Xianchao Liang ,Limin He,Xiaoming Luo ,Qingping Li ,Yuanpeng You ,Yiqiu Xu

1 College of Pipeline and Civil Engineering,China University of Petroleum (East China),Qingdao 266580,China

2 Offshore Oil Engineering Co.Ltd,Tianjin 300461,China

Keywords:Slug catcher Particle size distribution Separation efficiency Sedimentation Cyclone separator Baffle

ABSTRACT Sand production often leads to the failure of production equipment on offshore platform.Therefore,a new idea has been put forward,which is installing cyclone or baffle in the internal of the slug catcher for better sand control.In this paper,an experimental study is presented,which mainly includes sand particles accumulation shape,migration law and separation performance.The results suggest that the accumulation area is mainly divided into two zones:the crowded settlement zone and the free settlement zone.The crowded settlement zone has a special shape,which can be characterized by two parameters:accumulation length and accumulation angle.Axial sampling analysis shows obvious particle classification.Median particle size decreases with the increase of the axial distance,and the range of particle size distribution narrows gradually.The separation experiment shows that the gas velocity has the greatest influence on the separation efficiency.When the gas velocity is 14 m﹒s-1,the separation efficiency drops sharply,which can be abated by installing cyclone separator.In addition,the separation efficiency tends to be a constant under different gas velocities by installing baffle with appropriate height.Then the effectiveness and rationality of installing internal components can be strongly proved.All these provide important guidance for maximizing the sand control function of the slug catcher.

1.Introduction

With the continuous exploitation of oil and gas fields,the rock structure of the reservoir is destroyed due to the low rock strength or the unreasonable exploitation method [1] and operation method.This finally leads to the existence of sand production[2,3].Although sand production is a common phenomenon in the production process,it will lead to serious consequences such as pipeline blockage,valve elbow abrasion[4,5],and separator failure.Therefore,it brings huge threat to the safety of production.Taking an offshore gas field as an example,the sand particles follow the gas to the offshore platform,and a large amount of them eventually settle in the downstream equipment of slug catcher.For example,the separation effect of the electric coalescence separator is seriously deteriorated,and the filter element in the sewage treatment system often requires to be replaced due to dirty blockage.The final result is the decrease of process safety and the increase of operation cost.Therefore,the research on sand control is of great significance.

At present,there are two main ideas for sand control.The first one is to reduce the sand production from the source,mainly about some downhole sand control measures or improving the mode of production.The other is based on the fact that sand has been taken out of the wellbore,so it is necessary to control sand at the wellhead and downstream through various sand removal devices.In order to reduce sand production from the source,it is necessary to have a thorough understanding of sand production mechanism.Davood et al.[6]conducted a large number of experimental studies on sand production behavior and permeability changes.He believed that cumulative quality of sand production is closely related to permeability changes.Finally,a new function model is established to predict sand production,which can be used by petroleum engineers to improve production planning.Song et al.[7]studied the influence of fluid pressure,particle size distribution,fluid type and other factors on sand production process by numerical simulation.The results showed that the main causes of disastrous sand production are the collapse of thin inner layers of stable sand arches and complete collapse of sand body.Finally,the critical parameters were given to predict the disastrous sand production.According to Shabdirova et al.[8],the volume of sand production is approximately the same as the volume of plastic zone of failure material around the perforating channel.Therefore,the plastic zone is modeled and the sand production of weak sandstone reservoir in Kazakhstan is predicted by numerical simulation.The results showed a great agreement with the actual measured sand production of local oil field.It can be seen that the sand production mechanism is extremely complex,which is related to a variety of comprehensive factors.In addition,there is a big difference in rock conditions for different types of oil and gas fields.This really brings enormous difficulties to analyze the cause of sand production and predict the sand production rate.The most commonly used sand control method is to monitor sand production in the whole production cycle.Once it reaches the unacceptable level,it is necessary to take downhole sand control measures or adjust the production mode in time.Mechanical screen has become one of the most commonly used downhole sand control tools due to its advantages of low cost,great mechanical properties and simple construction[9,10].Enormous research on the material and structure of the screen [11,12,13] have been done.The design criterion of screen parameters for sand control is basically mature.The key point is to select the most appropriate screens according to the actual situation and characteristics of oil and gas fields.Some oil and gas fields are not suitable for installing downhole sand control tools due to their special production and operation conditions,or even if downhole sand control measures are taken,it is difficult to ensure that no sand particles are brought up with the well flow.At this time,it is necessary to install some sand removal equipment in the gathering and transportation process for sand control.

