TY - JOUR
AU - Wang,, Changhui
AB - Abstract Some high water-cut crude oils can flow in the temperature below the oil gel point, while oil particles may adhere to the pipe wall as paste; this process is known as ‘wall sticking’. This can cause partial or even total blocking of the transportation pipe. Several experiments using a laboratory flow loop were conducted to study the wall sticking characteristics of high water-cut crude oils. The experimental results indicated that the predominant influencing factors of wall sticking included shear stress, water-cut and differences between gel point and wall temperature. The wall sticking rate and occurrence temperature decrease with the increase of water-cut and shear stress. The criterion for the wall sticking occurrence temperature (WSOT), and the regression formula of the wall sticking thickness for high water-cut crude oil were then established. Typical case studies indicated that the prediction results obtained from the WSOT criterion and the wall sticking thickness regression formula were in accordance with the measured values. The wall sticking rate and WSOT vary widely under different conditions and it is necessary to consider its non-uniformity in production. gel point, high water-cut, wall sticking occurrence temperature, wall sticking rate 1. Introduction Certain highly viscous crude oils all reach the same high gel point, when heated during transporting, because of poor flow capability at ambient temperatures. However, high energy and risk for oil gelling during the pipeline shutdown can cause issues when the pipeline is heated. Moreover, with the decrease of crude oil in some oilfields, water flooding is widely used to enhance oil recovery, causing high water-cut oil during production, forming water–oil suspension. Finding safe and energy efficient transportation is necessary for oil with a high water-cut, high viscosity and high gel point. (Water-cut in this paper refers to the water volume fraction of well mixed water–oil suspension in a steady state). Research has shown that crude oils with a high water-cut can move at or even below the gelling temperature as being reported by Li et al (2014) and Chen et al (2007). Milind (2008) found that if all other factors remained constant,sticking in unheated pipes (i.e. in cold conditions) is weaker than if it is flowing under heated conditions. Properties of water–oil suspension, factors that affect the stability of its system, and the microstructure of crude oils with a high gel-point and a high viscosity at low temperatures were studied by El-Gamal et al (1997), Ruffier-Meray et al (1998), Ribeiro (1997), Huang et al (2013), Yu et al (2013) and Singh et al (2000). The non-heating oil-gathering process and blended water pipelining may be a feasible solution to decrease energy consumption during transportation of high water-cut, highly viscous, high gel-point crude oil, even though its temperature without heating may be around or below the gel point. Viscosity of oil below the gel point is high while the viscosity of water is low. Water as a continuous phase can push oil particles in the suspension flow in the pipeline. Because of the ambient temperatures, the oil dispersion drops will coalesce and stick on the pipe wall due to the adhesive force of the pipe wall being stronger than the force of the water phase working on the oil drops (known as ‘wall sticking’). This only appears in water–oil suspensions with a high water-cut, and it is easy to form emulsion with them. Then the water phase cannot push high content and highly viscous oil flow satisfactorily, and plugging will quickly appear. In the application of practical engineering, the non-heating oil-gathering process and blended water pipelining are applied to oil with a water-cut higher than 80%. (Here, a high water-cut means water volume fraction of water–oil suspension over 80%.) Wall sticking is caused by adhesion, while deposition is caused by concentration difference and molecular diffusion. Components of wall sticking are similar to oil particles, but the carbon number distribution of deposition is different from the pipe fluid oil. Wall sticking also causes pipeline plugging or blocking as described by Cosultchi et al (2001, 2003). Research on low-temperature gathering,transferring technology, and temperature allowance has been studied by Wang et al (2011) and Zhang et al (2004). Eskin (2011) studied the problem in depth of particles sticking on the wall and Li (2009) researched the effects of shear stress on the structural properties of Daqing gelled crude oil. Although previous studies can offer valuable guidance on wall sticking, the feasibility of low-temperature gathering and blended water pipelining still cannot be determined. Here, experiments were conducted to observe the effect of water-cut and shear stress on the wall sticking occurrence temperature (WSOT) and wall sticking rate. Then a criterion for the WSOT and wall sticking rate regression formula was established to provide a theoretical guidance for the transportation of high water-cut crude oils in certain oilfields. 