A New Method for Predicting Wall Sticking Occurrence Temperature of High Water C

Cui Yue; Huang Qiyu; Zhang Yan; Zhao Jiadi; Zheng Haimin; Cheng Xianwen

(1. Surface Engineering Pilot Test Center/National Engineering Laboratory for Pipeline Safety/Beijing Key Laboratory of Urban Oil and Gas Distribution Technology,China Uniνersity of Petroleum-Beijing, Beijing 102249;2. China Huanqiu Contracting & Engineering Co., Ltd., Beijing 100012;3. CNOOC Research Institute Co., Ltd., Beijing 100027)

Abstract: In crude oil transportation, adhesion of oil on pipe wall can cause partial or total blockage of the pipe. This process is signi ficantly affected by wall sticking occurrence temperature (WSOT). In this work, an efficient approach for estimating WSOT of high water-cut oil, which can agree well with the actual environment of multiphase transportation pipeline, is proposed. Based on the energy dissipation theory, it is possible to make comparison of average shear rates between the stirred vessel and the flow loop. The impacts of water content and shear rate on WSOT are investigated using the stirred vessel and the flow loop. Good agreement has been observed between the stirred vessel and the flow loop results with the maximum and the average absolute deviations equating to 3.30 °C and 2.18 °C, respectively. The development of gathering scheme can enjoy some bene fits from this method.

Key words: high water-cut oil; wall sticking occurrence temperature; adhesion; energy dissipation; low-temperature gathering

1 Introduction

In the middle and late stage of oil field development, the comprehensive water content of most produced fluids has increased to 70%. In order to reduce the energy consumption of oil transportation, the oil gathering temperature is usually reduced[1-2]. At this time, if operating parameters of the pipeline are not set properly,the gelled oil will stick to the pipe wall. On one hand, the gelled oil will increase the frequency of pipeline pigging operation. On the other hand, an excessive amount of gelled crude oil adhering to the pipe can even lead to the pipe blockage[3-4]. However, the composition of oil is very complex, and its properties are affected by flow conditions (temperature, shear condition, and degree of emulsi fication, etc) and flow history (thermal history, and shear history, etc), which makes it more difficult to study the adhesion behavior between oil and pipe wall[5-6].

The Fogler’s team discovered the initial condensate layer on the wall of the tube during the study of wax deposition[7]. However, the mechanism of wall sticking is different to that of wax deposition which is caused by molecular diffusion occurring under a temperature difference between the oil and the pipeline wall[8]. To determine the low-temperature operation parameters of the pipeline with high water content, Zheng, et al. studied the influencing factors of the adhesion behavior, such as the water content, the shear condition, the properties of crude oil[1]. Liu, et al. concerned about the complex layout problem of large-scale oil-gas gathering system,and the valuable optimization methodologies have been obtained[9]. Research shows that waxy crude oil with high water content can maintain flow at temperatures below the gel point. As the oil cools down to temperatures below the wax appearance temperature (WAT), the wax crystals begin to precipitate and interweave with each other, and the oil can forms a gel system[10-11]. Oil cluster will adhere to the pipe wall when the pipe flow stripping force is less than the adhesion force of the pipe wall. However, a universal theory for predicting the adhesion process has not been identified. Researchers have always relied on empirical data and experiments to predict the adhesion behavior of speci fic oil. Zheng[1]studied the effect of flow conditions on oil-wall adhesion behavior in a gathering pipe via an indoor flow loop, and the wall sticking occurrence temperature was found. This parameter provides an important basis for the determination of oil field gathering boundary with high water content at low temperature and has a great significance for preventing pipeline condensation and adhesion. In view of the current situation, it is evident that more work is needed to establish a consistent method for predicting the WSOT,which is the focus of the present study.

At present, the experimental research methods of oil-wall adhesion behavior and WSOT mainly include the field pipeline, the indoor flow loop, and the self-made device.Building multi-phase flow devices in the field is arguably the most intuitive way of conducting experiments, with the produced liquid from the well being fed directly into the loop, and the pipes constructed from transparent quartz glass tubes being used for observing the flow patterns inside. However, the construction of experimental pipelines on site is expensive, and the pigging operations carried out before and after the experiments are complex.An indoor flow loop with operating conditions being close to those of the actual multiphase transportation pipeline is widely used for field experiments, which have the advantage of ease of operation and pigging[1,9]. Meanwhile,the device structure is relatively complex and expensive to fabricate, and experiments are time-consuming. In contrast, a custom-made small stirred vessel device not only can occupy a smaller area but also is more convenient to operate. Nowadays, a detailed comparison between the experimental conditions and results of the stirred vessel and flow loop has not yet been carried out in previous studies.In response, this work develops a WSOT measurement technique, featuring a small-sized apparatus, short experimentation duration, and small test sample amount, as compared to conventional methods. A model based on the energy dissipation theory is used to compare the shearing rates of stirred vessel and flow loop. The impacts of water content and shear rate on WSOT are also investigated.

