CN112000916A - Oil pipeline axial wax deposition distribution rule judgment method based on mass flow change - Google Patents

Abstract

The invention provides a method for judging the distribution law of axial wax deposition in oil pipelines based on changes in mass flow, belonging to the field of oil pipeline transportation, comprising: selecting oil pipelines, measuring oil pipelines and testing relevant parameters of oil products; Temperature data; calculate the actual wall temperature T _wall when there is no deposition in the early stage of the oil pipeline; calculate the wax molecule diffusion coefficient D _wo,wall under the condition of the actual pipe wall temperature T _wall ; calculate the concentration difference of wax molecules at the oil wall C _oil -C _wall ; Calculate the mass flow of wax molecules J _wax , if the mass flow increases, the thickness of the deposition layer increases, otherwise the thickness of the deposition layer decreases. The method realizes instant online rapid assessment of wax deposition distribution along the pipeline, improves the efficiency of formulating the pigging plan, shortens the calculation time of the pigging cycle of the oil pipeline, and improves the operation safety of the oil pipeline.

Description

Method for judging distribution law of axial wax deposition in oil pipeline based on mass flow change

technical field

The invention belongs to the technical field of oil pipeline transportation, in particular to a method for judging the distribution law of axial wax deposition in oil pipelines based on mass flow changes.

Background technique

The occurrence of wax deposition in the pipeline transportation system will reduce the effective flow area of the pipeline, increase the transportation frictional resistance, increase the transportation pressure, and reduce the transportation capacity of the pipeline. The difficulty and risk of transportation cause huge economic losses. Therefore, pipe-flow wax deposition has always been the focus and hotspot of petroleum industry research at home and abroad.

At present, pigging on engineering is an effective method to alleviate the problem of excessively thick wax deposits in pipeline transportation. However, how to quickly and reasonably determine the pigging cycle is the key to the safe and economical operation of the pipeline. To determine a reasonable pigging period, first of all, it is necessary to fully grasp the thickness distribution of wax deposits along the pipeline, so it is particularly important to predict the thickness distribution of wax deposits along the pipeline. The existing wax deposition prediction methods mainly focus on the wax deposition kinetic model, that is, based on the molecular diffusion theory, considering the influence of temperature, flow rate and other factors in the wax deposition process, the relevant wax deposition prediction model is established.

Whether it is the actual pipeline transportation of waxy crude oil or the indoor and outdoor experimental simulation systems, operating temperature is an important factor affecting the formation and thickness of wax deposits, and it is also a hot issue in wax deposition research. Therefore, domestic and foreign scholars have conducted a large number of laboratory experiments on the effect of temperature on wax deposition, and have achieved some results. Although many scholars have studied the effect of temperature on the wax deposition law, there are still differences and no unified conclusion: some of the formulas believe that the increase of the oil wall temperature difference will lead to an increase in the amount of wax deposition, and some believe that the increase of the oil wall temperature difference will increase the amount of wax deposition. reduce. The main reasons for this situation are as follows: (1) Differences in experimental devices. At present, the research on wax deposition mainly adopts three experimental devices: cold finger, cold plate and ring channel. The ring channel device can approximately simulate the wax deposition under actual pipe flow conditions, but the cold finger and cold plate device are easy to operate, so The characteristics of less oil samples to be tested are also widely used; (2) the properties of oil products are different. Foreign researchers generally use light oil or simulated oil prepared in different proportions, while domestic scholars generally use on-site waxy crude oil with higher wax content; (3) The deposition time is different. Different experimenters set different deposition times according to oil products or experimental requirements, but the deposition process is a slowly changing process, so different deposition times will lead to different deposition amounts of wax; (4) Different experimental temperatures are selected. This phenomenon also leads to bias in engineering estimates of the distribution of wax deposits along oil pipelines.

Therefore, the present invention proposes a method for judging the distribution law of axial wax deposition in oil pipelines based on mass flow changes.