2. Experiment 2.1. Materials Oil samples were collected from three different oilfields in China. Their viscosity–temperature curves under different shear rates and basic physical properties are listed in figure 1 and table 1, respectively. Table 1. Basic physical properties of experimental oil samples. Sample . Gel point (°C) . Pour point (°C) . Density 20 °C (kg m-3) . Water-cut (vol.%) . Wax appearance temperature (°C) . Wax content (wt.%) . Oil 1 34 37 869.78 30 44 22.5 Oil 2 14 17 791.41 25 23 13.4 Oil 3 28 31 858.28 20 47 23.6 Sample . Gel point (°C) . Pour point (°C) . Density 20 °C (kg m-3) . Water-cut (vol.%) . Wax appearance temperature (°C) . Wax content (wt.%) . Oil 1 34 37 869.78 30 44 22.5 Oil 2 14 17 791.41 25 23 13.4 Oil 3 28 31 858.28 20 47 23.6 Open in new tab Table 1. Basic physical properties of experimental oil samples. Sample . Gel point (°C) . Pour point (°C) . Density 20 °C (kg m-3) . Water-cut (vol.%) . Wax appearance temperature (°C) . Wax content (wt.%) . Oil 1 34 37 869.78 30 44 22.5 Oil 2 14 17 791.41 25 23 13.4 Oil 3 28 31 858.28 20 47 23.6 Sample . Gel point (°C) . Pour point (°C) . Density 20 °C (kg m-3) . Water-cut (vol.%) . Wax appearance temperature (°C) . Wax content (wt.%) . Oil 1 34 37 869.78 30 44 22.5 Oil 2 14 17 791.41 25 23 13.4 Oil 3 28 31 858.28 20 47 23.6 Open in new tab Figure 1. Open in new tabDownload slide Viscosity-temperature curves of oil sample 1(a), oil sample 2(b), and oil sample 3(c). Figure 1. Open in new tabDownload slide Viscosity-temperature curves of oil sample 1(a), oil sample 2(b), and oil sample 3(c). 2.2. Experimental procedures A small-scale flow loop as shown in figure 2 was used to conduct the wall sticking experiments of gelling oil. This experimental apparatus can be operated under tightly controlled wall shear rates and temperature gradient conditions that reflect actual pipeline operating conditions. Further detailed introductions of this small-scale flow loop have been introduced previously by Huang (2000). Figure 2. Open in new tabDownload slide Flow loop configuration. Figure 2. Open in new tabDownload slide Flow loop configuration. Emulsion with a 30% water-cut was prepared first for the three oil samples. Then water–oil suspension with a high water-cut was prepared with a 30% water-cut emulsion and pure water, this was manipulated in the tank until it was well mixed. In a typical flow loop run, the fluid in the stirred oil tank was driven by the pump through the test section, then the reference section, and finally back to the oil tank. Temperatures of the outlet of the pump, the oil tank, and the inlet and outlet of the test and reference sections were monitored by thermocouples. The wall temperature of the test section was set below the oil temperature and gel point in order to induce the gel stick on the cold pipe wall. The reference section temperature was kept high enough to prevent wall sticking. Then the system was heated up to 60 °C and suspension was removed by compressed air. 3. Experiment results and discussion 3.1. Wall sticking occurrence temperature The WSOT is defined as the temperature below which the growth rate of thickness or the pressure drop appears to increase rapidly. It can be a standard to determine the feasibility of water–oil suspension transported in the pipelines. The effects of the water-cut on the WSOT can be seen in figures 3(a)–(c). The gel points of three oil samples with a 30%, 25% and 20% water-cut are shown in table 1. As can be seen in figure 3, the WSOT for samples with a higher water-cut were lower than the oil sample gel point with just a 30%, 25%, and 20% water-cut. It can be assumed that the high water-cut crude oil can be gathered and transferred below the oil gel point. The three oil samples have the same tendency for the WSOT to decrease as the water-cut increases. The reason