2 Experimental

2.1 Materials

Two different crude oil samples (named as #1 and #2 oil samples) were collected from oil fields in China and were used to prepare oil samples for adhesion experiments.The pour point (determined according to ASTM D5853-11), the density (obtained using an immersion densimeter with a measuring range of 800—1 000 kg/m3), the wax appearance temperature and wax content (measured using a differential scanning calorimeter TA Q20, USA), and the asphaltenes and resins content (measured according to ASTM D4124-09) of the oil samples are listed in Table 1. The viscosity-temperature curves under different shear rates (measured by using a Anton Paar Rheolab QC rheometer, Germany) of the oil samples are shown in Figures 1 and 2, respectively. For obtaining a better repeatability of experimental results, the samples were pretreated as follows to remove the memory effects.Samples were filled into reagent bottles evenly. Then, the sealed samples were heated up to 80 °C and were held for 2 h. At last, the samples were cooled quiescently and were kept at room temperature for at least 48 h before being used for tests.

Table 1 Properties of the original crude oil

2.2 Apparatus

2.2.1 Stirred vessel

As shown in Figure 3, the experimental apparatus, a stirred vessel, consists of: (1) an oil vessel with an ID of 75 mm and a length of 105 mm equipped with a bottom discharge valve and a bilayer structure connected to an AC200 Thermo Fisher thermal bath (U.S.A) with a control stability of ± 0.01 °C for temperature control; (2)an electric overhead stirrer equipped with an impeller and a tachometer, with an effective stirring speed ranging from 10 r/min to 1600 r/min; and (3) a V-shaped measuring tube for determining the free water volume of the oil-water mixture.

Figure 1 The wax amount precipitated from oil samples

Figure 2 Viscosity-temperature curves of oil samples

2.2.2 Flow loop

Figure 4 shows a schematic of the small-scale indoor flow loop apparatus. The flow loop mainly includes a test section, a reference section, a mass flow meter, a peristaltic pump, a thermostatic vessel controlled by a water bath, two pressure sensors, and a data acquisition system. The test and reference sections are the core components of the whole system, in which the wall sticking phenomenon occurs. These sections are structured as coaxial pipes with a length of 1.5 m including an inner pipe with Φ14 mm×1 mm in dimension and an outer pipe with Φ30 mm×1 mm in dimension. The annulus serveing as a water jacket is connected to the thermal water baths through rubber hoses enclosed with an insulating layer to manipulate the pipe wall temperature. A peristaltic pump is used to drive the fluid through the pipeline, which can avoid shear effects in the flow process that may change the crude oil sample. The flow rate is measured using a mass flow-meter with a measuring range of 0—600 kg/h.

Figure 3 Schematic diagram of the stirred vessel apparatus[5]

Figure 4 Schematic diagram of the flow loop apparatus[1]

2.3 WSOT measurement procedures

2.3.1 Stirred vessel experiment

The key aim of the present study is to determine the adhesion quality as a function of temperature. The emulsions were prepared in advance to eliminate the influence of the emulsification process itself on the behavior of the mixture during testing. At the start of the experiment, water was poured into the loop mixing vessel, followed by the prepared emulsion. The required volumes of water and emulsion were fed into the stirred vessel at a speci fied temperature and were then allowed to stay for 10 min to ensure that the temperature was uniform. The system was stirred at a constant speed while the temperature was lowered to the test temperature. The mixed oil-water system was discharged from the valve under the vessel and the water drop adhesion on the vessel wall was sucked out. The gelled emulsion adhering to the vessel wall following this process was removed using a filter paper and weighed.

2.3.2 Flow loop experiment

In this work, the differential pressure method is used in the flow loop experiments. This method is based on the principle that the increase of pressure drop over the test section stems from the reduction of the hydraulic diameter when the gelled oil adhesion appears. The adhesion thickness can be calculated by contrasting the pressure drop of the reference section and the test section under the same conditions.

