A calculation method for critical liquid-carrying velocity in multi-undulating pipe sections

Abstract

The present invention relates to a method for calculating the critical liquid-carrying flow rate of a multi-undulating pipe section, which mainly includes the following steps: first, according to the design parameters, operating parameters and longitudinal section data of the gathering and transportation pipeline, the design diameter, operating pressure, wellhead temperature and molar water content of the pipeline are determined, and basic data such as the maximum inclination angle of the pipeline, the total number of upslopes and the total height of the undulations are counted, and the critical liquid-carrying flow rate corresponding to the maximum inclination undulating pipe section is calculated, and the critical liquid-carrying flow rate is corrected by different actual critical liquid-carrying flow rate calculation formulas with the number of undulations of 8 as the boundary. The invention is beneficial in that it takes into account the multi-undulation characteristics of the ground gathering and transportation pipeline, determines the calculation method of the critical liquid-carrying flow rate under different undulation conditions, has a wide range of application, is verified by field data, has good accuracy, and can provide a theoretical basis for the prediction of liquid accumulation in the gathering and transportation pipeline under multi-undulating terrain conditions.

Description

Technical Field

[0001] The invention relates to the technical field of liquid accumulation in natural gas pipelines, and in particular to a method for calculating the critical liquid-carrying velocity of a gathering and transportation pipeline on undulating ground. Background Art

[0002] Yan'an Gas Field is located in the Loess Plateau in northern Shaanxi, with complex terrain, many undulating pipelines and high elevations. The ground gathering and transportation process uses wet gas transportation. During the production of gas wells, some free water is carried from the wellbore to the ground gathering and transportation pipeline. The transportation form of the ground gathering and transportation pipeline is gas-liquid two-phase flow. Under the conditions of undulating terrain, it is very easy to cause the liquid phase in the pipe to gather at the low point, which can cause slug flow in severe cases, resulting in large pressure drop in the pipeline, frequent tripping of the emergency shut-off valve at the wellhead, and corrosion of the low point of the pipeline.

[0003] At present, the research on the liquid carrying characteristics of gas-liquid two-phase flow is mainly focused on the exploitation of gas wells such as vertical wells and horizontal wells, and is mostly based on the assumptions of droplet model and liquid film model. However, there are few studies on the liquid carrying law of natural gas in ground gathering and transportation pipelines. Usually, the pipeline liquid accumulation problem is studied from the side, mainly including flow pattern prediction, liquid holdup and pressure drop calculation. Li Guohao established a calculation model for the critical gas velocity of liquid accumulation in wet gas pipelines based on the minimum cross-sectional shear stress criterion of stratified flow; Wang Baochun built an air-water two-phase flow undulating loop and used an ultrasonic Doppler velocimeter (UDV) to measure the critical liquid carrying velocity; Xing Peng used CFD software to simulate the flow state of multiphase flow in the undulating pipe section to determine the critical liquid carrying velocity, critical liquid holdup and critical unit pressure drop; Chen Jianlei analyzed the influencing factors of the critical liquid carrying velocity of the undulating wet gas pipeline, and established a calculation model for the critical liquid carrying gas velocity of the ground gathering and transportation pipeline based on the WALLIS liquid flooding empirical formula. However, the above models did not consider the influence of terrain fluctuations.

[0004] Therefore, determining the critical liquid-carrying velocity of surface gathering and transportation pipelines under undulating terrain conditions is of great significance for the prevention and control of liquid accumulation and long-term safe and stable operation of gas field surface gathering and transportation systems. Summary of the invention

[0005] The present invention aims to solve the above problems and proposes a method for calculating the critical liquid-carrying velocity of a pipeline taking into account the fluctuation of the pipeline shape.

[0006] The technical solution of the present invention is:

[0007] A method for calculating the critical liquid-carrying velocity of a multi-undulating pipe section comprises the following steps:

[0008] The gathering and transportation pipeline is divided according to the total number of ups and downs of the ups and downs section, with 8 times as the boundary; different actual critical liquid carrying flow rate calculation formulas are used respectively;

[0009] If the total number of ups and downs in the ups and downs section n≥8, the actual critical liquid carrying velocity is calculated as follows:

[0010]

[0011] Where: v cr is the actual critical liquid carrying velocity, m / s;

[0012] V θmax is the maximum inclination angle of the undulating pipe section θ max The corresponding critical liquid carrying velocity, m / s;

[0013] θ max is the maximum inclination angle of the undulating pipe section, °;

[0014] H is the total elevation of the undulating pipe section, km;

[0015] n is the total number of times the undulating pipe section tilts upward, times;

[0016] If the total number of ups and downs in the ups and downs section n is less than 8, the actual critical liquid carrying velocity is calculated as follows:

[0017]

[0018] in,

[0019] The maximum inclination angle θ of the undulating pipe section max The calculation process of the corresponding critical liquid carrying velocity is:

[0020]

[0021] Where: l is the density of liquid phase in wet natural gas, kg / m 3 ρ g is the gas phase density in wet natural gas, kg / m 3 ;

[0022] D is the diameter of the gathering and transportation pipeline, mm;

[0023] w is the molar water content of wet natural gas transported by the gathering and transportation pipeline, %.