for this is that as the water-cut increases, the volume concentration of the continuous phase (water) increases and the volume concentration of the dispersed phase (oil) decreases. Then, the space between the highly viscous oil particles increases. This reduces the probability of the oil particles aggregation, bombarding and sticking on the pipe wall at the same temperature. The viscosity of the oil particles around WSOT is very high as can be seen in figure 1. As such, suspensions with a low water-cut are not capable of pushing high content and highly viscous oil particles flow well. So, it may not be a good solution to adopt non-heating transportation for suspension with a low water-cut. Figure 3. Open in new tabDownload slide Relationship between WSOT and water-cut of oil sample 1(a), sample 2(b), and sample 3(c), and shear stress of oil sample 1(d), sample 2(e), and sample 3(f). Figure 3. Open in new tabDownload slide Relationship between WSOT and water-cut of oil sample 1(a), sample 2(b), and sample 3(c), and shear stress of oil sample 1(d), sample 2(e), and sample 3(f). The effects of shear stress on the WSOT are shown in figures 3(d)–(f). The WSOT decreased with the increase of shear stress. Due to the various sample constituents, these three oil samples have different variation gradients but the same trend. During the pipeline transportation of crude oils, the scouring action of the fluid on the pipe wall grows with an increasing shear stress, which makes it more difficult for gelled oil to stick on the pipe wall. Moreover, it is more likely for gelled oil to be removed from the pipe wall by a stronger shear stress. Due to the fact that the volume concentration of the high water-cut oil decreases to a lesser volume compared to those oils with a lesser water-cut, the WSOT decreases more rapidly against shear stress for the water–oil suspension with a higher water-cut. 3.2. Wall sticking rate 3.2.1. Determination of the wall sticking thickness. In the flow loop experiment, instead of direct measurement, the pressure-drop method was used to measure the wall sticking thickness of the fluid flowing in the pipe. This can be performed online without interrupting the experiment. The test and reference sections, geometric dimensioning and fluid velocity of the pipe are set at the same values. Ignoring the minor temperature difference, the wall sticking thickness can be calculated as follows. The pressure drop is calculated by the Darcy–Weisbach formula as equation (1): ΔP=8λLQ2ρπ2D51 where ΔP is the pressure drop (Pa) across the test or reference section, L is the length (m) of the test or reference section, ρ is the density (kg m-3) of suspension, Q is the volume flow rate (m3 s-1), and λ is the hydraulic friction coefficient, either under laminar flow λ=64/Re ⁠, or turbulent flow λ=0.3164/Re0.25 ⁠. The variables of velocity, volume rate of flow Q, density, and length have the same values in the test and reference section, and the minor viscosity difference can be ignored. The ratio of the test pressure drop and reference drop is expressed as follows: ΔPtΔPr=(RrRt)a2 where parameter a is 4 under laminar flow and 4.75 under turbulent flow. The wall temperature of the reference section was kept high enough to prevent wall sticking and its diameter Rr remained a constant value. The wall sticking thickness can be calculated as follows: δ=R0−Rt3 where ΔPt is the pressure drop (Pa) across the test section, ΔPr is the pressure drop (Pa) across the reference section, Rt is the pipe radius (m) of the test section, Rr is the pipe radius (m) of the reference section, and R0 is the initial pipe radius (m) of the test section. 3.2.2. Effects of shear stress, water-cut and temperature on the wall sticking rate. With any water-cut and shear stress, the thickness initially increased rapidly, followed by a gradual increase. Reasons for this might be that in the beginning, the pipe wall is rough and it is easy for gelled oil to stick. Then oil particles fill in the cavities of the pipe wall, creating smoother surface. Simultaneously, shear stress becomes stronger with the increasing wall sticking thickness (decreasing pipe diameter). Wall sticking stress becomes weaker because of the increased smoothness of the wall and stronger shear stress. The effects of water-cut on the wall sticking rate are shown in figures 4(a)–(c). It can be seen from this figure that the initial wall sticking rate diminished gradually with the increasing of water-cut