, the bottomhole mud temperature decreases after the same cycle time. And the temperature of the mud and the formations increases significantly near and above the position where the well-kick takes place. When the well-kick takes place at the bottom of the well, and the well-leakage occurs at a specific location in the borehole, compared with the ordinary cycle condition, the temperature of the bottom hole mud increases after the same cycle time and the mud temperature increases rather than decreases at the well-leakage location. Compared with other situations, it is found that when the well-kick occurs at the bottomhole, it has a more significant impact on the temperature distribution of the entire borehole.

Improved Method for Testing Integrity Loss of Wells with Sustained Casing Pressure

The pressure bleed-down/build-up (B/B) testing is enforced by the Bureau of Safty and Environmental Enforcement (BSEE) regulations in the US for gas-leaking wells with recurrent casing head annular pressure dubbed “Sustained Casing Pressure” (SCP). The bleed down test involves bleeding the pressure with a needle valve. Once the pressure reached to zero or stabilized, the valve is closedand a 24-h pressure monitoring starts. Analysis of the tests mandated by regulations involves merely a qualitative assessment of the low or high level of environmental risk measured by leak size. A quantitative analysis—based on mathematical models—has been already proposed and used for sizing cement leaks but the models give ambiguous results due to oversimplifying assumptions of the cement leak system and disregard for the testing procedure. This work addresses shortcomings of the current B/B testing and analysis methods by approaching the testing procedure as a source of useful information about the system of annular well leakage. We demonstrate that using a mathematical model for globally matching all stages of the B/B test data would prioritize the pressure bleed-down stage over the buildup stage thus making the latter stage mostly irrelevant in providing information on the whole gas migration system. We verify the hypothesis that a stage-wise analysis would improve the results by separately considering three stages of the B/B test: pressure bleed down, constant flow (steady-state), and pressure buildup, and, then, comparatively reconciling values of the system parameters. The stage-by-stage approach is feasible because—as shown by the sensitivity analysis—each stage is controlled mostly by a single parameter. Also presented is the development of an improved procedure for performing the B-B test. The study shows that the operational parameters controlled by the test operator—duration and size of the needle valve opening, and pressure recording time step, might significantly change values of the test output—the minimum bleed down, and the maximum 24-h buildup pressures. It is also found that the zero bleed-down pressure does not necessarily indicate that the leak is small as it also depends upon the length and properties of the annular fluid column above the cement top.

Annular Creep Barrier Evaluation and Qualification Using Ultrasonic Measurements

Summary It is well-known that formations that exhibit active creep behavior under downhole conditions, such as reactive shales and mobile salts, can form annular barriers across uncemented or poorly cemented annular sections behind casing strings. Such creep barriers can simplify well abandonments, particularly in high-cost offshore environments. Evaluation and qualification of creep barriers in the field, however, have proven challenging and labor-intensive when casing is perforated, and annular rock material is pressure-tested to verify its sealing ability. This work seeks to eliminate the need for pressure testing by allowing the barrier to be qualified using only casedhole log measurements. Sophisticated rock mechanical laboratory experiments under realistic downhole conditions were conducted to investigate the formation of creep barriers by North Sea Lark shale. The experiments evaluated barrier formation while varying annular fluid chemistry and temperature. Measurement parameters included creep rate, pressure transmission across newly formed barriers, pressure breakthrough through the newly formed barriers as well as ultrasonic responses by the shale. It was found that the Lark/Horda shale has a distinct anisotropic ultrasonic wave velocity profile that uniquely characterizes it. This can be used to identify its presence in an annular space when contacting the casing. A main conclusion is that a Lark shale barrier can be qualified through casedhole sonic and ultrasonic logging alone without the need for pressure testing if (1) the magnitude of the wave propagation velocity of the shale behind casing can be confirmed (2077 m/s for Lark shale); (2) the characteristic velocity anisotropy profile, unique to the shale (~10.1% for Lark shale), can be verified; (3) good contact with/bonding to the casing is observed; and optionally (4) anisotropy in the time behavior of the shale contacting the pipe is observed when the barrier is stimulated artificially. If these conditions are met, then our experiments show that the barrier will have excellent hydraulic sealing ability, with a permeability of a few microdarcies at most and a breakthrough pressure that approaches the minimum horizontal effective stress value. Additional findings are that shale heating will accelerate barrier formation but may damage the shale formation in the process. Extraordinary fast annular closure and barrier formation with evident shale rehealing was observed by using a concentrated KCl solution as pore fluid, showing the merits of barrier stimulation by chemical means. This result can be explained by considering the effect of solutes on shale hydration forces.