SUMMARY OF THE INVENTION

In order to overcome the above-mentioned deficiencies of the prior art, the present invention provides a method for judging the distribution law of axial wax deposition in an oil pipeline based on mass flow changes.

In order to achieve the above object, the present invention provides the following technical solutions:

A method for judging the distribution law of axial wax deposition in oil pipelines based on changes in mass flow includes the following steps:

Step 1. Select the oil pipeline, measure the relevant parameters of the oil pipeline and test oil;

Step 2. Combine the actual working conditions and collect the temperature data of the oil based on the on-site SCADA system;

Step 3, bring the collected temperature data into the heat transfer relation formula (1), calculate the actual wall surface temperature T _wall when deposition does not occur in the early stage of the oil pipeline;

In the formula, U—the total heat transfer coefficient of the test section, W/(m ² ·℃); α _in —the heat release coefficient of the oil flowing to the wall, W/(m ² ·℃); T _o —the temperature of the oil flow, °C; T _c - temperature of cooling liquid, °C;

Step 4, in conjunction with relevant parameters, calculate the wax molecule diffusion coefficient D _wo,wall under the actual wall temperature T _wall condition according to the actual wall temperature formula (2);

μ—油流在温度为T₀时的粘度值，Pa·s；

—蜡分子的平均相对分子量，ρ_w—蜡分子的平均密度，kg/m³；In the formula,

μ—viscosity value of oil flow at temperature T ₀ , Pa s;

-average relative molecular weight of wax molecules, _{ρw -average} density of wax molecules, kg/m ³ ;

Step 5, obtain the concentrations C _oil and C _wall of the wax molecules at the oil flow and the pipe wall according to the solubility curve of the oil product, and then calculate the concentration difference C _oil -C _{wall of the wax molecules at the oil wall} ;

Step 6, calculate the mass flow J _wax of wax molecules according to formula J _wax = D _{wo, wall} (C _oil -C _wall );

If the mass flow increases, the thickness of the deposited layer increases, otherwise the thickness of the deposited layer decreases.

Preferably, the acquisition of the actual wall temperature of the oil pipeline in the step 3 includes the following steps:

Step A. According to the classical molecular diffusion equation (3), the formation of wax deposition mainly depends on the molecular diffusion coefficient - the diffusion efficiency of wax molecules, the temperature concentration gradient (the slope of the solubility curve of wax in crude oil) - the deposition ability of wax molecules , temperature gradient - three factors of the diffusion kinetics of wax molecules;

—单位时间内扩散到管壁处的蜡分子质量，kg/s；ρ_w—蜡分子的平均密度，kg/m³；D_m—原油中蜡分子的扩散系数，m²/s；A—沉积面积，m²；

—蜡在原油中的溶解度系数，1/℃；

—径向温度梯度，℃/m；where:

—mass of wax molecules diffused to the pipe wall per unit time, kg/s; _{ρw —average} density of wax molecules, kg/m ³ ; D _m — diffusion coefficient of wax molecules in crude oil, m ² /s; A— Sedimentation area, m ² ;

- Solubility coefficient of wax in crude oil, 1/℃;

—Radial temperature gradient, °C/m;

Step B, formula (3) is simplified to obtain the mass flow J _wax of wax molecules,

In the formula, T _interface refers to the temperature at the interface, and interface refers to the interface;

Step C, the growth of the thickness of the wax deposition layer is determined by the convective diffusion mass flow J _wax of the wax molecules at the wall surface, and when calculating the mass flow before the deposition layer is formed, the deformation of formula (4) is simplified to formula (5) to obtain Calculate the mass flow when no deposition occurs,

J _wax =D _m,wall (C _oil -C _wall ) (5)

where D _m,wall is the diffusion coefficient of wax molecules at the actual wall temperature T _wall ;

Step D. Calculate by Hayduk-Minhas relational formula, as shown in formula (6),

—蜡分子的平均相对分子量；In the formula, C _oil is the concentration of wax molecules at the main body temperature of oil flow T ₀ ; C _wall is the concentration of wax molecules at the actual wall temperature T _wall ; μ is the viscosity value of oil flow when the temperature is T ₀ , Pa·s;

