Fluid Density Research Articles

Stability of Saltwater‐Freshwater Mixing Zones in Beach Aquifers With Geologic Heterogeneity

AbstractSaltwater‐freshwater mixing zones in beach aquifers support biogeochemical reactions that moderate chemical loads in fresh groundwater discharging to marine ecosystems. Existing laboratory and numerical modeling studies have demonstrated that fluid density gradients in the mixing zone can lead to free convection and the formation of density instabilities, or salt fingers, under a range of hydrologic, morphologic, and hydrogeologic conditions. However, salt fingers have rarely been observed in real‐world beach aquifers despite a growing body of field studies investigating intertidal mixing zones. In this study, we used geostatistical methods to generate randomly distributed assemblages of fine and medium sand and incorporated those geologic realizations into variable‐density variably‐saturated flow and salt transport simulations to explore the influence of geologic structure on mixing zone stability in tidally‐influenced beaches. Ensemble‐averaged model results show that geologic heterogeneity inhibits salt finger formation and promotes a stable intertidal mixing zone due to enhanced dispersion. This effect is highest for high degrees of heterogeneity and for more laterally connected geologic architecture. Compared to hydraulically equivalent homogeneous models, sediments with moderate to high heterogeneity produce mixing zones that are on average 19%–29% smaller and 3–10 times more stable due to the absence of the downward convection and seaward movement of salt fingers. The models indicate that geologic heterogeneity may explain the lack of field observations of salt fingers in real‐world intertidal mixing zones. The findings have implications for predicting the onset of free convection in beaches and for understanding intertidal pore water biogeochemistry and chemical fluxes to the ocean.

Study on the suspending mechanism of gangue particle in coal-based solid waste slurry

In China's coal mine backfill mining projects, certain mines demonstrate an extensive filling distance that exceeds 3000 m. However, there are few researches concerning the theoretical underpinnings for slurry long-distance pipeline transportation. Gangue particles suspending in filling slurry is the prerequisite for successful long-distance pipeline transportation. This paper conducts theoretical researches on the heightened sensitivity issue of particle settling within coal-based solid waste filling slurry (CSWFS) for long-distance pipeline transportation, which can easily lead to unstable slurry concentration then pipe blockage or bursting accidents. The mechanical model for particle settling and critical non-settling particle diameter for long-distance pipeline transportation are developed based on the "solid-liquid two-phase carrier suspension fluid", and reveal the intrinsical mechanism of particle settling within CSWFS for long-distance pipeline transportation. Through the calculation of the mechanical model for critical non-settling particle diameter and particle settling, coupled with the particle diameter screening experiment, the value of critical non-settling particle diameter is determined to be 0.0067 m, and the density of solid-liquid two-phase carrier suspension fluid is 1712.37 kg/m3. The CSWFS is classified as a semi-stable slurry, lacking time-dependent suspending state required for long-distance pipeline transportation, which could lead to pipe blockage accidents. Further research into effective methods is urgently required to ensure adequate safety and adaptability for long-distance pipeline transportation of CSWFS.

Real-time dynamic and structural behavior estimation of a steel lazy wave riser through finite-element-based digital twin and hull-motion sensor

A digital twin (DT) method is proposed to estimate the dynamic and structural behaviors of a Steel Lazy Wave Riser (SLWR) under environmental conditions. This method uses a 1D finite element (FE) riser model that can include various loads and boundary conditions measured from sensors. We selected a spread-moored Floating Production Storage and Offloading (FPSO) vessel to validate the proposed method, which hosts multiple Production and Injection SLWRs. Using the time-domain dynamics simulation program, we built a reference model considering the fully coupled FPSO with mooring lines and risers in various wind/wave/current conditions. The synthetic sensor data, which represents the motions and internal fluid density changes, were generated from the simulation of the reference model. Then, two riser DT models, the Top Oscillation Model (TOM) and the Two-Point Oscillation Model (TPOM), were built, considering only a single riser made of the 1D FE model. TOM employed vessel motions as boundary conditions while internal fluid density and ocean current could be inputted as external loads. TPOM incorporated an additional motion sensor attached to the riser. The motion data obtained from the reference model were first inputted into the respective riser DT models. Next, we further evaluated their performance with and without the estimated ocean currents and slugs of internal fluid. The time histories, spectra, and statistical data of displacements and stresses along the riser were systematically compared and analyzed for various test cases and environments. While the hull-motion sensor alone can provide practically acceptable results using the developed methodology, additional consideration of current load and internal fluid's density variation improves the overall riser monitoring performance. The present method can significantly reduce the number of sensors compared to traditional riser monitoring methods.

