CN104462684A - Method for predicting generation of hydrate in nature gas pipeline - Google Patents

Abstract

The invention discloses a method for predicting hydrate formation in natural gas pipelines, and belongs to the technical field of oil and gas storage and transportation engineering. With the continuous development of my country's economy, people's consumption demand for natural gas continues to increase. The industrial field needs to continuously improve the technology of natural gas transportation, and the natural gas hydrate formed in the process of pipeline transportation of natural gas is an important obstacle restricting the development of natural gas transportation technology. The invention solves the blindness in choosing the pipeline position when dealing with natural gas hydrate in previous projects. By analyzing the pressure and temperature at any point in the pipeline, the pressure-temperature curve of the pipeline is synthesized, and the pressure-temperature curve of the pipeline is compared with the hydrate phase equilibrium curve to find out the starting point of hydrate formation in the pipeline. The place where hydrate is generated in the natural gas transportation pipeline can be treated in a targeted manner, thus saving the amount of natural gas hydrate inhibitor used.

Description

A Prediction Method for Natural Gas Pipeline Hydrate Formation

technical field

The invention belongs to the technical field of oil and gas storage and transportation engineering, and in particular relates to a method for predicting the generation position of natural gas hydrate in a gas transmission pipeline by using computer software.

Background technique

During oil and natural gas extraction, transportation and processing, under relatively high pressure and low temperature conditions, certain components in natural gas and liquid water form ice-snow complexes, which are called natural gas hydrates. Since Davy first discovered gas hydrate in 1810, people have done a lot of research on the phase equilibrium conditions and formation mechanism of gas hydrate. At present, there are mainly three methods to form the phase equilibrium curve of natural gas hydrate: the first is to use the experimental method to obtain the required model according to the conditions when the hydrate is formed; the second is to use the actual field to establish the statistical thermodynamic model of natural gas hydrate The third is to test the pressure and temperature of hydrate formation in indoor experiments or on-site by sampling outdoors, and then numerically simulate to obtain a semi-empirical or empirical model. The phase equilibrium theoretical models of natural gas hydrates are mostly developed on the basis of the Van der Waals Platteeuw model. The theoretical basis is very reliable and has high calculation accuracy, but the calculation process is complicated and difficult to grasp and understand; and the most widely used The model is a statistical thermodynamic model, but it is difficult to be popularized; the graphical method and empirical semi-empirical model are simple to calculate and easy to use for working environments that do not require high precision, and they can especially play a good role in predicting.

During the long-distance pipeline transportation of natural gas, the blockage of natural gas hydrate is one of the important factors restricting the efficient and rapid development of natural gas pipeline transportation. The formation of hydrates in natural gas pipelines is related to natural conditions such as landform and climate, as well as the conditions of the pipeline itself. They are suspended in the gas flow and gradually gather together, adsorbed on the pipe wall, and eventually lead to pipeline blockage and gas transmission difficulties. In severe cases It will cause failure of separation equipment and instruments, which will affect the entire production process and cause immeasurable economic losses. Therefore, in the process of natural gas pipeline transportation, determining the hydrate formation conditions in the pipeline, predicting the initial hydrate formation location, and taking effective preventive measures are of great significance to ensure the safe operation of long-distance pipelines.

There are two main conditions for the formation of hydrates: ① natural gas must be at a relatively low temperature and high pressure; ② the temperature of natural gas must be lower than the water dew point temperature, and free water will appear. Therefore, when the composition of natural gas is constant, at a given pressure, there is a temperature corresponding to the formation of hydrates. If the temperature is equal to or lower than this temperature, hydrates will be formed, and if the temperature is higher than this temperature, hydrates will not be formed. As the pressure increases, the temperature corresponding to the formation of hydrates also increases continuously. At the same time, without free water, hydrates will not be produced.

In addition, there are three secondary conditions for the formation of hydrates: ①The flow rate of the gas is very fast, or the gas is in a violent disturbance such as pulsation and turbulence, and there is a crystallization center; ②The pressure is constantly changing; ③The natural gas contains volatile H2S or CO2 forming hydrates. At the same time, the critical temperature for the formation of hydrates is the highest temperature at which hydrates exist. No matter how high the pressure is above this temperature, hydrates will not form.

