Objective The SED oilfield is a remote block operated by PetroChina in Central Asia, located approximately 20.2 km from the pre-dehydration station FWKO6 of another oilfield. The area features significant terrain undulations that necessitate the predominance of downward inclined pipeline sections along the transport route between the two locations. Conventional oilfield surface engineering practices for long-distance gas or liquid single-phase transmission require the construction of pre-dehydration stations to pressurize and transport gas and liquid separately. This approach results in substantial investments in gas-liquid pipeline surface engineering, along with elevated production, operation, and maintenance costs. This simulation analysis explores the feasibility of leveraging wellhead pressure for the long-distance mixed transportation of oil, gas, and water to another major oilfield for treatment.

Methods The geographic information system (GIS) was integrated with the GIS function of PIPESIM software to automatically capture the elevation along the pipeline route, greatly improving the modeling speed and calculation accuracy for the multiphase mixed transmission pipeline. A comparison of the calculation results from different formula combinations for horizontal and vertical pipes in PIPESIM revealed that the simulation calculations based on Mukherjee & Brill for horizontal pipes and other formula combinations for vertical pipes, yielded relative errors of less than 14.6％ for pressure drop in the oil-gas-water multiphase mixed pipeline, all indicating negative deviations. Additionally, relative errors of less than 5.3％ for temperature drop were observed, all indicating positive deviations. After introducing equivalent diameters to modify the multiphase transportation formulas for downward inclined pipeline sections, the relative errors for pressure drop calculations of the oil-gas-water mixed pipeline fell below 5.4％, while those for temperature drop calculations declined to below 4.9％.

Results The pre-dehydration station FWKO6 at the major oilfield currently operates at an actual load rate of only 42.7％. Based on the simulation results for the long-distance multiphase mixed transmission pipeline, the original design plan to build a pre-dehydration station and a pump station at the SED oilfield for oil-gas mixed transmission has been canceled. Instead, the revised plan leverages the residual wellhead pressure at the SED oilfield for the long-distance multiphase mixed transmission of oil, gas, and water, with subsequent processing of the fluids produced from the SED oilfield occurring at the existing pre-dehydration station FWKO6 at the major oilfield. This design reduces the investment in ground engineering construction for an oil and gas gathering, transportation, and processing system at the SED oilfield by 200×10⁴ US dollars.

Conclusion The calculation results for the long-distance downward-inclination-dominated oil-gas-water multiphase mixed transmission pipeline, based on Mukherjee & Brill for horizontal pipes and other formula combinations for vertical pipes, show small deviations from actual production parameters. These errors are significantly lower than those obtained using Beggs & Brill’s formula and Dukler II’s formula, which are traditionally understood to produce small relative errors. The research findings establish a basis for predicting the operational safety of downward-inclination-dominated multiphase mixed pipelines.