Is bouncing easier with a negative effective dark fluid density ?

Is bouncing easier with a negative effective dark fluid density ?

Mapping hydrodynamic structure with sparse or no well data

Hydrodynamic traps are usually mapped using well pressure data to transform structural depth maps, but if well data are sparse then hydrodynamic maps produced this way may have large uncertainties. An alternative approach that does not rely on well data is described here, utilizing simplified, locally planar representation of the potentiometric surface. Ranges of hydraulic gradient magnitudes and azimuths representing different potentiometric surface orientations, together with a range of possible density contrasts between the flowing and trapped fluids, define a three-dimensional array, termed here ‘hydrodynamic space’. This array can be constrained and simplified by reasonable assumptions and the introduction of an additional new concept that combines the hydraulic gradient magnitude and fluid density contrast into a single parameter termed ‘potentiometric transform’. Ranges of these parameters yield a set of hydrodynamic structural maps. The fineness of sampling the hydrodynamic space parameters is limited only by the resources available to support the workflow. The array can be automatically assessed in terms of hydrodynamic trap volumes by applying a structural closure algorithm that isolates and characterizes dip-closed structure. Closure volumes across the map set are ranked by spatial distribution analysis that informs exploration programs relevant to any subsurface fluid management application. The method is described for the first time here and illustrated by application to a real structural dataset.

Going beyond an old shockwave conjecture for improving upon Navier-Stokes.

Nonequilibrium molecular dynamics (NEMD) computer simulations of steady shockwaves in dense fluids and rarefied gases produce detailed shockwave profiles of mechanical and thermal properties. The Boltzmann equation, under the assumption of local thermodynamic equilibrium (LTE), leads to the first-order (linear) continuum theory of hydrodynamic flow: Navier-Stokes-Fourier (NSF). (Expansion of the LTE Boltzmann equationin higher powers of gradients yields so-called Burnett second-order terms, etc.) NEMD simulations of strong shockwaves with high gradients are not well modeled by NSF theory. Many years ago, a conjecture for going "beyond Navier-Stokes" was proposed, applying the empirical observation of anisotropic thermal enhancement in the shock front to the temperature dependence of the NSF transport coefficients, whose dissipation determines the slope at the center of the shock profile: for weak shocks, the actual coefficients in NEMD simulations appear to be smaller than in NSF predictions, leading to steeper gradients being observed, while for strong shocks, the NEMD coefficients appear to be larger, leading to less steep shock rises than predicted by NSF calculations. In this paper, we show that adding significant Burnett nonlinearity into an LTE continuum theory reproduces the early shock rise and slope of NEMD profiles, for both weak and strong shocks in dense fluids, as well as strong shockwaves in the ideal gas. Moreover, we show that "Holian's conjecture" incorporates significant Burnett nonlinearity, but like all the other LTE continuum theories, it fails to describe the slow NEMD return to equilibrium beyond the shock front. We show that Maxwell relaxation has to be applied to the hydrodynamic variables themselves (rather than attempting indirect relaxation of their gradients) in order to more accurately model the entire shockwave profile. Non-LTE Maxwell relaxation is the only way to bring the entire profile into agreement with NEMD, most noticeably for strong shockwaves.

Self-diffusion coefficient as a function of the thermodynamic factor.