Simplifying Well Abandonments Using Shale as a Barrier

Summary Well abandonment is one of the biggest challenges in the oil and gas industry, both in terms of cost and effort as well as the technical hurdles associated with wellbore sealing for an indefinite term. A mechanism that may be exploited to simplify well abandonments is using natural shale or salt formations for the creation of annular barriers. Currently, uncemented annuli often require casing cutting and pulling or milling before abandonment plugs can be set, which necessitates the use of a drilling rig. This is an expensive, time- and labor-intensive process, particularly offshore. However, shale or salt creep may naturally form a barrier behind uncemented casing sections. With a qualified annular shale barrier in place, the well may only require the setting of abandonment plugs within the existing casing string(s), a task that can often be done rigless and with significantly less effort. The work described in this paper presents the results of a rock mechanical investigation into the creep behavior of North Sea shales and their ability to form effective annular barriers. Field core from the Lark shale, a member of the Hordaland Group, was used to conduct dedicated, customized experiments that simulated the behavior of shale confined under downhole effective stress, pressure, and temperature conditions to fill in an annular space behind a simulated casing string. Full-scale triaxial rock mechanics equipment was used for testing cylindrical shale samples obtained from a well-preserved field core in a setup that mimicked an uncemented casing section of a well. The deformation behavior of the shale was monitored for days to weeks, and the formation of the annular barrier was characterized using dedicated strain measurements and pressure pulse decay probing of the annular space. The large-scale laboratory results clearly show that the Lark shale will form competent low permeability annular barriers when left uncemented, as confirmed using pressure-pulse decay measurements. They also show that experimental conditions influence the rate of barrier formation; higher effective stress, higher temperature, and beneficial manipulation of the annular fluid chemistry all have a significant effect. This then opens up the possibility of activating shale formations that do not naturally create barriers by themselves into forming them (e.g., by exposing them to low annular pressure, elevated temperature, different annular fluid chemistry, or a combination of these). The results are in very good agreement with field observations reported earlier by several North Sea operators.

Coupling effect of annular volume change and non-linear fluid thermal property on annular pressure buildup in multi-layer annuluses of deepwater well

Coupling effect of annular volume change and non-linear fluid thermal property on annular pressure buildup in multi-layer annuluses of deepwater well

Investigation of thermohydraulic characteristics of a novel triple concentric pipe minichannel heat exchanger

In this paper, a novel triple concentric pipe minichannel heat exchanger (TCPMHE) is presented and built, which is a combination of concentric pipe heat exchanger and minichannel heat transfer technology. A range of experimental investigations are further systematically performed by varying the volume flow rate of hot water (Vh), inner annulus size (Si) and hot fluid inlet temperature (th, i) to evaluate the heat transfer and flow characteristics of TCPMHE. And the results are comprehensively compared with the double concentric pipe minichannel heat exchanger (DCPMHE), aiming to offer some references for the concentric pipe heat exchanger and minichannel heat transfer technology in further industry application. Results indicate that the friction factor (f) values for both DCPMHE and TCPMHE are almost equal under the same operating conditions. Although the Nusselt number (Nu) values of DCPMHE are 21.8%-50.7% higher than those of TCPMHE, the effectiveness (ε) values of TCPMHE are all much higher than those of DCPMHE under the same conditions, and the enhancing factor (η) ranges from 1.45 to 1.65. Compared with the DCPMHE, the volumetric heat transfer capacity of the TCPMHE is improved by up to 65.4% at Si = 0.5 mm, th,i = 60 ℃ and Vh = 209 L·h−1. In addition, the dimensionless correlations for inner annulus hot fluid Nu, ε, and f of TCPMHE were established, and the maximum deviations are ± 8%, ± 5% and ± 11% for Nu, f, and ε, respectively, which indicates that the predicted values from the correlations agree well with the measured results.

Experimental investigation on the water hammer characteristic of stalling fluid in eccentric casing-tubing annulus

This study aims to analyze the water hammer characteristics of water hammer on tubing offset center in oil project for providing reference for water injection and well washing. An analysis is conducted by utilizing a test apparatus, particle image velocimetry, and a transient pressure measurement system. The characteristic of dynamic fluid loss is investigated in eccentric annulus flow channel and tubing and in flow line in terms of velocity, vorticity, and pressure field. Results show that water hammer occurs, and the pressure of oil casing annulus increases at the inside and outside walls with the increase in eccentricity. The water hammer pressure forms double imbalance in the internal and external walls of eccentric annulus tubing. With the increase in eccentricity, flow velocity decreases in the small flow passage, and flow velocity attenuation is fast at oil casing annulus.