for the three samples. A decreasing concentration of oil drops make it more difficult for gelled oil particles to aggregate. Also, the viscosity becomes smaller with the increase of the water-cut and the formed gelation joint of the network becomes weaker. This may be the reason for the decreasing wall sticking rate. Figure 4. Open in new tabDownload slide Relationship between wall-sticking rate and water-cut of oil sample 1(a), sample 2(b), and sample 3(c), and shear stress of oil sample 1(d), sample 2(e), and sample 3(f), and temperature difference of oil sample 1(g) and sample 3(h). Figure 4. Open in new tabDownload slide Relationship between wall-sticking rate and water-cut of oil sample 1(a), sample 2(b), and sample 3(c), and shear stress of oil sample 1(d), sample 2(e), and sample 3(f), and temperature difference of oil sample 1(g) and sample 3(h). The effects of shear stress on the wall sticking rate are shown in figures 4(d)–(f). The three oil samples have the same overall trend in that the wall sticking rate decreased with the increase of shear stress. The pipe walls become smoother because of the oil particles sticking on the small cavities in the wall surface. Moreover, the scouring action of the fluid on the pipe wall increases with the decreasing pipe inner diameter due to wall sticking. The gelled oil therefore has difficulty adhering to the pipe wall and the wall sticking rate becomes slower. The influence of the surface roughness on the particle deposition is critical for Eskin (2011). Stronger shear stress can scour the pipe wall more effectively and lead to a smoother pipe wall, therefore reducing the wall sticking rate. Generally, as the flow rate increases, more gelled oil is able to be transported through the pipe per unit of time and more gel should be left on the pipe wall due to the increased volume. In contrast, the shear stress escalates with the increased flow rate, causing substantial scouring action. Thus, the wall sticking rate decreases with an increasing flow rate and shear stress. The WSOT is lower than the gel point and wall sticking only appears under temperatures lower than TGP. The effects of the temperature difference between the gel point and the pipe wall are shown in figures 4(g) and (h). It can be seen from these figures that the wall sticking rate increases with decreasing temperature difference between the gel point and pipe wall. As the pipe wall temperature reduces the fluid flowability of oil particles becomes weaker. Then it becomes easier for oil particles to stick to the pipe wall. Moreover, it can be seen from figures 4(g) and (h) that different oil samples have various temperatures at which wall sticking appears. Adding more water to gelled oil and using higher velocity are good for the transportation of high water-cut, highly viscous, and high gel-point crude oil. 4. Prediction of WSOT and wall sticking rate 4.1. Prediction of WSOT As can be seen from the results of the experiment, water-cut and shear stress are the main factors that influence the WSOT; an increase in either factor causes a decrease in the WSOT. Based on the comprehensive investigation and experimental data given above, an empirical formula was established to calculate the WSOT for high water-cut, highly viscous, and high gel-point crude oils in unheated pipelines: TN=TGP−kφmτwn4 TN is the WSOT (°C) of water–oil suspension in pipelines, TGP is the gel point (°C) of oil–water emulsion with stable water-cut of 30%, ϕ is the water-cut of water–oil suspension with range of 80%–99% (vol.), τw is shear stress (Pa) on pipe wall, and k, m, and n are the regression coefficients. The shear stress of the fluid under laminar and turbulent flow on the pipe wall is expressed as: τw=μ8vD5 Under turbulent flow it is expressed as: τw=4.94×10-3⁢ Re0.758vDμ6 The dynamic viscosity of the prepared water–oil suspension μ can be calculated by: μ=πΔPR48QL7 where ΔP is the initial stable pressure drop (Pa) across the test section, R is the initial pipe radius (m) of the test section and L is the length (m) of the test section. The parameters in equation (4) for the three samples were regressed. For the three oil samples, k, m and n are different because of composition differences. The WSOT of water–oil suspension for oil samples 1, 2 and 3 under various water-cut and different shear stresses were calculated by equation (4). The calculated values are consistent with the experimental values, and the absolute error is less than 0.8 °C. So the established criterion is reliable to calculate the WSOT allowance for the transportation of high water-cut water–oil suspension in un-heated pipeline. 