Much effort has been put into developing theories for dense fluids; as a result of these efforts many theories work for a certain type of particle or in a certain concentration regime. Rosenfeld proposed a dependence of the self-diffusion coefficient on the excess entropy. Our proposal is similar to Rosenfeld's in that it also attempts to describe diffusion in terms of a thermodynamic function but, instead of the excess entropy, we use the thermodynamic factor or the excess chemical potential. Simulations were taken for hard spheres and our model was fitted with two free parameters. Simulations were then carried out for a Lennard-Jones gas and our model correctly described the new data with the value of the free parameters that we had obtained for hard spheres. This is a feature of our model that we wish to emphasize, since the usual situation is that parameters have to be readjusted for different interaction potentials. An experimental xenon self-diffusion data set was used as an example of where the model can be applied, especially in the high-density regime.

Ultrasonic Sensor: A Fast and Non-Destructive System to Measure the Viscosity and Density of Molecular Fluids.

This study presents the design and development of an ultrasonic sensor as a fundamental tool for characterizing the properties of fluids and biofluids. The analysis primarily focuses on measuring the electrical parameters of the system, which correlate with the density and viscosity of the solutions, in sample volumes of microliters and with high temporal resolution (up to 1 data point per second). The use of this sensor allows the fast and non-destructive evaluation of the viscosity and density of fluids deposited on its free surface. The measurements are based on obtaining the impedance versus frequency curve and the phase difference curve (between current and voltage) versus frequency. In this way, characteristic parameters of the transducer, such as the resonance frequency, phase, minimum impedance, and the quality factor of the resonant system, can characterize variations in density and viscosity in the fluid under study. The results obtained revealed the sensor's ability to identify two parameters sensitive to viscosity and two parameters sensitive to density. As a proof of concept, the unfolding of the bovine albumin protein was studied, resulting in a curve that reflects its unfolding kinetics in the presence of urea.

Challenges in the diagnosis and treatment of pure non-gestational uterine choriocarcinoma in a child: a case report

BackgroundDiagnosing non-gestational uterine choriocarcinoma in children is challenging because of its rarity and nonspecific imaging findings. Herein, we report a case of non-gestational uterine choriocarcinoma in a child, which was unexpectedly found during exploratory laparotomy and confirmed by histopathological findings. However, the tumor did not respond to chemotherapy.Case presentationA 4-year-old Indonesian female patient was brought into the emergency unit with chief complaint of vaginal bleeding. She had suffered from vaginal spotting 4 months before being admitted to the hospital. Physical examination revealed a distended abdomen in the left lumbar region and a palpable fixed mass with a smooth surface. Abdominal computed tomography scans revealed a large mass (10 × 6 × 12 cm) with fluid density and calcification. Thus, we suspected left ovarian teratoma. The patient’s luteinizing hormone, follicle-stimulating hormone, and lactate dehydrogenase levels were 25.2 mIU/ml, 0.1 mIU/ml, and 406 U/l, respectively. According to the clinical and radiological findings, we decided to perform an exploratory laparotomy and found a tumor originating from the uterus, not the ovarium. We did not observe liver nodules and any enlargement of abdominal lymph nodes. Subsequently, we performed hysterectomy. The histopathological findings supported the diagnosis of choriocarcinoma. The patient was discharged uneventfully on postoperative day 5. Thereafter, the patient underwent nine cycles of chemotherapy, including carboplatin (600 mg/m2 IV), etoposide (120 mg/m2 IV), and bleomycin (15 mg/m2 IV). However, on the basis of the clinical findings of a palpable mass and partial intestinal obstruction, the tumor relapsed soon after the ninth cycle of chemotherapy. Currently, the patient is undergoing chemotherapy again.ConclusionsAlthough pure non-gestational uterine choriocarcinoma is rare, it should be considered as one of the differential diagnoses for intraabdominal tumors in a child, so as to better guide and counsel families regarding the surgical plan and prognosis, respectively. In the present case, the patient’s response to chemotherapy was poor, implying that the treatment of non-gestational choriocarcinoma is still challenging, particularly in the pediatric population.

