A PRELIMINARY STUDY ON HYDRATE ANTI-AGGLOMERANT APPLIED IN MULTIPHASE FLOW PIPELINE

Transcription

1 Proceedings of the 7th International Conference on Gas Hydrates (ICGH 011), Edinburgh, Scotland, United Kingdom, July 17-1, 011. A PREIMINARY STUDY ON HYDRATE ANTI-AGGOMERANT APPIED IN MUTIPHASE FOW PIPEINE Haiyuan Yao, Qingping i China National Offshore Oil Corporation Research Center No.6, Dong-Zhi-Men-Wai Xiaojie, Dongcheng District, Beijing, CHINA Guangjin Chen, Jing Gong China University of Petroleum, Beijing No.18, Fuxue Road, Changping District, Beijing, 1049 CHINA ABSTRACT When new types of hydrate chemical inhibitor (such as hydrate anti-agglomerant) are used in offshore fields, some difficulties will be encountered in the pipe transportation processing design. In view of these difficulties, a horizontal flow experiment pipe loop of transparent polypropylene pipe (5.4 mm inner diameter, 0 m long) was constructed, and some experiments were conducted. The working fluids were the mixed paraffin hydrocarbons, water and condensate oil mixtures obtained from offshore oil field. Analyzing the experimental results and adopting relevant theories of liquid-solid two-phase flow, the computational method of hydrate slurry transportation in pipe was developed. For the operating conditions of a certain multiphase pipeline, by using OGA software, this method can be used to calculate pressure drop of the multiphase flow pipelines using anti-agglomerant, which can provide support for hydrate antiagglomerant application. Keywords: hydrate, slurry, pressure drop NOMENCATURE a constant A liquid area A G gas area b constant C parameter D diameter D iquid equivalent diameter E S volume fraction of solid phase E w volume fraction of water phase E h volume fraction of hydrate phase g c dimension conversion coefficient (g c =1 kg m/n s ) H liquid holdup k absolute roughness ΔP ΔP h ΔP h p Q G Q O Q W Re s S i V V M V S length of pipeline pressure drop pressure drop gradient caused by hydrate particles pressure drop gradient caused by oil phase pressure flow rate of gas phase flow rate of oil phase flow rate of water phase Reynolds number of liquid phase ratio of liquid and solid phases density wetted length of the interface average velocity of liquid phase average velocity of mixture phase average velocity of solid phase Corresponding author: Phone: Fax yaohy@cnooc.com.cn