- the average relative molecular weight of the wax molecules;

Step E. In the calculation of heat transfer, the energy transfer of the test section is a steady-state process. The heat lost to the air in the test section is not considered. The heat lost by the oil flow is equal to the heat absorbed by the coolant, regardless of the oil flow. Frictional heat is generated in the flow process. From the heat balance relationship, the calculation formula of the total heat transfer coefficient of the test section can be obtained as follows:

In the formula, U—the total heat transfer coefficient of the test section, W/(m ² ·°C); α _in —the heat release coefficient of the oil flowing to the wall, W/(m ² ·°C); α _out —the outer wall of the inner tube to the cooling Heat release coefficient of liquid, W/(m ² ·℃); λ _p —thermal conductivity of pipe, W/(m·℃), thermal conductivity of steel pipe is between 46-50W/(m·℃); d _in - the inner diameter of the inner tube, m; d _out - the outer diameter of the inner tube, m;

The temperature gradient between the oil flow and the wall can be obtained according to the heat balance relationship of the test pipe section,

—油流与壁面沿径向的温度梯度，℃/m；T_o—油流的温度，℃；T_c—冷却液的温度，℃；λ_oil—油流的导热系数，W/(m·℃)；In the formula,

—The temperature gradient between the oil flow and the wall along the radial direction, °C/m; T _o —The temperature of the oil flow, °C; T _c —The temperature of the coolant, °C; λ _oil —The thermal conductivity of the oil flow, W/(m· °C);

The temperature at the wall can be calculated from the overall heat transfer coefficient of the test section:

In the formula, T _wall - the actual temperature at the wall, °C.

Preferably, the relevant parameters of the oil pipeline include pipe diameter and thermal conductivity; the relevant parameters of the oil product include density, molecular weight, solubility, and viscosity.

The method for judging the distribution law of axial wax deposition in oil pipelines based on mass flow changes provided by the present invention utilizes the classical molecular diffusion theory, based on the temperature data collected by the on-site SCADA system, and by calculating the mass flow of wax molecular diffusion, proposes how to judge the effect of temperature on deposition The method and steps of the influence of layer regularity can realize the instant online rapid assessment of wax deposition distribution along the pipeline, improve the efficiency of pigging plan formulation, shorten the calculation time of the pigging cycle of the oil pipeline, and improve the safety of the oil pipeline operation.

Description of drawings

1 is a flowchart of a method for judging the distribution law of axial wax deposition in an oil pipeline based on mass flow variation according to Embodiment 1 of the present invention;

Fig. 2 is the wax concentration-temperature curve of waxy crude oil;

Figure 3 shows the viscosity-temperature curve of waxy crude oil.

Detailed ways

In order to enable those skilled in the art to better understand the technical solutions of the present invention and implement them, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. The following examples are only used to illustrate the technical solutions of the present invention more clearly, and cannot be used to limit the protection scope of the present invention.

Example 1

The present invention provides a method for judging the distribution law of axial wax deposition in oil pipelines based on mass flow changes, as shown in FIG. 1 , including the following steps:

Step 1. Select the oil pipeline, measure the relevant parameters of the oil pipeline and test oil;

In this embodiment, the relevant parameters of the oil pipeline include pipe diameter and thermal conductivity; the relevant parameters of the oil product include density, molecular weight, solubility, and viscosity;

Step 2. Combine the actual working conditions and collect the temperature data of the oil based on the on-site SCADA system;

Step 3, bring the collected temperature data into the heat transfer relation formula (1), calculate the actual wall surface temperature T _wall when deposition does not occur in the early stage of the oil pipeline;

In the formula, U—the total heat transfer coefficient of the test section, W/(m ² ·℃); α _in —the heat release coefficient of the oil flowing to the wall, W/(m ² ·℃); T _o —the temperature of the oil flow, °C; T _c - temperature of cooling liquid, °C;