Three-pressure prediction method of jointing well-seismic data in JT1 well area of Sichuan Basin in China

With the high yield of many wells represented by Well JT1 in the Maokou Formation, has catalyzed a surge in exploration activities along the platform margin facies of the Maokou Formation in central Sichuan and further showed the significant exploration potential of the Maokou Formation in the northern slope. However, the fracture cave body of the Maokou Formation exhibits a high degree of development, strong longitudinal and horizontal heterogeneity, large formation pressure differences, and drilling events such as gas kicks and lost circulation occur frequently, which seriously affects the efficient implementation of drilling. Understanding the spatial distribution of the three-pressure in the formation can help better deal with and solve the above problems. Therefore, in order to help the safe, high-quality and rapid drilling of the Maokou Formation in the study area, and enhance the efficiency of oil and gas development, this paper explores the research on the prediction method of the three-pressure of jointing well-seismic data based on the geomechanical experimental data and the actual drilling data. In the process of prediction of pore pressure, this study found that the pore pressure and formation velocity in the study area have an exponential relationship. In order to enhance the applicability of the Filippone’s method in the study area and improve the prediction accuracy of pore pressure, the linear relationship between pore pressure and formation velocity in the Filippone’s method is modified to an exponential relationship, and a pore pressure prediction model suitable for the work area was established. Based on the Mohr–Coulomb criterion and Huang's model, the prediction models of collapse pressure and fracture pressure applicable to the study area were established, respectively. Then, the elastic parameters were obtained through pre-stack inversion, and the three-pressure bodies were calculated based on the elastic parameter bodies. The results indicate that: (1) The three-pressure prediction method of the jointing well-seismic data in this paper can predict the formation's longitudinal and transverse pressure anomaly zones in advance. (2) The Maokou Formation in the study area is characterized by abnormally high pressure, to balance the pressure of the high-ground formation, high-density drilling fluid is necessary. (3) The prediction results of three-pressure in this paper are highly consistent with the actual drilling engineering events, which verifies the reliability of the three-pressure prediction results presented in this study. The results of the study can provide a basis for decision-making in drilling geological design, such as the determination of drilling fluid density, the evaluation of borehole stability and other engineering problems that require support from three-pressure data.

On a similarity solution for lock-release gravity currents affected by slope, drag and entrainment

We consider the long-time propagation of a Boussinesq inertia–buoyancy (large-Reynolds- number) gravity current released from a lock over a downslope of angle $\gamma$ , affected by entrainment and drag. We show that the shallow-water (depth-averaged) equations with a Benjamin-type front-jump condition admit a similarity solution $x_N(t) = K t^{2/3}$ while $h, \phi, u$ change like $t$ to the power of $2/3, -4/3, -1/3$ , respectively; here $x_N, h, \phi, u$ and $t$ are the position of the nose (distance from backwall), thickness, concentration of dense fluid, velocity and time, respectively, and K is a constant. Assuming that $\gamma$ and the coefficients of entrainment and drag are constant, we derive an analytical exact solution for the similarity profiles and show that $K \propto (\tan \gamma )^{1/3}$ ; the driving of the slope is balanced by entrainment and/or drag. The predicted $t^{2/3}$ propagation is in agreement with previously published experimental data but a conclusive quantitative assessment of the present theory cannot be performed due to various uncertainties (discussed in the paper) that must be resolved by future work.