Typical desander mainly includes two types[17,20],one is filter desander,and the other is gravity separator,mainly including cyclone desander [18,19] and simple settling vessel.Each type of desander has its advantages and disadvantages.The resistance of the filter desander increases rapidly,and the filter screen and filter material need to be replaced frequently,which increase the operation cost and make it really difficult to ensure continuous production.The gravity settling vessel has the advantages of large capacity and high reliability,but it is bulky and difficult to install.The research on cyclone desander about its separation theory[14,15] and structure form [16] is the earliest and most sophisticated because of the advantage of high separation efficiency.Certainly,it is also the most widely used sand removal equipment in various oil and gas fields.

In view of the special conditions of narrow space of offshore production platform,gravity settling vessel is not suitable.Although the cyclone desander has high separation efficiency and is more flexible,there is a big disadvantage of the lower work capacity.Once it exceeds the range of the applicable working conditions,the separation efficiency could drops sharply.Therefore,in consideration of the flexibility,reliability and economy of platform equipment operation,a new idea is put forward.The slug catcher,which is located at the upstream of the electric coalescer and the production separator,can be modified to achieve the sand control function.By installing cyclone or baffle inside the slug catcher,it is not only unnecessary to add additional separation device,but also can protect the downstream production separator and other devices.Specifically,this kind of sand removal method belongs to gravity sand removal,which mainly utilizes the huge volume of the vessel to settle,with large processing capacity and good stability.However,it may be difficult to ensure stable and high separation efficiency,so this paper is of great significance to study it clearly.

2.Materials and Methodology

2.1.Experimental medium

The experimental medium include air,water and quartz sand particles.All the experimental parameters are shown in the Table 1.

Table 1 Experimental parameters

The classical mandelhan flow pattern map is adopted to analyze the gas–liquid condition in this paper,so as to accurately identify the flow pattern in the inlet pipe.As shown in Fig.1,all conditions are in the region of stratified wave flow.

In addition,the flow state of liquid phase is determined by calculating Reynolds number.The formula for calculation is as follows:

where:Q is the volume flow(m3﹒s-1),d is the pipe diameter(m),v is the kinematic viscosity (m2﹒s-1).

The results show that all Reynolds numbers are less than 2000.In particular,the maximum Reynolds number is 0.53.Therefore,according to the criterion of flow state,the liquid flow state under all conditions belongs to laminar flow.

2.2.Experimental prototype

The conventional slug catcher is an empty cylindrical shell without any internals.In order to optimize the sand control performance and improve the separation efficiency,a cyclone separator can be added in the inner inlet of the slug catcher for gas preseparation,and a baffle can be installed to control sand migration.As shown in Fig.2,the length of the cylinder is 1100 mm and the diameter is 400 mm.The diameter of both inlet and outlet pipes is 50 mm.In order to observe and measure the accumulation and migration of the sand particles,the prototype and all the internals are made of PMMA (Polymethyl Methacrylate) material.The PMMA has the advantages of high transmittance(92%),strong tensile and impact resistance,light weight(the density is 1.18 kg﹒m-3),and convenient processing.

The inlet and outlet pipes are equipped with standard flange,which is convenient for connection with other standard fittings.In this experiment,the gas outlet and the liquid outlet which are farthest from the inlet are selected for discharging gas and liquid,while the rest outfall are blocked.