[0024] in,

[0025] The total number of ups and downs of the ups and downs pipe section n is the total number of upslopes of all the ups and downs pipe sections along the gathering and transportation pipeline, calculated based on the terrain data along the gathering and transportation pipeline.

[0026] in,

[0027] The calculation process of the total elevation H of the undulating pipe section is:

[0028]

[0029] Where: i is the number of times the undulating pipe section tilts upward, times;

[0030] h i is the undulating elevation corresponding to the i-th undulating pipe section, m;

[0031] According to the terrain data along the gathering and transportation pipeline, the number of upslopes of all undulating pipe sections along the gathering and transportation pipeline is counted, the corresponding undulating elevations of all undulating pipe sections are calculated, and the sum is calculated.

[0032] in,

[0033] The maximum inclination angle θ of the undulating pipe section max The calculation process is:

[0034]

[0035] Where: l i is the pipeline length corresponding to the i-th undulating pipe section, m.

[0036] in,

[0037] The gas phase density ρ in wet natural gas g The calculation process is:

[0038]

[0039] Where: m is the mass of natural gas, kg; V is the volume flow rate of natural gas, m 3 / s;

[0040] P is the operating pressure, MPa; M is the molar mass of natural gas, kg / m 3 ;

[0041] R is the gas constant, which is 8.314KJ / (mol·K); T is the natural gas temperature, K;

[0042] t is the natural gas temperature, °C;

[0043] in:

[0044]

[0045] Where: M i is the molar mass of each component in natural gas, kg / m 3 ;

[0046] x i is the molar content of each component in natural gas, %.

[0047] in,

[0048] The calculation process of the molar water content w of the wet natural gas transported by the gathering and transportation pipeline is:

[0049]

[0050] in,

[0051]

[0052]

[0053] Where: N gas is the molar content of natural gas produced by the gas well, dimensionless;

[0054] N water is the molar content of water produced from the gas well, dimensionless;

[0055] m h20 is the mass flow rate of water produced from the gas well, kg / s; M h20 is the molar mass of water, kg / mol;

[0056] V g is the volume flow rate of natural gas under standard conditions, m 3 / s; V m is the molar volume of natural gas produced by the gas well, m 3 / mol.

[0057] The technical effects of the present invention are:

[0058] The benefit of the present invention lies in that it takes into account the multi-undulating characteristics of the ground gathering and transportation pipeline, determines the calculation method of the critical liquid-carrying flow rate under different undulating conditions, has a wide range of applications, has good accuracy as verified by field data, and can provide a theoretical basis for the prediction of liquid accumulation in gathering and transportation pipelines under undulating terrain conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 Schematic diagram of the calculation process of the critical liquid-carrying velocity in a multi-undulating pipe section.

[0060] Figure 2 This is a longitudinal section diagram along a gathering and transportation pipeline.

[0061] Figure 3 Schematic diagram of the evaluation of the critical liquid-carrying velocity model for multi-undulating pipe sections. DETAILED DESCRIPTION

[0062] A method for calculating the critical liquid-carrying velocity of a multi-undulating pipe section, taking a gathering and transportation pipeline in a well area of ​​Yan'an gas field as an example, includes the following steps:

[0063] Step 1: Collect the gas quality component data of the well area (see Table 1). The operating pressure P of the gathering and transportation pipeline is 5.6 MPa, and the natural gas temperature t is 20 °C. According to formula (6), the density of the natural gas under the operating condition (gas phase density ρg ) is 34.47kg / m 3 , the water content of natural gas is 0.6m 3 / ten thousand cubic meters, equivalent to molar water content w is 7.27%, output liquid density (liquid phase density ρ l ) is 900kg / m 3 .

[0064] Table 1 Natural gas composition

[0065]

[0066] Step 2: Collect the designed diameter, wall thickness and longitudinal section data of the gathering and transportation pipeline. The longitudinal section diagram of the gathering and transportation pipeline is shown in Figure 2 , determine the following basic design parameters:

[0067] (1) The diameter D of the gathering and transportation pipeline is 80 mm;

[0068] (2) According to the elevation h corresponding to each undulating pipe section i and pipeline length l i Calculate the corresponding inclination angle θ and determine the maximum inclination angle θ of the undulating pipe section according to formula (5) max is 22.5°;

[0069] (3) According to the elevation h corresponding to each undulating pipe section i and pipeline length l i The total elevation of all undulating sections along the gathering and transportation pipeline and the total number of upslopes were 682.06m and 15 times respectively.

[0070] Step 3: Use formula (3) to calculate the maximum inclination angle θ of the undulating pipe section max The critical liquid carrying velocity v corresponding to 22.5° θmax It is 4.53m / s.

[0071] Step 4: Since the total number of ups and downs of the ups and downs section is 15>8 times, the critical liquid-carrying velocity under the maximum inclination angle calculated above is corrected by formula (1). The corrected actual critical liquid-carrying velocity v cr It is 4.23m / s.