The Effects of Liquid Density and Impeller Size with Volute Clearance on the Performance of Radial Blade Centrifugal Pumps: An Experimental Approach

This study was carried out to experimentally investigate the effect of impeller size with volute clearance and liquid density on the performance of radial blade centrifugal pumps. Three impeller sizes of 121.54mm, 109.38mm and 97.23mm with respective volute clearance of 6mm, 12mm and 18mm were considered, and five different liquid blend which comprise of water, two liquids that are denser than water and two other liquids that are less-dense than water (with respective liquid densities of: 1197 kg/m3 , 1097 kg/m3 , 1000kg/m3 , 898.2 kg/m3 and 798.4 kg/m3 ). In the experimental set-up, the pump flow capacity were varied from 12m3 /hr to 62m3 /hr at interval of 5m3 /hr and the corresponding pump power consumption, attained pump head were recorded and pump efficiency were determined. Microsoft excel was used to evaluate the trend, performance trend was used to develop the pump performance model showing the relationship between the various parameters. The results from the investigation revealed that with the various impeller sizes of 121.54mm, 109.38mm and 97.23mm of respective volute clearance of 6mm, 12mm and 18mm, the attained optimum efficiency were 74.42%, 54.78 and 33.54% respectively at a correspondence optimum pump head of 23.66m, 20.60m and 18.87m. The results also showed that there is a direct relationship between pump power consumption and process liquid density, while showing an inverse linear relationship between the pump instantaneous start up power and impeller diameter. It was therefore concluded among others that liquid with higher density will usually require higher power to initiate and maintain flow at constant flow rate and impeller size. It is then recommended among others that pump designers, application engineers and users of centrifugal pumps should consider possible increase in pump power consumption when working with a process application that has higher tendency for dynamic increase on the process fluid density.

The effects of wellbore stone materials and fluid density on the shock wave induced by plasma pulse system propagation in rock: An analytical and experimental study.

A pulsed plasma system is a highly efficient tool for linking wells to reservoirs to repair near-wellbore damage and restore production. This research aims to create a computational model and experimental setup to delve into the generation and spread of shock waves from electrical discharge. It also explores the influence of different stone wall materials such as limestone, sandstone, and dolomite, along with varied fluid densities such as saline water and drilling mud. Results show that when an electrical current passes through a metal wire, the wire explosively disintegrates, creating a plasma pulse that applies pressure shock to the stone walls, leading to crack formation. It is important to note that the explosion's energy can be increased by raising the capacitor's energy and modifying the circuit's inductance. Analyzing the impact of fluid density on the shock wave reveals that enhancing the plasma pulse system and attaining higher energy levels can be achieved by using materials with higher viscosity. In terms of stone wall materials, limestone demonstrates superior mechanical stability, dolomite exhibits moderate stability, and sandstone shows relatively lower stability in crack growth.

Exploring the impact of compressibility on reconstructed porous materials: A numerical study

This study underscores fluid density's significance in CFD simulations for porous materials, addressing its impact on accuracy and computational efficiency. The paper proposes a tailored form of Navier-Stokes equations that accounts for fluid density's influence on CFD analyses of porous materials in industrial contexts, including cases where the solid phase is deformable. Numerical analyses demonstrate fluid density's significance (ρ≠constant) and explore the importance of the energy equation in governing equations. The energy equation is essential in setting up the governing equations, as it calculates thermal characteristic length based on cell temperatures. By examining various porous material samples, the study suggests a streamlined approach: employing a single coupled CFD-FEM simulation to directly determine each geometrical parameter. Additionally, the study investigates the capability to accurately simulate turbulent fluid motion at the pore scale and analyze the flow field characterization within porous media. Computational cost analyses underscore the advantages of coupled simulations, establishing their profitability over separate parameter-specific simulations.

Investigation of the characteristics of optically dense gray mixed convection fluid flow along a cantilever shape

The goal of the work under consideration is to inspect the steady buoyancy driven flow for a viscous and density independent fluid along a cantilever shape, by considering the optically dense gray fluid. This study examines the characteristics of buoyancy driven convective heat and fluid flow adjacent to the cantilever shape. The governing equations constituting the aforementioned problem are converted into ordinary differential equations by utilizing the stream function formulation. The shooting technique BVP4C along with the MATLAB program is then used to obtain the numerical solutions. The impact of dimensionless cantilever shape parameter n, Prandtl number Pr, Richardson parameter λ, and thermal radiation parameter F on the velocity and temperature profile of the cantilever shape is demonstrated graphically. The tables provide the results for the skin friction and rate of heat transfer coefficient due to the impact of a number of evolving parameters.