3 Theoretical Modeling

3.1 Average shear rate in stirred vessel

It is known that the shear strength of a fluid varies spatially in a stirred vessel — specifically, the shear strength is greater near the impeller. In the present study, the average shear rate in the stirred vessel was used to characterize the shear strength at different rotational speeds. The average shear rate over the whole stirred vessel can be calculated using a model based on the relationship between the energy dissipation rate and the shear rate[12]:

whereNis the rotating speed of the vane, r/s;Vis the volume of the fluid in the vessel, m3;Mis the torque of the mixing propeller, N·m; andμis the dynamic viscosity of the fluid, Pa·s. This model is derived based on fluid mechanics theory, and its application is not limited by the fluid flow state and impeller type.

3.2 Average shear rate in pipeline

whereKis the consistency coefficient of power fluid,Pa·sn;nis the flow behavior index;Qis the volume rate of flow, m3/s;his the friction loss of fluid in the pipe, m;LandRis the length and radius of the pipe, m, respectively;fis the coefficient of friction resistance;ρis the density of the fluid inside the pipe, kg/m3;vis the average flow rate of the fluid, m/s;Dis the internal diameter of the pipe, m.If the fluid in the pipe is a Newtonian one,n= 1 andK=μ;hence, Eq. (3) becomes:

3.3 Corresponding expressions of stirred vessel and flow loop

The principle of the present study is to simulate the shear of pipe flow by the shear of stirring flow. The differential pressure can be calculated from the Bernoulli equation:

wherep1andp2are the pressure values at the beginning and the end of the pipe, Pa; andZ1andZ2are the elevation at the beginning and the end, m. In the flow loop,Z1=Z2.The average shear rate can then be determined by using:

whereμis the dynamic viscosity of the medium in the pipe, Pa·s.

The dynamic viscosity (μ) of the prepared emulsion in the test section with different water fractions can be calculated according to[13]:

where ΔPis the initial stable pressure drop of the test section, Pa;Ris the initial pipe radius of the test section,m; andLis the length of test section, m. When the average shear rate of the fluid in the stirred vessel is the same as that of the fluid in the flow loop, the operating parameter can be determined using the following expression:

If the pressure drop in the pipeline is unknown, the operating parameters can also be obtained by the following formula:

In the present study, the waxy crude oil-water system with high water content was taken as a Newtonian fluid for the purpose of simplifying the preliminary calculations.This simplified calculation method has been verified by Zheng[1]in the measurement of boundary temperature of high water-cut crude oil, and the experimental results are consistent with the field experiments. It is planned to extend the model for incorporating the non-Newtonian fluids in future studies.

4 Results and Discussion

4.1 Experimental determination

It has been shown that the content of wax, resins, and asphaltenes in the adhering oil was a little higher than that of crude oil through microscopic morphology observation and component test, resulting in higher density and viscosity of the system[2]. The water content of the adhering emulsion includes the free water, the adsorbed water, the capillary water, and the internal emulsified water. Generally, the network structure constructed by wax, resins and asphaltenes contains oil, mechanical impurities, and water droplets, which constitute a kind of soft material. The gelled oil cluster is subjected to the Van der Waals forces, capillary forces, buoyancy and gravity in the vertical direction, the frictional resistance from the pipe wall, the peeling action of the flow medium,and soon. When the shear and peel action is greater than the friction resistance and adhesion force, no obvious wall adhesion behavior is observed. With a decrease in the operating temperature, the viscosity and hence the structural strength of waxy crude oil increases. If the peeling action is less than or equal to the frictional resistance and adhesion force, the mass of adhering oil begins to increase[14]. The critical temperature is de fined as the WSOT and the mass of adhering oil is de fined as the wall sticking mass.

Figure 5 shows the result of the experiment of the oil sample with a water content of 90% under a shear rate of 72 s-1. The figure shows that a uniform thin oil layer adheres to the vessel wall when the temperature is greater than 26 °C. After the temperature is lowered to 25 °C, the mass of gelled oil on the wall noticeably increases and exhibits as a blocky structure. Thus, the WSOT (To) under this condition is 25 °C, as con firmed by Figure 6.

In the flow loop, with the decrease in temperature, the inner diameter of the pipe decreased, and consequently the pressure drop of the test section increased rapidly. As shown in Figure 7, the WSOT is 24 °C.