[0072] Step 5:

[0073] Calculate the actual flow rate of natural gas in the gathering and transportation pipeline. When the actual flow rate of natural gas is ≥ the actual critical liquid carrying velocity v cr When the actual flow rate of natural gas is <v cr It is predicted that the gathering and transportation pipelines will be seriously flooded.

[0074] Taking 7 gathering and transportation pipelines in a well area of ​​Yan'an gas field as an example, the density of natural gas under operating conditions (gas phase density ρg ) is 34.47kg / m 3 , molar water content w7.27%, output liquid density (liquid phase density ρ l ) is 900kg / m 3 .

[0075] Statistics of the diameter D of each gathering and transportation pipeline, the total elevation H of the undulating pipe section, the total number of upslopes n of the undulating pipe section, and the maximum inclination angle θ max and the corresponding critical liquid carrying velocity V θmax , actual critical liquid carrying velocity v cr Other basic parameters are shown in Table 2.

[0076] Table 2 Comparison of calculation results of critical liquid carrying velocity of different pipelines

[0077]

[0078] Through the model evaluation of 30 gas production pipelines under the jurisdiction of Yan 439 well area, the calculation results of the modified model are compared and analyzed with those of the OLGA two-fluid model. Figure 3 It can be seen that the calculated values ​​of the model are highly consistent with the experimental values, both reaching more than 90%. The prediction results show that 24 gas production pipelines in the well area fall in the liquid accumulation area, which is consistent with the actual situation of on-site operation. The critical liquid-carrying flow rate calculated by the model is reasonable and can guide the safe production of the surface gathering and transportation system of the Yan'an gas field.

Claims

1. A method for calculating the critical liquid-carrying velocity of a multi-undulating pipe section, characterized in that: The following steps are involved: The gathering and transportation pipeline is divided according to the total number of ups and downs of the ups and downs section, with 8 times as the boundary; different actual critical liquid carrying flow rate calculation formulas are used respectively; If the total number of ups and downs in the ups and downs section n≥8, the actual critical liquid carrying velocity is calculated as follows: Where: v cr is the actual critical liquid carrying velocity, m / s; V θmax is the maximum inclination angle of the undulating pipe section θ max The corresponding critical liquid carrying velocity, m / s; θ max is the maximum inclination angle of the undulating pipe section, °; H is the total elevation of the undulating pipe section, km; n is the total number of times the undulating pipe section tilts upward, times; If the total number of ups and downs in the ups and downs section n is less than 8, the actual critical liquid carrying velocity is calculated as follows:

2. The method for calculating the critical liquid-carrying velocity of a multi-undulating pipe section according to claim 1, characterized in that: The maximum inclination angle θ of the undulating pipe section max The calculation process of the corresponding critical liquid carrying velocity is: Where: l is the density of liquid phase in wet natural gas, kg / m 3 ; ρ g is the gas phase density in wet natural gas, kg / m 3 ; D is the diameter of the gathering and transportation pipeline, mm; w is the molar water content of wet natural gas transported by the gathering and transportation pipeline, %.

3. The method for calculating the critical liquid-carrying velocity of a multi-undulating pipe section according to claim 2, characterized in that: The total number of ups and downs of the ups and downs pipe section n is the total number of upslopes of all the ups and downs pipe sections along the gathering and transportation pipeline, calculated based on the terrain data along the gathering and transportation pipeline.

4. The method for calculating the critical liquid-carrying velocity of a multi-undulating pipe section according to claim 3, characterized in that: The calculation process of the total elevation H of the undulating pipe section is: Where: i is the number of times the undulating pipe section tilts upward, times; h i is the undulating elevation corresponding to the i-th undulating pipe section, m; According to the terrain data along the gathering and transportation pipeline, the number of upslopes of all undulating pipe sections along the gathering and transportation pipeline is counted, the corresponding undulating elevations of all undulating pipe sections are calculated, and the sum is calculated.

5. The method for calculating the critical liquid-carrying velocity of a multi-undulating pipe section according to claim 4, characterized in that: The maximum inclination angle θ of the undulating pipe section max The calculation process is: Where: l i is the pipeline length corresponding to the i-th undulating pipe section, m.

6. The method for calculating the critical liquid-carrying velocity of a multi-undulating pipe section according to claim 5, characterized in that: The gas phase density ρ in wet natural gas g The calculation process is: Where: P is the operating pressure, MPa; M is the molar mass of natural gas, kg / mol; R is the gas constant, which is 8.314KJ / (mol·K); t is the natural gas temperature, °C.

7. The method for calculating the critical liquid-carrying velocity of a multi-undulating pipe section according to claim 6, characterized in that: The calculation process of the molar water content w of the wet natural gas transported by the gathering and transportation pipeline is: in, Where: N gas is the molar content of natural gas produced by the gas well, dimensionless; N water is the molar content of water produced from the gas well, dimensionless; m h20 is the mass flow rate of water produced from the gas well, kg / s; M h20 is the molar mass of water, kg / mol; V g is the volume flow rate of natural gas under standard conditions, m 3 / s; V m is the molar volume of natural gas produced by the gas well, m 3 / mol.

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