Similarity solution for the magnetogasdynamic shock wave in a self-gravitating and rotating ideal gas under the influence of radiation heat flux

The Lie invariance method is used to analyze the one-dimensional, unsteady flow of a cylindrical shock wave in a rotating, self-gravitating, radiating ideal gas under the influence of an axial or azimuthal magnetic field, with an emphasis on adiabatic conditions. The analysis assumes a stationary environment just ahead of the shock wave and considers variations in fluid velocity, magnetic field, and density within the perturbed medium just behind the shock front. In the governing equations, the impact of thermal radiation under an optically thin limit is integrated into the energy equation. Utilizing the Lie invariance method, the set of partial differential equations governing the flow in this medium is transformed into a system of nonlinear ordinary differential equations (ODEs) using similarity variables. Two distinct cases of similarity solutions are obtained by selecting different values for the arbitrary constants associated with the generators. Among these cases, one yields similarity solutions assuming a power-law shock path and the other an exponential-law shock path. For both cases, the resulting set of nonlinear ODEs are numerically solved using the 4th-order Runge–Kutta method in MATLAB software. The article thoroughly explores the influence of various parameters, including γ (adiabatic index of the gas), Ma−2 (Alfvén–Mach number), σ (ambient density exponent), l1 (rotational parameter), and G0 (gravitational parameter) on the flow properties. The findings are visually presented to offer a comprehensive insight into the effects of these parameters.

Reverse cementing: How can it work?

Primary cementing of casing strings is a necessary well construction operation where the annulus behind the string is displaced to a cement slurry. Cementing by reverse-circulation involves injecting cementing fluids directly to the annulus from surface, which can reduce the circulation pressure exerted onto the open-hole section during placement. We study density-unstable reverse-circulation displacement flows involving non-Newtonian fluids in near-vertical, eccentric annuli and use a semi-analytical displacement model to assess whether the interface between displaced and displacing fluids will elongate (unsteady displacement), or remain compact as cementing fluids are pumped down the well (steady displacement). We use the displacement model to study how casing eccentricity and fluid viscosity hierarchy impact the displacement classification. We find that typical field values tend to associate with unsteady displacements, particularly in regions of poor casing centralization, and for displacements involving “traditional” drilling mud, spacer and cement slurry properties. Improving casing centralization, increasing the displacing fluid viscosity and reducing the fluid density difference all act to stabilize the displacement. Similarly, increasing the imposed flow rate can also reduce the rate at which the interface elongates by enhancing the viscous stresses during placement. We compare selected scenarios to displacement simulations performed with the 2-dimensional gap-averaged (2DGA) model, and find qualitative agreement between the 2DGA results and our displacement classification predictions. While our study shows that it is possible to make density-unstable reverse-circulation displacements steady, we note that the identified measures are likely to increase the circulation pressures, which may offset an important benefit of reverse-circulation cementing.

Model establishment and flow field simulation analysis of high torque screw motor in ultra-deep well

Abstract To explore the distribution law of the flow field in the inner annular flow channel of the stator-rotor of the high-torque screw drill tool and determine the key performance parameters of the tool under different drilling fluid densities and flow rates, a physical model for fluid simulation was established with the 7/8 head screw motor as the research object to study the distribution law of the flow field and the output torque of the rotor at different flow rates and densities, and a comparative analysis was made with the actual test. The results show that the pressure of the internal flow channel is mainly concentrated at the entrance of the flow channel and the wall of the flow channel. The flow rate in the thick fluid domain is high, the flow rate in the thin fluid domain is low, and the flow rate in the thinnest area is about zero. The relationship between displacement pressure drop and output torque presents a positive correlation. When displacement increases, the output torque of the screw increases, and the relationship between displacement and torque presents a linear increase. The torque and displacement of fluid simulation under ideal conditions are larger than the actual test parameters. In the actual test, the drilling fluid temperature and experimental equipment factors affect the performance of the screw motor. In the process of fluid simulation, the setting of boundary conditions should refer to the real situation as much as possible to improve the accuracy of simulation results. The difference between fluid simulation and actual test parameter values is small and the trend is the same, so fluid simulation can guide practical application and reduce cost to a certain extent.

