Drag reducing agents (DRA) are commonly used in the industry to increase pipelines' throughput by reducing the frictional pressure drop along the pipeline segment. The application of DRA has been primarily for single phase flow; however, a multiphase DRA was introduced and trial tested in an offshore gas-oil separation plant (GOSP) in Saudi Arabia to maximize the production of heavy oil from the GOSP without the need to flare the excess produced gas. This initiative was considered to avoid a major GOSP upgrade.

The objective of conducting the trial test is to determine the effectiveness of the DRA in maximizing GOSP fluid output utilizing the existing equipment and layout without having to go into flaring mode. To achieve this, a multiphase DRA was injected into the GOSP discharge line or trunkline to reduce pump discharge pressure by lowering line differential pressure.

This DRA was tested twice on the same facility with contradicting outcomes from each test mainly due to the different procedures used. Analysis of the inconsistency in the different test results sheds some light on the optimum environment of the multiphase DRA and on the best practices to be followed in a test procedure. This paper provides results and analysis of both DRA trial tests along with detailed procedures of each test. It elaborates on the causal factors of the discrepancies noticed, and the optimum environment for the multiphase DRA application.

Multiphase DRA Principles

Drag reducers are any material that reduces frictional pressure during fluid flow in a conduit or pipeline. Using drag reducing agents (DRAs) allows increased flow using the same amount of energy or decreased pressure drop for the same flow rate of fluid in pipelines.

With only parts per million levels in the pipeline fluid, drag reducing polymer subdues the turbulence in a pipeline, which can focus the hydraulic energy on moving the fluid down the pipeline rather than in a chaotic, random motion. DRAs in multiphase flow work primarily to reduce frictional pressure losses caused by the liquid fraction, and in some cases, change the flow regime to a more favorable flow regime.

Drag reducers injected in a single-phase system can cause substantial reductions in the frictional pressure losses; however, in multiphase systems reduction is lower due to the presence of gas and water. The friction losses caused by the free gas, especially cannot be drag reduced, therefore the reason why drag reduction can never be as high in multiphase systems as they are in single phase systems; only the liquid portion of the total frictional pressure losses in the system can be treated. The DRA effectiveness depends on many parameters such as oil viscosity, pipe diameter, gas and liquid velocities, oil composition, water cut, pipe inclination, DRA concentration, type of DRA and shear degradation of the DRA. Drag reducers are the most effective in pipelines with a gas-oil ratio (GOR) of less than 2,000 scf/bbl3.

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