2 V S superficial velocity of the liquid phase [V S = (1-E S ) V ] V SS superficial velocity of the solid phase [V SS = E S V S ] z pipe length Z parameter β scale factor ε equivalent absolute roughness θ inclination angle λ H friction coefficient of hydrate phase λ friction coefficient of liquid phase λ * S friction coefficient of solid phase λ S hydrate friction coefficient ρ iquid phase density ρ s solid phase density τ i interfacial sheer stress between gas and liquid shear stress between liquid and pipe wall τ S However, there is no example with the application of anti-agglomerant in China. According to the experiments carried out in the hydrate slurry flow loop, a hydraulic calculation model combined with OGA software wa s presented to calculate the pressure drop in hydrate slurry pipelines using anti-agglomerant. It was technical assistance for the application of anti-agglomerant in petroleum industry. EXPERIMENTA FACIITY AND MEDIUM The experimental facility was built in reservoirs phase key laboratory of China University of Petroleum (Beijing), as described in Figure 1[3]. R esitance thermocouple detector Scanning Thermome te r Recirculation Tube Diffe rnetial Pressure Transducer ase r Scattering De vic e Hydrate Tube Swagelok Union INTRODUCTION Hydrates formed high pressure and low temperature condition are ice-like, cage-like, crystalline crystals. With the development of the marginal and deepwater gas oil fields, multiphase flow was a better method to enhance the economic efficiency. However, hydrates may form in the high pressure and low temperature severe condition in the submarine pipeline. If the measures taken in the subsea pipelines are inappropriately, hydrate plug will happen in the multiphase pipeline flow. There are many problems of the traditional injecting thermodynamic inhibitors methods to prevent hydrate formation in pipeline, such as large required quantity, must be recovered and high operation cost. Thus, to develop new low dosage hydrate inhibitors, such as antiagglomerant, and to make them into application have been paid more and more attention in petroleum industry. The basic principle is to prevent hydrates from agglomerating together and to keep hydrates to transport in slurry status, which contains hydrates, oil and water [1,]. The cost of this hydrate slurry flow technology is lower, and the pollution is less. It has good application for the medium or small gas oil fields and the deepwater gas oil fields. So far, anti-agglomerant has been successfully applied in part of the gas oil fields by some international companies such as SHE, BP. Flowmeter Pump Condens or Compre ssor P mixing tank Cooling System Gas Cylinder Data Ac quistion System Circulating Bath Figure1 Hydrate slurry flow experimental loop U-bend This facility was the first experimental loop to study the hydrate slurry in China. The circulating amount of this experimental loop was 4 ton/hour, and the total volume of its reaction vessel reached 30. The total length of the loop was 0 m and the inner diameter of pipe was 5.4 mm. The operating pressure wa s 1~40 bar and the temperature limit wa s -10~+50 ºC. The characteristics of hydrate formation and hydrate slurry flow could be researched in this flow loop. The purity of Methane used in our experiments wa s greater than 99.9 %. The anti-agglomerant wa s developed by University of petroleum, Beijing and China National Offshore Oil Corporation (CNOOC) research center. It could prevent hydrates from growing to plug in the pipeline effectively. The mass percent of this antiagglomerant added in the experiments was % of the mass content of water. The oil phase was condensate oil obtained from the offshore gas oil fields. The physical properties of this condensate oil were listed in Table 1. The composition of the gas phase was 80 % of CH 4, 15% of C H 6 and 5 % of C 3 H 8. The liquid phase was mixture of J

3 condensate oil, water and anti-agglomerant. The water cut was changed, which was 5%, 10%, 15%, 0%, 5% and 30% respectively. Table 1. Physical properties of the oil Density (g/cm 3 ) Viscosity at 5 (mpa s) JZ0- condensate oil PRESSURE DROP CACUATION MODE FO R HYDRATE SURRY FOW Generally, the hydraulic calculation for hydrate slurry flow wa s based on the single-phase fluid calculated model. According to the flow loop experiment pressure drop and flow velocity data, the friction coefficient or liquid viscosity could be calculated. Thus, a model for predict the friction coefficient could be established, or a modified slurry viscosity model could be presented. Based on these models, the pressure drop of hydrate slurry flow could be predicted. However, the lack of experimental data made the accuracy of the models unsatisfactory. The limitation of Its application increased. The hydrate slurry wa s liquid-solid two-phase flow, so it was more reasonable to calculate pressure drop with the theories of liquid-solid two-phase flow. According to the experimental researches of the water-solid two-phase system, Gaessler (1967) [3] proposed a pressure drop calculated model, which wa s defined as follows: g V SS VS * VV S( V SS) P ( s1) S S (1) gcvs gd ( VS VSS) VSS gd The scale factor β depended on the solid fraction, the velocity of the solid phase, and the friction coefficient in the solid-phase sedimentary layer. For hydrate slurry flow, the hydrate slurry could be regarded as homogeneous fluid. There should not be a solid-phase sedimentary layer in the pipeline. Otherwise, it would plug the pipeline. Therefore, the first part in Eq. (1) was equaled to zero for hydrate slurry flow. The slip between the different phases was not considered for hydrate slurry flow, so the average velocities of liquid, solid and mixture phase were equal. It was important to calculate the friction coefficient of solid phase λ * S to define the second part of Eq. (1). Considered the shear-thinning behavior of the hydrate slurry, the friction coefficient of solid phase λ * S included not only the fraction between the particles and the pipe, but also included the influences of the changeable particles concentration and shear to the physical property of the hydrate slurry. Thus, Eq. (1) could be simplified as follows: P VM * ses S (1 ES) D () The density of the hydrate slurry was bet ween 0.8~1. g/cm 3. It depended on the type of the hydrate former gas and hydrate formation conditions. According to the density table of typical hydrate slurry, the density of the hydrate could be predicted. s Considered that water converted to hydrate was a volume expansion process, the volume of hydrate would expand 1.5 times of water for I hydrate. Therefore, the volume fraction of the hydrates wa s given by: E S 1.5E (3) W The liquid phase friction coefficient could be calculated by referring to the method used in homogeneous liquid. In order to calculate the pressure drop of the hydrate slurry in pipeline, all the parameters, except the solid phase friction coefficient λ * S, could be define based on the discussion mentioned above. The solid phase friction coefficient λ * S could be inverse calculated by Eq. (1) easily based on the experiment data, which was given by: * 1 PD S (1 ) ES SEE V m (4) It was found that there was a relationship among the solid phase friction coefficient, hydrate fraction and liquid phase Reynolds number, according to the analysis of the experiment data with the same experimental medium, which wa s written as follows:

4 E Re (5) * a b S S The values of a, b were constant, which related with the experiment medium. However, for the same experiment medium, it also related with flow conditions of pipelines. This paper regressed the values of a, b in different conditions listed in Table.. Table. The values of a, b in Eq. (5) Hydrate Condensate oil fraction a b Table showed that the solid phase friction coefficient decreases with the rise of the hydrate fraction and the Reynolds number, during the range of the initial water content under experiment. The values of a, b were still different with different hydrate fraction, even though the experiment medium wa s identical. After all the parameters discussed above were input to Eq. (), the pressure drop of hydrate slurry flow could be obtained. CACUATION EXAMPE It was important to choose the appropriate calculation formulas during the process of technological design in the offshore pipelines. In this case, Beggs & Brill model was used to calculate the liquid holdup and pressure drop in the hydrate slurry flow pipelines. The field data of a condensate oil pipeline was used to be an example to calculate the pressure drop in the hydrate slurry pipeline. The specific calculated conditions and calculated steps were described as follows: The relative density of the nature gas was 0.8. The densities of the condensate oil and water were kg/m 3 and 1000 kg/m 3, respectively. The viscosity of the oil phase was 1.3 mpa.s. The mass flow rate was kg/s and the whole length was m. The inner diameter of pipe was m. Firstly, the hydrate fraction along the pipeline should be simulated based on the above flow conditions by using OGA software, which was shown in Figure. Figure Hydrate fraction along the pipeline The Figure showed that water consumed completely at 5000 m away from the inlet of the pipeline under this flow conditions, because of the lower environment temperature. Based on the discussion mentioned above, the pressure drop calculation of this pipeline was divided into two parts. The first part of the pressure drop calculation was simulated by OGA software in the oil-gas-water multiphase flow from the inlet to the location, where water consumed completely. And the second part of the pressure drop calculation was predicted by the model mentioned above in gas-hydrate slurry multiphase flow with the length of m. The specific calculated steps for the pressure drop in the gasoil-hydrate slurry flow were described as follows: Flow rate. It was known that the inlet flow rate of the gas phase and oil phase were m 3 /h and 16.5 m 3 /h, respectively. The initial water cut wa s 5% and the flow rate of the water phase was m 3 /h. Pressure and temperature. The inlet pressure wa s 5.64 MPa and the inlet temperature was 13.6 ºC. Gas consumed. Generally, If 1 m 3 gas hydrate decomposed, 164 m 3 gas and 0.8 m 3 water wo uld be released in standard state. Therefore, 1 m 3 water wo uld consume about 05 m 3 gas in standard state to form hydrate, which was m 3 under the transportation state. Gas flow rate. After the water completely consumed at the initial condition, the residual gas