Operation of downhole pumps under sanding conditions

One of the unfavourable factors of downhole pumping equipment operation at the fields at the final stage of development is high content of mechanical impurities in the pumped fluid. Similar problems are observed in fields with friable, weakly cemented formations, the fracture products of which flow downhole, resulting in lower extraction rates, equipment damage and higher operating costs. In oil production practice various technological measures and technical means are used to reduce the negative impact of sand penetration - regulation and optimisation of pumping equipment operation mode to ensure a rational rate of fluid withdrawal, application of technical devices in the form of sand filters and sand anchors and others. However, these measures and recommendations, for the most part, work successfully in the modes of continuous operation of wells. With the growing number of oil fields at the final stage of development, the issue of sanding becomes more and more urgent, as due to low flow rate wells at such fields are often switched to the mode of periodic operation, which favours the formation of sand plugs in tubing above the pump due to sufficient time for sedimentation of mechanical impurities. This can lead to pump jamming or rod breakage during subsequent well pump start-up. The experience of using sand filters (frame, slotted and others) in the arrangement of downhole pumps has shown that it does not always effectively solve the problem of sand penetration due to clogging of their cells with mechanical impurities and increased hydraulic resistance. This negatively affects the pumping equipment operation due to the shift of characteristics into the zone of suboptimal modes with associated problems for the electric drive. This makes it necessary to carry out underground well workover with removal of deep well pumping equipment for replacement or washing of the filter in surface conditions, as due to their peculiarities in the well pump layout (the filter is located at the intake of the pumping section), washing in well conditions is not always possible. These problems require not only the creation of effective downhole devices to prevent sand ingress to the pump intake or devices to prevent sand plugs above it, but also acceptable in practice calculation methods for predicting the sedimentation time of mechanical impurities in tubing or determining the minimum required speed of upward oil flow for stable sand removal. These calculation methods would also allow to reasonably regulate the time of technological interruptions while waiting for the wells to start up in periodic operation modes or to predict the time of activation of sand control devices for discharging the sand accumulated above the pump. This is the subject of this article. As a result of calculation analysis it was established that the main factors affecting the sedimentation process of mechanical impurities in the well are the speed and mode of fluid movement, its viscosity, density and size of the impurities themselves. Moreover, the most sensitive for the calculation accuracy is to take into account the change of fluid viscosity as it moves along the tubing as a result of heat exchange and the change of dissolved gas content due to pressure drop. Although at this stage the proposed methodology does not take into account some factors that can make additional adjustments to the process of sand sedimentation, however, it allows to predict with acceptable accuracy the time of technological interruptions in the well operation to prevent the formation of sand plugs, or the rational mode of operation of pumping equipment, providing the necessary speed of upward flow in the pipe for sand removal.

Densitaxis: Active particle motion in density gradients

Organisms often swim through density-stratified fluids. Here, we investigate the dynamics of active particles swimming in fluid density gradients and report theoretical evidence of taxis as a result of these gradients (densitaxis). Specifically, we calculate the effect of density stratification on the dynamics of a force- and torque-free spherical squirmer and show that density gradients induce reorientation that tends to align swimming either parallel or normal to the gradient depending on the swimming gait. In particular, swimmers that propel by generating thrust in the front (pullers) rotate to swim parallel to gradients and hence display (positive or negative) densitaxis, while swimmers that propel by generating thrust in the back (pushers) rotate to swim normal to the gradients. This work could be useful to understand the motion of marine organisms in ocean or be leveraged to sort or organize a suspension of active particles by modulating density gradients.