Fig.1.Mandhane flow pattern map.

Fig.2.Schematic diagram and physical drawing of slug catcher prototype.

2.3.Experimental program

Three-phase gas–liquid–solid experiment system is designed as seen in Fig.3.Because the particle size is small and easy to be fluidized,the liquid and solid phase can be mixed first through the stirred tank numbered 1,and then liquid–solid mixture can be mixed with the gas phase in a static mixer numbered 14,thus forming a three-phase mixture.

Fig.3.Schematic of three-phase gas–liquid-solid experiment.1.stirred tank 2.cut-off valve 3.centrifugal pump 4.electromagnetic flowmeter 5.regulating valve 6.water tank 7.filter 8.centrifugal pump 9.mass flow meter 10.compressor 11.buffer tank 12.orifice flowmeter 13.vortex flowmeter 14.static mixer 15.slug catcher 16.cyclone filter 17.settling tank.

The speed of the agitator is controlled by the frequency converter,so that the liquid–solid is stirred evenly in the tank,and the consistency of the particle concentration is ensured.The liquid–solid flows through the centrifugal pump to pressurize and then enters the static mixer after being measured by the electromagnetic flowmeter.At the same time,the compressed air from the buffer tank is metered by the vortex flowmeter,and then enters the static mixer to mix with the liquid–solid to form a stable three-phase flow.In addition,when the required amount of liquid is too large,the water from water tank can be additionally pumped to the static mixer.

The three-phase flow first enters into the separation module of slug catcher,and then the separated gas phase enters into the cyclone filter to capture sand particles,while the separated liquid phase enters into the settling tank for gravitational settling.Sampling ports are set at the inlet (A) and the liquid outlet (B) of the slug catcher respectively for particle size analysis.

Rosemount high precision pressure sensors are set at inlet (A)and gas phase outlet (C) for pressure monitoring.High-speed acquisition card Pci-6071e of national instrument company is installed and processing software LabVIEW is adopted to collect temperature,flow and other parameters.

2.4.Evaluation indexes

2.4.1.Separation efficiency ?

The ratio of the mass of the sand settling in the slug catcher to the total mass of the sand is defined as the mass separation efficiency,which is:

where:msis the mass of sand deposited in the slug catcher,and minis the total mass of the sand entering the experimental system.

2.4.2 Median particle size d50

Median particle size is often used to represent the average size of particle swarm.It means that particles larger than it or smaller than it all account for 50% in total.

3.Results and Discussion

In order to fully understand the mechanism of adding cyclone separator and baffle,the sand particles accumulation parameters and separation parameters under the empty cylinder and with internals were tested.If there is no special indication for the working condition with inlet cyclone separator or baffle,the default condition is empty cylinder.

3.1.Sand particles accumulation shape

In order to ensure the accuracy of the experiment,each working condition has been tested three times in the same experiment cycle,and the similar sand accumulation shape is obtained as shown in Fig.4.It can be seen that there is obvious partition characteristics.In order to explain the particles settlement characteristics and accumulation law in different areas,the whole area can be divided into two zones,numbered 1 and 2.Firstly,zone 1 is a typical crowded settlement area.The incoming flow impinging on the free liquid surface causes strong disturbances,producing many dead zones and vortices of different sizes.This indeed enhances the ability of dispersed-phase particles to follow the vortex of liquid and the possibility of collision between different sand particles.Zone 2 can be called the free settlement zone,which is far away from disturbances at the entrance.The flow field is stable,so the sand particles can settle freely.The entire area of zone 2 is similar to a uniform settlement area,and that is why it shows a regular rectangular shape.In addition,another significant difference between these two zones is the local concentration of sand particles.Area 1 is a typical high-concentration area,and the concentration of sedimentation layers at different heights is distinct,so it can also be called the uneven settlement area.However,the concentration of sand particles in area 2 is relatively small,and most of the particles are in a suspended state with a nearly uniform concentration.