Step 4, in conjunction with relevant parameters, calculate the wax molecule diffusion coefficient D _wo,wall under the actual wall temperature T _wall condition according to the actual wall temperature formula (2);

μ—油流在温度为T₀时的粘度值，Pa·s；

—蜡分子的平均相对分子量，ρ_w—蜡分子的平均密度，kg/m³；In the formula,

μ—viscosity value of oil flow at temperature T ₀ , Pa s;

-average relative molecular weight of wax molecules, _{ρw -average} density of wax molecules, kg/m ³ ;

Step 5, obtain the concentrations C _oil and C _wall of the wax molecules at the oil flow and the pipe wall according to the solubility curve of the oil product, and then calculate the concentration difference C _oil -C _{wall of the wax molecules at the oil wall} ;

Step 6, calculate the mass flow J _wax of wax molecules according to formula J _wax = D _{wo, wall} (C _oil -C _wall );

If the mass flow increases, the thickness of the deposited layer increases, otherwise the thickness of the deposited layer decreases.

In the above calculation results, the maximum value of the mass flow J _wax corresponds to the maximum value of the thickness of the deposited layer. In the actual pipeline transportation, the temperature of the first station of the oil product is known, and the temperature distribution along the route is obtained through the on-site data acquisition system SCADA, and then the mass flow of wax molecules along the oil pipeline without sedimentation is obtained according to the above calculation process, and then from the Find out the temperature point or temperature zone corresponding to the maximum mass flow, and the corresponding position along the route is the position of the thickest sedimentary layer in the pipeline transportation process, and the corresponding pigging plan can be formulated according to the sedimentation amount at this point.

Further, in this embodiment, the acquisition of the actual wall temperature of the oil pipeline in step 3 includes the following steps:

Step A. According to the classical molecular diffusion equation (3), the formation of wax deposition mainly depends on the molecular diffusion coefficient - the diffusion efficiency of wax molecules, the temperature concentration gradient (the slope of the solubility curve of wax in crude oil) - the deposition ability of wax molecules , temperature gradient - three factors of the diffusion kinetics of wax molecules;

—单位时间内扩散到管壁处的蜡分子质量，kg/s；ρ_w—蜡分子的平均密度，kg/m³；D_m—原油中蜡分子的扩散系数，m²/s；A—沉积面积，m²；

—蜡在原油中的溶解度系数，1/℃；

—径向温度梯度，℃/m；where:

—mass of wax molecules diffused to the pipe wall per unit time, kg/s; _{ρw —average} density of wax molecules, kg/m ³ ; D _m — diffusion coefficient of wax molecules in crude oil, m ² /s; A— Sedimentation area, m ² ;

- Solubility coefficient of wax in crude oil, 1/℃;

—Radial temperature gradient, °C/m;

Step B, formula (3) is simplified to obtain the mass flow J _wax of wax molecules,

In the formula, T _interface refers to the temperature at the interface, and interface refers to the interface;

Step C, the growth of the thickness of the wax deposition layer is determined by the convective diffusion mass flow J _wax of the wax molecules at the wall surface, and when calculating the mass flow before the deposition layer is formed, the deformation of formula (4) is simplified to formula (5) to obtain Calculate the mass flow when no deposition occurs,

J _wax =D _m,wall (C _oil -C _wall ) (5)

where D _m,wall is the diffusion coefficient of wax molecules at the actual wall temperature T _wall ;

Step D. Calculate by Hayduk-Minhas relational formula, as shown in formula (6),

—蜡分子的平均相对分子量；In the formula, C _oil —the concentration of wax molecules at the main body temperature T _O of the oil stream, calculate the solubility curve of wax molecules in crude oil according to the DSC data of the waxy crude oil, as shown in Figure 2; C _wall —the actual wall temperature T The concentration of wax molecules under the _wall ; μ—viscosity value of oil flow when the temperature is T ₀ , Pa s, the viscosity curve of waxy crude oil is shown in Fig. 2, and the viscosity-temperature curve of waxy crude oil is shown in Fig. 3;