4.2. Regression formula of the wall sticking rate As can be seen from the experiment, the predominant factors which influence the wall sticking rate include water-cut, shear stress on the pipe wall, and differences between gel point and wall temperature. Based on the above comprehensive investigation, a regression formula was established for the wall sticking rate: dδdt=aτ0b(1-ϕ)cexp(dϕgτf⁢ )(TGP-Tbi)g.8 The wall sticking thickness can be expressed as: δ=dδdtt9 where dδ/dt is wall sticking rate (m s-1) on the pipe wall, τ0 is the initial shear stress (Pa) on the pipe wall, ϕ is total water-cut, Tbi is the temperature (°C) of the pipe wall, and a, b, c, d, e, f, and g are the regression coefficients, δ and t are the wall sticking thickness and sticking time respectively. For the three oil samples, these coefficients have different values because of their composition differences. The calculation values under different experimental conditions obtained through the novel regression formula were compared with the experimental values, and the relative errors were all under 30%. It is therefore acceptable to practical engineering calculations and it can estimate the wall sticking thickness of pipelines on site and provide guidance for optimizing the pigging frequency. 5. Conclusions In this paper, a series of laboratory flow loop experiments with high water-cut, highly viscous and high gel-point crude oil were conducted. Experimental results showed that it was feasible to transfer high water-cut water–oil suspension without heating below the gel point, but wall sticking could occur on the pipe wall. Water-cut and shear stress are the prominent factors that affect the WSOT. This decreased as the water-cut and shear stress increased. The predominant factors that affect the wall sticking rate are the water-cut, shear stress and temperature difference between the gel point and pipe wall. The wall sticking rate increased as the water-cut and shear stress decreased. Moreover, the wall sticking rate increased with the increase of temperature difference between the gel point and pipe wall. In the process of wall sticking, the thickness of the sticking oil increased rapidly initially and later gradually because of the smoothing of the pipe wall and the increase in shear stress. The WSOT criterion and wall sticking rate regression formula were put forward based on obtained experimental data. The absolute error of the WSOT was less than 0.8 °C and the relative error of wall sticking rate was less than 30%. This study provides a collection of data detailing the wall sticking problem. But some questions and necessary future research remain pertaining to wall sticking. Research should be undertaken relating to the deposit sediment of wall sticking, for example the water-cut and carbon number distribution of sediment. Further study is needed into the relationship between the crude oil gel point and the WSOT. A micro-structure analysis is necessary as part of any further wall sticking study. Acknowledgment The authors are grateful for the support from the projects of the National Natural Science Foundation of China No. 51374224 for this research. References Chen Y , Tang L H , Wang X B . , 2007 Study on ultimate temperature in low temperature transportation of water cut oil , Offshore Oil , vol. 27 (pg. 60 - 63 ) OpenURL Placeholder Text WorldCat Cosultchi A , Garciafigueroa E . , 2001 Petroleum solid adherence on tubing surface , Fuel , vol. 80 (pg. 1963 - 1968 ) 10.1016/S0016-2361(01)00042-4 Google Scholar Crossref Search ADS WorldCat Crossref Cosultchi A , Rossbach P . , 2003 XPS analysis of petroleum well tubing adherence , Surf. Interface Anal. , vol. 35 (pg. 239 - 245 ) 10.1002/sia.1516 Google Scholar Crossref Search ADS WorldCat Crossref El-Gamal I M , Gad E A M . , 1997 Low temperature rheological behavior of Umbarka waxy crude and influence of flow improver , Oil Gas Sci. 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TI - Wall sticking of high water-cut crude oil transported at temperatures below the gel point
JF - Journal of Geophysics and Engineering
DO - 10.1088/1742-2132/12/6/1008
DA - 2015-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/wall-sticking-of-high-water-cut-crude-oil-transported-at-temperatures-yjva29Xe9M
SP - 1008
VL - 12
IS - 6
DP - DeepDyve
ER -