In order to analyze the influencing factors of the morphological changes in the crowded settlement area,two characterization parameters are defined,namely the accumulation length Ltand the accumulation angle α.The accumulation length mainly represents the extended range of turbulent flow field and impact on free surface.When the liquid volume flow increases,the impact of liquid flow on the free surface also be intensified,and the churning range of the entire turbulence field is expanded.For the same reason,the liquid flow would have a larger initial momentum to impact the liquid surface as increasing the superficial gas velocity.And the disturbance of the gas on the liquid surface will also cause the entire free surface to move forward.Finally,the range of the crowded settlement zone is increased.Therefore,the accumulation length has the same tendency to increase as shown in Fig.5.It should be noted that compared with the change of liquid volume,gas velocity makes the accumulation length change more steeply.Specifically,when the gas velocity increases from 10 to 16 m/s,the accumulation length increases by about 19 cm,while the liquid volume increases from 30 to 75 L﹒h-1,the accumulation length only increases by 8 cm.

Fig.4.Sand particles accumulation shape.

Fig.5.Influence of gas velocity and liquid volume flow on the length and angle of particle accumulation.

The accumulation angle mainly represents the accumulation effect brought by the local concentration and size of the sand particles.First of all,it can be seen from the basic shape that the accumulation area far from the entrance area is contracted,which causes the existence of the accumulation angle.With the continuous sedimentation process of sand particles,the area concentration in the axial direction gradually decreases from the entrance position,resulting in the existence of a narrowed area.It is found from Fig.5 that the influence of gas velocity and liquid flow on the angle of accumulation α is opposite.This is because the increase of the liquid flow rate means that more particles are brought into the slug catcher within a certain time interval,which exacerbates the difference in the local concentration of particles in the axial direction.Therefore,the increase in the width difference between the front and rear areas leads to an increase in the stacking angle.The increase of the air velocity strengthens the purging effect on the entire liquid surface,and the tendency of the liquid flow to move forward is strengthened,so that more particles are carried away from the entrance,weakening the concentration difference in different regions in the axial direction and making α decreases.

In addition,due to the shape characteristics of the cylindrical structure,the particles accumulated on the cylinder surface are easy to slide to the bottom,which also contributes to the existence of the accumulation angle.In order to analyze the mechanism of the sliding motion and the size of the sliding particles,samples were taken from different positions on the cross section of the slug catcher.The schematic diagram of the sampling points is shown in Fig.6.Point B is set as the coordinate origin.

In an ideal state,the particle sedimentation velocity in laminar flow can be expressed by the Stokes formula:

where u is the particle sedimentation speed(m﹒s-1),ρsis the particle density (kg﹒m-3),ρ is the liquid fluid density (kg﹒m-3),d is the particle size(m),μ is the fluid viscosity(Pa﹒s),g is the gravitational acceleration (9.81 m﹒s-2).

Fig.6.Schematic diagram of sampling points on the cross section of the slug catcher.

Therefore,a basic rule is that particles with large diameters have a large sedimentation speed and will settle down near the entrance first.As a result,the median particle size of the sampling point gradually decreases with the increase of the axial distance as shown in Fig.7.It can also be seen that the median particle size of the two axially symmetrical sampling points are basically the same.This is because the symmetrical structure of the cylinder restricts the gas–liquid flow field,so that the development law of the flow field is basically consistent on both sides of the axis.In particular,it can be seen that the median particle size of the sampling point off the axis is generally smaller than the median particle size at the axis.For example,the medium particle size of point E is about 10 μm larger than that of point D.

In addition,the particle size distribution of D,E and F is shown in Fig.8.The particle size distribution at point D and F are basically same.Compared with the other two points,the proportion of small-sized particles of point E is reduced and the proportion of large-sized particles is increased.This also implies that there is a large possibility that some large-size particles slip from the sides to the axis,and eventually promote the formation of the accumulation angle.

Fig.7.Median particle size in the different sampling point.