From breakup to coiling and buckling regimes in buoyant viscoplastic injections

We study the injection flow of a heavy viscoplastic fluid into a light Newtonian fluid, via modelling and experiments. The injection is carried out downward, via an eccentric inner pipe inside a vertical closed-end outer pipe. This configuration results in a core viscoplastic fluid surrounded by an annular Newtonian fluid. The flow is structured and mixing is negligible. As the injection rate increases in a typical experiment, we observe three distinct flow regimes, associated with the core fluid behaviour, namely the breakup, coiling and buckling (bulging) regimes. In the breakup regime, the core fluid is yielded due to the extension caused by buoyancy, while in the buckling regime the yielding occurs due to the compression promoted by the pressure and the interfacial shear stress applied by the upward flow of the annular fluid. For the coiling regime, the core fluid remains largely unyielded until it exhibits a coiling behaviour. We develop a lubrication approximation model, using the Herschel–Bulkley constitutive equation, with dimensionless flow parameters including the Bingham number, the power-law index, the buoyancy number, the viscosity ratio, the diameter ratio, the eccentricity and the aspect ratio. Based on a reasonable prediction to the yielding onset, the model allows us to classify the flow regimes versus an elegant combination of the dimensionless numbers.

Lessons Learned in Horizontal Well Workovers and Sand Cleanouts, in Cerro Negro Field, Venezuela

Summary The Orinoco Oil Belt in Venezuela contains the largest known deposits of heavy oil in the world, and Cerro Negro is part of the fields in this area. However, the sand production is a major issue which affects the oil production in this field. There are some major issues that stand in the way of the proper cleanout of these wells. These problems include low reservoir pressure, geometry of the wells, and extremely high oil viscosity. Another major problem is the presence of big pieces of progressing cavity pumps (PCP) elastomers, some metallic parts from pressure and temperature sensors configuration, and other metallic parts from previous workover rig operations, encountered during cleanout operations. As a result of these difficulties, some of the past jobs performed in this field were unsuccessful. After these unssuccessful jobs, a campaign of recent multiple sand cleanout jobs was planned in the Cerro Negro field. Numerous difficulties arose during these jobs. These issues were largely due to the presence of big pieces of PCP elastomers and metallic parts. An additional issue was the low reservoir pressure. Due to the size of the casing/liner and the horizontal section, the annulus fluid velocity obtained was insufficient. Therefore, several techniques were applied to overcome these problems. One of these methods was to use reverse circulation for the sand cleanout in the 9-5/8-in. slanted casing section, while circulation was still possible to obtain a higher fluid velocity. Afterward, once the circulation was lost, a special stroking pump was used alongside a large string of tubing to store large amounts of sand and debris. This tool was also used to recover pieces of PCP elastomers and other metallic parts. This tool uses a piston pump to create a vacuum effect, and fluid circulation is not needed for the cleanout. Without this innovative stroking pump, it would have been almost impossible to recover these large pieces. This was the first time this innovative tool was used in these types of wells. When the open horizontal 7-in. production zone was reached, coiled tubing (CT) was used with the proper proportion of diesel and nitrogen to obtain enough fluid velocity and the proper equivalent density. In the end, when necessary, a smaller reliner was run depending on the information obtained during the analysis of the grain sizes. Some techniques were applied to run the reliner successfully in an old and worn out casing, where the geometry of the well and high doglegs caused difficulties. The challenges to complete the cleanout of the well with lost circulation problems included a large annulus area in the highly slanted and horizontal sections, the extremely high oil viscosity, and the presence of large pieces of PCP and metallic parts. These challenges were overcome by using different methods such as the special stroking pump, reverse circulation in the 9.625-in. section, and CT with the proper diesel and nitrogen proportions. Additionally, to keep sand production under control in the cleaned wells, a smaller reliner was run. Finally, these workover jobs were finalized to bring these wells back into production successfully. The oil production results of these operations are shown in this paper, and demonstrated that it is possible to retrieve the production in the sanded wells of the Orinoco Belt in a profitable way.