Contents of the invention

The purpose of the present invention is to provide a method for predicting hydrate formation in natural gas pipelines. The problem of blindness in the use of natural gas hydrate inhibitors is solved, and the automatic processing method of computer is realized, the use of natural gas hydrate inhibitors is reduced, and the degree of automation of the production process is improved.

Technical scheme of the present invention is as follows:

① The phase equilibrium conditions of natural gas hydrates have been deeply studied at home and abroad, and the pressure-temperature curves of the natural gas hydrate phase equilibrium have been formed. At the same time, the natural gas hydrate phase equilibrium P-T curves can be adjusted for specific natural gas components (L1);

② When the natural gas pipeline inlet temperature TQ, outlet temperature TZ, ambient temperature T0 of the pipeline along the line, thermal conductivity K of the pipeline along the line, density of natural gas, compression factor Z of natural gas, specific heat CP of natural gas, distance L of the pipeline, Under the condition of the pipe diameter D of the conveying pipeline, the temperature-length distribution curve (L2) along the pipeline can be obtained by programming in C language;

③Under the conditions of the inlet pressure PQ of the natural gas pipeline, the outlet pressure PZ, the density of natural gas, the compression factor Z of natural gas, the distance L of the transmission pipeline, the diameter D of the transmission pipeline, the roughness of the inner wall of the pipeline, and the friction resistance of the pipeline, The pressure-length distribution curve (L3) along the pipeline can be obtained by programming in C language;

④ After obtaining the T-L and P-L distribution curves along the pipeline, C language programming can be used to realize the pressure and temperature values at the same pipeline position in a Cartesian coordinate system in which the vertical axis is pressure P and the horizontal axis is temperature T It is shown that the pressure and temperature values at a certain point in the pipeline form a point in the Cartesian coordinate system, while the pressure and temperature on the entire pipeline form a P-T curve (L4) in the Cartesian coordinate system;

⑤ It is known that the lower right side of the gas hydrate phase equilibrium P-T curve (L1) is the region where no gas hydrate is formed, and the upper left side of the gas hydrate phase equilibrium P-T curve (L1) is the region where gas hydrate is formed , using computer technology to put the P-T curve (L4) on the entire pipeline and the gas hydrate phase equilibrium P-T curve (L1) in the same Cartesian coordinate system;

⑥Theoretically, there may be three situations: ⑴The P-T curve (L4) on the entire pipeline is on the lower right side of the natural gas hydrate phase equilibrium P-T curve (L1), then the entire pipeline will not produce hydrates; (2) If the P-T curve (L4) on the entire pipeline is on the upper left side of the natural gas hydrate phase equilibrium P-T curve (L1), then the entire pipeline will produce hydrates; (3) when the P-T curve (L4) on the entire pipeline is in line with the natural gas When the hydrate phase equilibrium P-T curves (L1) intersect, it is divided into two cases: (a) when the high-pressure section of the P-T curve (L4) on the entire pipeline is in the natural gas hydrate phase equilibrium P-T curve (L1 ) on the upper left side, hydrates will be produced from the starting point of the pipeline to the intersection point of the two curves, and the intersection point is the starting point where no hydrates will be produced; (b) when the high-pressure section of the P-T curve (L4) on the entire pipeline is in the natural gas On the lower right side of the hydrate phase equilibrium P-T curve (L1), no hydrate will be produced from the starting point of the pipeline to the intersection point of the two curves, and the intersection point is the starting point of hydrate generation;

⑦When the P-T curve (L4) on the entire pipeline intersects with the natural gas hydrate phase equilibrium P-T curve (L1), the pressure and temperature values at the intersection point are the pressure and temperature values along the pipeline, and then from the temperature along the pipeline - From the length distribution curve (L2) and the pressure-length distribution curve (L3) along the pipeline, the location of hydrate generation in the pipeline can be detected.

The phase equilibrium curve of gas hydrate is obtained by inductively drawing a graph based on a large number of gas field experimental data, while the traditional empirical formula is an approximate description of the graph, and we use graphs instead of formulas to improve the calculation accuracy.

The predecessors have already had mature software for the generation of the pressure distribution diagram and temperature distribution diagram on the gas pipeline. The software is used to analyze the specific location of hydrate formation in the pipeline by comparing the pressure-temperature distribution curve of the pipeline with the phase equilibrium diagram of natural gas hydrate.