Fig.8.Particle size distribution of point D,E and F.

To prove that large-size particles are more likely to fall off during the particles accumulation process,the first thing to explain is the important factors that affect the stability of the accumulation body.A frame with certain strength is necessary to maintain the static stability.This strength is determined by the frictional strength of all particles,and the frictional strength can be divided into sliding friction and occlusal friction.The sliding friction refers to the surface friction between particles,which is mainly related to the surface properties of the particles.The occlusal friction refers to the occlusal force generated by the embedding and interlocking effects between particles,which is mainly related to the shape of the particles.Generally,the flatter the surface,the smaller the occlusal friction,such as needle-like or flaky particles.

By screening the particles at point D,E and F,the particles with large size and small size are obtained respectively.Then the picture taken by high power microscope is shown in Fig.9.It can be seen that the main shape of the large-sized particles is flake or rhombus,and the lateral dimension is larger than its thickness,showing obvious two-dimensionality.The flat surface results in small occlusal friction,which is easy for the particles to lose stability and slide down to the axis.The small-sized particles are mostly threedimensional spheroids,and the surface has a large number of pits.Interlocking structures are easily formed between the particles,which has good stability.

3.2.Sand particles migration law

After the sand particles enter the slug catcher,the movement state mainly includes irregular diffusion and vertical gravity settlement.Due to the impact of the liquid on the free surface,local turbulence is so strong,so the fine particles are suspended under the turbulent fluctuation and diffuse around the liquid flow field.At the same time,in the vertical direction,there is a density difference between the sand particles and the liquid.The particles with larger sizes can overcome the buoyancy and pulse dynamics by gravity,and then settle downward.Within the effective settlement distance,once the sand particles settle to the bottom of the cylinder,it is difficult to be wrapped up by the liquid flow again.In order to investigate the migration distance of particles with different particle sizes and evaluate their ability to follow the turbulent field diffusion,sampling points were set at axial distances of 20 cm,40 cm,60 cm,and 80 cm,respectively.The particle size distribution of the experimental sand sample in the incoming flow is shown in Fig.10.It has not been sieved and has a wide range of size distributions,ranging from a few microns to hundreds of microns.This is very helpful for exploring the migration characteristics of particles with different sizes.

3.2.1.Median particle size in axial direction

According to the general law of particle sedimentation,the larger the particle size,the easier it is to overcome the pulsation force of the turbulent field and the entrainment force brought by various micro-vortexes,so as to settle at the front area faster.As shown in Fig.11,under different gas velocity and liquid flow rates,the median particle size in the axial distance shows a clear particle screening phenomenon.With the increase of the axial distance,the median particle size gradually decreases,as if it has passed through the sieving of a machine,but it is the Stokes settlement law that works here.

Fig.9.Picture of sand particles under microscope.

Fig.10.Particle size distribution of experimental sand sample.

Fig.11.Median particle size of different axial distance.

The increase of liquid volume or gas velocity will undoubtedly increase the turbulent pulsation and the disorder of the flow field near the entrance,which will increase the fluctuation of the free surface,increase the number of internal micro vortex,and greatly enhance the particle-carrying and diffusion capacity of the flow field.As a result,more large-sized particles are taken away from the entrance,so the median particle size measured at four axial distances has increased in different levels.However,it can also be seen that as the axial distance increases,the effect of increasing liquid volume or gas velocity on the median particle size gradually has been weakened.This is because the gas–liquid flow field is strongly affected by the change in operating conditions near the entrance.And the distinction of local concentration between different areas of the crowded zone is the largest.Then the effect of changing the flow field conditions on the particles classification can be amplified.When the axial distance is 20 cm,the maximum difference of medium particle size is 35 μm at different gas velocities.At the same time,the maximum difference of medium particle size is about 16 μm under different liquid volume.When the axial distance is 80 cm,the sand particles are completely in the free settlement zone,and the particle concentration becomes very small,belonging to a sparse distribution.The effect of gas–liquid flow field change is difficult to reflect,so the change in median particle size is not obvious.