5 flow rate Q G under the transportation condition wa s 0.1 m 3 /s. Hydrate fraction and hydrate flow rate. The volume fraction of hydrate in liquid phase calculated by Eq. (3). The flow rate of hydrate slurry wa s summarized the flow rate of oil Q O and the expended water (1.5Q W ), which was calculate as m 3 /s. Superficial velocity. The superficial velocities of liquid phase V S and gas liquid V SG were m/s and m/s. iquid holdup. The inlet liquid holdup wa s calculated by the superficial velocities of liquid phase V S and gas liquid V SG wa s Beggs & Brill model to predict liquid holdup wa s recognized the most precise model for the low liquid gas-liquid horizontal multiphase flow. The liquid holdup calculated by this model was under this flow condition. The annular flow was predicted by the Beggs & Brill's method. Combined with the flow behaviors of annular flow, the corresponding pressure drop calculation model was described as follows. The schematic diagram of annular flow was shown in Figure 3. P V g V V Er Er S d P S Figure 3 Schematic diagram of annular flow Ignored the influence of the droplets entrainment by the gas, the momentum equation of the annular liquid was given by: A S D i S i A l g sin 0 (6) The momentum equation of the gas phase wa s given by: i A G i S i A Gg g sin 0 (7) Combined Eq. (6) and Eq. (7), the interfacial shear stress was eliminated. So the pressure drop calculation equation was written as follows: SD ( Al AGg)sin (8) A For horizontal steady flow, Eq. (8) wa s deduced as: D A S (9) The shear stress between the liquid and pipe wall wa s defined as follows: S V (10) 8 The pressure gradient of gas-hydrate slurry in annular flow also was pressure gradient of the liquid phase. The Reynolds number of the liquid phase should be calculated first. However, in annular flow, there was no relationship with the liquid Reynolds number and the liquid holdup. The liquid Reynolds was defined as follows: Re DV S (11) O Based on Eq. (5) with the initial water cut of 5%, the hydrate friction coefficient was calculated as follows: E R (1) S S e The liquid phase friction coefficient was described as follows: 1.4 Z C C 1 (13) Z 68 Re (14) C 10 ( 8 Z) (15)

6 k (16) D It was obvious that the liquid friction coefficient wa s relative to the liquid Reynolds number. In horizontal pipelines, the pressure drop gradient of annular flow included the pressure drop gradient caused by hydrate particles ΔP h and the pressure drop gradient caused by the flow of the oil phase ΔP o. Generally, the pressure drop gradient caused by gas-liquid accelerated flow was relatively less than others and was ignored. Ignored the fraction between gas phase and liquid phase, the liquid equivalent diameter was defined as follows: D A D D H D 4 DH (17) The pressure drop gradient caused by hydrate particle was defined as follows: E v E v P (18) h h S h H S D DH The pressure drop gradient caused by the flow of the oil phase wa s defined as follows: (1 E ) v (1 E ) v P (19) o h h S D DH Therefore, the pressure drop gradient in horizontal pipes in annular flow was given by: MPa. However, the assumed inlet pressure wa s 5.64 MPa. The inlet pressure should be assumed again and the calculated steps mentioned above should be repeated. Then, the inlet pressure of this pipe calculated wa s MPa, and the pressure drop was MPa. The OGA software was used to calculate the pressure drop of the first part, and the outlet pressure was kept as MPa. The other conditions were unchanged. The site of hydrate started to form would change, which should be recalculated by OGA software. Then repeated all the calculation mentioned above. Finally, it was concluded that the inlet pressure wa s 6.98 MPa and the fluid flow as oil-gas-waster multiphase flow from inlet to 5000 m, where water consumed completely. Then the fluid flow wa s gas-oil-hydrate slurry multiple phase flow. REFERENCES [1] Marita Wolden, Are und, Nita Oza, etc. Cold Flow Black Oil Slurry Transport of Suspended Hydrate and Wax Solids[A]. Proceedings of the Fifth International Conference on Gas Hydrates[C]. Trondheim, Norway. June 1-16, 005 [] I Qing-ping,YAO Hai-yuan, CHEN Guangjin. An Experimental Study on the Flow Characteristic of Hydrate Slurry Adding Antiagglomerant. Journal of Engineering [3] Thermophysics [J] (1): G.W.GOVIER, K.AZIZ. The Flow of Complex Mixture in Pipes (II) [M]. Beijing: Petroleum Industry Press. 1986: lv PEhh (1 Eh) l (0) DH sl l The Reynolds number of the oil phase was Based on the Table, the friction coefficient of the hydrates particles predicted was.48. According to the Eq. (13), The Fan-Ning friction coefficient of oil phase calculated wa s The total pressure drop gradient calculated was Pa/m. The pressure drop of gas-hydrate slurry in this condition was up to.575 MPa. Because the outlet pressure was kept as 5 MPa, the inlet pressure wa s