Advantages of the invention

Using graphical method instead of complicated calculation formula, the phase equilibrium condition of natural gas hydrate is represented by pressure-temperature distribution diagram, and the pressure and temperature conditions along the pipeline are also represented by pressure-temperature distribution diagram, and the intersection point of the two diagrams is used The method replaces two complex calculation formulas for equal calculations. At the same time, the specific position along the pipeline can be directly investigated from the intersection point, while the calculation method can only approximate the position of the hydrate formation point through the discrete line method.

Description of drawings

Below in conjunction with accompanying drawing and embodiment the present invention is described in further detail:

Fig. 1 is a flow chart of the invention of a method for predicting hydrate formation in natural gas pipelines.

Detailed ways

The steps of a method for predicting hydrate formation in natural gas pipelines are as follows:

① The phase equilibrium conditions of natural gas hydrates have been deeply studied at home and abroad, and the pressure-temperature curves of the natural gas hydrate phase equilibrium have been formed. At the same time, the natural gas hydrate phase equilibrium P-T curves can be adjusted for specific natural gas components (L1);

② When the natural gas pipeline inlet temperature TQ, outlet temperature TZ, ambient temperature T0 of the pipeline along the line, thermal conductivity K of the pipeline along the line, density of natural gas, compression factor Z of natural gas, specific heat CP of natural gas, distance L of the pipeline, Under the condition of the pipe diameter D of the conveying pipeline, the temperature-length distribution curve (L2) along the pipeline can be obtained by programming in C language;

③Under the conditions of the inlet pressure PQ of the natural gas pipeline, the outlet pressure PZ, the density of natural gas, the compression factor Z of natural gas, the distance L of the transmission pipeline, the diameter D of the transmission pipeline, the roughness of the inner wall of the pipeline, and the friction resistance of the pipeline, The pressure-length distribution curve (L3) along the pipeline can be obtained by programming in C language;

④ After obtaining the T-L and P-L distribution curves along the pipeline, C language programming can be used to realize the pressure and temperature values at the same pipeline position in a Cartesian coordinate system in which the vertical axis is pressure P and the horizontal axis is temperature T It is shown that the pressure and temperature values at a certain point in the pipeline form a point in the Cartesian coordinate system, while the pressure and temperature on the entire pipeline form a P-T curve (L4) in the Cartesian coordinate system;

⑤ It is known that the lower right side of the gas hydrate phase equilibrium P-T curve (L1) is the region where no gas hydrate is formed, and the upper left side of the gas hydrate phase equilibrium P-T curve (L1) is the region where gas hydrate is formed , using computer technology to put the P-T curve (L4) on the entire pipeline and the gas hydrate phase equilibrium P-T curve (L1) in the same Cartesian coordinate system;

⑥Theoretically, there may be three situations: ⑴The P-T curve (L4) on the entire pipeline is on the lower right side of the natural gas hydrate phase equilibrium P-T curve (L1), then the entire pipeline will not produce hydrates; (2) If the P-T curve (L4) on the entire pipeline is on the upper left side of the natural gas hydrate phase equilibrium P-T curve (L1), then the entire pipeline will produce hydrates; (3) when the P-T curve (L4) on the entire pipeline is in line with the natural gas When the hydrate phase equilibrium P-T curves (L1) intersect, it is divided into two cases: (a) when the high-pressure section of the P-T curve (L4) on the entire pipeline is in the natural gas hydrate phase equilibrium P-T curve (L1 ) on the upper left side, hydrates will be produced from the starting point of the pipeline to the intersection point of the two curves, and the intersection point is the starting point where no hydrates will be produced; (b) when the high-pressure section of the P-T curve (L4) on the entire pipeline is in the natural gas On the lower right side of the hydrate phase equilibrium P-T curve (L1), no hydrate will be produced from the starting point of the pipeline to the intersection point of the two curves, and the intersection point is the starting point of hydrate generation;

⑦When the P-T curve (L4) on the entire pipeline intersects with the natural gas hydrate phase equilibrium P-T curve (L1), the pressure and temperature values at the intersection point are the pressure and temperature values along the pipeline, and then from the temperature along the pipeline - From the length distribution curve (L2) and the pressure-length distribution curve (L3) along the pipeline, the location of hydrate generation in the pipeline can be detected.