Electrokinetic energy conversion of core-annular flow in a slippery nanotube

In this paper, the electrokinetic energy conversion of core-annular flow (CAF) is investigated theoretically in a slippery nanotube. Two immiscible and undisturbed fluids are full of the whole nanotube, consisting of a two-layer fluid system. The core fluid is assumed to be surrounded by the annular fluid with an ideal flow pattern. Based on the Debye–Hückel linearization, the analytical expressions of flow velocity, streaming potential and electrokinetic energy conversion efficiency have been obtained. Result shows that the CAF can be enhanced obviously by the permittivity and viscosity ratios. Compared with the classical single-layer fluid system, large streaming potential strength can be obtained by altering the annular fluid property in the present two-layer fluid system. This implies that more streaming current can be obtained. Besides, as a significant application of electrokinetic flow, the optimal ratios can also be obtained in the two-layer fluid system and the conversion efficiency attains the maximum at these optimal ratios.

Conjugate Natural Convection: A Study of Optimum Fluid Flow and Heat Transfer in Eccentric Annular Channels

Laminar natural convection-conduction heat transfer in vertically positioned annular channels with inherent eccentricity is investigated for optimum solid-fluid thermal conductivity ratio and optimum cylinder walls thicknesses allowing maximum induced fluid flow rate and heat transfer under varying geometry parameters, i.e., annulus eccentricity and radius ratio. A finite-difference technique is employed to solve the coupled momentum and energy equations for the cylindrical annulus walls and the annular fluid with Prandtl number 0.7. As part of the results, fluctuations in the induced fluid flow rate and heat transfer in the eccentric annular channel due to the conjugate effect, governed by the ratios of the solid and fluid thermal conductivities and thicknesses of outer and inner circular cylinder walls, are obtained for the boundary conditions of one wall heated isothermally and the other kept adiabatic. Commonly encountered ratios of the solid and fluid thermal conductivities and cylinder walls thicknesses are utilized in the present analysis. Results reveal that the optimum conductivity ratio and cylinder walls thicknesses increase nonlinearly with eccentricity and radius ratio. Such results can be very useful in effectively designing the heat transfer equipment for optimum performance.

An improved thermal model for predicting wellbore temperature distribution in deep-water gas well

Accurate prediction of wellbore temperature plays an extremely important role in the production design of deep-water gas wells. This study overcomes the shortcomings of the previous literature in temperature calculations, comprehensively studies the heat transfer mechanism in the annulus and tubing, and an improved thermal model for predicting wellbore temperature in deep-water gas well is established. This model includes two important innovations: the natural convection heat transfer model in the annulus and the gas–liquid two-phase heat transfer model in the tubing. Firstly, the mechanism of heat transfer through natural convection in annulus is revealed by a numerical study, and a new calculation method for the annulus convective-heat-transfer coefficient, which can be applied to non-Newtonian fluids, is proposed. Secondly, considering that the two-phase flow in the tubing often presents an annular-mist flow in water-bearing gas well, a heat transfer model of the annular-mist flow in the tubing is developed considering the gas–liquid distribution characteristics and droplet dynamics. The results obtained using this improved thermal model are in good agreement with the field data of deep-water gas well, and the model prediction error is within 10%. Further, the sensitivity analysis is carried out on the parameters of annulus testing fluid that affect the wellbore temperature, and a novel solution for the annular insulation testing fluid was designed. Compared with ordinary annulus testing fluids, the wellbore temperature are more than doubled after adopting the new solution, thereby providing effective guidance for the development of insulation testing fluid system during deep-water gas well testing.

Vibro-acoustic analysis of double-walled cylindrical shells through a novel semi-analytic method

A novel semi-analytic model is developed for vibro-acoustic analysis of immersed double-walled cylindrical shells coupled by annular plates and annular fluid between two shells. For structural model, it is firstly divided to inner shell, outer shell and annular plates, and the inner and outer shells are further divided to narrow shell segments. By utilizing Flügge shell theory and power series method, all shell segments and annular plates are uniformly analyzed as conical shells, and they are rapidly assembled to the overall structural model via boundary conditions and continuity conditions between segments. For acoustic model, interior annular fluid region and exterior infinite fluid region are separately analyzed via the Helmholtz surface integral equation. Two surface integrals are simplified to two line integrals after expanding surface sound pressure and velocity as Fourier series, so the pressure is further expressed as displacements of all segments. Through adding the pressure to the continuity conditions, the structural model is coupled with acoustic model. High efficiency and accuracy are demonstrated via comparing vibro-acoustic results of present method with ones in the literature or calculated by coupled finite element-boundary element method. Furthermore, effects of annular plates, annular fluid and excitation on vibro-acoustics of double-walled shells are discussed.