Figure 5 Gelled #2 oil stuck on the wall in the stirred vessel

4.2 Effect of water content

The WSOT determined by the stirred vessel experiments is shown in Figure 8. The gel point of #1 oil, #2 oil, #1 oil with 10% of water, and #2 oil with 30% of water is 27 °C, 34 °C, 30 °C, and 35 °C, respectively. All the WSOT values of the samples are lower than the gel points of oil and emulsion samples, which can confirm the feasibility of the low temperature gathering process. The two crude oil samples both showed a negative correlation between the WSOT and the water content. This fact may be explained as follows: An increase in the water content increases the continuous water phase in the conveying medium, causing a relative decrease in the number of gelled crude oil cluster; hence, the probability of contact between the oil and the wall is reduced.

Figure 6 WSOT determination using the stirred vessel

Figure 7 WSOT determination using the flow loop

Figure 8 Relationship between WSOT and water content of oil samples

4.3 Effect of shear rate

The effect of shear rate on the WSOT of gelled oil is shown in Figure 9. With a water content ranging from 70% to 95%, the shear rate appears to be negatively correlated with the WSOT. This is due to the fact that the cluster of gelled crude oil is dispersed in the aqueous phase at high water content. When the oil cluster comes into contact with the pipe wall, they are mainly subjected to three different forces, viz.: the buoyancy, the adhesion force between the gelled oil cluster and the pipe wall, and the shear by the aqueous phase. However, at the same temperature and water content, the former two forces are constant; therefore, the greater the shear stress, the stronger the peeling effect on the gelled oil. In addition,the increase of shear stress will destory the reticular structure fornred by wax crystal, which will weaken the structural strength of oil agglomerate and make it easier to be washed away by flowing water. Thus, it is less likely to stick to the wall, and the WSOT will be lower.

5 Comparison between Stirred Vessel and Flow Loop Results

Both the indoor loop and the simulation vessel are used to conduct the wall sticking experiments at different water contents and shear rates. The comparison and error analysis between the results obtained with the two systems are shown in Table 2.

As Table 2 shows, there is a good agreement between the two methods. The absolute errors between the two methods are less than 3.30 °C for all data points, with the average errors being equal to 2.10 °C and 2.26 °C for each oil sample, respectively, and the average absolute deviation is 2.18 °C. This fact indicates that the shear rate correspondence method using the stirred vessel apparatus can work reasonably well as a simpli fied method for lowtemperature wall sticking experiments with high watercut crude oil as compared to the flow loop method.

Possible reasons for the discrepancy between the two experimental methods are as follows: Firstly, the simulation vessel is a non-sealed system. During the experiment, despite the use of a cover plate over the vessel, the evaporation of the water phase could not be completely avoided. This structure could reduce the water content of oil in the system slightly, thus weakening the scouring effect of water relative to gelled oil, making the simulation vessel experimental results generally higher than those of the corresponding loop experiments.Secondly, the indoor annular device was not a completely horizontal straight pipe section, since it contained some elbows and measuring instruments. These components could impede the flow of the gelled crude oil in the system, causing some oil clusters to adhere to these locations instead of sticking to the tube wall[15-16]. This phenomenon could result in an increased water content of the system to reduce the WSOT.

Figure 9 Relationship between WSOT and wall shear rate of oil samples

Table 2 Comparison between experimental results of the two test systems

6 Conclusions

A convenient and efficient method based on the energy dissipation theory for measuring WSOT of high watercut oil in the gathering system has been developed. Then,the model corresponding to the average shear rate of stirred vessel and flow loop is derived. The operating parameters of the two systems calculated by the model are verified by experimentation. It was found that the average absolute deviations between two test systems were equal to 2.10 °C and 2.26 °C for each oil sample,respectively, and the maximum absolute deviation was 3.30 °C for experimental parameters during measuring oil samples with a water content in the range of 70%—90% under a shear rate of 40—72 s-1. Therefore,under these conditions, this technique featuring a short experimentation duration, a small amount of test samples,and a small-sized apparatus can serve as a reasonable replacement for the actual multiphase transportation pipeline method for measuring WSOT.

At high water contents, the waxy crude oil-water mixture can be transported at a temperature lower than the gel point of crude oil, which verifies the feasibility of lowtemperature transportation of crude oil with high water content.

Acknowledgements:This work was financially supported by the National Natural Science Foundation of China (NNSF, Grant No. 51534007).