- the average relative molecular weight of the wax molecules;

Step E. In the calculation of heat transfer, the energy transfer of the test section is a steady-state process. The heat lost to the air in the test section is not considered. The heat lost by the oil flow is equal to the heat absorbed by the coolant, regardless of the oil flow. Frictional heat is generated in the flow process. From the heat balance relationship, the calculation formula of the total heat transfer coefficient of the test section can be obtained as follows:

In the formula, U—the total heat transfer coefficient of the test section, W/(m ² ·°C); α _in —the heat release coefficient of the oil flowing to the wall, W/(m ² ·°C); α _out —the outer wall of the inner tube to the cooling Heat release coefficient of liquid, W/(m ² ·℃); λ _p —thermal conductivity of pipe, W/(m·℃), thermal conductivity of steel pipe is between 46-50W/(m·℃); d _in - the inner diameter of the inner tube, m; d _out - the outer diameter of the inner tube, m;

The temperature gradient between the oil flow and the wall can be obtained according to the heat balance relationship of the test pipe section,

—油流与壁面沿径向的温度梯度，℃/m；T_o—油流的温度，℃；T_c—冷却液的温度，℃；λ_oil—油流的导热系数，W/(m·℃)；In the formula,

—The temperature gradient between the oil flow and the wall along the radial direction, °C/m; T _o —The temperature of the oil flow, °C; T _c —The temperature of the coolant, °C; λ _oil —The thermal conductivity of the oil flow, W/(m· °C);

The temperature at the wall can be calculated from the overall heat transfer coefficient of the test section:

where T _wall is the actual temperature at the wall, °C;

By calculating and comparing the variation of mass flow J _wax with temperature, the variation law of wax deposition layer thickness with temperature can be obtained.

The method for judging the distribution law of axial wax deposition in oil pipelines based on mass flow changes provided in this embodiment uses the classical molecular diffusion theory, based on the temperature data collected by the on-site SCADA system, and calculates the mass flow of wax molecular diffusion. The method and steps of the influence of sedimentary layer regularity can realize the real-time online rapid assessment of wax deposition distribution along the pipeline, improve the efficiency of pigging plan formulation, shorten the calculation time of the pigging cycle of the oil pipeline, and improve the safety of the oil pipeline operation.

The above-mentioned embodiments are only preferred specific embodiments of the present invention, and the protection scope of the present invention is not limited thereto. Any person skilled in the art can obviously obtain the simplicity of the technical solution within the technical scope disclosed in the present invention. Changes or equivalent replacements all belong to the protection scope of the present invention.

Claims (3)

1. The method for judging the axial wax deposition distribution rule of the oil pipeline based on mass flow change is characterized by comprising the following steps of:

step 1, selecting an oil pipeline, and measuring related parameters of the oil pipeline and an oil product to be tested;

step 2, collecting temperature data of the oil product based on an on-site SCADA system by combining with actual working condition conditions;

and 3, substituting the acquired temperature data into a heat transfer relation formula (1), and calculating the actual wall temperature T when no deposition occurs at the initial stage of the oil pipeline_wall；

In the formula, U-total heat transfer coefficient of the test section, W/(m)²·℃)；α_inCoefficient of heat release of oil flow to the wall, W/(m)²·℃)；T_o-temperature of the oil stream, ° c; t is_c-temperature of the cooling liquid, ° c;

step 4, calculating the actual wall temperature T according to the actual wall temperature formula (2) by combining the relevant parameters_wallDiffusion coefficient of wax molecules under conditions D_wo,wall；

In the formula (I), the compound is shown in the specification,

mu-oil stream at temperature T₀Viscosity value in time, Pa · s;

average relative molecular weight of wax molecules, p_wAverage density of wax molecules, kg/m³；

Step 5, obtaining the concentration C of the wax molecules at the oil flow and the pipe wall according to the solubility curve of the oil product_oil、C_wallAnd then calculating to obtain the concentration difference C of the wax molecules at the oil wall_oil－C_wall；

Step 6, according to a formula J_wax＝D_wo,wall(C_oil－C_wall) Calculation of the Mass flow J of wax molecules_wax；

If the mass flow increases, the deposited layer thickness increases, otherwise the deposited layer thickness decreases.