Therefore, first obtain the natural gas hydrate phase equilibrium curve (L1), then obtain the temperature distribution curve along the pipeline (L2), then obtain the pressure distribution curve along the pipeline (L3), and finally synthesize the entire pipeline pressure-temperature distribution curve (L4 ). Then compare the natural gas hydrate phase equilibrium curve (L1) with the entire pipeline pressure-temperature distribution curve (L4), and four results will be obtained as shown in the figure:

①If the graph (L4) is on the lower right side of the graph (L1), hydrate will not be produced.

②If the graph (L4) is on the upper left side of the graph (L1), hydrate will be produced.

③If the high-pressure section area of the graph (L4) is on the upper left side of the graph (L1), the intersection point is the starting point where the pipeline stops producing hydrates.

④ If the high-pressure section of the graph (L4) is on the lower right side of the graph (L1), the intersection point is the starting point for the pipeline to produce hydrates.

At the same time, according to the pressure and temperature at the intersection point, the specific location of the pipeline can be found from the temperature distribution curve (L2) along the pipeline and the pressure distribution curve (L3) along the pipeline.

Claims (1)

1. a Forecasting Methodology for natural gas line hydrate generation, it is characterized in that, the step of method is:

1. the domestic and international phase balance condition to gas hydrate has darker research, and define the pressure-temperature curve map of natural gas hydrate phase balance emulation, simultaneously can for concrete gas component adjustment natural gas hydrate phase balance emulation P-T curve map (L1);

2., under the condition of specific heat CP, the distance L of conveyance conduit of the compressibility factor Z of the density of the coefficient of heat conductivity K of the environment temperature T0 of known natural gas line inlet temperature TQ, outlet temperature TZ, pipeline along the line, pipeline along the line, rock gas, rock gas, rock gas, the caliber D of conveyance conduit, the method that C language can be utilized to programme obtains pipeline temperature along the line-length distribution curve figure (L2);

3., under the condition of the inner wall roughness of the caliber D of the distance L of the compressibility factor Z of the density of known natural gas line intake pressure PQ, top hole pressure PZ, rock gas, rock gas, conveyance conduit, conveyance conduit, pipeline, pipe friction, the method that C language can be utilized to programme obtains pipeline pressure along the line-length distribution curve figure (L3);

4. after obtaining pipeline T-L, P-L scatter chart along the line, the method that C language can be utilized to programme realizes the pressure on same pipeline position, temperature value to be pressure P at a longitudinal axis, to show in the transverse axis rectangular coordinate system that is temperature T, in pipeline, certain any pressure and temperature value define a point in rectangular coordinate system, and the pressure on whole piece pipeline and temperature then define a P-T curve map (L4) in rectangular coordinate system;

5. the known lower right side at natural gas hydrate phase balance emulation P-T curve map (L1) is the region not generating gas hydrate, be the region that gas hydrate generate in the upper left side of natural gas hydrate phase balance emulation P-T curve map (L1), utilize computer technology to be placed in same rectangular coordinate system by the P-T curve map (L4) on whole piece pipeline and natural gas hydrate phase balance emulation P-T curve map (L1);

6. three kinds of situations may be there are in theory: the P-T curve map (L4) (1) on whole piece pipeline is in the lower right side of natural gas hydrate phase balance emulation P-T curve map (L1), then whole piece pipeline all can not produce hydrate; (2) the P-T curve map (L4) on whole piece pipeline is in natural gas hydrate phase balance emulation P-T curve map (L1) upper left side, then whole piece pipeline all can produce hydrate; (3) when the P-T curve map (L4) on whole piece pipeline is crossing with natural gas hydrate phase balance emulation P-T curve map (L1), be divided into again two kinds of situations: (a) is when the high-pressure area section of the P-T curve map (L4) on whole piece pipeline is in natural gas hydrate phase balance emulation P-T curve map (L1) upper left side, then all can produce hydrate from the intersection point of pipeline starting point to two curve, intersection point is the starting point not producing hydrate; B () is when the high-pressure area section of the P-T curve map (L4) on whole piece pipeline is in natural gas hydrate phase balance emulation P-T curve map (L1) lower right side, then all can not produce hydrate from the intersection point of pipeline starting point to two curve, intersection point is the starting point producing hydrate;

7. when the P-T curve map (L4) on whole piece pipeline is crossing with natural gas hydrate phase balance emulation P-T curve map (L1), pressure on intersection point, temperature value are pipeline pressure and temp value along the line, then can find the position producing hydrate in pipeline from pipeline temperature-length distribution curve figure (L2) along the line, pipeline pressure-length distribution curve figure (L3) along the line.