3.2.2.Particle size distribution in axial direction

The median particle size only represents the macroscopic size of a particle group,and it is impossible to know the proportion of each particle size at a certain position.Therefore,it is necessary to analyze the particle size distribution.As shown in Fig.12(vsg=10 m﹒s-1,QL=60 L﹒h-1),as the axial distance increases,the particle size distribution range gradually narrows.Taking the axial distance of 20 cm and 80 cm for illustration,the particle size distribution range at 80 cm is about 0 μm-60 μm,and the particle size distribution range at 20 cm is about 0 μm-120 μm.The proportion of small-sized particles at 20 cm is significantly smaller than that at 80 cm.This shows that most of the small-sized particles are difficult to settle in the front settlement zone,but are brought to the further area.

Fig.12.Particle distribution of different axial distance.

The particle size distribution range at each sampling point is smaller than the entrance experimental sand sample.At different axial distances,it seems that there is a blocking force with different strength,which can block sand particles with different size to form the particle classification.At a certain position,when the sand particles of a certain size range are intercepted,the proportion of it is increased compared with that of the entrance experimental sand sample,otherwise the proportion is decreased.The change of volume fraction is defined as the volume fraction of a particle size at a certain axial distance minus the volume fraction in the entrance experimental sand sample.A positive value means an increase,while a negative value means a decrease.

As shown in Fig.13,there is a turning point at each axial distance,at which the percentage change value turns from positive to negative.Therefore,the particle size corresponding to this turning point can be called critical intercepted particle size,which means the maximum particle size that can be intercepted.And the axial distance corresponding to the particle size is exactly the maximum distance that particles can migrate.As the axial distance increases,the interception critical particle size becomes gradually smaller.It proves that small-sized sand particles are difficult to be trapped in the front area and have a larger migration distance,so that it is easy to follow the liquid flow to migrate downstream and even flow out from the liquid outlet.This is also the main reason for the poor effect of sand control.

Fig.13.Fractional change in volume of different axial distance.

3.3.Separation efficiency

In order to verify the rationality and effectiveness of adding internals inside the slug catcher,the evaluation on particle separation efficiency of the whole equipment was performed for the three conditions:empty cylinder,only with cyclone separator,and only with baffle.In addition,the sand control mechanism of different internals was analyzed,and the basic rules of sand control were clarified.

3.3.1.Separation efficiency under empty cylinder

Fig.14.Influence of liquid volume flow or gas velocity on separation efficiency.

All conditions under the empty cylinder belong to high gas–liquid ratio conditions [21],so the disturbance of gas phase to the internal flow field is dominant compared to the liquid phase.On the one hand,larger gas velocity can really increases the initial turbulent kinetic energy of the liquid phase,thereby strengthening the impact on the free liquid surface,and making the range of sand particle diffusion wider.On the other hand,the influence of gas channeling and purging inside the cylinder also causes the free liquid surface to slosh,aggravating the turbulence of the liquid flow field,which is not conducive to the particles sedimentation.Therefore,as shown in Fig.14,with the increase of liquid volume,the separation efficiency only slowly decreases by less than 5%.However,the increase of the gas velocity caused a significant decrease in the separation efficiency,especially a steep drop by about 20%at the gas velocity of 16 m﹒s-1.So this also gives an important hint.To maximize the function of sand control,the gas phase should be the most important focal point.

3.3.2.Separation efficiency with cyclone separator only

The inlet cyclone separator is installed separately for gas and liquid pre-separation.The direction of the liquid flow entering the cylinder is changed from axial forward to radial downward,which reduces the initial inertial migration distance in the axial direction,thereby reducing the migration distance of the particles to the downstream liquid outlet.In the end,the separation efficiency is improved to a certain extent,as shown in Fig.15(a).Certainly,when the gas velocity is 16 m﹒s-1,the increase of separation efficiency is the biggest,reaching about 16%.However,it should also be noted that the liquid phase is tangentially thrown out of the separator after the swirling action of the inlet cyclone separator,which increases the contact range of the liquid flow with the entire free liquid surface.This makes the area of impact and disturbance larger,which is not conducive to particle sedimentation.Therefore,it can be seen that the separation efficiency in Fig.15(b) does not increase to a great extent and the maximum change value of it was about 2%.