2. The method for judging the distribution rule of the axial wax deposition of the oil pipeline based on the mass flow change according to the claim 1, wherein the step 3 of obtaining the actual wall temperature of the oil pipeline comprises the following steps:

step A, according to a classical molecular diffusion equation (3), the formation of wax deposition mainly depends on three factors of a molecular diffusion coefficient, a temperature concentration gradient and a temperature gradient;

in the formula:

-the mass of wax molecules diffusing to the wall of the tube per unit time, kg/s; rho_wAverage density of wax molecules, kg/m³；D_mDiffusion coefficient of wax molecules in crude oil, m²S; a-area of deposition, m²；

-solubility coefficient of wax in crude oil, 1/° c;

-radial temperature gradient, c/m;

step B, simplifying the formula (3) to obtain the mass flow J of wax molecules_wax，

In the formula, T_interfaceRefers to the temperature at the interface, interface refers to the interface;

step C, increasing the thickness of the wax deposition layer through convection diffusion mass flow J of wax molecules at the wall surface_waxDetermining and calculating the mass flow when deposition does not occur by simplifying the deformation of the formula (4) to the formula (5) when calculating the mass flow before deposition is not formed,

J_wax＝D_m,wall(C_oil-C_wall) (5)

in the formula, D_m,wallIs the actual wall temperature T_wallLower wax molecular diffusion coefficient;

step D, calculating through a Hayduk-Minhas relational expression, as shown in a formula (6),

in the formula, C_oilOil stream bulk temperature T_OThe wax molecule concentration; c_wallActual wall temperature T_wallThe wax molecule concentration; mu-oil stream at temperature T₀Viscosity value in time, Pa · s;

-average relative molecular weight of wax molecules;

step E, in the calculation of heat transfer, the energy transfer of the test section is a steady-state process, the heat dissipated to the air by the test section is not considered, the heat dissipated by the oil flow is equal to the heat absorbed by the cooling liquid, the frictional heat generation in the flowing process of the oil flow is not considered, and the calculation formula of the total heat transfer coefficient of the test section is obtained by the heat balance relation and is as follows:

in the formula, U-total heat transfer coefficient of the test section, W/(m)²·℃)；α_inCoefficient of heat release of oil flow to the wall, W/(m)²·℃)；α_outCoefficient of heat release from the outer wall of the inner tube to the coolant, W/(m)²·℃)；λ_pThe thermal conductivity of the pipe, W/(m.DEG C.), the thermal conductivity of the steel pipe is between 46 and 50W/(m.DEG C); d_in-the internal pipe diameter of the internal pipe, m; d_out-the outer pipe diameter of the inner pipe, m;

the temperature gradient at the oil flow-wall can be determined from the thermal equilibrium relationship of the test tube section,

in the formula (I), the compound is shown in the specification,

-temperature gradient of the oil flow in radial direction to the wall surface, c/m; t is_o-temperature of the oil stream, ° c; t is_c-temperature of the cooling liquid, ° c; lambda [ alpha ]_oil-thermal conductivity of the oil stream, W/(m.degree.C.);

the temperature at the wall can be calculated from the total heat transfer coefficient of the test section:

in the formula, T_wall-actual temperature at the wall, ° c.

3. The method for judging the axial wax deposition distribution rule of the oil pipeline based on the mass flow change according to claim 1, wherein the relevant parameters of the oil pipeline comprise pipe diameter and heat conductivity coefficient; relevant parameters of the oil include density, molecular weight, solubility, viscosity.

Images