Gathering and transportation seamless pipelines include the oil (gas) pipeline from a single oil (gas) well to the oil (gas) treatment plant (or processing device), as well as the pipeline from the treatment plant to the oil (gas) depot or the first station of the long-distance pipeline. Oil (gas) pipelines are managed by oilfields in my country. Gathering and transportation pipelines are characterized by small diameter and low pressure, and seamless pipes are mostly used as pipe materials.

Influencing factors

Coalbed methane fields are characterized by relatively pure coalbed methane (coalbed methane) components, low single-well production, dense well pattern distribution, and a large number of wellheads. The wellhead pressure is generally around 0.5 bar, which is relatively low. Common coalbed methane field gathering and transportation processes are generally low-pressure gas gathering, multi-well series connection, and centralized pressurization. Due to the low gathering and transportation pressure of coalbed methane fields, a large number of pipelines need to be built, and the investment cost is high. It is of great significance to study the influencing factors and laws of the design of coalbed methane gathering and transportation pipelines for the design and optimization of pipeline parameters and to reduce the overall investment of pipelines. In this paper, the multiphase flow simulation software OLGA is used to establish the hydraulic calculation model of the coalbed methane gathering and transportation pipeline, and simulate and analyze the influence of pipeline flow, pipe diameter, water content and inlet pressure on the pressure drop parameters of the pipeline.

1. Basic parameters

The inlet pressure of the pipeline gas gathering station is 0.5 bar, the wellhead flow rate is 150 kg/h, and the pipe diameter is 150 mm. The coalbed methane used in the simulation has a small amount of free water, and the water content is 0.50%.

2. Analysis of calculation results

2.1 Flow impact calculation results

The inlet pressure of the gas gathering station is set to 0.5 bar, and the total length of the pipeline is 5 kilometers. Since the maximum elevation difference of the pipeline is less than 100 m, it can be regarded as a horizontal pipeline of the gas gathering pipeline, and the pipe diameter is set to 150 mm. The output of a single well of coalbed methane is relatively low, therefore, this calculation selects 8 kinds of wellhead flow rates ranging from 50 kg/h to 400 kg/h, which can include the actual wellhead flow values of most coalbed methane fields.
When the pipe diameter and inlet pressure are fixed, the pipeline pressure drop increases with the increase of flow rate. When the flow rate is 50 kg/h, the pressure along the pipeline decreases from 0.509 bar to 0.5 bar, and the pressure drop is 0.18×10-5 bar/m; when the flow rate increases to 400 kg/h, the pressure along the pipeline decreases from 0.783 bar To 0.5 bar, the pressure drop is 5.66×10-5 bar/m. This is because under the condition of the same pipe diameter, the flow rate is proportional to the flow velocity, and the frictional resistance is proportional to the square of the flow velocity. Therefore, under the action of the inner wall resistance of the pipe, the gradual increase of the flow rate leads to an increasing pressure drop.

2.2 Calculation results of pipe diameter influence

The inlet pressure of the gas gathering station is set to 0.5 bar, the total length of the pipeline is 5 kilometers, the flow rate of the pipeline is 150 kg/h, and 8 kinds of pipe diameters are selected. Under the same flow conditions, the pipeline pressure drop decreases significantly with the increase of the pipeline. The pipe diameter has a great influence on the pressure drop of the pipeline. When the pipe diameter is 80 mm, the pressure along the pipeline decreases from 1.244 bar to 0.5 bar, and the pressure drop is 14.88×10bar/m; when the pipe diameter increases to 300 mm, the pressure along the pipeline The pressure is reduced from 0.502 bar to 0.5 bar, and the pressure drop is 0.04×10bar/m. Therefore, the increase in pipe diameter will significantly reduce the pressure drop in the pipe. The pressure drop of pipes with a diameter of 80 mm to 150 mm varies greatly, and the pressure drop of pipes with a diameter of 150 mm to 300 mm does not change much. Therefore, when the pipe diameter increases to a certain extent, increasing the pipe diameter has little effect on reducing the pressure drop of the pipe. This is because when the flow rate is constant, the larger the pipe diameter, the larger the cross-sectional area of the pipe, the smaller the flow velocity, the smaller the friction resistance of the pipe, and the smaller the change in pressure drop.

2.3 Effect of moisture content on calculation results

The inlet pressure of the gas gathering station is set to 0.5 bar, the total length of the pipeline is 5 kilometers, the flow rate of the pipeline is 150 kg/h, the diameter of the pipeline is set to 150 mm, and different water contents are selected for calculation. The change of water content in coalbed methane has no obvious influence on the change of pipeline pressure drop. When the moisture content is 0.20%, the pressure along the pipeline decreases from 0.558 7 bar to 0.5 bar, and the pressure drop is 1.175×10bar/m; when the moisture content increases to 1.00%, the pressure along the pipeline decreases from 0.558 6 bar to 0.5 bar , the pressure drop is 1.172×10bar/m.

2.4 Incoming pressure affects calculation results

The length of the pipeline is set to 5 km, the flow rate of the pipeline is 150 kg/h, and the diameter of the pipeline is set to 150 mm, and the calculation is carried out according to the different pressure conditions of the pipeline entering the station. When the inlet pressure is 0.5 bar, the pressure along the pipeline decreases from 0.558 7 bar to 0.5 bar, and the pressure drop is 1.175×10bar/m; when the inlet pressure increases to 0.6 bar, the pressure along the pipeline decreases from 0.601 9 bar to 0.6 bar, the pressure drop is 0.039×10bar/m. The pressure drop is more obvious. But as the inlet pressure continues to increase to 1 bar, the pressure along the pipeline decreases from 1.001 2bar to 1 bar, and the pressure drop is 0.024×10bar/m. The reduction in pressure drop is not significant.

3. Analysis of sensitive factors

According to the change degree of pipeline pressure gradient, the sensitivity of pipeline flow, pipe diameter, water content and inlet pressure to the influence of pipeline parameters is judged. When the range of pressure drop exceeds 5×10bar/m, the pipeline is considered to be sensitive to this parameter. From the above calculation results, the sensitivity of the parameters of the coalbed methane gas gathering pipeline can be obtained.

4. Conclusions and recommendations

(1) Pipe diameter and flow rate are the key factors affecting the parameter design of coalbed methane gathering and transportation pipelines, and pipe diameter has the greatest influence. The influence of pipe diameter should be considered in order to achieve the optimization goal in the design process.
(2) Although increasing the pipe diameter has a great influence on the calculation of the pipeline, when the pipe diameter increases to a certain value, continuing to increase the pipe diameter will no longer significantly affect the pressure drop of the pipeline. Therefore, in the design of the gathering and transportation pipeline, there is an optimal pipe diameter.
(3) The coalbed methane gas is relatively pure, and the water content is not high, and with the increase of water content, the pressure drop of the pipeline changes very little. However, when the water content is too high, the increase of liquid accumulation in the pipeline should be considered pipe drainage problems.