3.3.3.Separation efficiency with baffle only

A baffle is installed in front of the downstream liquid outlet to prevent sand particles from migrating to the liquid outlet,thereby reducing the possibility that the sand particles follow the liquid phase and flow out of the slug catcher.Experiments are performed on four working conditions:without baffle,baffle height 5 cm,10 cm,and 15 cm,as shown in Fig.16.Firstly,as the height of the baffle increases,the range of the baffle in the cross section becomes wider,and the possibility of the particle group colliding with the baffle becomes larger,so the particle separation efficiency naturally increases.In addition,the baffle can also play the role of a wave preventer.It reduces the degree of sloshing of the liquid surface,and provides a relatively stable flow field for the free settlement of sand particles.In particular,it can be seen from Fig.16(a) that when the height of the baffle is increased to 15 cm,theseparation efficiency at different gas velocity is basically the same and all of them reach more than 85%.The effect of the baffle completely offsets the impact of the change in gas velocity.This shows that even if the gas velocity is large enough to take more sand particles to a place with a larger axial distance,as long as the height of the baffle is set appropriately,there is no need to worry about too much sand particles flowing out.

Fig.15.Influence of cyclone separator on separation efficiency.

Fig.16.Influence of baffle height on separation efficiency.

However,it can be seen in Fig.16 (b) that even with the baffle height of 15 cm,the separation efficiency at large liquid volume flow is still 3% lower than at small liquid volume flow.This is because a large liquid volume flow means a shorter residence time,while the sand particles near the baffle have a relatively small size and better following behaviors.Then the particles can quickly flow out of the outlet without colliding with the baffle.As an extreme example,if the liquid flow has a sufficiently long residence time and the baffle height is appropriate,almost all particles moving toward the outlet near the baffle will collide with the baffle and eventually settle down

4.Conclusions

(1) Based on three repeated experiments,the accumulation shape of sand particles at the bottom of the slug catcher was identified.According to the settlement mode and the local concentration of sand particles,the entire accumulation area is divided into crowded settlement zone and free settlement zone.The two morphological parameters of the crowded zone,including the accumulation length and accumulation angle,are easily affected by the gas–liquid operating conditions.The increase of gas velocity and liquid flow has a synergistic effect on the expansion of the accumulation length and an antagonistic effect on the accumulation angle.

(2) The results of axial sampling analysis show that there is an obvious particle screening phenomenon.The median particle size gradually decreases with the increase of axial distance,and the particle distribution range gradually gets narrow.In addition,there is a critical intercepted particle size at each sampling point.The larger the axial distance is,the smaller the critical intercepted particle size is.Therefore,it proves that the migration distance of large particle size is shorter and is easier to be intercepted.

(3) Experiments under an empty cylinder show that the effect of gas velocity on the separation efficiency is much greater than the liquid flow rate.When the gas velocity reaches 16 m/s,the separation efficiency drops sharply,while the change of the liquid flow rate has a weaker effect on the separation efficiency.The cyclone separator completes the gas phase pre-separation,which largely improves the separation efficiency at large gas velocity,but does not improve the separation efficiency at large liquid volume.The installation of a baffle with an appropriate height can basically offset the impact and turbulence caused by the large gas velocity,so eventually the separation efficiency is consistent at different gas velocities.Most importantly,when the baffle height is 15 cm,the separation efficiency is not less than 84% under different gas velocity and liquid volume,and the highest separation efficiency is up to 88%.Finally,the positive effect of installing cyclone and baffle on sand control was fully confirmed.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.